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**Correction of Astigmatism** 

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Howes FW. (2009). Chapter 5.3: Patient work-up for cataract surgery. In: *Ophthalmology*, 3rd

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Mandell R, Horner D. (1993). Alignment of videokeratoscopes. In: *An Atlas of Corneal Topography*, Sanders D, Cock D, eds, pp. 197-204, Slack Inc, Thorofare. Rabbetts RB. (1998). Chapter 20: Measurement of ocular dimensions, In: *Bennett & Rabbetts* 

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Shankar H, Taranath D, Santhirathelagan CH, Pesudovs K. (2008). Anterior segment

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automated measurements. J Cataract Refract Surg 34 (), pp. 103-113. Shirayama M, Wang L, Weikert MP, Koch DD. (2009). Comparison of corneal powers obtained from 4 different devices. *Am J Ophthalmol* 148 (4), pp. 528-535. Wilson S, Verity S, Conger D. (1992). Accuracy and precision of the corneal analysis system

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ring based corneal topography devices using surfaces with aspheric profiles. Invest

biometry with the Pentacam: Comprehensive assessment of repeatability of

EyeMap (Visioptic EH-270) Corneal Topographer on normal human corneas.

## **Cataract Surgery in Keratoconus with Irregular Astigmatism**

Jean-Louis Bourges

*Université Sorbonne Paris Cité, Paris Descartes, Faculté de médecine Assistance Publique-Hôpitaux de Paris, Hôtel-Dieu, Department of Ophthalmology France* 

## **1. Introduction**

Keratoconus generates highly irregular corneal astigmatism. While age is well known to slow down the progression of keratoconic ectasia and tends to fix the subsequent irregular astigmatism, the natural onset of cataract contributes to further decrease vision in already disabled patients.

To offer these patients an optimal strategy for cataract treatment, different options on how to manage irregular astigmatism of a keratoconic patient with surgical cataract have been proposed and are reviewed.

The stage of keratoconus and the history of the patient are both critical to orient the strategy. However, combined parameters should be considered for patients with highly irregular astigmatism due to keratoconus, to anticipate refractive results close to those obtained on patients with normal corneas. Contact lens equipment, intracorneal segment rings, lamellar or penetrating keratoplasties and, more generally, therapeutics which are usually applied for keratoconus, can be opportunely combined with the whole range of solutions offered by modern cataract surgery. Different methods of keratometry and formulas for intraocular lens (IOL) calculation have been proposed to improve as much as possible the predictability of the final refractive status, which still remains far from the standards of classical cataract surgery. So far, multifocal IOLs are still not suitable when associated with irregular corneal astigmatism, but toric intraocular lenses (IOL) could be selectively considered as an option in these patients.

## **2. Spherical intraocular lens (IOL) power calculation**

All formulas for intraocular lens calculation are mainly based on keratometric values. Precisely estimating the mean keratometry is therefore mandatory to define the closest IOL refractive power to the desired postoperative refraction. In keratoconus, however, standard deviations of differences between steepest and flattest keratometric reading vary greatly depending on the category of patients, from 1 up to more than 5 D for severe keratoconus, according to the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study (Zadnik et al., 1998). Moreover, once a clear corneal incision has been performed during the procedure, keratometric readings from keratoconic corneas may turn unstable after cataract surgery and evolve in an unforeseeable manner, even when patients have been operated on at a non-progressing preoperative state. The resulting change in corneal curvature should, though, be estimated prior to the surgery. Complex mathematical algorithms have been elaborated to predict lens power better in such difficult cases (Langenbucher et al., 2004), but they remain of restricted use in current clinical practice.

Finally, in most keratoconus, the corneal apex is decentered. For IOL power calculation, keratometric readings should therefore be taken in the central cornea, where the optical zone corresponds to the projection of the visual axis. How large the central optical zone should be is still to be clinically appreciated, as the balance observed between corneal curvatures of the two corresponding hemi-meridians depends on corneal apex decentration. Large optical zones create a significant hazard in IOL power calculation by overweighting high values taken from the apex of the ectasia, instead of averaging curvatures that are relevant for visual acuity in the optical axis.

Whatever method is eventually used to calculate the IOL power, the patient should be aware of the possible miscalculation induced by keratoconus on his/her intended postoperative refraction status.

#### **2.1 Formulas for IOL power calculation**

No 1 or 2 level of evidence-based medical data is available today to determine whether one particular calculation method will perform better than another for accuracy or reproducibility in IOL power estimation. Based on a retrospective analysis of a small cohort of nine patients (12 eyes) including various stages of keratoconus, Thebpatiphat *et al.*  (Thebpatiphat et al., 2007) observed that the SRK-II formula provided the more predictable results than SRK or SRK-T. Still, it remains unclear whether one formula should be preferred to another. For instance, the SRK-T formula is reputed to achieve better results than SRK-II on myopic eyes (Brandser et al., 1997, Sanders et al., 1990), while keratoconus and myopia are frequently associated (Ernst et al., 2011).

Besides the dilemma of calculation formula and keratometry, it is critical to use clinically relevant data for axial length, which are challenging to evaluate in keratoconus. The decentered apex of keratoconic corneas creates unpredictable parallax errors in the visual axis estimation. For this reason, the axial length measurement should be perfectly aligned with the manifest visual axis, and optical measurements are often preferred to other manual or ultrasound (US) techniques to ensure patients' fixation easily, although US achieves better predictability in myopic eyes with normal corneas (Pierro et al., 1991).

#### **2.2 Keratometry based on the manifest refraction**

Careful manifest refraction contributes to refine highly irregular keratometric values. The Jackson cylinder method at best refines the manifest axis and the optimal power of the cylinder. Ideally, the difference between the two keratometric values should match with the value of the manifest cylinder. However, the mean value of objective astigmatism based on measured keratometry (K2-K1) is usually reduced to more subjective values. It is not rare that the power for manifest cylinder is half measured values. Although favoring values that are clinically relevant, this method is somewhat empiric and lacks reproducibility. It should also be pointed out that accurate manifest refraction may not be possible in patients with cataract.

#### **2.3 Topography-based keratometry**

94 Astigmatism – Optics, Physiology and Management

surgery and evolve in an unforeseeable manner, even when patients have been operated on at a non-progressing preoperative state. The resulting change in corneal curvature should, though, be estimated prior to the surgery. Complex mathematical algorithms have been elaborated to predict lens power better in such difficult cases (Langenbucher et al., 2004),

Finally, in most keratoconus, the corneal apex is decentered. For IOL power calculation, keratometric readings should therefore be taken in the central cornea, where the optical zone corresponds to the projection of the visual axis. How large the central optical zone should be is still to be clinically appreciated, as the balance observed between corneal curvatures of the two corresponding hemi-meridians depends on corneal apex decentration. Large optical zones create a significant hazard in IOL power calculation by overweighting high values taken from the apex of the ectasia, instead of averaging curvatures that are

Whatever method is eventually used to calculate the IOL power, the patient should be aware of the possible miscalculation induced by keratoconus on his/her intended

No 1 or 2 level of evidence-based medical data is available today to determine whether one particular calculation method will perform better than another for accuracy or reproducibility in IOL power estimation. Based on a retrospective analysis of a small cohort of nine patients (12 eyes) including various stages of keratoconus, Thebpatiphat *et al.*  (Thebpatiphat et al., 2007) observed that the SRK-II formula provided the more predictable results than SRK or SRK-T. Still, it remains unclear whether one formula should be preferred to another. For instance, the SRK-T formula is reputed to achieve better results than SRK-II on myopic eyes (Brandser et al., 1997, Sanders et al., 1990), while keratoconus and myopia

Besides the dilemma of calculation formula and keratometry, it is critical to use clinically relevant data for axial length, which are challenging to evaluate in keratoconus. The decentered apex of keratoconic corneas creates unpredictable parallax errors in the visual axis estimation. For this reason, the axial length measurement should be perfectly aligned with the manifest visual axis, and optical measurements are often preferred to other manual or ultrasound (US) techniques to ensure patients' fixation easily, although US achieves

Careful manifest refraction contributes to refine highly irregular keratometric values. The Jackson cylinder method at best refines the manifest axis and the optimal power of the cylinder. Ideally, the difference between the two keratometric values should match with the value of the manifest cylinder. However, the mean value of objective astigmatism based on measured keratometry (K2-K1) is usually reduced to more subjective values. It is not rare that the power for manifest cylinder is half measured values. Although favoring values that are clinically relevant, this method is somewhat empiric and lacks reproducibility. It should also be pointed out that accurate manifest refraction may not be possible in patients with

better predictability in myopic eyes with normal corneas (Pierro et al., 1991).

but they remain of restricted use in current clinical practice.

relevant for visual acuity in the optical axis.

**2.1 Formulas for IOL power calculation** 

are frequently associated (Ernst et al., 2011).

**2.2 Keratometry based on the manifest refraction** 

cataract.

postoperative refraction status.

Elevation topographs take advantage of analyzing both anterior and posterior corneal curvatures to generate true net power maps (Kim et al., 2009). Irregular astigmatism, in keratoconus patients for instance, changes anterior curvature and posterior/anterior ratio. Standard IOL calculation formulas are not sufficiently accurate to predict IOL power. True net power maps provide significantly different values for estimating the corneal power within a specific corneal area by assuming paraxial imaging and combining two lenses separated by the central corneal thickness through Gaussian formulas (**Figure 1**). This feature is of particular interest in keratoconus, where the corneal thickness varies with a non-linear pattern from the center to the periphery of the cornea. The keratometric index is refined with elevation topographs (Ho et al., 2008). Moreover, where keratometers assume that keratometry derives from a constant corneal refractive power, elevation topographs measures the true power of the cornea (Eryildirim et al., 1994). They provide "optical" keratometries closer to the manifest refraction than specular values. This objective method is more reproducible to prevent IOL power miscalculation, although it should be stressed that elevation topographs have their own limits in reproducibility and their data are not interchangeable for analysis (Bourges et al., 2009, Quisling et al., 2006).

Fig. 1. **Refractive power and true net power maps of patient CYS, 63-y-o female with keratoconus**. Within a single acquisition, the elevation topograph (Pentacam, Oculus) provides both the anterior refractive power and the true net power of the cornea, which vary significantly for this keratoconic patient. Notice that with a simple topograph-based classification(Zadnik, 1998), the keratoconus could either be classified as severe (maximal K reading>52 D), referring to refractive power map, or mild, based on a true net power map (45 D<maximal K reading≤52 D).

More recently, Oculus released a new device associating Pentacam (Oculus) with the Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmics) to generate total corneal power maps (TCP). It uses ray tracing technology, which propagates incoming parallel rays and uses Snell's law to refract these rays through the anterior and posterior corneal surfaces and determine corneal power. In eyes that have irregular astigmatism, in the near future, the use of TCP values might be superior to corneal power calculations based on Gaussian formula and contribute to further refine accuracy in IOL calculation. This remains to be validated in the clinical setting.

## **2.4 Equivalent K-Readings (EKR)**

Equivalent K-Readings (EKR) are values provided by the Holladay Report and powered by the Pentacam (Oculus software). They are based on elevation topography maps. Equivalant K Readings correct keratometric values, focusing on the central cornea and balancing irregularities of the corneal curvature observed between steeper and flatter hemi-meridians. The accuracy of keratometric values thus obtained to calculate pseudophakic IOL on keratoconic cornea is still under investigation, but the preliminary results obtained on patients with irregular astigmatisms are encouraging. For example, **Figure 2** shows keratometric values obtained by various methods. The closest value from the manifest refraction is obtained with the topograph after EKR correction and is approximately half the value obtained using other means.

Fig. 2. **Keratometric readings obtained by automatic keratometer, by topography true net power map, and by topographic map after EKR correction on a single patient with keratoconus**. This case illustrates how close to the manifest values of the cylinder EKR values can be in keratoconus as compared to other keratometric readings.

## **3. Toric intraocular lens (IOL) implantation**

Although the indication of toric intraocular lens (T IOL) implantation in keratoconus is still not fully admitted, and does not belong to laboratory recommendations because of the irregularity of astigmatism, it appears to be an emerging practice. The first T IOLs were inserted in phakic eyes for purely refractive purposes on stable keratoconus (Alfonso et al., 2011, Budo et al., 2005, Kamburoglu et al., 2007, Kamiya et al., 2008, Kamiya et al., 2011, Navas et al., 2009, Sauder et al., 2003, Sokel et al., 1973). Toric IOLs have been considered for pseudophakic implantation since 2003 (SauderJonas, 2003) and their worthiness is currently under investigation (Jaimes et al., 2011). If the surgeon's choice is to place a toric IOL in a keratoconic eye, minimizing the corneal irregularity should be considered, as well as the eventuality of a possible keratoplasty. The latter generally rarely applies, as cataract occurs in the elderly and toric IOL should preferentially be proposed for stabilized keratoconus.

## **3.1 Reducing irregular astigmatism**

Reducing the irregularity of astigmatism in keratoconus is a key factor to achieve good results in predictability and accuracy for IOL pseudophakic implantation in cataract surgery. Various methods have been proposed. The most popular are intracorneal segment rings (ICSR) placement and corneal collagen cross-linking (CXL) with or without PTK. At the margin, relaxing incisions and conductive keratoplasty have also been tested.

Since Colin *et al.* proposed ICSR placement to reduce irregular astigmatism in keratoconus (Colin et al., 2000, Colin et al., 2001), the technique has become extremely popular among ophthalmic surgeons treating a wide range of ectatic corneal diseases (Dauwe et al., 2009, Ertan et al., 2007, Pinero et al., 2011). Despite that is unusual that irregular astigmatism of ectasia to be fully compensated by the procedure, it dramatically reduces differences in curvatures between opposite corneal hemi-meridians and usually makes toric IOL consecutive placement reasonably conceivable.

## **3.2 Case reports**

96 Astigmatism – Optics, Physiology and Management

(TCP). It uses ray tracing technology, which propagates incoming parallel rays and uses Snell's law to refract these rays through the anterior and posterior corneal surfaces and determine corneal power. In eyes that have irregular astigmatism, in the near future, the use of TCP values might be superior to corneal power calculations based on Gaussian formula and contribute to further refine accuracy in IOL calculation. This remains to be validated in

Equivalent K-Readings (EKR) are values provided by the Holladay Report and powered by the Pentacam (Oculus software). They are based on elevation topography maps. Equivalant K Readings correct keratometric values, focusing on the central cornea and balancing irregularities of the corneal curvature observed between steeper and flatter hemi-meridians. The accuracy of keratometric values thus obtained to calculate pseudophakic IOL on keratoconic cornea is still under investigation, but the preliminary results obtained on patients with irregular astigmatisms are encouraging. For example, **Figure 2** shows keratometric values obtained by various methods. The closest value from the manifest refraction is obtained with the topograph after EKR correction and is approximately half the

Fig. 2. **Keratometric readings obtained by automatic keratometer, by topography true net power map, and by topographic map after EKR correction on a single patient with keratoconus**. This case illustrates how close to the manifest values of the cylinder EKR

values can be in keratoconus as compared to other keratometric readings.

the clinical setting.

**2.4 Equivalent K-Readings (EKR)** 

value obtained using other means.

**Patient 1** was a 65-year-old women with combined keratoconus, high myopia, and a senile bilateral cataract. Her axial length was obtained by mode B echography and was measured at 31.40 mm OD and 31.10 mm OS. The IOL powers were calculated using various methods


Table 1. **IOL power calculation obtained with EKR for Patient 1.** The IOL power targeting a postoperative refraction of -3 D varied significantly for the patient, from 3 to 4.5 D and - 11.5 to 0.5 D respectively for OD and OS, depending on the keratometric value taken for calculation, on the IOL calculation formula and on the diameter of the central corneal area selected for EKR. K 1 and K2 = keratometric readings. Km = mean keratometry. 65%Km = median keratometry (most represented values).

Fig. 3. **Corneal topography (Pentacam Oculus) of Patient 1.** Her keratoconus was considered moderate for OD (> 45 D) and severe for OS (> 52D). The topography shows fairly preserved corneal thickness.

Fig. 3. **Corneal topography (Pentacam Oculus) of Patient 1.** Her keratoconus was considered moderate for OD (> 45 D) and severe for OS (> 52D). The topography shows

fairly preserved corneal thickness.

and the post-operative refraction was estimated based on each of them. The resulting powers of IOL are shown in **table 1**. Her elevation topographies, the latter of which is displayed in **Fig. 3**, demonstrated that corneal curvature was not progressing over a 2 years period. She underwent bilateral cataract surgery. The left eye was first operated on based on an automated keratometry with the SRK-T formula. The IOL power calculation for the right eye was based on the 4.5 mm EKR using SRK-T. The 4.5 mm zone was chosen considering less than 3 D irregularities between two opposite meridians. Thus, a 3D IOL was placed in OD and a -9 D IOL placed in OS, planning a -2.50 D post-operative refraction for OD and emetropia for OS. She finally achieved a -2 D OD postoperative spherical equivalent and +1.25 D for OS. The manifest refraction was stable two months after surgery.

The poor predictability observed for the left postoperative refraction could be explained both by the uncertainty drawn by high myopia regarding axial length calculation and by miscalculations. At the time of the first operation, EKRs were not available and could not be used for IOL power estimation. However, the resulting manifest refraction clearly demonstrated that videotopographic true net powers would have better predicted refractive outcome in this case.

In the opposite eye, EKR performed nicely to predict the appropriate IOL power in the right eye despite uncertainties in axial length measurement linked to high myopia.

## **4. Additional considerations**

#### **4.1 Optical aberrations induced by keratoconus**

No cataract surgery is able to reduce high order aberrations (HOA) induced by keratoconus significantly, so far. With appropriate light delivery settings, Light Adjustable Lenses (LAL) are attempt, at least in part, to address this issue on stable etasia, since this post-insertion method for the correction of refractive errors has been used successfully to correct astigmatism after cataract surgery (Hengerer et al., 2011, Lichtinger et al., 2011, Sandstedt et al., 2006, Schwartz et al., 2001).

Topo-guided phototherapeutic keratectomy (PTK) has also been proposed after corneal collagen cross-linking to both stabilize the corneal ectasia and reduce the residual ametropia and other relevant HOA (Krueger et al., 2011, Kymionis et al., 2011).

#### **4.2 Biomechanical outcome of the cornea after clear corneal incision**

Although keratoconus is usually stable at the age of cataract surgery, any corneal wound or surgical event is at risk for progression of irregular astigmatism. Subsequently, surgeons should customize their postoperative follow-up toward such patients. As keratoconic corneas demonstrate deteriorated Young modulus, it is advised not to rely on self-sealing clear corneal incisions but rather to perform sutured incision, not only the safety issue, but because it opportunely offers an additional chance to improve refraction and regularize astigmatism.

#### **5. Conclusion**

Intraocular lens power calculation still demonstrates better accuracy in eyes with regular optical disorders compared to keratoconic corneas. However, modern videotopographies and relevant formulas now significantly contribute to enhance the predictability of the manifest refraction after uneventful cataract surgery.

#### **6. References**


and relevant formulas now significantly contribute to enhance the predictability of the

Alfonso, J. F., Fernandez-Vega, L., Lisa, C. et al. (2011). Collagen copolymer toric posterior

Bourges, J. L., Alfonsi, N., Laliberte, J. F. et al. (2009). Average 3-dimensional models for the

Brandser, R., Haaskjold, E. and Drolsum, L. (1997). Accuracy of IOL calculation in cataract

Budo, C., Bartels, M. C. and van Rij, G. (2005). Implantation of Artisan toric phakic

Colin, J., Cochener, B., Savary, G. et al. (2001). INTACS inserts for treating keratoconus: one-

Dauwe, C., Touboul, D., Roberts, C. J. et al. (2009). Biomechanical and morphological

Ertan, A.Colin, J. (2007). Intracorneal rings for keratoconus and keratectasia. J Cataract

Eryildirim, A., Ozkan, T., Eryildirim, S. et al. (1994). Improving estimation of corneal

Hengerer, F. H., Hutz, W. W., Dick, H. B. et al. (2011). Combined correction of sphere and

Ho, J. D., Tsai, C. Y., Tsai, R. J. et al. (2008). Validity of the keratometric index: evaluation by

Kamburoglu, G., Ertan, A. and Bahadir, M. (2007). Implantation of Artisan toric phakic

Kamiya, K., Shimizu, K., Ando, W. et al. (2008). Phakic toric Implantable Collamer Lens

Kamiya, K., Shimizu, K., Kobashi, H. et al. Clinical outcomes of posterior chamber toric

surgery. Acta Ophthalmol Scand, Vol. 75, No. 2 (Apr), pp.162-5

rings. J Cataract Refract Surg, Vol. 26, No. 8 (Aug), pp.1117-22

year results. Ophthalmology, Vol. 108, No. 8 (Aug), pp.1409-14

biometric study. Eye Contact Lens, Vol. 37, No. 1 (Jan), pp.2-5

Refract Surg, Vol. 33, No. 7 (Jul), pp.1303-14

Refract Surg, Vol. 20, No. 2 (Mar), pp.129-31

Surg, Vol. 33, No. 3 (Mar), pp.528-30

(Jan), pp.137-45

Cataract Refract Surg, Vol. 37, No. 2 (Feb), pp.317-23

keratoconus. J Refract Surg, Vol. 24, No. 8 (Oct), pp.840-2

with keratoconus. J Refract Surg, Vol. 21, No. 3 (May-Jun), pp.218-22 Colin, J., Cochener, B., Savary, G. et al. (2000). Correcting keratoconus with intracorneal

keratoconus. J Cataract Refract Surg, Vol. 35, No. 10 (Oct), pp.1761-7 Ernst, B. J.Hsu, H. Y. (2011). Keratoconus association with axial myopia: a prospective

chamber phakic intraocular lens in eyes with keratoconus. J Cataract Refract Surg,

comparison of Orbscan II and Pentacam pachymetry maps in normal corneas.

intraocular lenses for the correction of astigmatism and spherical errors in patients

corneal response to placement of intrastromal corneal ring segments for

refractive power by measuring the posterior curvature of the cornea. J Cataract

astigmatism using the light-adjustable intraocular lens in eyes with axial myopia. J

the Pentacam rotating Scheimpflug camera. J Cataract Refract Surg, Vol. 34, No. 1

intraocular lens following Intacs in a patient with keratoconus. J Cataract Refract

implantation for the correction of high myopic astigmatism in eyes with

phakic intraocular lens implantation for the correction of high myopic astigmatism

manifest refraction after uneventful cataract surgery.

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**6. References** 

in eyes with keratoconus: 6-month follow-up, In: Graefes Arch Clin Exp Ophthalmol, Oct 16, Available from:

<http://www.springerlink.com/content/271mh27m11r84136/>


Zadnik, K., Barr, J. T., Edrington, T. B. et al. (1998). Baseline findings in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Invest Ophthalmol Vis Sci, Vol. 39, No. 13 (Dec), pp.2537-46

## **Aspheric Refractive Correction of Irregular Astimatism**

Massimo Camellin1 and Samuel Arba-Mosquera2,3

*1SEKAL Rovigo Microsurgery Centre, Rovigo, 2Grupo de Investigación de Cirugía Refractiva y Calidad de Visión, Instituto de Oftalmobiología Aplicada, University of Valladolid, Valladolid, 3SCHWIND eye-tech-solutions, Kleinostheim, 1Italy 2Spain 3Germany* 

## **1. Introduction**

102 Astigmatism – Optics, Physiology and Management

Zadnik, K., Barr, J. T., Edrington, T. B. et al. (1998). Baseline findings in the Collaborative

Vol. 39, No. 13 (Dec), pp.2537-46

Longitudinal Evaluation of Keratoconus (CLEK) Study. Invest Ophthalmol Vis Sci,

In irregular astigmatism, the two meridians may be located at something other than 90 degrees apart (principal meridians are not perpendicular); or there are more than two meridians.

Irregular astigmatism is that in which the curvature varies in different parts of the same meridian or in which refraction in successive meridians differs irregularly. Irregular astigmatism is associated with loss of vision.

Irregular astigmatism occurs when the orientation of the principal meridians changes from one point to another across the pupil, or when the amount of astigmatism changes from one point to another.

The further distinction of irregular astigmatism includes regularly or irregularly irregular astigmatism and relates to the presence of pattern recognition on computerized topography.

Irregularly irregular astigmatism is rough or uneven, and shows no recognizable pattern on topography.

Irregular astigmatism with defined pattern (macroirregular, or regularly irregular astigmatism) in which there is a steep or flat area of at least 2 mm of diameter, which is the primary cause of the astigmatism.

Irregular astigmatism with undefined pattern (microirregular, or irregularly irregular astigmatism) in which multiple irregularities; big and small, steep and flat, and profile maps are almost impossible to calculate.

Irregular astigmatism may appear in irregular but stable corneas (e.g., irregular scar surface), in which, cornea is irregular because of local geography, or in irregular but unstable corneas (biomechanical decompensation), in which, cornea is irregular because of global corneal weakness.

The astigmatism we will refer during this chapter is corneal astigmatism. In particular we will analyse corneal astigmatism and the effects on and influences from:

1. Aspheric Optical Zones and The Effective Optical Zone


## **2. Aspheric Optical Zones and the Effective Optical Zone**

The required ablation depth in corneal laser refractive surgery increases with the amount of ametropia to be corrected and the diameter of the optical zone selected1. Therefore, the smallest diameter optical zone should be used compatible with normal physiologic optics of the cornea2.

Complaints of ghosting, blur, haloes, glare, decreased contrast sensitivity, and vision disturbance3,4 have been documented with small optical zones, especially when the scotopic pupil dilates beyond the diameter of the surgical optical zone5, and these symptoms may be a source of less patient satisfaction6. This is supported by clinical findings on night vision with small ablation diameters7,8 as well as large pupil sizes5,8 and attempted correction9.

Laser refractive surgery generally reduces low order aberrations (defocus and astigmatism), yet high-order aberrations, particularly coma and spherical aberration, may be signicantly induced10,11.

In recent years, the increasing the size of the planned ablation zone and the use of new techniques to measure aberrations12 opened the possibility to correct, or at least reduce the induction, some of the high-order aberrations. Excimer laser refractive surgery has evolved from simple myopic ablations to the most sophisticated topography-guided13 and wavefront-driven14, either using wavefront measurements of the whole eye (obtained, e.g., by Hartmann-Shack wavefront sensors) or by using corneal topography-derived wavefront analyses15,16, customised ablation patterns. Special ablation patterns were designed to preserve the preoperative level of high-order aberrations17,18,19, if the best corrected visual acuity, in this patient, has been unaffected by the pre-existing aberrations20. Thus to compensate for the aberrations induction observed with other types of profile definitions21, some of those sources of aberrations are those ones related to the loss of efficiency of the laser ablation for non-normal incidence22,23,24.

Methods for determining functional optical zones have been used previously. Independently developed ray-tracing programs8,25 have been used to determined Functional Optical Zone after refractive surgery. A direct approach to measure Functional Optical Zone after refractive surgery has been proposed by manually determining the transition region between treated and untreated areas from corneal topography maps26.

Fig. 1 shows what it can be considered an intuitive definition of the optical zone for both myopic and hyperopic treatments. The actual definition of the optical zone reads "the part of the corneal ablation area that receives the full intended refractive correction" (Drum B. The Evolution of the Optical Zone in Corneal Refractive Surgery. 8th International Wavefront Congress, Santa Fe, USA; February 2007). However, operational definition of the Optical Zone consists of the part of the corneal ablation area that receives the treatment that is designed to produce the full intended refractive correction. Finally, Effective Optical Zone can be defined as the part of the corneal ablation area that actually conforms to the theoretical definition. However, the definition implies that the optical zone need not be circular.

The required ablation depth in corneal laser refractive surgery increases with the amount of ametropia to be corrected and the diameter of the optical zone selected1. Therefore, the smallest diameter optical zone should be used compatible with normal physiologic optics of

Complaints of ghosting, blur, haloes, glare, decreased contrast sensitivity, and vision disturbance3,4 have been documented with small optical zones, especially when the scotopic pupil dilates beyond the diameter of the surgical optical zone5, and these symptoms may be a source of less patient satisfaction6. This is supported by clinical findings on night vision with small ablation diameters7,8 as well as large pupil sizes5,8 and attempted correction9. Laser refractive surgery generally reduces low order aberrations (defocus and astigmatism), yet high-order aberrations, particularly coma and spherical aberration, may be signicantly

In recent years, the increasing the size of the planned ablation zone and the use of new techniques to measure aberrations12 opened the possibility to correct, or at least reduce the induction, some of the high-order aberrations. Excimer laser refractive surgery has evolved from simple myopic ablations to the most sophisticated topography-guided13 and wavefront-driven14, either using wavefront measurements of the whole eye (obtained, e.g., by Hartmann-Shack wavefront sensors) or by using corneal topography-derived wavefront analyses15,16, customised ablation patterns. Special ablation patterns were designed to preserve the preoperative level of high-order aberrations17,18,19, if the best corrected visual acuity, in this patient, has been unaffected by the pre-existing aberrations20. Thus to compensate for the aberrations induction observed with other types of profile definitions21, some of those sources of aberrations are those ones related to the loss of efficiency of the

Methods for determining functional optical zones have been used previously. Independently developed ray-tracing programs8,25 have been used to determined Functional Optical Zone after refractive surgery. A direct approach to measure Functional Optical Zone after refractive surgery has been proposed by manually determining the transition region between treated

Fig. 1 shows what it can be considered an intuitive definition of the optical zone for both myopic and hyperopic treatments. The actual definition of the optical zone reads "the part of the corneal ablation area that receives the full intended refractive correction" (Drum B. The Evolution of the Optical Zone in Corneal Refractive Surgery. 8th International Wavefront Congress, Santa Fe, USA; February 2007). However, operational definition of the Optical Zone consists of the part of the corneal ablation area that receives the treatment that is designed to produce the full intended refractive correction. Finally, Effective Optical Zone can be defined as the part of the corneal ablation area that actually conforms to the theoretical definition. However, the definition implies that the optical zone need not be

2. Correction of aberrations and refractive errors in irregular astigmatism

**2. Aspheric Optical Zones and the Effective Optical Zone** 

3. TransPRK

the cornea2.

induced10,11.

circular.

4. Corneal Wavefront Epi-LASEK

laser ablation for non-normal incidence22,23,24.

and untreated areas from corneal topography maps26.

5. Pathologic TransPRK

Fig. 1. Intuitive definition of the optical zone.

In order the analyse Effective Optical Zone in our treatments in a systematic way consistent with formal definitions, we decided to base our analysis upon previous knowledge. Since wavefront aberration describes properly optical quality, it seems adequate to use wavefront aberration for determining Effective Optical Zone. Since we were applying the analysis to corneal laser refractive surgery, it seems adequate to use corneal wavefront aberration for determining Effective Optical Zone. Since corneal refractive surgery increases wavefront aberration, it seems adequate to analyse the change of the corneal root-mean-square (RMS) for determining Effective Optical Zone. Since the most induced term is spherical aberration, it seems adequate to analyse the change of the corneal spherical aberration for determining Effective Optical Zone. Since AMARIS Aberration-Free profiles aim being neutral for High order aberration, it seems adequate to analyse the root-mean-square of the change of the corneal wavefront aberration for determining Effective Optical Zone.

The measurement technique actually imposes restrictions on optical zone size that may underestimate it for decentrations. On the other hand, data not fit by the Zernike polynomials up to the seventh radial order (36 Zernike coefficients). It is known that the residual irregularity of the cornea not fit by Zernike's may have a significant impact on visual quality. Ignoring this effect might bias the effective optical zone size determined leading to an overestimate that can be significant.

Uozato and Guyton2 were the first to calculate the optical zone area needed to obtain glarefree distance vision in emmetropia. They stated that, "for a patient to have a zone of glare-

Fig. 2. Formal definition of the functional optical zone.

free vision centered on the point of fixation, the optical zone of the cornea must be larger than the entrance pupil (apparent diameter of the pupil)." Not only must this optical zone be without scarring and irregularity, but it must also be of uniform refractive power.

Biomechanical changes after Myopic Astigmatism treatments contribute to an oblate contour, increasing spherical aberration and shrinking the effective optical zone. Healing response27, radial ablation efficiency losses28 and biomechanical effects29 all reduce the effective ablation in the outer portion of the nominal optical zone. These effects shrink the actual zone of full refractive correction, i.e., the effective optical zone. They also distort attempted cylindrical ablations by flattening the cornea along the astigmatic axis, introducing an unintended spherical correction component and reducing the cylindrical correction.

The shrinking effect is larger for major corrections, i.e. larger optical zones should be used for major corrections, but larger optical zones result in deeper and longer ablations increasing the potential risks of keratectasia30,31.

A similar approach was used by Tabernero et al.32, applied in a different way. They analysed directly on the cornea the functional optical zone in patients pre and postoperatively, instead applied to the differential map. They wanted to determine the Functional Optical Zone of the cornea, whereas we aimed to determine the Effective Optical Zone of the treatments. But essentially the methods are equivalent.

*RMS HOAb EOZ D* 0.375

free vision centered on the point of fixation, the optical zone of the cornea must be larger than the entrance pupil (apparent diameter of the pupil)." Not only must this optical zone be

Biomechanical changes after Myopic Astigmatism treatments contribute to an oblate contour, increasing spherical aberration and shrinking the effective optical zone. Healing response27, radial ablation efficiency losses28 and biomechanical effects29 all reduce the effective ablation in the outer portion of the nominal optical zone. These effects shrink the actual zone of full refractive correction, i.e., the effective optical zone. They also distort attempted cylindrical ablations by flattening the cornea along the astigmatic axis, introducing an unintended spherical correction component and reducing the cylindrical

The shrinking effect is larger for major corrections, i.e. larger optical zones should be used for major corrections, but larger optical zones result in deeper and longer ablations

A similar approach was used by Tabernero et al.32, applied in a different way. They analysed directly on the cornea the functional optical zone in patients pre and postoperatively, instead applied to the differential map. They wanted to determine the Functional Optical Zone of the cornea, whereas we aimed to determine the Effective Optical Zone of the

without scarring and irregularity, but it must also be of uniform refractive power.

Fig. 2. Formal definition of the functional optical zone.

increasing the potential risks of keratectasia30,31.

treatments. But essentially the methods are equivalent.

correction.

Maloney5 described the consequences of a decentered optical zone and discussed methods to ensure centering.

Effective Optical Zone correlates positively with Planned Optical Zone, declines steadily with increasing Defocus corrections; and Effective Optical Zone depends strongerly on Planned Optical Zone than on Spherical equivalent.

On average, and simplifying the relationship to only Effective Optical Zone and Planned Optical Zone we observed that Planned Optical Zone larger than 6.75 mm result in Effective Optical Zone, at least, as large as Planned Optical Zone. For Optical Zone smaller than 6.75 mm, a nomogram for Optical Zone can be applied.

Mok and Lee33 reported that larger optical zones decrease postoperative high-order aberrations. They found the measured high-order aberrations to be less in eyes with larger optical zones. Assessing the quality of vision (rather than the quality of the optical zone) after a refractive procedure is a separate issue. The relationship between pupil size and vision after refractive surgery is critically important and this relationship cannot be evaluated accurately with a measurement of aberrations through a predetermined aperture with an aberrometer. Pupil sizes vary considerably among patients depending on light level and age34. Mok and Lee have shown a strategy for planning optical zone size based on patient pupil size. However, an aberration analysis that takes into account variations in planned optical zone size may provide more insight as to the quality of the outcome obtained.

Partal and Manche35 using direct topographic readings observed over a large sample of eyes in moderate compound myopic astigmatism, a reduction from Planned Optical Zone of 6.50 mm to Effective Optical Zone of 6.00-mm. Noteworthy and opposed to our findings, they did not find a greater contraction of Effective Optical Zone for increasing myopic corrections.

Qazi et al.36 using a different approach observed over a sample of eyes similar to ours, a reduction from Planned Optical Zone of 6.50-mm to Effective Optical Zone of 5.61-mm.

To extend our methodology for the analysis of customised corrections can be quite simple if we consider that customised corrections in their intrinsic nature aim to reduce aberrations (either from the corna only, or from the complete ocular system) to a zero level. In this way, the corresponding formulations would be:

$$RMSho\_{CW} \left(EOZ\right) = 0.3D \tag{1}$$

$$\text{RMSlho}\_{\text{ON}} \text{(EOLZ)} = 0.3D \tag{2}$$

for corneal (CW) and ocular wavefront (OW) corrections respectively.

It is possible that the Effective Optical Zone could be larger than the Planned Optical Zone if it encompasses some portions of the Transition Zone, or even larger than the Total Ablation Zone. Although Planned Optical Zone, Transition Zone, and Total Ablation Zone are parameters defined by the laser treatment algorithms, Effective Optical Zone must be determined postoperatively (from the differences to the baseline) and may change with time because of healing and biomechanical effects. In the same way, it would be possible that the Functional Optical Zone were larger postoperatively than it was preoperatively, or that the Functional Optical Zone could be larger than the Planned Optical Zone or even than the Total Ablation Zone.

For our analysis, the concept of equivalent defocus (DEQ) has been used as a metric to minimise the differences in the Zernike coefficients due to different pupil sizes. Seiler et al.37 described an increase in spherical aberration with pupil dilation in corneas that have undergone photorefractive keratectomy but not in healthy corneas.

In conclusion, wavefront aberration can be a useful metric for the analysis of the effective optical zones of refractive treatments or for the analysis of functional optical zones of the cornea or the entire eye by setting appropriate limit values. In particular, the method seems to be a rigorous analysis accounting for any deviation from the attempted target for the wavefront aberration.

The profiles etched onto the cornea and their optical influence greatly differ between myopic and hyperopic corrections38,39. Biomechanical changes after Hyperopic Astigmatism treatments contribute to a hyperprolate contour, decreasing spherical aberration to negative values, and shrinking the effective optical zone. In our own experience (data submitted for publication), comparing Effective Optical Zone in myopic and hyperopic astigmatism, we observed that Effective Optical Zone is significantly smaller in hyperopic astigmatism compared to myopic astigmatism. In myopic astigmatism, we observed a mean Effective Optical Zone of 6.74-mm analyzed with the RMSho method and 6.42-mm analyzed with the RMS(HOAb) method, whereas in hyperopic astigmatism the values were 6.47-mm for the RMSho method and 5.67-mm analyzed with the RMS(HOAb) method. The mean relative ratio between Effective Optical Zone and Planned Optical Zone diameters was 0.97±0.06 for myopia and 0.90±0.12 for hyperopia, whereas the mean relative ratio between Effective Optical Zone and Planned Optical Zone surfaces was 0.95±0.12 for myopia and 0.81±0.26 for hyperopia. Determined Effective Optical Zone for hyperopic astigmatism were more scattered than the ones for myopic astigmatism. For equivalent corrections, mean Effective Optical Zone were smaller for hyperopia than for myopia by -8%±8% in diameter, or by -15%±13% in surface. As well, the impact of the defocus correction in reducing the size of the Effective Optical Zone is much stronger in hyperopia than in myopia.

For our analysis the threshold value of 0.3 D for determining Effective Optical Zone was arbitrarily chosen, since with simple spherical error, degradation of resolution begins for most people with errors of 0.3 D. If other value was used, the general conclusions derived in this study will still hold. However, the numerical values can be a bit larger for threshold values larger than 0.3 D, and smaller for values below 0.3 D. We have actually re-run the analyses for 0.2D and 0.5D thresholds, and found -18% smaller Effective Optical Zone and +10% larger Effective Optical Zone respectively.

Our search algorithm is an "increasing diameter" analysis, this ensures that the smallest Effective Optical Zone condition is found. Finally, our search was set to start from 4-mm upwards, i.e. 3.99 mm is the smallest Effective Optical Zone that could be found. We have done that because for very small analysis diameters, the Zernike fit seems to be less robust, mostly due to the decreasing sampling density within the unit circle.

The magnitude of astigmatism corrected could affect the diameter at which the EQ of RMSho is greater than 0.375D. For example, an eye with 1 DS/+3 D of hyperopia vs. 2.5 D of hyperopia would have different Effective Optical Zone and Functional Optical Zones based on the definitions, despite the same SE.

Although it is generally accepted and well known that the effective optical zone diameter is less than the intended optical zone diameter, the specific results in this study are possibly

For our analysis, the concept of equivalent defocus (DEQ) has been used as a metric to minimise the differences in the Zernike coefficients due to different pupil sizes. Seiler et al.37 described an increase in spherical aberration with pupil dilation in corneas that have

In conclusion, wavefront aberration can be a useful metric for the analysis of the effective optical zones of refractive treatments or for the analysis of functional optical zones of the cornea or the entire eye by setting appropriate limit values. In particular, the method seems to be a rigorous analysis accounting for any deviation from the attempted target for the

The profiles etched onto the cornea and their optical influence greatly differ between myopic and hyperopic corrections38,39. Biomechanical changes after Hyperopic Astigmatism treatments contribute to a hyperprolate contour, decreasing spherical aberration to negative values, and shrinking the effective optical zone. In our own experience (data submitted for publication), comparing Effective Optical Zone in myopic and hyperopic astigmatism, we observed that Effective Optical Zone is significantly smaller in hyperopic astigmatism compared to myopic astigmatism. In myopic astigmatism, we observed a mean Effective Optical Zone of 6.74-mm analyzed with the RMSho method and 6.42-mm analyzed with the RMS(HOAb) method, whereas in hyperopic astigmatism the values were 6.47-mm for the RMSho method and 5.67-mm analyzed with the RMS(HOAb) method. The mean relative ratio between Effective Optical Zone and Planned Optical Zone diameters was 0.97±0.06 for myopia and 0.90±0.12 for hyperopia, whereas the mean relative ratio between Effective Optical Zone and Planned Optical Zone surfaces was 0.95±0.12 for myopia and 0.81±0.26 for hyperopia. Determined Effective Optical Zone for hyperopic astigmatism were more scattered than the ones for myopic astigmatism. For equivalent corrections, mean Effective Optical Zone were smaller for hyperopia than for myopia by -8%±8% in diameter, or by -15%±13% in surface. As well, the impact of the defocus correction in reducing the size of the Effective Optical

For our analysis the threshold value of 0.3 D for determining Effective Optical Zone was arbitrarily chosen, since with simple spherical error, degradation of resolution begins for most people with errors of 0.3 D. If other value was used, the general conclusions derived in this study will still hold. However, the numerical values can be a bit larger for threshold values larger than 0.3 D, and smaller for values below 0.3 D. We have actually re-run the analyses for 0.2D and 0.5D thresholds, and found -18% smaller Effective Optical Zone and

Our search algorithm is an "increasing diameter" analysis, this ensures that the smallest Effective Optical Zone condition is found. Finally, our search was set to start from 4-mm upwards, i.e. 3.99 mm is the smallest Effective Optical Zone that could be found. We have done that because for very small analysis diameters, the Zernike fit seems to be less robust,

The magnitude of astigmatism corrected could affect the diameter at which the EQ of RMSho is greater than 0.375D. For example, an eye with 1 DS/+3 D of hyperopia vs. 2.5 D of hyperopia would have different Effective Optical Zone and Functional Optical Zones based

Although it is generally accepted and well known that the effective optical zone diameter is less than the intended optical zone diameter, the specific results in this study are possibly

undergone photorefractive keratectomy but not in healthy corneas.

Zone is much stronger in hyperopia than in myopia.

+10% larger Effective Optical Zone respectively.

on the definitions, despite the same SE.

mostly due to the decreasing sampling density within the unit circle.

wavefront aberration.

only relevant for patients operated with the actual specific excimer laser and software. In the event of wavefront-guided treatments, results may differ.

This study was focused on corneal aberrations, but the optical quality of the post-operative eyes depends on aberrations in the whole eye. We are confident that the conclusions will still hold when ocular aberrations are considered for the analysis.

## **3. Correction of aberrations and refractive errors in irregular astigmatism**

At the SEKAL Micro Chirurgia in Rovigo, we basically perform surface treatments in the form of LASEK40 or Epi-LASEK41 techniques42. These in combination with the SCHWIND AMARIS offer advantages particularly in the high safety of these methods because no preparation of a corneal flap takes place, thus no enduring weakening of the cornea, and the demonstrated accuracy of the AMARIS treatments, as well as the efficient control over corneal aberrations. The treatments are in nearly every case painless. Quality of vision is restored within 10 days and remains stable over the long-term.


Fig. 3. Classic induction of aberration may differ between stromal and surface ablations.

Fig. 3 shows the modelled induction of aberration observed clinically using Munnerlyn based ablation profiles. It can be seen, that this induction of aberrations may differ between stromal and surface ablations in a relevant way. The origins of these differences is probably multifold: The creation of a corneal flap weakens the corneal structure prior to the ablation, so that by the creation of this flap (irrespective with the technique with which it is created) may induced optical alterations of the cornea. Further, the ablation takes place at different corneal depths (more superficially in PRK and deeper in LASIK). Since the cornea is structured in well organized layers, but each layer has its own entity and layers differ their composition in depth, it may be inferred that different aberrations and aberration patterns may be induced. Taking this into account, specific compensation for different biomechanical effects in the different techniques shall be considered and implemented in the treatment devices.

## **3.1 Centration aids**

Mainly, two different centration references can be detected easily and measured with currently available technologies. Pupil centre may be the most extensively used centration method for several reasons. First, the pupil boundaries are the standard references observed by the eye-tracking devices. Moreover, the entrance pupil can be well represented by a circular or oval aperture, and these are the most common ablation areas. Centering on the pupil offers the opportunity to minimize the optical zone size. The pupil centre considered for a patient who fixates properly defines the line-of-sight, which is the reference axis recommended by the Optical Society of America for representing the wavefront aberration.

The corneal vertex in different modalities is the other major choice as the centration reference. In perfectly acquired topography, if the human optical system were truly coaxial, the corneal vertex would represent the corneal intercept of the visual axis. Despite the human optical system is not truly coaxial, the cornea is the main refractive surface. Thus, the corneal vertex represents a stable preferable morphologic reference. Ablations can be centered using the pupillary offset, the distance between the pupil centre and the normal corneal vertex, which corresponds to the angle between the line of sight and the visual axis.

For aspherical, or, in general, non-wavefront-guided treatments, in which the minimum patient data set (sphere, cylinder, and axis values) from the diagnosis is used, it is assumed that the patient's optical system is aberration-free or that those aberrations are not clinically relevant (otherwise a wavefront-guided treatment would have been planned). For those reasons, the most appropriate centering reference is the corneal vertex; modifying the corneal asphericity with an ablation profile neutral for aberrations, including loss of efficiency compensations. For wavefront-guided treatments, change in aberrations according to diagnosis measurements, a more comprehensive data set from the patient diagnosis is used, including the aberrations, because the aberrations maps are described for a reference system in the centre of the entrance pupil. The most appropriate centering reference is the entrance pupil as measured in the diagnosis.

Due to the smaller angle kappa associated with myopes compared with hyperopes, centration issues are less apparent43. However, angle kappa in myopes may be sufficiently large to show differences in results. A pupillary offset of 0.25 millimeters seems to be sufficiently large to be responsible for differences in aberrations.

We prefer using aberration-free treatments centred in the pupil in cases where the pupil centre differs less than 0.1 mm from the corneal vertex, and aberration-free treatments centred in the corneal vertex in cases where the pupil centre differs more than 0.1 mm and less than 0.5 mm from the corneal vertex.

We prefer using corneal wavefront for hyperopia in combination with astigmatism in cases where the pupil centre differs more than 0.5 mm from the centre of the astigmatism (= corneal vertex).

composition in depth, it may be inferred that different aberrations and aberration patterns may be induced. Taking this into account, specific compensation for different biomechanical effects in the different techniques shall be considered and implemented in the treatment

Mainly, two different centration references can be detected easily and measured with currently available technologies. Pupil centre may be the most extensively used centration method for several reasons. First, the pupil boundaries are the standard references observed by the eye-tracking devices. Moreover, the entrance pupil can be well represented by a circular or oval aperture, and these are the most common ablation areas. Centering on the pupil offers the opportunity to minimize the optical zone size. The pupil centre considered for a patient who fixates properly defines the line-of-sight, which is the reference axis recommended by the Optical Society of America for representing the

The corneal vertex in different modalities is the other major choice as the centration reference. In perfectly acquired topography, if the human optical system were truly coaxial, the corneal vertex would represent the corneal intercept of the visual axis. Despite the human optical system is not truly coaxial, the cornea is the main refractive surface. Thus, the corneal vertex represents a stable preferable morphologic reference. Ablations can be centered using the pupillary offset, the distance between the pupil centre and the normal corneal vertex, which corresponds to the angle between the line of sight and the

For aspherical, or, in general, non-wavefront-guided treatments, in which the minimum patient data set (sphere, cylinder, and axis values) from the diagnosis is used, it is assumed that the patient's optical system is aberration-free or that those aberrations are not clinically relevant (otherwise a wavefront-guided treatment would have been planned). For those reasons, the most appropriate centering reference is the corneal vertex; modifying the corneal asphericity with an ablation profile neutral for aberrations, including loss of efficiency compensations. For wavefront-guided treatments, change in aberrations according to diagnosis measurements, a more comprehensive data set from the patient diagnosis is used, including the aberrations, because the aberrations maps are described for a reference system in the centre of the entrance pupil. The most appropriate centering

Due to the smaller angle kappa associated with myopes compared with hyperopes, centration issues are less apparent43. However, angle kappa in myopes may be sufficiently large to show differences in results. A pupillary offset of 0.25 millimeters seems to be

We prefer using aberration-free treatments centred in the pupil in cases where the pupil centre differs less than 0.1 mm from the corneal vertex, and aberration-free treatments centred in the corneal vertex in cases where the pupil centre differs more than 0.1 mm and

We prefer using corneal wavefront for hyperopia in combination with astigmatism in cases where the pupil centre differs more than 0.5 mm from the centre of the astigmatism (=

reference is the entrance pupil as measured in the diagnosis.

sufficiently large to be responsible for differences in aberrations.

less than 0.5 mm from the corneal vertex.

corneal vertex).

devices.

**3.1 Centration aids** 

wavefront aberration.

visual axis.

Fig. 4. Principal reference axes of the human eye.

In this way, results and centration are improved.

#### **3.2 Optical zone**

Another important point for the control of aberrations is the appropriate selection of the optical zone. The use of large optical zones (with smart blending zones) (to avoid edge effects, especially in coma and spherical aberration) shall be considered.

In general, optical zone size shall be at least the size of the scotopic pupil diameter plus twice the pupil-vertex offset plus the eye-tracker resolution.

The optical zone should normally be at least 7 mm and correspond with the mesopic pupil diameter; we never go below 6.5 mm. In hyperopia, an optical zone of 7.5 mm is preferred in order to minimize the risk of regression and possible halos at night. In hyperopia, we never go below 7 mm. Whenever necessary, we protect the hinge with a spatula.

Differences between effective optical zone and planned optical zone are larger for smaller planned optical zone or larger corrections44. Planned optical zones >6.75 mm result in effective optical zones at least as large as planned optical zones. For optical zones <6.75 mm, a nomogram should be applied.

#### **3.3 Epi-LASEK technique**

The only difference in the Epi-LASEK technique compared with LASEK is the use of an epikeratome (nasal hinge) for separation of the epithelium. This is our preferred technique because the epithelium is easily separated, excellent hinge width is achieved and putting the epithelium back in place is easier than with LASEK or Epi-LASIK.

We use to apply LASEK for myopia up to -3 D, and Epi-LASEK for myopia up to -12 D, hyperopia up to +5 D, or astigmatism up to -6 D. The use of mitomycin C significantly decreases subepithelial haze45.

If the pupil can get larger than 8 mm, we place a limit on the treatment spectrum of -4 to +1.5 D.

In myopia, a central residual corneal thickness of at least 350μm including the epithelium must be considered. In hyperopia, the peripheral residual stromal thickness shall remain thicker than in the center.

The postoperative corneal curvature should be ≥ 32 D to ensure achievement of good vision quality. Additionally, the postoperative corneal curvature should be around 49 D, on the other side pay attention to preoperative very flat corneas (i.e. 40-42 D), because there might be a bad peripheral transition in case of high corrections (i.e. a significant step).

Following this rules, in a series of 20 consecutive patients treated for myopic astigmatism, we merely want to outline that both, the Spherical equivalent and the cylinder were significantly reduced to subclinical values at six months postoperatively (mean residual defocus refraction was -0.05±0.43 D (range -1.00 to +0.62 D) (p<.0001) and mean residual astigmatism magnitude 0.21±0.54 D (range, 0.00 to 1.50 D) (p<.001)) and that 90% of eyes were within ±0.50 D of the attempted correction. For these cases, preoperative corneal coma aberration (C[3,±1]) was 0.26±0.23 µm RMS, corneal spherical aberration (C[4,0]) (SphAb) was +0.28±0.15 µm, and corneal RMSho was 0.45±0.12 µm RMS. Postoperatively, corneal coma magnitude changed to 0.30±0.25 µm RMS (p<.05), corneal SphAb to +0.38±0.24 µm (p<.005), and corneal RMSho changed to 0.56±0.28 µm RMS (p<.01).

Another important point for the control of aberrations is the appropriate selection of the optical zone. The use of large optical zones (with smart blending zones) (to avoid edge

In general, optical zone size shall be at least the size of the scotopic pupil diameter plus

The optical zone should normally be at least 7 mm and correspond with the mesopic pupil diameter; we never go below 6.5 mm. In hyperopia, an optical zone of 7.5 mm is preferred in order to minimize the risk of regression and possible halos at night. In hyperopia, we never

Differences between effective optical zone and planned optical zone are larger for smaller planned optical zone or larger corrections44. Planned optical zones >6.75 mm result in effective optical zones at least as large as planned optical zones. For optical zones <6.75 mm,

The only difference in the Epi-LASEK technique compared with LASEK is the use of an epikeratome (nasal hinge) for separation of the epithelium. This is our preferred technique because the epithelium is easily separated, excellent hinge width is achieved and putting the

We use to apply LASEK for myopia up to -3 D, and Epi-LASEK for myopia up to -12 D, hyperopia up to +5 D, or astigmatism up to -6 D. The use of mitomycin C significantly

If the pupil can get larger than 8 mm, we place a limit on the treatment spectrum of -4 to

In myopia, a central residual corneal thickness of at least 350μm including the epithelium must be considered. In hyperopia, the peripheral residual stromal thickness shall remain

The postoperative corneal curvature should be ≥ 32 D to ensure achievement of good vision quality. Additionally, the postoperative corneal curvature should be around 49 D, on the other side pay attention to preoperative very flat corneas (i.e. 40-42 D), because there might

Following this rules, in a series of 20 consecutive patients treated for myopic astigmatism, we merely want to outline that both, the Spherical equivalent and the cylinder were significantly reduced to subclinical values at six months postoperatively (mean residual defocus refraction was -0.05±0.43 D (range -1.00 to +0.62 D) (p<.0001) and mean residual astigmatism magnitude 0.21±0.54 D (range, 0.00 to 1.50 D) (p<.001)) and that 90% of eyes were within ±0.50 D of the attempted correction. For these cases, preoperative corneal coma aberration (C[3,±1]) was 0.26±0.23 µm RMS, corneal spherical aberration (C[4,0]) (SphAb) was +0.28±0.15 µm, and corneal RMSho was 0.45±0.12 µm RMS. Postoperatively, corneal coma magnitude changed to 0.30±0.25 µm RMS (p<.05), corneal SphAb to +0.38±0.24 µm

be a bad peripheral transition in case of high corrections (i.e. a significant step).

(p<.005), and corneal RMSho changed to 0.56±0.28 µm RMS (p<.01).

effects, especially in coma and spherical aberration) shall be considered.

go below 7 mm. Whenever necessary, we protect the hinge with a spatula.

epithelium back in place is easier than with LASEK or Epi-LASIK.

twice the pupil-vertex offset plus the eye-tracker resolution.

In this way, results and centration are improved.

**3.2 Optical zone** 

a nomogram should be applied.

**3.3 Epi-LASEK technique** 

decreases subepithelial haze45.

thicker than in the center.

+1.5 D.

**Effective Optical Zone vs. Achieved Deofucs correction**

Fig. 5. Influence of the planned optical zone and defocus correction on the effective optical zone for myopia.

In hyperopia, in a similar case cohort of 20 consecutive patients, the Spherical equivalent and the cylinder were significantly reduced to subclinical values at six months postoperatively (mean residual defocus refraction was -0.04±0.44 D (range -1.00 to +0.63 D) (p<.0001) and mean residual astigmatism magnitude 0.22±0.55 D (range, 0.00 to 1.50 D) (p<.001)) and 90% of eyes were within ±0.50 D of the attempted correction. Preoperative corneal coma aberration (C[3,±1]) was 0.27±0.24 µm RMS, corneal spherical aberration (C[4,0]) (SphAb) was +0.29±0.16 µm, and corneal RMSho was 0.46±0.13 µm RMS. Postoperatively, corneal coma magnitude changed to 0.34±0.26 µm RMS (p<.05), corneal SphAb to -0.01±0.25 µm (p<.005), and corneal RMSho changed to 0.64±0.29 µm RMS (p<.01).

## **4. TransPRK**

In our own clinical experience, TransPRK in combination with corneal wavefront is the treatment of choice for retreatments after a radial keratotomy or corneal transplants46. At SEKAL, it is also used for haze, scarred corneal tissue and for keratoconus after crosslinking. In keratoconus, we aim at minimizing the ablation of tissue and smoothing the existing astigmatism.

The TransPRK technique makes sense in all cases where a difficult epithelial flap is expected or where the epithelium covers corneal irregularities of the stromal tissue.

Our approach is treating refracto-therapeutic problems by sequentially performing Corneal-Wavefront guided aspheric ablation profiles followed by a defined epithelial thickness profile, without masking fluid, performed to take away the rest of epithelium that can be present in the center or in the periphery of the treated area. The new evolution of the SCHWIND AMARIS has implemented this technique (since September 2009) in a single step procedure. Thanks to this improvement the procedure is now faster and the amount of epithelial tissue is optimized to avoid the myopic like correction (about -0.75D). This new single-step approach is treating refracto-therapeutic problems by superimposing a defined epithelial thickness profile (~55 µm at the centre and ~65 µm at the periphery 4 mm radially from centre) with aspheric ablation profiles. The system analytically creates a single ablation volume, which is then discretised into laser pulses sorted spatially and temporally in a pseudo-random fashion. Further, there is a pseudo-sequentialization of the Corneal-Wavefront guided and epithelial thickness profile components, but both components elapse without breaks.

The TransPRK (Transepithelial Photorefractive Keratectomy) with the SCHWIND AMARIS is an "all laser" version of surface treatments. Thereby the epithelium, which is the regenerative surface of the eye, is ablated by the laser system.

The TransPRK is the only surface treatment where the eye does not require contact with an instrument.

Furthermore, the epithelium is removed more precisely and more easily than through manual abrasion. Because the wound surface is smaller than, for example, with manual PRK, the healing process is shorter. Additionally, both the epithelium and the stroma are ablated in a single procedure. This shortens the overall treatment time significantly and minimises the risk of corneal dehydration.

Fig. 6. Comparison between the PRK and TransPRK profiles of the same treatment.

In our own clinical experience, TransPRK in combination with corneal wavefront is the treatment of choice for retreatments after a radial keratotomy or corneal transplants46. At SEKAL, it is also used for haze, scarred corneal tissue and for keratoconus after crosslinking. In keratoconus, we aim at minimizing the ablation of tissue and smoothing the

The TransPRK technique makes sense in all cases where a difficult epithelial flap is expected

Our approach is treating refracto-therapeutic problems by sequentially performing Corneal-Wavefront guided aspheric ablation profiles followed by a defined epithelial thickness profile, without masking fluid, performed to take away the rest of epithelium that can be present in the center or in the periphery of the treated area. The new evolution of the SCHWIND AMARIS has implemented this technique (since September 2009) in a single step procedure. Thanks to this improvement the procedure is now faster and the amount of epithelial tissue is optimized to avoid the myopic like correction (about -0.75D). This new single-step approach is treating refracto-therapeutic problems by superimposing a defined epithelial thickness profile (~55 µm at the centre and ~65 µm at the periphery 4 mm radially from centre) with aspheric ablation profiles. The system analytically creates a single ablation volume, which is then discretised into laser pulses sorted spatially and temporally in a pseudo-random fashion. Further, there is a pseudo-sequentialization of the Corneal-Wavefront guided and epithelial thickness profile components, but both components elapse

The TransPRK (Transepithelial Photorefractive Keratectomy) with the SCHWIND AMARIS is an "all laser" version of surface treatments. Thereby the epithelium, which is the

The TransPRK is the only surface treatment where the eye does not require contact with an

Furthermore, the epithelium is removed more precisely and more easily than through manual abrasion. Because the wound surface is smaller than, for example, with manual PRK, the healing process is shorter. Additionally, both the epithelium and the stroma are ablated in a single procedure. This shortens the overall treatment time significantly and

Fig. 6. Comparison between the PRK and TransPRK profiles of the same treatment.

or where the epithelium covers corneal irregularities of the stromal tissue.

regenerative surface of the eye, is ablated by the laser system.

minimises the risk of corneal dehydration.

**4. TransPRK** 

existing astigmatism.

without breaks.

instrument.

In our group of therapeutic patients, we had a remarkable decrease in corneal aberrations at 6-mm. Residual defocus averages about -0.6 D, and residual cylinder about 0.9 D, with 71% within 1.0 D of the target correction in defocus and astigmatism simultaneously. The mean decrease in astigmatism magnitude is 78%, representing a moderate undercorrection of astigmatism. Analyzing our mean postoperative defocus component, no hyperopic shift was observed.

In our experience, the percentage of eyes with UCVA of 20/32 or better is 60% and 20% achieve a UCVA of 20/20 or better. 47% of the eyes gain two or more lines of BSCVA, with 23% of the eyes showing an increase of more than four lines (especially after penetrating keratoplasty).

Despite large defocus, astigmatism and HOA magnitudes, high order aberrations are drastically reduced after simultaneous aspheric Corneal-Wavefront guided TransPRK profiles using SCHWIND AMARIS system among eyes with refracto-therapeutic problems after radial keratotomy or keratoplasty. The correction of the most relevant aberrations correlates well with intended values.

## **5. Corneal wavefront epi-LASEK**

The SCHWIND AMARIS offers different levels of aberrations control, in the form:


influence the measuring results. Mention is made that in this way forcing a fixed asphericity quotient (Q) on the eyes through the treatment is avoided. Instead, this strategy employs a dynamic postoperative expected asphericity quotient.



Table 1. Level of detail available at AMARIS.

We apply corneal wavefront based profiles for all retreatments in order to eliminate (or at least reduce) higher order aberrations.

## **6. Pathologic transPRK**

Refracto-therapeutic surgery with excimer laser has evolved from simple cylindric ablations47 to correct severe, disabling astigmatism after keratoplasty, although substantial regression limited its effectiveness, initially under the form of PRK48 and later on LASIK49. The correction of classical ametropias (myopia and astigmatism) after penetrating keratoplasty using PRK50 was less effective and less predictable than PRK for naturally occurring myopia and astigmatism, corneal haze and refractive regression were more prevalent. Refracto-therapeutic ablations evolved to the most sophisticated topographyguided customized corneal ablations for irregular corneal astigmatism after keratoplasty51,52.


> **Aberration-free Treatment**

Aspherical ablation profile Yes Yes Yes

aberrations (biomechanical effect) Yes Yes Yes

+ Cylinder Yes Yes Yes

aberrations Preserved Yes Yes

laser beam Yes Yes Yes

We apply corneal wavefront based profiles for all retreatments in order to eliminate (or at

Refracto-therapeutic surgery with excimer laser has evolved from simple cylindric ablations47 to correct severe, disabling astigmatism after keratoplasty, although substantial regression limited its effectiveness, initially under the form of PRK48 and later on LASIK49. The correction of classical ametropias (myopia and astigmatism) after penetrating keratoplasty using PRK50 was less effective and less predictable than PRK for naturally occurring myopia and astigmatism, corneal haze and refractive regression were more prevalent. Refracto-therapeutic ablations evolved to the most sophisticated topographyguided customized corneal ablations for irregular corneal astigmatism after keratoplasty51,52.

**Corneal Wavefront Treatment**

Yes Yes Yes

**Ocular Wavefront Treatment**

strategy employs a dynamic postoperative expected asphericity quotient.

America standard.

Simultaneous correction of Sphere

Correction of high order

Compensation by microkeratome usually induced aberrations (biomechanical effect)

Compensation of ablation induced

Compensation of energy loss of the

least reduce) higher order aberrations.

**6. Pathologic transPRK** 

Table 1. Level of detail available at AMARIS.

influence the measuring results. Mention is made that in this way forcing a fixed asphericity quotient (Q) on the eyes through the treatment is avoided. Instead, this

Fig. 7. CW guided Epi-LASEK ablation volume.

2-step procedures were proposed either in the form PTK + customized PRK or Flap + customized LASIK53 as well as wavefront-driven customised ablation patterns mostly using corneal topography-derived wavefront analyses54. Another proposal is the use of simultaneous customized transepithelial PRK + PTK55, which combines the refractive effect of the PRK with the therapeutic effect of a laser-assisted epithelium removal. A similar evolution can be observed in the resolution of residual myopia in eyes following radial keratotomies, with lower predictability56 using PRK associated with greater corneal haze and regression of refractive correction than in previously unoperated eyes, and encouraging early postoperative results of the correction by LASIK of a hyperopic shift after RK57. However, specific LASIK risks after RK exist in the form of uncontrolled shearing forces in lifting the corneal flap and extension of radial keratotomy wound dehiscence, which could lead to epithelial ingrowth and loss of best-corrected vision58.

Our approach is treating refracto-therapeutic problems by sequentially performing a Corneal-Wavefront guided aspheric ablation profiles followed by a defined epithelial thickness profile59 (60 to 70 µm depth in our case) in the form of a PTK, without masking fluid, performed to take away the rest of epithelium that can be present in the center or in the periphery of the treated area. For both sequential ablations, the system analytically creates coresponding ablation volumes, which are then discretised into laser pulses sorted spatially and temporally in a pseudo-random fashion.

The advantage of this ablation profile is that aims reducing the corneal wave aberration (within Optical Zone) together with the sphere and cylinder components. PTK removes an epithelial thickness profile that could be considered a little myopic like treatment (about

Fig. 8. CW guided TransPRK ablation volume.

1 D60), being epithelium thinner in the center59,61,62. We decided not to take into account this possible error because usually these corneas have a slight regression and this can lead to a compensating factor. We have thought to link the treatment directly over the epithelium as it acts as a smoothing agent. A rough stromal surface becomes smoother when epithelium re-grows but when we perform a topographical analysis we really assess the topography of the outer part (epithelium) and not the stromal surface.

We know epithelium is thicker in valleys and thinner in peaks and if we want a perfect result in these cases, we should perform a topographical analysis of the stromal surface, before ablation, but we know it is almost impossible due to the poor reflectivity.

The transepithelial approach allows a perfect correspondence between the topography and the cornea and the only error we can achieve is the difference in the photoablative rate between stroma and epithelial tissue63. This difference (~20% higher in epithelium) is partly compensated at the AMARIS, and, anyway, negligible for small amount of tissue. The need to perform a PTK (with parallel layer) is in order to remove the possible rest of epithelium in the centre or in the periphery of the treatment. Usually this PTK has a depth of 60-70 µm according to the ablation of the epithelium easy to check under microscope.

In our group of patients, we have remarkable decreases in corneal aberrations at 6-mm.

Analyzing our mean postoperative defocus component, no hyperopic shift was observed despite no nomogram adjustments or coupling effects were accounted for.

In our study, the percentage of eyes with UCVA of 20/32 or better was 60% and 20% had a UCVA of 20/20 or better. No single eye had a loss even one line of BSCVA, and sixteen eyes had gained two or more lines of BSCVA (p<.05), 8 eyes (23%) had an increase of more than

1 D60), being epithelium thinner in the center59,61,62. We decided not to take into account this possible error because usually these corneas have a slight regression and this can lead to a compensating factor. We have thought to link the treatment directly over the epithelium as it acts as a smoothing agent. A rough stromal surface becomes smoother when epithelium re-grows but when we perform a topographical analysis we really assess the topography of

We know epithelium is thicker in valleys and thinner in peaks and if we want a perfect result in these cases, we should perform a topographical analysis of the stromal surface,

The transepithelial approach allows a perfect correspondence between the topography and the cornea and the only error we can achieve is the difference in the photoablative rate between stroma and epithelial tissue63. This difference (~20% higher in epithelium) is partly compensated at the AMARIS, and, anyway, negligible for small amount of tissue. The need to perform a PTK (with parallel layer) is in order to remove the possible rest of epithelium in the centre or in the periphery of the treatment. Usually this PTK has a depth of 60-70 µm

before ablation, but we know it is almost impossible due to the poor reflectivity.

according to the ablation of the epithelium easy to check under microscope.

despite no nomogram adjustments or coupling effects were accounted for.

In our group of patients, we have remarkable decreases in corneal aberrations at 6-mm. Analyzing our mean postoperative defocus component, no hyperopic shift was observed

In our study, the percentage of eyes with UCVA of 20/32 or better was 60% and 20% had a UCVA of 20/20 or better. No single eye had a loss even one line of BSCVA, and sixteen eyes had gained two or more lines of BSCVA (p<.05), 8 eyes (23%) had an increase of more than

Fig. 8. CW guided TransPRK ablation volume.

the outer part (epithelium) and not the stromal surface.

four lines. Especially at the KP group, 14 of 18 treated eyes gained two or more lines of BSCVA after simultaneous aspheric CWg TransPRK + PTK with AMARIS.

As shown from the data presented, simultaneous aspheric Corneal-Wavefront guided TransPRK + PTK profiles using SCHWIND AMARIS system among eyes with refractotherapeutic problems after radial keratotomy or keratoplasty are safe and effective. This is an improvement relative to previous laser platforms, and may be related to the high-speed AMARIS system reduces variability from stromal hydration effects, which increase with time of treatment64,65.

Despite large defocus, astigmatism and HOA magnitudes, high order aberrations are drastically reduced after simultaneous aspheric Corneal-Wavefront guided TransPRK + PTK profiles using SCHWIND AMARIS system among eyes with refracto-therapeutic problems after radial keratotomy or keratoplasty. The correction of the most relevant aberrations correlates well with intended values. The refractive results in this clinical setting show a trend toward slight undercorrection in astigmatism, we believe that with some slight adjustment for astigmatic correction, the percentage of eyes within ±0.50 D of intended correction will increase significantly. The same applies for the difference observed between the rate of aberration correction. Although this small series of treated eyes does not allow for definitive conclusions or evidence-based statements, our preliminary results are promising.

In a previous study66 we have checked the stability of these corrections following RK and demonstrated that in three years, refraction and mean corneal power remained stable. We have seen that hyperopic shift following RK reaches a value and seems to stop at a certain time point. Our feeling is that probably the variation is partially due to an increased thickness of the cornea in the area of the incisions, as we have observed with a Scheimpflug camera, and partially due to an ectasia. The long term stability we have observed seems to claim for a major effect of the increased thickness as to explain the hyperopic shift. The reason of the increased thickness can be an augment in the hydration of the stroma.

As for corneal transplants, the problem is more complex since we know that astigmatism is a long term side effect and, for keratoconus, involves the inferior area of the cornea. We can suppose a massage effect of the upper lid that both in RK eyes and in transplant ones determines a bulging effect in the inferior area of the cornea. In both cases, when the defect is too high, it is not safe to approach with a laser treatment because we should ablate too much tissue weakening the structure. We must therefore take into account the thickness and the amount of tissue to remove before choosing a laser procedure. As a rule of thumb, we leave at least 300 µm of untouched stroma.

Haze is not a problem anymore thanks to MMC and only one case (with traces) following transplant have shown this problem in our analysis.

We are aware that a laser treatment on an unstable cornea could lead to a dehiscence in future but we must balance the advantage of a fast and easy procedure with the possible (not certain risk) variation of the astigmatism in the future.

We think these cases have no other solutions apart a new corneal transplant so this approach can be considered safer and faster.

Particularly, a delayed regression may occur at least up to one year, when MMC is used. Despite these limitations, simultaneous aspheric CWg TransPRK + PTK ablation profiles with AMARIS are superior to other ablation profiles for the correction of refractotherapeutic problems after radial keratotomy or keratoplasty.

In summary, simultaneous aspheric CWg TransPRK + PTK ablation profiles with AMARIS yield very good visual, optical, and refractive results for the correction of refractotherapeutic problems after radial keratotomy or keratoplasty. Preoperative astigmatisms are postoperatively reduced to subclinical values in the RK group and to moderate values in the KP group, with important reduction of High-Order-Aberrations (which influence contrast sensitivity). Simultaneous aspheric CWg TransPRK + PTK ablation profiles with AMARIS have, therefore, the potential to replace currently used algorithms for the correction of refracto-therapeutic problems after radial keratotomy or keratoplasty.

## **7. References**


In summary, simultaneous aspheric CWg TransPRK + PTK ablation profiles with AMARIS yield very good visual, optical, and refractive results for the correction of refractotherapeutic problems after radial keratotomy or keratoplasty. Preoperative astigmatisms are postoperatively reduced to subclinical values in the RK group and to moderate values in the KP group, with important reduction of High-Order-Aberrations (which influence contrast sensitivity). Simultaneous aspheric CWg TransPRK + PTK ablation profiles with AMARIS have, therefore, the potential to replace currently used algorithms for the correction of

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## **Treating Mixed Astigmatism – A Theoretical Comparison and Guideline for Combined Ablation Strategies and Wavefront Ablation**

Diego de Ortueta1, Samuel Arba Mosquera2 and Christoph Haecker3 *1Medical Director Augenlaserzentrum Recklinghausen, Consultant AURELIOS Augenzentrum, 2Schwind Eye-Tech Solutions, Kleinhostheim 3Independent Physiscist* 

*Germany* 

## **1. Introduction**

124 Astigmatism – Optics, Physiology and Management

[66] Camellin M. Lasek & Asa History Technique Long-term Results. Fabiano Editore,

The goal of laser refractive surgery is to achieve predictable and stable correction of myopia, hyperopia, and astigmatism. New, sophisticated diagnostic instruments such as topographers and aberrometers offer potential for improved results in terms of treatment efficiency and visual quality.( MaRae t al. 2000, Seiler et al. 2000, Manns et al. 2002, Mrochen et al. 2004)

Many articles have been published concerning laser correction of myopia and hyperopia with and without astigmatism, but few dealing with mixed astigmatism (Chayet et al 1998, Chayet et al 2001, Hasabla et al. 2003, Albarran-Diego et al. 2004, De Ortueta&Haecker 2008, Stonecipher et al. 2010). In the late 90's Chayet (Chayet et al 1998, Chayet et al 2001), and Vinciguerra (Vincinguerra et al. 1999) published toric ablation techniques, which apply a myopic cylinder and a hyperopic cylinder (90 degrees away).

Azar (Azar&Primack 2000) and Gatinel (Gatinel et al. 2002) compared the theoretical ablation profiles and depths of tissue removal for all kinds of astigmatism using various ablation strategies such as combined hyperopic spherical and myopic cylindrical treatments, combined spherical (plus or minus) and hyperopic cylindrical treatments, combined cylindrical treatments, and combined Cross-Cylinder and spherical equivalent (SEQ) treatments.

Both authors concluded that combined spherical and hyperopic cylindrical or combined cylindrical approaches result in reduced ablation depth for treating compound hyperopic and mixed astigmatism whereas applying a hyperopic sphere combined with a myopic cylinder incurs the largest amount of central and peripheral corneal tissue ablation. (Azar&Primack 2000) Despite these important theoretical publications, the definitions and differences between Bitoric and Cross-Cylinder treatments remain unclear in various publications. (Hasaballa et al. 2003, Gatinel et al 2002, Doane&Slade 2003) .

For this reason, we attempt to provide a guideline for refractive surgeons including a subclassification of mixed astigmatism and a generalised Bitoric formula. Furthermore, we compare and contrast Bitoric, Cross-Cylinder and combined spherical and cylindrical (Sequential) ablation strategies with 2nd order wavefront ablation for the correction of mixed astigmatism. We want to know which ablation strategie uses less ablation depht. In order to compare our results with the findings of Azar (Azar&Primack 2000) and Gatinel (Gatinel et al. 2002) we expand the theoretical comparison by pure myopic and hyperopic as well as compound myopic and hyperopic astigmatism. For treating mixed astigmatism we differentiate between cases of zero or negative spherical equivalent (SEQ 0 D) and positive SEQ.

## **2. Materials and methods**

## **2.1 Sub-classification of mixed astigmatism**

Optically the spherical equivalent (SEQ) of an astigmatic eye represents the circle of least confusion (conoid of Sturm), which has two main focal lines, each one parallel to one of the principal meridians of a spherocylindrical lens (American Academy Ophthalmology 2002- 2003) The location of these focal lines leads to the classification of astigmatism:


Fig. 1. Three-dimensional 2nd order wave front maps

(Sequential) ablation strategies with 2nd order wavefront ablation for the correction of mixed astigmatism. We want to know which ablation strategie uses less ablation depht. In order to compare our results with the findings of Azar (Azar&Primack 2000) and Gatinel (Gatinel et al. 2002) we expand the theoretical comparison by pure myopic and hyperopic as well as compound myopic and hyperopic astigmatism. For treating mixed astigmatism we differentiate between cases of zero or negative spherical equivalent (SEQ 0 D) and positive

Optically the spherical equivalent (SEQ) of an astigmatic eye represents the circle of least confusion (conoid of Sturm), which has two main focal lines, each one parallel to one of the principal meridians of a spherocylindrical lens (American Academy Ophthalmology 2002-

Mixed astigmatism: one focal line is in front of the retina and one is behind the retina

2003) The location of these focal lines leads to the classification of astigmatism:

 Compound myopic astigmatism: both focal lines are in front of the retina Compound hyperopic astigmatism: both focal lines are behind the retina

Simple or pure astigmatism: one focal line is on the retina

Fig. 1. Three-dimensional 2nd order wave front maps

SEQ.

**2. Materials and methods** 

**2.1 Sub-classification of mixed astigmatism** 

Three dimensional wavefront maps illustrate the circle of least confusion in reference to the retina and help to sub-classify mixed astigmatism. Figure 1 represents outgoing 2nd order wavefront maps over the exit pupil plane. The green frame surrounding the wavefront maps indicates an aberration-free plane wavefront. For a purely myopic eye (W1, figure1) the optical path is shorter for rays passing near the pupil margin compared to rays passing through the pupil center (chief ray) (Thibos&Applegate 2001). Hence, the back reflected wavefront arrives earlier in the periphery (red color) compared to the chief ray (center of the pupil), indicated by the blue color. Unlike the regular bowl shaped pattern of W1 the wavefront error of myopic astigmatism (W2) indicates two concave meridians of different radii. For hyperopic astigmatism (W3) rays of light in the periphery travel a longer distance compared to the chief ray (red area), which is represented by the convex shaped wavefront map. In contrast to previous maps mixed astigmatism, represented by W4, W5 and W6 (Figure 1), indicates that the cross section along one principle meridian is concave whereas the other meridian is curved convex.

The centre (blue colour) of wavefront map W4 (1.00 –3.00 X 180, SEQ = –0.50 D) is located behind the green frame, indicating a myopic eye, whereas the central yellow colour of W5 (2.00 –2.50 X 180, SEQ = 0.75 D) represents a hyperopic eye. Wavefront map W6 (Figure 1) depicts mixed astigmatism (2.00 –4.00 X 180) with a SEQ of 0.00 D. Hence, the conoid of Sturm is in the retinal plane (the centre of the map has the same colour as the surrounding green frame). In summary, whether using minus or plus cylinder convention, mixed astigmatism can be sub-characterised into:


## **2.1 Bitoric versus cross-cylinder**

In general, Arturo Chayet (Chayet et al. 2001) and Paolo Vinciguerra (Vincinguerra et al. 1999) describe methods to split the prescription into two cylindrical (toric) ablation patterns: applying a minus cylinder to flatten the steep meridian and a plus cylinder (90 degrees away) to steepen the flat meridian. The two authors have differing concepts. A major difference between the two concepts is the proportion used to "split" the spherocylindrical prescription. The Cross-Cylinder approach (P.Vinciguerra) proposes a three-stage treatment. The prescribed subjective cylinder (figure 2) is split into two halves Cneg (negative cylinder) and Cpos (positive cylinder) of equal magnitude and opposite sign (Figure 2). Where the initial prescription has a minus cylinder convention, the positive cylinder is treated at 90 degrees to the negative cylinder. As a third step the residual refractive error is compensated by a spherical treatment.

In contrast to the Cross-Cylinder approach, the Bitoric concept of Chayet (Chayet et al. 2001) proposes a two-stage treatment: Splitting the cylinder into two perpendicular components of opposite sign and differing magnitude. Additionally, Chayet`s concept considers a compensation for the coupling effect (hyperopic shift) which occurs when a myopic cylinder is treated (Chayet et al 1998, Chayet et al 2001, McDonell 1991)

The original Bitoric formula (figure 3), published by Chayet (Chayet et al. 2001) is used with prescriptions in minus cylinder convention. It was designed for Nidek Excimer lasers with a coupling effect of approximately 33%, which is an empirical factor based on clinical experience with the Nidek® laser.

$$\begin{aligned} \text{(1) } \begin{array}{l} \text{SEQ} &= \text{S}\_{\text{subj}} \quad + \quad 0.5 \cdot \text{C}\_{\text{subj}}\\ \text{(2) } \text{C}\_{\text{neg}} &= 0.5 \cdot \text{C} \\\\ \text{(3) } \text{C}\_{\text{pos}} &= 0.5 \cdot \begin{vmatrix} \text{C}\_{\text{subj}} \end{vmatrix} \\ \text{(3) } \text{C}\_{\text{pos}} &= 0.5 \cdot \begin{vmatrix} \text{C}\_{\text{subj}} \end{vmatrix} \\ \text{(4) } \text{S}\_{\text{pos}} &: \text{positive cylinder} \\ \text{C}\_{\text{pos}} &: \text{positive cylinder} \\ \text{C}\_{\text{subj}} &: \text{subjectv cylinder} \\ \text{Example: +1.50} &= \text{S}\_{\text{00}} \text{X } \text{180} \\ \text{SEQ} &= 1.5 \text{ D} + 0.5 \cdot (-\text{S} \text{D}) = -1 \\\\ \text{C}\_{\text{neg}} &= 0.5 \cdot (-5 \text{D}) = -2.50 \text{ X } 180 \\\\ \text{C}\_{\text{pos}} &= 0.5 \cdot 5 \text{D} = +2.50 \text{ X } 90 \end{aligned}$$

#### Fig. 2. Cross-Cylinder formula (Vinciguerra)

Furthermore, it might be confusing that the result always becomes a positive figure (example, figure 3) although a negative cylinder will be applied. For these reasons, we developed a general Bitoric formula for individual Excimer laser systems, provided in Figure 4. It may be used for both, minus and plus cylinder convention. However, it is important to apply the correct axis for each cylinder. Considering minus cylinder convention, the negative cylinder will be applied according to the axis of the prescription. The plus cylinder is treated at 90 degrees to the negative cylinder. Considering plus cylinder convention, the positive cylinder is treated according to the axis of the prescription and the negative cylinder is treated at 90° to the positive cylinder.

$$\begin{aligned} \text{(4)} \quad \text{C\_{neg}} &= \begin{vmatrix} \text{S}\_{\text{subj}} + \text{C\_{subj}} \end{vmatrix} / 1.33\\ \text{(5)} \quad \text{C\_{pos}} &= \begin{vmatrix} \text{C\_{subj}} \end{vmatrix} - \text{C\_{neg}}\\ \text{C\_{neg}} &: \begin{array}{l} \text{C\_{pos}} \end{array} : \text{Positive cylinder} \\ \text{C\_{subj}} &: \text{subjective sphere} \qquad \begin{array}{l} \text{C\_{pos}} \ : \text{positive cylinder} \\ \text{C\_{subj}} \ : \text{subjective cylinder} \end{array} \\ \text{Example: +1.50 } &= \text{5.00 X 180} \\ \text{C = 3.5 D / 1.33 } &= +2.63 \text{ X 180} \\ \text{neg} &= 5 \text{ D} - 2.63 \text{ D } = +2.37 \text{ X 90} \\ \text{pos} &\end{aligned}$$

Fig. 3. Original Bitoric formula (Chayet) showing a coupling factor of 33% which is the specifically for the Nidek laser.

#### **2.2 Sequential method**

Another approach to correction of mixed or compound astigmatism is to treat spherical and cylindrical components sequentially (Sequential method) as prescribed (minus or plus

$$\begin{array}{l} \text{(3)} \quad \text{SEQ} &=\text{S}\_{\text{subj}} \quad + \text{ } \text{0.5}\cdot\text{C}\_{\text{subj}}\\ \text{(4)} \quad \text{C}\_{\text{neg}} &= \frac{\text{SEQ} - \left| \begin{array}{l} \text{0.5}\cdot\text{C}\_{\text{subj}} \right|}{1 + \text{CF}\left[ \text{\%} \right]} \right|}\\ \text{(5)} \quad \text{C}\_{\text{neg}} &= \frac{\text{S} \, \text{S} \, \text{\textdegree C}}{1 + \text{CF}\left[ \text{\%} \right]} \frac{1}{100} \end{array}$$
 
$$\begin{array}{l} \text{(5)} \quad \text{C}\_{\text{pos}} = \text{SEQ} + \left| \begin{array}{l} \text{0.5}\cdot\text{C}\_{\text{subj}} \right| \, + \left| \begin{array}{l} \text{CF}\left[ \text{\%} \right] \text{\%} \end{array} \right|}{100} \right| \\\\ \text{C}\_{\text{pos}} \text{\textdegree; negative cylinder} & \quad \text{C}\_{\text{un}} \text{\textdegree; positive cylinder} \\ \text{(5)} \quad \text{Simplicity} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; conjugate cylinder} \\ \text{[5]: } \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\textdegree; } \text{\%} \, \text{\%} \, \text$$

Fig. 4. General Bitoric formula

subj 0.5 Csubj

subj

SEQ : spherical equivalent Cneg : negative cylinder Cpos : positive cylinder Ssubj : subjective sphere

90X2.50D50.5

1D)5(0.5D 1.5SEQ

180X2.50D)5(0.5

Furthermore, it might be confusing that the result always becomes a positive figure (example, figure 3) although a negative cylinder will be applied. For these reasons, we developed a general Bitoric formula for individual Excimer laser systems, provided in Figure 4. It may be used for both, minus and plus cylinder convention. However, it is important to apply the correct axis for each cylinder. Considering minus cylinder convention, the negative cylinder will be applied according to the axis of the prescription. The plus cylinder is treated at 90 degrees to the negative cylinder. Considering plus cylinder convention, the positive cylinder is treated according to the axis of the prescription and the

subjCsubj / 1.33

Fig. 3. Original Bitoric formula (Chayet) showing a coupling factor of 33% which is the

90X2.37D2.63D5

1.33/D3.5 180X2.63

Cneg : negative cylinder Cpos : positive cylinder Ssubj : subjective sphere Csubj : subjective cylinder

Another approach to correction of mixed or compound astigmatism is to treat spherical and cylindrical components sequentially (Sequential method) as prescribed (minus or plus

Fig. 2. Cross-Cylinder formula (Vinciguerra)

pos C

neg C

(1) SEQ S

(2) Cneg 0.5 C

Csubj : subjective cylinder **Example: 1.50 – 5.00 X 180** 

(3) Cpos 0.5 Csubj

specifically for the Nidek laser.

**2.2 Sequential method** 

negative cylinder is treated at 90° to the positive cylinder.

(4) Cneg S

pos C

neg C

(5) Cpos Csubj Cneg

**Example: 1.50 – 5.00 X 180** 

cylinder convention). For example the prescription of 1.00 –3.00 X 90 would be corrected by combining a hyperopic sphere of 1.00 D with a myopic cylinder of –3.00 X 90 or after converting to plus cylinder convention (–2.00 3.00 X 180) as follows: –2.00 D sphere combined with a hyperopic cylinder of 3.00 X 180.

### **2.3 Wavefront ablation (2nd order)**

Traditionally combined ablation strategies are used to correct the refractive error with LASIK, LASEK or PRK by means of additive optical correction. Hence, the overall correction is achieved by sequentially ablating spherical and/or cylindrical lenticules.

Since the introduction of wavefront analysis and Zernike decomposition, the refractive error of an eye may be described in terms of deviations from an ideal plane wavefront. Unlike traditional concepts, the opposed wavefront can directly correct these so-called aberrations on the cornea with a single step ablation.

In this study, MathCAD2000 Professional® is used to calculate and visualize ablation patterns for different ablation concepts. According to Mrochen et al. 4 Zernike coefficients Z [2,0] (defocus), Z [2, –2] and Z [2, +2] (astigmatism) are derived from sphere (S), cylinder (C) and axis (A) in order to describe 2nd order wavefront errors W(x,y) (Formula 6).

$$W(\mathbf{x}, \mathbf{y}) = Z \left[ \mathbf{2}, -\mathbf{2} \right] \cdot \left( \mathbf{2} \mathbf{x} \cdot \mathbf{y} \right) \\
+ Z \left[ \mathbf{2}, 0 \right] \cdot \left[ \mathbf{2} \cdot \left( \mathbf{x}^2 + \mathbf{y}^2 \right) - 1 \right] \\
+ Z \left[ \mathbf{2}, + \mathbf{2} \right] \cdot \left( \mathbf{x}^2 - \mathbf{y}^2 \right) \tag{6}$$

After sign reversal of the wavefront error W(x,y) further factors have to be taken into account to allow for the correction on the cornea (Equation 7): removing one micron of corneal tissue reduces the wavefront retardation by the difference of the refractive indices (nstroma - nair). Secondly, because no tissue can be added onto the cornea W(x,y) has to be shifted by the smallest constant C to keep the ablation A(x,y) from becoming negative anywhere: (Huang 2001)

$$\mathbf{A(x,y) = [C - W(x,y)] \cdot [1 \ / \ (n\_{\rm stroma} - n\_{\rm air})]} \tag{7}$$

#### **2.4 Comparison of ablation strategies in terms of ablation depth**

For objective, theoretical comparison of different ablation strategies, it is necessary to neglect variables due to individual surgical techniques (e.g. nomogram adjustments). As well varying ablation profiles of different laser systems such as design of transition zone, coupling factors etc. must be excluded. High order aberrations (HOA) are excluded and optical zones (OZ) are kept constant at 6 mm for all calculations. Because we assume wavefront ablation to be the most direct way of refractive correction, it is chosen as the reference ablation volume for all examples.

To compare the traditional concepts with 2nd order wavefront ablation, the spherocylindrical components of the combined ablation concepts (Bitoric, Cross-Cylinder and Sequential methods) are first derived from the subjective refraction. Then the wavefront error W(x,y) for each spherocylindrical component (different for each concept) is transposed (huang 2001) into the corresponding ablation profile A(x,y) (Equation 7).

The total ablation for a combined ablation concept is calculated by adding (superimposing) its elementary optical components (Figure 5). Finally, the difference in shape and elevation is revealed by subtracting the 2nd order wavefront ablation pattern (always considered as the reference) from the total ablation of the combined ablation concept.

#### **3. Results**

For simplicity, we illustrate with a single example (Figure 5) comparing the traditional ablation strategies with 2nd order wavefront ablation. Further results for all ablation strategies and astigmatic corrections are shown in Table 1.

Using Bitoric ablation (Figure 5, first row) to correct mixed astigmatism (3.00 –4.00 X 180) delivers positive Cylinder (PC1) (3.00 X 90) and a negative cylinder (NC1) (–1.00 X 180). Superimposing PC1 and NC1 equates to TB1(total bitoric). The difference (DBW1) between total ablation (TB1) and wavefront ablation (WA) is DBW1 = TB1 – WA.

The Cross-Cylinder ablation (Figure 5, second row) suggests three steps: Sphere (S2 )(1.00), positive cylinder (PC2) (2.00 X 90) and negative cylinder (NC2) (–2.00 X 180). Subtracting TC2 (total ablation Cross-Cylinder) by the wavefront ablation equates to DCW2.

The sequential method in minus cylinder convention ablates corneal lenticules S3 (3.00) and NC3 (–4.00 X 180). The difference between sequential method (TSN3 = S3 NC3) and wavefront ablation (WA) is represented by DSNW3 (Figure 5, third row). The sequential ablation in plus cylinder convention (Figure 5, last row) removes corneal lenticules S4 (– 1.00) and PC4 (4.00 X 90). The difference between the sequential method (TSP4 = S4 PC4) and the wavefront ablation is represented by DSPW4.

After sign reversal of the wavefront error W(x,y) further factors have to be taken into account to allow for the correction on the cornea (Equation 7): removing one micron of corneal tissue reduces the wavefront retardation by the difference of the refractive indices (nstroma - nair). Secondly, because no tissue can be added onto the cornea W(x,y) has to be shifted by the smallest constant C to keep the ablation A(x,y) from becoming negative

For objective, theoretical comparison of different ablation strategies, it is necessary to neglect variables due to individual surgical techniques (e.g. nomogram adjustments). As well varying ablation profiles of different laser systems such as design of transition zone, coupling factors etc. must be excluded. High order aberrations (HOA) are excluded and optical zones (OZ) are kept constant at 6 mm for all calculations. Because we assume wavefront ablation to be the most direct way of refractive correction, it is chosen as the

To compare the traditional concepts with 2nd order wavefront ablation, the spherocylindrical components of the combined ablation concepts (Bitoric, Cross-Cylinder and Sequential methods) are first derived from the subjective refraction. Then the wavefront error W(x,y) for each spherocylindrical component (different for each concept) is transposed (huang

The total ablation for a combined ablation concept is calculated by adding (superimposing) its elementary optical components (Figure 5). Finally, the difference in shape and elevation is revealed by subtracting the 2nd order wavefront ablation pattern (always considered as

For simplicity, we illustrate with a single example (Figure 5) comparing the traditional ablation strategies with 2nd order wavefront ablation. Further results for all ablation

Using Bitoric ablation (Figure 5, first row) to correct mixed astigmatism (3.00 –4.00 X 180) delivers positive Cylinder (PC1) (3.00 X 90) and a negative cylinder (NC1) (–1.00 X 180). Superimposing PC1 and NC1 equates to TB1(total bitoric). The difference (DBW1) between

The Cross-Cylinder ablation (Figure 5, second row) suggests three steps: Sphere (S2 )(1.00), positive cylinder (PC2) (2.00 X 90) and negative cylinder (NC2) (–2.00 X 180). Subtracting

The sequential method in minus cylinder convention ablates corneal lenticules S3 (3.00) and NC3 (–4.00 X 180). The difference between sequential method (TSN3 = S3 NC3) and wavefront ablation (WA) is represented by DSNW3 (Figure 5, third row). The sequential ablation in plus cylinder convention (Figure 5, last row) removes corneal lenticules S4 (– 1.00) and PC4 (4.00 X 90). The difference between the sequential method (TSP4 = S4 PC4)

**2.4 Comparison of ablation strategies in terms of ablation depth** 

2001) into the corresponding ablation profile A(x,y) (Equation 7).

strategies and astigmatic corrections are shown in Table 1.

and the wavefront ablation is represented by DSPW4.

the reference) from the total ablation of the combined ablation concept.

total ablation (TB1) and wavefront ablation (WA) is DBW1 = TB1 – WA.

TC2 (total ablation Cross-Cylinder) by the wavefront ablation equates to DCW2.

A(x,y) = [C – W(x,y)] [1 / (nstroma – nair)] (7)

anywhere: (Huang 2001)

**3. Results** 

reference ablation volume for all examples.

Fig. 5. Ablation strategies for "hyperopic" mixed astigmatism (3.00 –4.00 X 180) PC (positive cylinder) , NC (negative cylinder), Total Bitoric (TBI), Total cross cylinder (TC), Total sphere and negative cylinder (TSN), Total sphere and positive cylinder (TSP), DB (difference to the Wavefront ablation)

For all methods, the right hand column of Figure 5 shows that the difference from 2nd order wavefront profile is either zero or a layer of tissue of uniform thickness (PTK or piston).

Hence, the final geometric shape is identical for all approaches. For this reason, it is possible to compare the ablation depth of all approaches only in the ablation centre. Table 1 shows representative astigmatic corrections (in plus and minus cylinder convention) and their central ablation depths for different ablation concepts. Summarising the theoretical result for different astigmatic corrections yields a qualitative overview of differences in ablation depth for various ablation strategies:

## **3.1 Pure myopic and pure hyperopic astigmatism**

For treating pure myopic astigmatism, all ablation strategies result in the same amount of tissue removal.

For pure hyperopic astigmatism, the Bitoric ablation is similar to wavefront ablation and the Sequential approach following the positive cylinder convention. The Cross-Cylinder technique ablates more tissue and the Sequential method applying a hyperopic sphere and a myopic cylinder ablates even more.

## **3.2 Compound myopic astigmatism**

Cross-Cylinder and both Sequential concepts are equal to wavefront ablation. Using the generalised Bitoric formula for myopic astigmatism leads to a special case of combining two crossed, myopic cylinders, which results in more tissue removal.

## **3.3 Compound hyperopic astigmatism**

Bitoric ablation and the Sequential approach of treating a hyperopic sphere and a hyperopic cylinder are identical to Wavefront ablation. The Cross-Cylinder technique ablates more tissue and the Sequential method applying a hyperopic sphere and a myopic cylinder ablates even more.

## **3.4 Mixed astigmatism (SEQ 0 D)**

The least tissue removal to correct mixed astigmatism with a SEQ equal or less than 0 dioptres (SEQ ≤ 0 D) is achieved by Wavefront ablation, Bitoric, Cross-Cylinder and sequential treatment of myopic sphere and hyperopic cylinder. Sequential treatment of hyperopic sphere and myopic cylinder removes more tissue.


Table 1. Central ablation depths of astigmatic corrections for different ablation concepts

## **3.5 Mixed astigmatism (SEQ > 0 D)**

For "hyperopic" mixed astigmatism (SEQ > 0 D) Wavefront ablation, Bitoric ablation and the combination of myopic sphere and hyperopic cylinder equally remove least tissue. The Cross-Cylinder technique ablates more tissue and sequential treatment of hyperopic sphere and myopic cylinder ablates even more.

## **4. Discussion**

132 Astigmatism – Optics, Physiology and Management

Cross-Cylinder and both Sequential concepts are equal to wavefront ablation. Using the generalised Bitoric formula for myopic astigmatism leads to a special case of combining two

Bitoric ablation and the Sequential approach of treating a hyperopic sphere and a hyperopic cylinder are identical to Wavefront ablation. The Cross-Cylinder technique ablates more tissue and the Sequential method applying a hyperopic sphere and a myopic cylinder

The least tissue removal to correct mixed astigmatism with a SEQ equal or less than 0 dioptres (SEQ ≤ 0 D) is achieved by Wavefront ablation, Bitoric, Cross-Cylinder and sequential treatment of myopic sphere and hyperopic cylinder. Sequential treatment of

> Wavefront ablation depth

astigmatism 0 / -4 x 60 47.9 m 47.9 m 47.9 m 47.9 <sup>m</sup>

astigmatism 3 / -3 x 180 0.0 m 0.0 m 18.0 m 35.9 <sup>m</sup>

astigmatism -3 / -1 x 90 47.9 m 83.8 m 47.9 m 47.9 <sup>m</sup>

astigmatism 4 / -2 x 45 0.0 m 0.0 m 12.0 m 23.9 <sup>m</sup>

SEQ < 0 1 / -3 x 90 23.9 m 23.9 m 23.9 m 35.9 <sup>m</sup>

SEQ = 0 2 / -4 x 60 23.9 m 23.9 m 23.9 m 47.9 <sup>m</sup>

SEQ > 0 3 / -4 x 45 12.0 m 12.0 m 23.9 m 47.9 <sup>m</sup>

Table 1. Central ablation depths of astigmatic corrections for different ablation concepts

Bitoric ablation depth


0 / 3 x 90 0.0 m 0.0 m 18.0 m 0.0 m


2 / 2 x 135 0.0 m 0.0 m 12.0 m 0.0 m




Cross-Cylinder ablation depth

Sequential ablation depth

**3.2 Compound myopic astigmatism** 

**3.3 Compound hyperopic astigmatism** 

**3.4 Mixed astigmatism (SEQ 0 D)** 

ablates even more.

Ablation concept

pure myopic

pure hyperopic

comp. myopic

comp. hyperopic

mixed astigmatism

mixed astigmatism

mixed astigmatism

crossed, myopic cylinders, which results in more tissue removal.

hyperopic sphere and myopic cylinder removes more tissue.

prescription [ D ]

We reaffirm that all correction strategies result in identical surface shape but differ in ablation depth. For treating astigmatism in general, 2nd order wavefront ablation and Sequential treatment of spherical and hyperopic cylindrical lenticules are the most tissue saving methods. Hence, these strategies may likely be most efficient and most predictable in order to achieve the desired refractive and visual outcome. As removing less tissue makes the results more predictable and therefore more efficient.

In contrast to the findings of Gatinel (Gatinel et al. 2002) this study demonstrates that 2nd order wavefront ablation results in minimum tissue removal, despite the fact of splitting the amount of astigmatism into 2 components (cardinal and oblique). In addition, we theoretically found that correction of mixed and compound hyperopic astigmatism using Bitoric ablation or using sequential ablation of spherical and hyperopic cylindrical components is identical to 2nd order wavefront ablation.

While agreeing with Azar (Azar&Primack 2000) and Gatinel (Gatinel et al 2002) that Vinciguerra`s cross-cylindrical approach for compound hyperopic and pure hyperopic astigmatism does not cause minimal tissue removal, our findings differ from those of Azar and Gatinel`s for mixed astigmatism. Using the Cross-Cylinder formula for mixed astigmatism with "hyperopic" SEQ removes more tissue, whereas in cases of zero or negative spherical equivalent (SEQ 0 D) minimum amount of tissue is removed equally to Bitoric ablation, wavefront ablation and sequentially treating a sphere together with a hyperopic cylinder.

Bitoric and Cross-Cylinder (for SEQ ≤ 0 D) ablations are appropriate methods to treat mixed astigmatism for Excimer lasers or software which do not allow 2nd order wavefront based ablation or the combined treatment of myopic sphere and hyperopic cylinder. For treating mixed astigmatism Bitoric ablation has advantages compared to Cross-Cylinder ablation, because it accounts for the hyperopic shift, applies only 2 treatment steps and it results in minimal ablation depth. The Bitoric formula (Figure 4) should not be used for myopic astigmatism, because it applies two crossed minus cylinders resulting in excessive tissue removal. Except for pure myopic and compound myopic astigmatism, the sequential treatment of sphere and myopic cylinder should be avoided.

The intention of this paper is to reveal that ablation profiles based on 2nd order Zernike polynomials lead to minimal tissue removal. However, ablation strategies, taking into account more variables (loss of efficiency, preoperative corneal asphericity, hyperopic shift etc.) might be the state-of-the-art technique to improve the visual outcome. Understanding the concept of wavefront ablation will lead to optimized photo-ablative standard treatments, especially when spherical aberrations are pre compensated for their induction. (Manns 2002) (Mrochen 2004) Then, in general the ablation depth will increase due to consideration of high order aberrations.

## **5. References**


## **Management of Post-Penetrating Keratoplasty Astigmatism**

Sepehr Feizi

*Ophthalmic Research Center and Department of Ophthalmology, Labbafinejad Medical Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran* 

## **1. Introduction**

134 Astigmatism – Optics, Physiology and Management

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Chayet AS, Magallanes R, Montes M, et al. (1998). Laser in situ keratomileusis for simple

Chayet AS, Montes M, Gómez L, et al. (2001). Bitoric laser in situ keratomileusis for the correction of simple myopic and mixed astigmatism. *Ophthalmology*, 108:303-308 De Ortueta D, Haecker C. (2008). Laser in situ keratomileusis for mixed astigmatism using a

Doane JF, Slade SG. (2003) Treatment of Astigmatism. *Custom Lasik: Surgical Techniques and* 

Gatinel D, Hoang-Xuan T, Azar DT. (2002). Three-dimensional representation and

Huang D. (2001). Physics of customized corneal ablation. *Customized corneal Ablation: the* 

MacRae SM, Schwiegerling J, Snyder R (2000). Customized corneal ablation and super

Manns F, Ho A, Parel JM, Culbertson W (2002). Ablation profiles for wavefront-guided

McDonnel PJ, Moreira H, Garbus J, et al. (1991). Photorefractive keratectomy to create toric ablations for correction of astigmatism. *Archives Ophthalmology 109*:710-3 Mrochen M, Donitzky C, Wüllner C, Löffler (2004) J. Wavefront-optimized ablation profiles: Theoretical background. *Journal of Cataract and Refractive Surgery,* 30:775-785 Seiler T, Kaemmerer M, Mierdel P, Krinke HE (2000). Ocular optical aberrations after

Stein R. (2003). Lasik for mixed Astigmatism. *Custom Lasik: Surgical Techniques and* 

Stonecipher KG, Kezirian GM, Stonecipher K. (2010). LASIK for mixed astigmatism using

Thibos LN, Applegate RA. (2001) Assessment of optical quality. *Customized corneal Ablation:* 

Vinciguerra P, Sborgia M, Epstein D, et al. (1999) Photorefractive keratectomy to correct

situ keratomileusis for compound hyperopic and mixed astigmatism. *Journal of* 

myopic, mixed, and simple hyperopic astigmatism. *Journal of Refractive Surgery*,

modified formula for bitoric ablation. *European Journal of Ophthalmology* 18(6):869-76

qualitative comparisons of the amount of tissue ablation to treat mixed and compound astigmatism. *Journal of Cataract and Refractive Surgery;*, 28:2026-2034 Hasaballa MA., Ayala MJ, Alío JL. (2003). Laser in situ keratomileusis correction of mixed

astigmatism by bitoric ablation. *Journal of Cataract and Refractive Surgery*, 29: 1889-1895

correction of myopia and primary spherical aberration. *Journal of Cataract and* 

photorefractive keratectomy for myopia and myopic astigmatism. *Archives* 

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**5. References** 

14:175-176

Penetrating keratoplasty (PK) has emerged as a relatively safe means of restoring vision in corneal opacities and irregularities. Astigmatism is the most common cause of suboptimal vision after corneal transplantation despite a clear corneal graft.1,2 Based on several studies,3- 615%–31% of patients undergoing PK may develop postoperative astigmatism greater than 5 diopters (D). The astigmatism can be irregular with associated higher-order aberrations that can ultimately limit the vision obtained and add to patient's inability to wear standard optical correction.7 This explains why visual acuity in 10%–20% of PK cases cannot be corrected satisfactorily by spectacles or contact lenses.8-10

Factors influencing the amount of astigmatism after PK include the severity of the underlying disorder (e.g. keratoconus), oval or eccentric trephination,11 graft size and donor–recipient disparity,12 corneal thickness mismatch between the donor and recipient,13 a poor suturing technique,13, 14-17 and time of suture removal or adjustment14-17.

Commonly practiced techniques to reduce post-PK astigmatism consist of postoperative suture manipulation including running suture tension adjustment and selective interrupted suture removal,14,18-21 optical correction consisting of spectacles and contact lenses,22 relaxing incisions,2,10 compression sutures,2,23 a combination of relaxing incisions and compression sutures (augmented relaxing incisions),24-26 laser refractive surgery,27-33 insertion of intrastromal corneal ring segments,34 wedge resection,9,35-39 toric phakic intraocular lenses,40- 42 and finally regrafting.43

## **2. General considerations**

The corneal graft-host junction typically heals by 1 year after transplantation and corneal surface stability is achieved 3 to 4 months after complete suture removal. However, this period can significantly vary due to patient's age, general health status (diabetes mellitus and collagen vascular disorders), and use of topical and systemic immunosuppressive medications. Given that, any surgical intervention for post-PK astigmatism should be postponed at least 3 to 4 months after complete suture removal. Previous rejection episodes should be noted and the patient should be stable on minimal immunosuppressive agents.44

Prior to any surgical intervention, a comprehensive ophthalmic examination including uncorrected (UCVA) and best spectacle-corrected visual acuity (BSCVA) should be performed. Slit-lamp biomicroscopy is used to evaluate graft size, centration, and clarity as well as detect any areas of haze or neovascularization. Attention should be paid to the grafthost interface for quality of apposition (override or underride) and stability of surgical wound.

Astigmatism should be evaluated through a combination of manifest (and sometimes cycloplegic) refraction, keratometry, corneal topography, and occasionally wavefront analysis. Central and peripheral pachymetry is required when laser or incisional refractive surgery is anticipated, respectively.

## **3. Intraoperative measurements**

During PK, attention should be paid to some critical points if a low postoperative astigmatism is to be obtained. A perfect surgical technique including round and central trephination of recipient and donor which should be large enough to cover abnormal areas (such as thin cornea in keratoconus) is required to achieve an acceptable refractive outcome postoperatively. Additionally, appropriate sutures with evenly distributed tension, and apposition make sure that patients experience a low amount of astigmatism. Suturing technique including interrupted, single or double running, and combined interrupted and running are comparable in terms of postoperative graft astigmatism as long as timely suture adjustment and/or removal are performed.45

#### **4. Suture tension adjustment and selective suture removal**

After PK, sutures should be kept for at least one year unless complications such as cheesewiring, loosening, and vascularization develop. During this period, astigmatism >4 D can be reduced by suture manipulation consisting of selective interrupted suture removal and tension adjustment of running sutures. Use of interrupted or combined running and interrupted sutures allows for the selective removal of interrupted ones, with the goal of reducing astigmatism. Successful visual rehabilitation therefore depends partially on accurate identification of the tight interrupted sutures. Refraction and keratometry can be used to determine which sutures have to be removed. Identifying the steep and flat corneal meridians 90° apart, however, refraction and manual keratometry could be misleading in patients undergoing keratoplasty in whom non-orthogonal and irregular astigmatism is common. Computerized corneal topography has the advantage of mapping subtle corneal power changes accurately over the entire optical zone and beyond allowing identification of steep meridians that can be attributed to specific sutures.21,46 In the interrupted suturing technique, selective suture removal can start as early as 2 months after PK provided that, the neighboring sutures are not to be removed at least 6 months postoperatively. That is because removal of adjacent sutures within this period is more likely to make the wound unstable than removal of alternate or non-adjacent sutures. After initial suture removal, non-adjacent sutures can be removed at an interval of 4-6 weeks, as seen necessary.19,20 It is better to remove only a single suture at a time as it yields better results in terms of astigmatism as compared to multiple suture removal at one time.14,21

If a combined running and interrupted suturing technique is used, then many of the interrupted sutures can be safely removed as early as 1 week postoperatively with minimal risk of wound problems.

Tension adjustment of running sutures should be done after 2 to 4 weeks when graft edema disappears but within 2 months when the reparative response does not completely take place at the graft-host interface. Every episode of suture removal has the added risk of infection and/or rejection and appropriate antibiotic and steroid cover is essential.

When, a small amount of astigmatism is achieved through suture manipulation, the sutures are left in as long as possible, until they fray or break.18,43

## **5. Optical corrections**

136 Astigmatism – Optics, Physiology and Management

Prior to any surgical intervention, a comprehensive ophthalmic examination including uncorrected (UCVA) and best spectacle-corrected visual acuity (BSCVA) should be performed. Slit-lamp biomicroscopy is used to evaluate graft size, centration, and clarity as well as detect any areas of haze or neovascularization. Attention should be paid to the grafthost interface for quality of apposition (override or underride) and stability of surgical

Astigmatism should be evaluated through a combination of manifest (and sometimes cycloplegic) refraction, keratometry, corneal topography, and occasionally wavefront analysis. Central and peripheral pachymetry is required when laser or incisional refractive

During PK, attention should be paid to some critical points if a low postoperative astigmatism is to be obtained. A perfect surgical technique including round and central trephination of recipient and donor which should be large enough to cover abnormal areas (such as thin cornea in keratoconus) is required to achieve an acceptable refractive outcome postoperatively. Additionally, appropriate sutures with evenly distributed tension, and apposition make sure that patients experience a low amount of astigmatism. Suturing technique including interrupted, single or double running, and combined interrupted and running are comparable in terms of postoperative graft astigmatism as long as timely suture

After PK, sutures should be kept for at least one year unless complications such as cheesewiring, loosening, and vascularization develop. During this period, astigmatism >4 D can be reduced by suture manipulation consisting of selective interrupted suture removal and tension adjustment of running sutures. Use of interrupted or combined running and interrupted sutures allows for the selective removal of interrupted ones, with the goal of reducing astigmatism. Successful visual rehabilitation therefore depends partially on accurate identification of the tight interrupted sutures. Refraction and keratometry can be used to determine which sutures have to be removed. Identifying the steep and flat corneal meridians 90° apart, however, refraction and manual keratometry could be misleading in patients undergoing keratoplasty in whom non-orthogonal and irregular astigmatism is common. Computerized corneal topography has the advantage of mapping subtle corneal power changes accurately over the entire optical zone and beyond allowing identification of steep meridians that can be attributed to specific sutures.21,46 In the interrupted suturing technique, selective suture removal can start as early as 2 months after PK provided that, the neighboring sutures are not to be removed at least 6 months postoperatively. That is because removal of adjacent sutures within this period is more likely to make the wound unstable than removal of alternate or non-adjacent sutures. After initial suture removal, non-adjacent sutures can be removed at an interval of 4-6 weeks, as seen necessary.19,20 It is better to remove only a single suture at a time as it yields better results in terms of astigmatism as

wound.

surgery is anticipated, respectively.

**3. Intraoperative measurements** 

adjustment and/or removal are performed.45

compared to multiple suture removal at one time.14,21

**4. Suture tension adjustment and selective suture removal** 

Spectacles and rigid gas-permeable (RGP) contact lenses are the simplest method of addressing postoperative refractive error even when sutures are still in place. However, the use of glasses may not be possible when a significant amount of astigmatic anisometropia is present. RGP contact lenses which may be effective in 80% of cases often provide superior visual acuity and are frequently required in eyes with moderate to severe astigmatism.22 Unfortunately, contact lenses are often difficult to fit, strictly dependent on a patient's tolerance and lifestyle, and may induce peripheral corneal neovascularization, leading to graft rejection and failure. Furthermore, many patients (the elderly in particular) are unable to handle or maintain contact lenses.47,48

## **6. Incisional keratotomy**

Relaxing incisions with or without counter-quadrant compression sutures is an effective, simple, and safe method to reduce high post-PK astigmatism.10,25,26,49-53 Patients with keratometric astigmatism > 4.0 D after complete suture removal can be considered for this procedure. Under topical anesthesia and direct visual inspection, relaxing incisions are made down to Descemet membrane usually on the both sides of the steepest meridian with an arc length of 45 degrees to 90 degrees. The site and extension of relaxing incisions are determined on the basis of corneal topography.54 The effect of these relaxing incisions is monitored intraoperatively with a hand-held keratoscope. If an adequate effect is not obtained through relaxing incisions alone, interrupted 10-0 nylon compression sutures are added to achieve overcorrection of astigmatism in the opposite meridian (90 degrees away) to reverse the axis of astigmatism as apparent by the keratoscopic mires. Postoperatively, selective suture removal is initiated 3-4 weeks after the procedure until an acceptable amount of astigmatism is achieved. Thereafter, further suture removal is postponed until no suture effect is observed.

The site of relaxing incision can be either in the donor cornea or at the graft-host interface. Incisions in the recipient cornea are not recommended as it is believed that the scarring at the graft-host junction changes the biomechanical state of the cornea. The keratoplasty wound is supposed to form a new limbus, blocking the effect of relaxing incisions in the recipient cornea.55

Using subtraction or vector analysis to calculate the reduction in astigmatism, a wide range of correction between 3.4 D and 9.7 D has been reported by this approach. 10,25,26,49-53 However, this procedure has a high incidence of recurrence of astigmatism and low predictability.9 Other disadvantages include overcorrection, corneal perforation, wound dehiscence, and prolonged instability of corneal topography.9,39,56 Additionally, there are no standardized nomograms to correlate the amount of keratometric astigmatism with the extension of incisions and those developed for congenital astigmatism can not be applied to the correction of post-PK astigmatism.

In an attempt to increase the safety and efficacy, femtosecond laser (FSL) technology has been recently introduced in the clinical practice. Nublie et al.57 confirmed the feasibility and efficacy of astigmatic keratotomy using FSL to treat post-keratoplasty astigmatism. They reported paired FSL incisions located on the steepest corneal meridian, peripherally inside the graft, at the intended depth of 90% of the local stromal thickness, provided a significant reduction of preoperative subjective astigmatism from 7.163.07 D to 2.231.55 D which remained stable for several months. Kumar et al.58 reported IntraLase-enabled astigmatic keratectomy was effective in reducing high post-PK astigmatism and significantly improved UCVA and BSCVA while, refraction became stable between 3 and 6 months postoperatively. Adverse effects encountered in these two studies, however, were overcorrection necessitating early resuturing and a higher rate of allograft rejection successfully treated with topical corticosteroids.57,58 Additionally, the procedure adversely affected higher-order aberrations which was similar to what reported after manual astigmatic keratectomy in PK corneas.57-59

In the majority of cases, relaxing incisions with or without counter-quadrant compression sutures are the only procedure performed at the time. However, it is sometimes combined with other interventions such as cataract extraction and intraocular lens (IOL) implantation or phakic IOL implantation to simultaneously address lens opacity or high refractive error, respectively. To choose the accurate power of IOLs in such cases, it is important to know the exact effect of the intervention on graft steepness. Any possible hyperopic or myopic shift caused by such interventions should be compensated for in the power of IOLs to achieve a reasonable refractive outcome after combined surgeries. Previously, a myopic shift of up to 1.5 D has been reported after relaxing incisions8,9,26 which should be taken into account for IOL power calculation during combined approaches.

## **7. Laser refractive surgery**

Excimer laser photoablation techniques are capable of treating astigmatism as well as coexisting spherical refractive error after corneal transplantation. The use of LASIK after PK was first reported by Arenas and Maglione in 1997.28 PRK has also been used to correct refractive errors after PK.29-33 A unique advantage of PRK is the lack of flap-related complications. However, PRK in post-PK patients is less predictable and less effective than for naturally occurring astigmatism.31 Other complications associated with Post-PK PRK are increased incidence of irregular astigmatism, significant regression, and late-developing corneal haze.31,60,61 There has been a decrease in the incidence of post-PRK haze in recent years because of improved laser, the intraoperative use of mitomycin-C, and better postoperative care.62 Additionally, the introduction of custom PRK wavefront ablation technique can further refine the outcomes of laser surgery in this complex group of eyes.63 As compared to PRK, LASIK has several advantages including fast visual rehabilitation, decreased stromal scarring, minimal regression, and the ability to treat a greater amount of

predictability.9 Other disadvantages include overcorrection, corneal perforation, wound dehiscence, and prolonged instability of corneal topography.9,39,56 Additionally, there are no standardized nomograms to correlate the amount of keratometric astigmatism with the extension of incisions and those developed for congenital astigmatism can not be applied to

In an attempt to increase the safety and efficacy, femtosecond laser (FSL) technology has been recently introduced in the clinical practice. Nublie et al.57 confirmed the feasibility and efficacy of astigmatic keratotomy using FSL to treat post-keratoplasty astigmatism. They reported paired FSL incisions located on the steepest corneal meridian, peripherally inside the graft, at the intended depth of 90% of the local stromal thickness, provided a significant reduction of preoperative subjective astigmatism from 7.163.07 D to 2.231.55 D which remained stable for several months. Kumar et al.58 reported IntraLase-enabled astigmatic keratectomy was effective in reducing high post-PK astigmatism and significantly improved UCVA and BSCVA while, refraction became stable between 3 and 6 months postoperatively. Adverse effects encountered in these two studies, however, were overcorrection necessitating early resuturing and a higher rate of allograft rejection successfully treated with topical corticosteroids.57,58 Additionally, the procedure adversely affected higher-order aberrations which was similar to what reported after manual astigmatic keratectomy in PK

In the majority of cases, relaxing incisions with or without counter-quadrant compression sutures are the only procedure performed at the time. However, it is sometimes combined with other interventions such as cataract extraction and intraocular lens (IOL) implantation or phakic IOL implantation to simultaneously address lens opacity or high refractive error, respectively. To choose the accurate power of IOLs in such cases, it is important to know the exact effect of the intervention on graft steepness. Any possible hyperopic or myopic shift caused by such interventions should be compensated for in the power of IOLs to achieve a reasonable refractive outcome after combined surgeries. Previously, a myopic shift of up to 1.5 D has been reported after relaxing incisions8,9,26 which should be taken into account for

Excimer laser photoablation techniques are capable of treating astigmatism as well as coexisting spherical refractive error after corneal transplantation. The use of LASIK after PK was first reported by Arenas and Maglione in 1997.28 PRK has also been used to correct refractive errors after PK.29-33 A unique advantage of PRK is the lack of flap-related complications. However, PRK in post-PK patients is less predictable and less effective than for naturally occurring astigmatism.31 Other complications associated with Post-PK PRK are increased incidence of irregular astigmatism, significant regression, and late-developing corneal haze.31,60,61 There has been a decrease in the incidence of post-PRK haze in recent years because of improved laser, the intraoperative use of mitomycin-C, and better postoperative care.62 Additionally, the introduction of custom PRK wavefront ablation technique can further refine the outcomes of laser surgery in this complex group of eyes.63 As compared to PRK, LASIK has several advantages including fast visual rehabilitation, decreased stromal scarring, minimal regression, and the ability to treat a greater amount of

the correction of post-PK astigmatism.

IOL power calculation during combined approaches.

**7. Laser refractive surgery** 

corneas.57-59

refractive errors.28,60,64-66 Factors that may influence the outcome of astigmatism treatment by LASIK other than the wound-healing process are the position of the hinge in relation to the location of the visual axis, flap diameter relative to the PK donor button diameter, and flap thickness.55,67 In addition, corneal graft thickness and the amount of refractive error may limit the efficacy of the procedure.68 The disadvantages include limited correction of astigmatism and potential for flap complications such as epithelial ingrowth, button hole, free or incomplete flaps28,68 as well as an increased risk of photoablation-induced graft rejection69-71. However, endothelial cell loss after LASIK is not higher than the normal postkeratoplasty decline.72,73 Furthermore, because the lamellar flap is larger than the corneal graft, thinning of the graft-host interface occurs after microkeratome cut which can lead to wound dehiscence.72,74,75

To improve outcomes, some authors propose performing the LASIK procedure in 2 steps (flap creation first followed by laser ablation 8 to 12 weeks later) because of a hinged lamellar keratotomy effect.76,77 Lamellar cuts may induce substantial changes in the graft shape as corneal stress caused by irregularities in wound shape and wound healing is removed from the graft center after creating a flap resulting in changes of up to 4.0 D of astigmatism.77

## **8. Intrastromal corneal ring segments**

In a small group of patients with post-PK astigmatism, Kerarings were implanted which significantly reduced mean keratometry values and significantly improved corneal topography and uncorrected visual acuity.34 However, several complications were encountered during and after Kerarings implantation including small dehiscence of grafthost interface during stroma tunnel dissection, an inflammatory infiltrate around the segment immediately after operation, stromal channel vascularization leading to ring explantation and night halos.34

#### **9. Wedge resection**

In this procedure, a wedge of corneal tissue including the recipient and/or donor cornea is excised from the flatter corneal meridian to correct high astigmatism (usually higher than 10.0 D) after PK.35-39 The length and width of a wedge resection and its proximity to the central cornea determine the amount of astigmatism to be corrected. Various nomograms have been used. As a general, approximately 0.05 to 0.1 mm of tissue is removed for every 1.0 D of preoperative astigmatism.36-38 Suture tightness and removal are important factors. The sutures should be tight enough to approximate the borders of the wound. Usually 6 to 8 sutures are placed on each wound and kept for 3 to 6 months. An initial overcorrection is the rule and should not induce premature suture removal. The procedure results in an increase in overall graft curvature hence, a myopic shift will generally be encountered.36,39

One surgical drawback of corneal wedge resection is difficulty in manually excising the exact amount of tissue in width and depth, which may account for the low predictability of the technique.36 Additionally, microperforations can occur during the course of the procedure which renders the eye soft and prevents completion of the procedure.

Recently, FSL has been used as a safe and effective alternative to the manual technique to perform a corneal wedge resection.78 This device can allow easier, more controlled, and more precise excision of tissue in width, length, and depth and reduce the risk of corneal perforation. Using this technique, Ghanem and Azar78 reported a reduction of 14.5 D in post-keratoplasty astigmatism.

## **10. Intraocular lens implantation**

In cases of high astigmatism after penetrating keratoplasty, implantation of a toric IOL (tIOL) offers a promising alternative to arcuate keratotomies with or without compression sutures. These kinds of IOLs are used during cataract extraction or in phakic eyes. Cataract extraction with implantation of tIOL is a new surgical option for correction of residual astigmatism following penetrating keratoplasty with only minimal direct manipulation of the graft. Viestenz et al.40 reported the refractive cylinder could be reduced from 7.02.6 D to 1.631.5 D after surgery. They recommended, however, regular and symmetric corneal topography be essential for successful implantation of tIOL.40

In phakic eyes, Artisan toric intraocular lens was implanted to correct refractive errors after keratoplasty.41,42 The use of the Artisan toric IOL, with a power range of 7.5 D of cylinder and -20.5 D of myopia to +12.0 D of hyperopia, provides a wide field for correction of postkeratoplasty astigmatism and ametropia. Tahzib et al.42 reported the spherical equivalent was reduced from -3.194.31 D (range, +5.5 to -14.25 D) preoperatively to - 1.031.20 D (range, +1.0 to -5.25 D) postoperatively and refractive cylinder from -7.062.01 D to -2.001.53 D at the last follow-up.42 After 36 months, the postoperative mean endothelial cell loss was 30.4%32.0%42 which is significantly higher than the reported cell loss in other studies of the natural endothelial cell loss after penetrating keratoplasty (between 4.2% and 7.8%)79',80 and than that in studies of Artisan lens implantation for correction of high myopia (between 0.78% and 9.1%)81-83 Probably, the higher cell loss is explained by the increased vulnerability of the corneal graft endothelium, which usually has low cell densities and may cause a higher rate of endothelial cell loss. Other potential complications of the Artisan tIOL for the correction of postkeratoplasty astigmatism include loss of >2 lines of BSCVA, surgically induced astigmatism by implantation of the rigid PMMA IOL through a 5.5- to 6.0-mm incision, reversible immunologic rejection, and irreversible corneal decompensation.41,42

## **11. Repeat keratoplasty**

This intervention should be considered as the last option for treating intractable high/irregular postkeratoplasty astigmatism in clear corneal grafts when other aforementioned interventions fail. Reporting a small group of patients who underwent repeat PK using the 193-nm Zeiss-Meditec MEL-60 excimer laser and employing double running sutures, Szentmary et al.43 observed a significant decrease in central graft power and an improvement in astigmatism with sutures in place. However, astigmatism increased significantly after second suture removal. They concluded with all-sutures-in, BSCVA and astigmatism improve significantly after repeat PK for high/irregular astigmatism. However, to prevent significant increase in astigmatism, final suture removal should be postponed as long as possible in such eyes.

## **12. Conclusion**

140 Astigmatism – Optics, Physiology and Management

Recently, FSL has been used as a safe and effective alternative to the manual technique to perform a corneal wedge resection.78 This device can allow easier, more controlled, and more precise excision of tissue in width, length, and depth and reduce the risk of corneal perforation. Using this technique, Ghanem and Azar78 reported a reduction of 14.5 D in

In cases of high astigmatism after penetrating keratoplasty, implantation of a toric IOL (tIOL) offers a promising alternative to arcuate keratotomies with or without compression sutures. These kinds of IOLs are used during cataract extraction or in phakic eyes. Cataract extraction with implantation of tIOL is a new surgical option for correction of residual astigmatism following penetrating keratoplasty with only minimal direct manipulation of the graft. Viestenz et al.40 reported the refractive cylinder could be reduced from 7.02.6 D to 1.631.5 D after surgery. They recommended, however, regular and symmetric corneal

In phakic eyes, Artisan toric intraocular lens was implanted to correct refractive errors after keratoplasty.41,42 The use of the Artisan toric IOL, with a power range of 7.5 D of cylinder and -20.5 D of myopia to +12.0 D of hyperopia, provides a wide field for correction of postkeratoplasty astigmatism and ametropia. Tahzib et al.42 reported the spherical equivalent was reduced from -3.194.31 D (range, +5.5 to -14.25 D) preoperatively to - 1.031.20 D (range, +1.0 to -5.25 D) postoperatively and refractive cylinder from -7.062.01 D to -2.001.53 D at the last follow-up.42 After 36 months, the postoperative mean endothelial cell loss was 30.4%32.0%42 which is significantly higher than the reported cell loss in other studies of the natural endothelial cell loss after penetrating keratoplasty (between 4.2% and 7.8%)79',80 and than that in studies of Artisan lens implantation for correction of high myopia (between 0.78% and 9.1%)81-83 Probably, the higher cell loss is explained by the increased vulnerability of the corneal graft endothelium, which usually has low cell densities and may cause a higher rate of endothelial cell loss. Other potential complications of the Artisan tIOL for the correction of postkeratoplasty astigmatism include loss of >2 lines of BSCVA, surgically induced astigmatism by implantation of the rigid PMMA IOL through a 5.5- to 6.0-mm incision, reversible immunologic rejection, and irreversible corneal decompensation.41,42

This intervention should be considered as the last option for treating intractable high/irregular postkeratoplasty astigmatism in clear corneal grafts when other aforementioned interventions fail. Reporting a small group of patients who underwent repeat PK using the 193-nm Zeiss-Meditec MEL-60 excimer laser and employing double running sutures, Szentmary et al.43 observed a significant decrease in central graft power and an improvement in astigmatism with sutures in place. However, astigmatism increased significantly after second suture removal. They concluded with all-sutures-in, BSCVA and astigmatism improve significantly after repeat PK for high/irregular astigmatism. However, to prevent significant increase in astigmatism, final suture removal should be postponed as

post-keratoplasty astigmatism.

**11. Repeat keratoplasty** 

long as possible in such eyes.

**10. Intraocular lens implantation** 

topography be essential for successful implantation of tIOL.40

Now, we have a large armamentarium of refractive surgery to correct post-keratoplasty astigmatism. However, none of them appear as a perfect option and corneal surgeons should tailor a specific plan, on the basis of patient's needs and clinical situations, to take advantages of each intervention. For example, when the astigmatism is too high to be corrected with excimer laser alone, it can be reduced by relaxing incisions to a level which is treatable by PRK or LASIK. Similarly, a combination of relaxing incisions followed by IOL implantation or IOL implantation followed by excimer laser can be considered to achieve a refractive outcome very close to emmetropia.

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## **Controlling Astigmatism in Corneal Marginal Grafts**

Lingyi Liang1 and Zuguo Liu2 *1Zhongshan Ophthalmic Center, Sun Yat-sen University, 2Xiamen Eye Institute, Xia-men University, China* 

## **1. Introduction**

146 Astigmatism – Optics, Physiology and Management

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Study. Ophthalmology 2004;111(2):309 –317.

eyes: an evaluation of the United States Food and Drug Administration Ophtec

Postoperative astigmatism is inevitable after corneal marginal grafts. Visual rehabilitation following corneal transplantation remains a formidable challenge. High degrees of irregular astigmatism can lead to poor functional vision despite a clear corneal graft. The average postoperative astigmatism of penetrating keratoplasty (PK) is approximately 4 to 5 diopters despite all the improved suturing techniques, and is more obvious in eyes with marginal grafts.(Jonas et al., 2001;Riedel et al., 2001;Varley et al., 1990;Chern et al.; 1997 Riedel et al., 2002;Kerényi & Süveges, 2003). The common causes of refractive error after anatomically successful marginal KP include preoperative corneal irregularity of the host and the donor, intraoperative surgical tissue alignment, uneven suture tension, and postoperative wound healing variability. Therefore, astigmatism after marginal corneal transplant is mostly irregular and unstable.

## **2. Detection of postoperative astigmatism**

The measurement of astigmatism following marginal corneal grafting is usually quite difficult. Maloney (Maloneyet al., 1993) described a method that uses mathematical algorithms to fit the measured corneal surface on the videokeratograph with the closest spherocylinder and then substracts the curvature of the spherocylinder from that of the actual corneal surface, the difference being the amount of irregular astigmatism. This helps the quantification of the irregular astigmatism. With development in Orbscan topography technology, we can obtain information on anterior and posterior corneal curvature as well as detailed sectorial pachymetry, which may help in the detection of corneal irregularities. (Kang et al., 2000;Seitz et al., 2002) Liang (Liang et al, 2008) and Kerényi (Kerényi et al, 2003) have reported the postoperative astigmatisam changes after marginal corneal grafting using Orbscan topography. The reading of Orbscan topography are consistent with the refractive cylinder. Recently, Pentacam Anterior Segment Topography has been employed in the diagnosis and monitoring of postoperative astigmatism, giving more structural and refractive information of the cornea.( Ho et al., 2009) The Pentacam is a recently introduced rotating Scheimpflug system. The Scheimpflug principle has already been established in the assessment of lens thickness and densitometry, corneal transparency, thickness, and curvature, anterior chamber depth, and in the detection of intraocular lens (IOL) tilting. However, Pentacam is the first Scheimpflug camera–based instrument that can capture images in multiple meridians in a single automated scan. Pentacam has been more commonly used in the evaluation of postoperative astigmatism after keratoplasty and further guiding the keratotomy procedure. (Buzzonetti, et al., 2009) Further evaluation of using Pentacam specifically in marginal corneal grafting is warranted.

## **3. Control of postoperative astigmatism**

## **3.1 Intraoperative control of astigmatism**

As previously mentioned, the postoperative astigmatism arises partly from intraoperative surgical tissue alignment and uneven suture tension; therefore, improvement in surgical skill is essential in minimizing postoperative astigmatism. Good matching of the graft and the corneal button is required. The thickness and size of the graft should fit the recipient site to restore the contour and curvature of the cornea. As illustrated in Table 1 and Figure 1, the shape and position of the graft and the placement of sutures should be designed to avoid involving the optical zone. (Table 1).(Liang et al., 2008; Huang et al., 2008)

 For marginal corneal diseases without perforation, semilunar, crescentic, and annular lamellar keratoplasties are commonly performed. The type of lamellar keratoplasty is determined by the size, depth, and location of the corneal lesion and its relationship with pupil (Table 1)


Table 1. Design of different types of lamellar keratoplasty.

Fig. 1. Schematic Illustration of Marginal Corneal Grafts with Different Shapes According to Different Location of the Corneal Disease. Semiluna (A), Crescentic (B), and Annular (C).

The surgical pearls for different types of marginal corneal grafts are listed as following: Semilunar (Fig 1A)/crescentic (Fig 1B) lamellar keratoplasty: the corneal epithelial side was dried, and the semilunar/crescentic shape to be excised was marked on the epithelium with

However, Pentacam is the first Scheimpflug camera–based instrument that can capture images in multiple meridians in a single automated scan. Pentacam has been more commonly used in the evaluation of postoperative astigmatism after keratoplasty and further guiding the keratotomy procedure. (Buzzonetti, et al., 2009) Further evaluation of

As previously mentioned, the postoperative astigmatism arises partly from intraoperative surgical tissue alignment and uneven suture tension; therefore, improvement in surgical skill is essential in minimizing postoperative astigmatism. Good matching of the graft and the corneal button is required. The thickness and size of the graft should fit the recipient site to restore the contour and curvature of the cornea. As illustrated in Table 1 and Figure 1, the shape and position of the graft and the placement of sutures should be designed to avoid

 For marginal corneal diseases without perforation, semilunar, crescentic, and annular lamellar keratoplasties are commonly performed. The type of lamellar keratoplasty is determined by the size, depth, and location of the corneal lesion and its relationship with

Fig. 1. Schematic Illustration of Marginal Corneal Grafts with Different Shapes According to Different Location of the Corneal Disease. Semiluna (A), Crescentic (B), and Annular (C). The surgical pearls for different types of marginal corneal grafts are listed as following: Semilunar (Fig 1A)/crescentic (Fig 1B) lamellar keratoplasty: the corneal epithelial side was dried, and the semilunar/crescentic shape to be excised was marked on the epithelium with

Pupillary area involved

No Semilunar

No Crescentic

No Annular

Types of PK

(Fig 1B)

(D shape, Fig 1A)

(Ring shape, Fig 1C)

using Pentacam specifically in marginal corneal grafting is warranted.

involving the optical zone. (Table 1).(Liang et al., 2008; Huang et al., 2008)

>6 Within the involved area Yes Total

Straight line between two ends of involved limbus

of the involved cornea

Table 1. Design of different types of lamellar keratoplasty.

<6 Adjucent to the interior side

<6 Incorporate involved area and normal cornea

>6 Incorporate involved area and normal cornea

**3. Control of postoperative astigmatism 3.1 Intraoperative control of astigmatism** 

pupil (Table 1)

Involved area (clock hours)

a marker pen. A diamond knife was used to cut the cornea along the epithelial marker line, reaching three-quarters of normal corneal depth. A razor blade was used for dissection parallel to the bed of stromal lamella. The thinnest area was the last to be dissected. Precise dissection of the lamellar recipient bed to form vertical margins and an even stromal bed depth is very important. Then the recipient bed was covered by the wholly dissected lamellar graft, with both limbi being precisely overlapped. The lamellar graft was secured with 10-0 nylon interrupted sutures at the limbus. The shape and border of the recipient bed could be viewed through the translucent lamellar graft. The diamond knife was used to incise the donor graft along the recipient bed border, and the remaining graft was cut off by corneal scissors. The 10-0 nylon suture was used to secure the graft with interrupted or continuous sutures at the interior side (pupil side) and interrupted sutures at the limbus.

Annular lamellar keratoplasty (Fig 1C): a 7 to 7.25 mm trephine was used to cut to a depth of three-quarters thickness of the normal cornea. A razor blade was used to dissect annularshaped entire corneal circumference. The thinnest area was the last to dissect. The button was prepared in an annular shape leaving the central normal cornea. A trephine with same size or 0.25mm smaller was used to cut the annular donor. The annular graft was inserted into the recipient button and secured with 16 10-0 nylon sutures at the outer border. Since the inner border of the annular graft perfectly fits the inner edge of the recipient bed, the inner border can be left unsutured in most cases. When there is suspected space between graft and the bed, then eight 10-0 nylon sutures should be applied at the inner border.

If the thickness of the peripheral foci is remarkably reduced and results in evident ectacia, the donor graft should be undersized by 0.25 to 0.5 mm as compared with the recipient bed. When this narrower donor is sutured onto the wider recipient bed, tightening of nonabsorbable polypropylene sutures results in a 'belt-tightening' effect with exertion of a compressive force on the recipient bed, resulting in flattening and reduction of ectasia of the cornea, and more importantly, in significant reduction in astigmatism.

In advanced cases, corneal perforation may occur before or during surgery. If the perforation is small with iris prolapsed, the anterior chamber is deep without aqueous leaking, lamellar keratoplasty mentioned above is still effective. If the perforation is small but the anterior chamber is shallow with aqueous leakage, a double lamellar keratoplasty is advocated (Fig 2). In double lamellar keratoplasty, beneath the anterior graft, a posterior endothelium graft with a same size as the perforation is sutured by interrupted 10-0 nylon. If the perforation is larger than 3 mm, penetrating keratoplasty should be considered.

Fig. 2. Schematic Illustration of double lamellar keratoplasty. A graft with endothelium and deep lamellar is underneath the anterior lamellar graft.

When compared penetrating keratoplasty with lamellar keratoplasty, it is believed that lamellar keratoplasty has less postoperative astigmatism. The lower astigmatism in lamellar keratoplasty is ascribed to the fact that the residual corneal lamellar bed provides support to maintain the normal corneal curvature during and after surgery, which guarantees that the above corneal graft can be placed and heal in an ideal position without corneal torsion.

Additionally, to create a smoother graft-host interface and more accurate incision depth in marginal lamellar keratoplasty, recently-developed techniques (such as excimer laser and femtosecond laser) can be applied to prepare the donor graft and recipient bed. These methods can extensively prevent high astigmatism as well as interlamellar opacification, and provide excellent refractive results (Yilmaz et al., 2007;Mian & Shtein, 2007; Sarayba et al., 2007; Schmitz et al., 2003; Soong et al., 2005).

Intraoperative keratometry is a simple device but very useful to guide adjusting sutures to create an even suture tension (Gross et al., 1997). Therefore the amount of postoperative stigmatism can be further reduced.

### **3.2 Postoperative control of astigmatism**

Several surgical and nonsurgical options now exist for the management of postoperative astigmatism after marginal corneal grafts, and a stepwise approach to disease severity and stability would represent a logical approach. The stability of surgical induced astigmatism mainly depends on the duration of postoperative period. Videokeratography such as Orbscan topography or Pentacam helps to detect and to monitor the changes of corneal astigmatism after surgery.

For low and stable corneal irregular astigmatism, patients are visually rehabilitated with glasses or contact lens. In cases of high but stable corneal irregular astigmatism, surgical intervention becomes an option. According to Guell's definition, "*low"* means 2 or fewer Snellen lines of difference between the best rigid gas permeable contact lens (RGP) visual acuity and best spectacle corrective visual acuity (BSCVA). When this difference is more than two lines we consider the case as high irregular astigmatism.(Guell & Velasco, 2003)

#### **3.2.1 Contact lens**

Although spectacles are the simplest method of addressing postoperative refractive error, its corrective effect for irregular astigmatism is limited. Contact lenses often provide superior visual acuity and are frequently required in eyes with evident irregular astigmatism. Designing a contact lens for a patient who has undergone keratoplasty will require the practitioner to carefully assess all the relevant features of the corneal graft. In this regard, there are many factors that need to be considered including the diameter of the graft zone, the topographical relationship between the host cornea and donor cornea, the corneal (graft) toricity and the location of the graft. (Szczotka & Lindsay, 2003)

The various types of contact lens tried in patients with postoperative astigmatism include soft toric lenses, scleral and toric PMMA contact lens, rigid gas permeable (RGP) contact lens, apex and toric RGP lenses, Softperm and scleral lenses. It is generally believed that a spherical rigid lens can correct up to 4D of corneal astigmatism and for higher astigmatism, a back toric or bitoric lens is preferred. Kastl and Kirby reported usefulness of bitoric rigid contact lens for high corneal astigmatism. The lenses can be manufactured in gas-permeable material for corneas with as much as 6D of toricity. (Kastl & Kirby, 1987)

Kompellar et al have reported that large-diameter RGP contact lenses are better tolerated and lead to significant improvement in visual acuity in patients with irregular astigmatism caused by marginal corneal ectacia. (Kompella et al., 2002) In advanced cases, the high asymmetric against-the-rule astigmatism makes soft lens and rigid lens fitting difficult. The

guarantees that the above corneal graft can be placed and heal in an ideal position

Additionally, to create a smoother graft-host interface and more accurate incision depth in marginal lamellar keratoplasty, recently-developed techniques (such as excimer laser and femtosecond laser) can be applied to prepare the donor graft and recipient bed. These methods can extensively prevent high astigmatism as well as interlamellar opacification, and provide excellent refractive results (Yilmaz et al., 2007;Mian & Shtein, 2007; Sarayba et

Intraoperative keratometry is a simple device but very useful to guide adjusting sutures to create an even suture tension (Gross et al., 1997). Therefore the amount of postoperative

Several surgical and nonsurgical options now exist for the management of postoperative astigmatism after marginal corneal grafts, and a stepwise approach to disease severity and stability would represent a logical approach. The stability of surgical induced astigmatism mainly depends on the duration of postoperative period. Videokeratography such as Orbscan topography or Pentacam helps to detect and to monitor the changes of corneal

For low and stable corneal irregular astigmatism, patients are visually rehabilitated with glasses or contact lens. In cases of high but stable corneal irregular astigmatism, surgical intervention becomes an option. According to Guell's definition, "*low"* means 2 or fewer Snellen lines of difference between the best rigid gas permeable contact lens (RGP) visual acuity and best spectacle corrective visual acuity (BSCVA). When this difference is more than two lines we consider the case as high irregular astigmatism.(Guell & Velasco, 2003)

Although spectacles are the simplest method of addressing postoperative refractive error, its corrective effect for irregular astigmatism is limited. Contact lenses often provide superior visual acuity and are frequently required in eyes with evident irregular astigmatism. Designing a contact lens for a patient who has undergone keratoplasty will require the practitioner to carefully assess all the relevant features of the corneal graft. In this regard, there are many factors that need to be considered including the diameter of the graft zone, the topographical relationship between the host cornea and donor cornea, the corneal (graft)

The various types of contact lens tried in patients with postoperative astigmatism include soft toric lenses, scleral and toric PMMA contact lens, rigid gas permeable (RGP) contact lens, apex and toric RGP lenses, Softperm and scleral lenses. It is generally believed that a spherical rigid lens can correct up to 4D of corneal astigmatism and for higher astigmatism, a back toric or bitoric lens is preferred. Kastl and Kirby reported usefulness of bitoric rigid contact lens for high corneal astigmatism. The lenses can be manufactured in gas-permeable

Kompellar et al have reported that large-diameter RGP contact lenses are better tolerated and lead to significant improvement in visual acuity in patients with irregular astigmatism caused by marginal corneal ectacia. (Kompella et al., 2002) In advanced cases, the high asymmetric against-the-rule astigmatism makes soft lens and rigid lens fitting difficult. The

toricity and the location of the graft. (Szczotka & Lindsay, 2003)

material for corneas with as much as 6D of toricity. (Kastl & Kirby, 1987)

without corneal torsion.

al., 2007; Schmitz et al., 2003; Soong et al., 2005).

**3.2 Postoperative control of astigmatism** 

stigmatism can be further reduced.

astigmatism after surgery.

**3.2.1 Contact lens** 

Softperm lens, which has a central RGP portion and a peripheral hydrophilic skirt, has been found useful in correcting irregular astigmatism. (Astin1994) The introduction of gaspermeable scleral contact lenses has generated a renewed interested. (Pullum & Buckley, 1997) These lenses offer advantages over other lens designs, such as easy maintenance and a scleral bearing surface that eliminates the need for close alignment between the cornea and the lens compared with a RGP contact lens (as required in RGP lenses).

### **3.2.2 Selective removal or augmentation of suture**

Selective suture removal should be waited at least 6-8 weeks after surgery if there is no loose suture. The suture removal should be based on central keratometry readings and corneal topography. The curvature, contour of the whole cornea and the amount of astigmatism, as well as the steepest and flattest axis are evaluated before suture removal. The suture in the steep axis is removed then. Vise Versa, the suture in the flat axis can be considered to be augmented for the same reason. The topographic changes induced by suture removal occurred immediately. However, continued shifting in corneal curvature did take place over the subsequent 4 to 6 weeks. Unpredictable shifts were more pronounced in patients whose surgery had been performed more than 20 months prior to suture removal.(Goren et al., 1997)

## **3.2.3 Surgical intervention**

Many patients who have undergone corneal transplantation are unable to achieve satisfactory visual acuity with spectacle and contact lens correction alone. On the other hand, contact lenses are sometimes difficult to fit, and they may induce peripheral corneal neovascularization, increasing the risk of graft rejection and failure. Furthermore, some patients, especially the elderly, those with bilateral poor vision, and those with severe dry eye, are unable to handle or to maintain contact lenses. For these patients, refractive surgery becomes a viable option to reduce the post-keratoplasty astigmatism. With the many recent advances in refractive surgery, new possibilities arise for application to improve the vision rehabilitation in patients after marginal keratoplasty. The indication for surgery in patients with postoperative astigmatism after marginal corneal graft is stable but remarkable astigmatism that cannot be corrected by conventional optical means or contact lens intolerance.

Prior to attempting refractive surgery after keratoplasty, there must be adequate tectonic, refractive, and immunogenic stability. The timing of surgery should generally be at least 12 months after keratoplasty and 3 months after suture removal. Since inflammation induced by surgery is a trigger factor of graft rejection, it is advocated that the patient should be stable on minimal immunosuppressive agents before and after surgery (Preschel et al., 2000). Astigmatism should be evaluated through a combination of refraction, keratometry, keratoscopy, corneal videokeratography, and wavefront analysis. Slit-lamp biomicroscopy should be used to evaluate graft location, size, and clarity, with attention to areas of haze or neovascularization. For cases of lamellar keratoplasty, the graft-host interface should be assessed for quality of apposition, override or underride, asymmetry, and edema. Pachymetry measurements should be performed centrally and on either side of the grafthost interface. Specular microscopy is helpful in determining the status of the endothelial cell layer (Preschel et al., 2000).

#### **3.2.3.1 Releasing incision or wedge excision**

Topographic guided releasing incision or wedge excision remains a common and simple method of reducing astigmatism after keratoplasty. The biomechanical effects of incisional keratotomy on post-keratoplasty corneas continue to be studied. The biomechanical response to contraction or relaxation of corneal tissue forms the basis of incisional keratotomy. Using the same principles as selective suture removal, radial and astigmatic keratotomies are rapid and feasible, but their refractive effects are highly variable. Relaxing incision should be designed in the steep axis; while the wedge excision should be designed in the flat axis. Relaxing incisions and compression sutures can correct an average of 4–5 D of astigmatism (Hardten & Lindstrom, 1997). In a recent study evaluating the refractive effect of a standardized incision, the astigmatic effect was found to be proportional to the magnitude of the preoperative cylinder. This suggests that nomograms for congenital astigmatism do not apply to the correction of post-keratoplasty astigmatism (Wilkins et al., 2005). The releasing incision or wedge excision can also be combined with selective adjustment of sutures (Javadi et al., 2009).

As for relaxing incision, two relaxing incisions of 3 clock hours, at 3/4 depth, are used. This procedure may be in combination with two sets of three compression sutures placed 90 degrees from the incisions. Selective removal of the compression sutures allows for a graded reduction in overcorrection. A mean reduction in astigmatism of 6-7 D can be achieved 3 months postoperatively.( Lustbader & Lemp, 1990)

As for wedge excision, a thin sliver of cornea measuring between 0.1 and 0.2 mm in thickness was excised from just inside the graft-recipient interface. The length of the incision centered at the axis of the flatter meridian of the cornea and was extended over a range of 60-90 degrees. The wound was closed with interrupted 10-0 nylon sutures placed every 15 degrees. The mean reduction in astigmatism in this method ranges from 6.3 to 25.4 D. (Ezra et al., 2007)

It should be noted that most of the studies on relaxing incision and wedge excision are carried on routine centric keratoplasty, studies on their potential effect on marginal corneal graft are warranted. According to author's experience, in cases of marginal corneal grafting, it will be better not to involve the graft when designing the incision or excision. Otherwise the wound of corneal graft may take the risk of dehiscence.

#### **3.2.3.2 Laser treatment**

Most authors wait at least 1 year after keratoplasty and 3 months after last suture removal or other refractive procedure prior to performing laser refractive surgeries. Good wound apposition with minimal graft override and underride is important. Adequate endothelial cell counts should also be assessed.

Photorefractive keratectomy has been used after keratoplasty since the early 1990s (Campos et al., 1992). Unfortunately, these studies demonstrated substantial regression, haze, and even severe scarring (Bilgihan et al., 2000). The adjunctive use of mitomycin C 0.02% (0.2 mg/ml) is a promising new method of scar prevention in eyes undergoing photorefractive keratectomy. (Gambato et al., 2005). Photorefractive keratectomy with mitomycin C was used to treat post-keratoplasty hyperopic astigmatism. It should be noted that with larger ablation zones and deeper peripheral ablation than myopic treatments, hyperopic treatments may particularly compromise the integrity of the graft-host junction. No complications have been reported with the adjunctive one-time use of mitomycin C at the time of photorefractive keratectomy.

Nowadays, LASIK has become a popular modality for correcting refractive error after corneal transplantation. This can be combined with arcuate keratotomies and wedge resections for optimal astigmatic control (Barraquer & Rodriguez-Barraquer, 2004;Buzard et al., 2004).

keratotomy on post-keratoplasty corneas continue to be studied. The biomechanical response to contraction or relaxation of corneal tissue forms the basis of incisional keratotomy. Using the same principles as selective suture removal, radial and astigmatic keratotomies are rapid and feasible, but their refractive effects are highly variable. Relaxing incision should be designed in the steep axis; while the wedge excision should be designed in the flat axis. Relaxing incisions and compression sutures can correct an average of 4–5 D of astigmatism (Hardten & Lindstrom, 1997). In a recent study evaluating the refractive effect of a standardized incision, the astigmatic effect was found to be proportional to the magnitude of the preoperative cylinder. This suggests that nomograms for congenital astigmatism do not apply to the correction of post-keratoplasty astigmatism (Wilkins et al., 2005). The releasing incision or wedge excision can also be combined with selective

As for relaxing incision, two relaxing incisions of 3 clock hours, at 3/4 depth, are used. This procedure may be in combination with two sets of three compression sutures placed 90 degrees from the incisions. Selective removal of the compression sutures allows for a graded reduction in overcorrection. A mean reduction in astigmatism of 6-7 D can be achieved 3

As for wedge excision, a thin sliver of cornea measuring between 0.1 and 0.2 mm in thickness was excised from just inside the graft-recipient interface. The length of the incision centered at the axis of the flatter meridian of the cornea and was extended over a range of 60-90 degrees. The wound was closed with interrupted 10-0 nylon sutures placed every 15 degrees. The mean reduction in astigmatism in this method ranges from 6.3 to 25.4 D. (Ezra

It should be noted that most of the studies on relaxing incision and wedge excision are carried on routine centric keratoplasty, studies on their potential effect on marginal corneal graft are warranted. According to author's experience, in cases of marginal corneal grafting, it will be better not to involve the graft when designing the incision or excision. Otherwise

Most authors wait at least 1 year after keratoplasty and 3 months after last suture removal or other refractive procedure prior to performing laser refractive surgeries. Good wound apposition with minimal graft override and underride is important. Adequate endothelial

Photorefractive keratectomy has been used after keratoplasty since the early 1990s (Campos et al., 1992). Unfortunately, these studies demonstrated substantial regression, haze, and even severe scarring (Bilgihan et al., 2000). The adjunctive use of mitomycin C 0.02% (0.2 mg/ml) is a promising new method of scar prevention in eyes undergoing photorefractive keratectomy. (Gambato et al., 2005). Photorefractive keratectomy with mitomycin C was used to treat post-keratoplasty hyperopic astigmatism. It should be noted that with larger ablation zones and deeper peripheral ablation than myopic treatments, hyperopic treatments may particularly compromise the integrity of the graft-host junction. No complications have been reported with the adjunctive one-time use of mitomycin C at the

Nowadays, LASIK has become a popular modality for correcting refractive error after corneal transplantation. This can be combined with arcuate keratotomies and wedge resections for optimal astigmatic control (Barraquer & Rodriguez-Barraquer, 2004;Buzard et al., 2004).

adjustment of sutures (Javadi et al., 2009).

et al., 2007)

**3.2.3.2 Laser treatment** 

cell counts should also be assessed.

time of photorefractive keratectomy.

months postoperatively.( Lustbader & Lemp, 1990)

the wound of corneal graft may take the risk of dehiscence.

Conductive keratoplasty is a radiofrequency-based technique that denatures and shrinks corneal stromal collagen from the heat generated secondary to tissue resistance to current flow. Although conductive keratoplasty is most often used for the reduction of low to moderate levels of hyperopia (McDonald et al., 2002), some have applied this technique to treat post-LASIK hyperopia (Comaish & Lawless, 2003). The potential effect of this modality in treating post-keratoplasty irregular astigmatism remains unknown.

#### **3.2.3.3 Intraocular refractive surgery**

As the understanding of post-keratoplasty biomechanics improves, the ability to apply incisional techniques can be refined with more accurate nomograms. More efficient methods of phacoemulsification are less traumatic to the cornea and warrant further study in this setting. New lens implants allow for the correction of high degrees of astigmatism. Modern cataract surgery appears to have lower incidences of graft rejection and failure. Developments in lens implantation technology continue to offer expanding options for intraocular refractive surgery.

A toric Artisan, or Verisyse, iris-fixated intraocular lens (Ophtec BV, Groningen, The Netherlands) has been used to correct spherocylindrical refractive error after penetrating keratoplasty. In Nuijts's study, the mean time from keratoplasty to lens implantation was 48.9 months, with a mean 21.3 months after suture removal. After implantation, mean refractive spherical equivalent decreased from 4.09 D to 0.96 D, and mean cylinder decreased from 6.66 D to 1.42 D. Eight eyes (50%) had a postoperative uncorrected visual acuity of 20/40 or better, and 94% of eyes were 20/80 or better. No eyes lost lines of bestcorrected visual acuity, and eight eyes (50%) gained two or more lines. Although the endothelial loss rate was 7.6% at 3 months and 21.7% at 6 months, there were no cases of graft failure during the study period (Nuijts et al., 2004).

## **3.2.4 Cross-linking**

In recent years, a variety of treatment modalities have emerged, and includes methods to increase corneal rigidity, such as a novel UVA/riboflavin collagen cross-linking approach. This treatment, however, requires longer term study, and is currently limited to few centres. This utilizes UVA at 370nm to activate riboflavin, generating reactive oxygen species that induce covalent bonds between collagen fibrils. The procedure involves first removing corneal epithelium within a central 6–7mm diameter zone. Riboflavin 0.1% solution is applied at 5-min intervals starting 5–20 min before UVA irradiation, which is provided by an array of two to seven ultraviolet emitting diodes (Vinciguerra et al., 2006). Irradiance is calibrated for 3mW/cm2 at a working distance of 1cm from the cornea. Exceeding this level of irradiance is contraindicated, and patients with corneal thicknesses of under 400mm should also be excluded as the cytotoxic threshold for the endothelium would be breached. In general, the effects of this treatment are limited in the anterior cornea (Spoerl & Seiler, 2004), and Seiler and Hafezi (Seiler & Hafezi, 2006) reported their findings of a demarcation line visible on slit lamp examination at approximately 60% corneal depth.

Clinically, ultraviolet cross-linking treatment appears to be able to slightly flatten of the cornea of up to 2D and increase the corneal symmetry. (Caporossi et al., 2006). Since this treatment alone does not normalize corneal curvature, attempts have been made to combine it with other surgical modalities. While these results appear promising, further studies evaluating safety, stability of effect, and in addition, combination of cross-linking technology with other modalities remain an intriguing possibility in the treatment of postoperative astigmatism after marginal corneal grafts.

Intrastromal ring implants is an alternative option in the treatment of corneal ectacia induced irregular astigmatism. However, it requires a high safe corneal margin which is absent in post marginal corneal grafts. Therefore, intrastromal ring implants is not advocated in these subset of patients.

## **4. References**


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*Ophthalmology.* Vol.112, No.2, pp.208-218.

*Am J Ophthalmol.* Vol.123, No.5, pp.636-43.

*Ophthalmic Surg Lasers.* Vol.28, No.3, pp.208-214.

**4. References** 

248.

436.

pp.819-825.


## **Contact Lens Correction of Regular and Irregular Astigmatism**

Raul Martín Herranz, Guadalupe Rodríguez Zarzuelo and Victoria de Juan Herráez *University of Valladolid, School of Optometry, Optometry Unit - IOBA Eye Institute Spain* 

## **1. Introduction**

156 Astigmatism – Optics, Physiology and Management

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pellucid marginal corneal degeneration. *Am J Ophthalmol.* Vol.110, No.2, pp.149-52.

Refractive, topographic, tomographic, and aberrometric analysis of keratoconic eyes undergoing corneal cross-linking. *Ophthalmology.* Vol.116, No.3, pp.369-378. Wilkins MR, Mehta JS, Larkin DF.(2005). Standardized arcuate keratotomy for postkeratoplasty astigmatism. *J Cataract Refract Surg.* Vol.31, No.2, pp.297-301. Yilmaz S, Ali Ozdil M, Maden A (2007) Factors affecting changes in astigmatism before and

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An astigmatism is an ametropia in which light rays do not focus at a single point (American Academy Ophthalmology, 2005) but form two focal lines. This image of a point is called a conoid of Sturm with two main focal or two primary meridians (Michaels D, 1988). If the primary meridians are always 90º apart, then it is a regular astigmatism. An irregular astigmatism occurs when the primary meridians are not perpendicular.

Astigmatisms can be classified as regular or irregular based on the contribution of the ocular component and by orientation (Benjamin W, 1998). Clinically, one of the most common criteria employed is with respect to the refractive error (i.e., myopia and hyperopia).


In the correction of the refractive errors, spectacles should be considered before contact lenses or refractive surgery (American Academy Ophthalmology, 2005). However, in some astigmatism cases, contact lenses will be the choice method, especially in cases with irregular astigmatisms. The astigmatism compensation with spectacle lenses is possible if the primary meridians are perpendicular because ophthalmic astigmatic lenses can only correct orthogonal astigmatisms. An irregular astigmatism is difficult to correct with standard spectacles, and subjects often complain of blurring (due to the loss of the corrected visual acuity), monocular diplopia or poliopia. In these cases, obliquely crossed cylinders and other techniques have been proposed (Benjamin W, 1998), although visual acuity reached with this method could be inferior to the best possible treatment. This outcome represents an important problem in patients with induced irregular astigmatisms related to a primary eye disease or secondary to some eye surgical procedure or traumatism.

Regular astigmatisms can be corrected with standard ophthalmic lenses, contact lenses and surgical procedures. However, irregular astigmatisms are more difficult to correct with glasses because the visual acuity could be lower than expected. Contact lens could be an elective way to improve visual acuity in these cases. Different surgical procedures have been proposed to correct irregular astigmatism.

In this chapter, we explore ways to correct regular and irregular astigmatism with contact lenses to improve visual function as compared with the visual acuity obtained with a standard ophthalmic spectacle lenses correction.

## **2. Regular and irregular astigmatism**

The cornea is the main refringent surface of the eye. It represents the largest change in the refractive index, and a small change in the corneal radius induces a large effect on power. For this reason, astigmatisms are most frequently produced by the toricity of the anterior corneal surface (Benjamin W, 1998).

The toricity of the lens surfaces or tilting of the lens can be responsible for an astigmatism, and this is referred to as lenticular astigmatism. However, the magnitude of a lenticular astigmatism is small and frequently in the direction opposite that of a corneal astigmatism (Benjamin W, 1998). The abnormal location of the fovea with respect to the optic axes could be also responsible of astigmatism. Lens and retina-induced astigmatisms are called internal astigmatism. Clinically, the most important astigmatisms are attributable to the cornea surface.

For this reason, astigmatisms are clinically classified based on the perpendicularity of the principal meridians of the cornea in regular and irregular corneas.

The clinical assessment of the cornea with keratometry and corneal topography is described in previous chapters of this book. A keratometer is one of the most commonly used instruments for corneal curvature measurements. Corneal topography has been a powerful advance in corneal assessment and permits full corneal exploration. The main disadvantages of corneal topography systems include errors in alignment, focusing, calibration and soft and hardware data interpretation (Hom M, 2000).

Corneal topography is very useful in corneal assessments to classify corneal astigmatisms. Although the different corneal topography devices are available, all of them generate a color-coded topographical map of the corneal curvature (Figure 1). In general, hot colors (red) are used to represent steeper points of the cornea (with high power and low corneal radius), and cold colors (blue) are used to represent the flatter regions of the cornea (with low power and high corneal radius). In a regular astigmatism, the corneal topography is similar to a tie with two perpendicular main meridians (Figure 1-A). In an irregular astigmatism, the corneal topography does not have the two perpendicular meridians (Figure 1-B).

#### **2.1 Regular astigmatism**

In regular astigmatisms, the meridians having the maximum and minimum refractive power are separated by a 90° angle. In these cases, the main meridians of the cornea are perpendicular, and the main focus lines must be orthogonal (Figure 1-A).

Regular astigmatisms may be classified as either with-the-rule or against-the-rule astigmatisms. In with-the-rule astigmatisms, the vertical meridian is the steepest. This type is the more common regular astigmatism, especially in children. In against-the-rule astigmatisms, the horizontal meridian is steeper than vertical one, and this is more frequent found in older adults. The term oblique astigmatism is used to describe a regular astigmatism in which the orientation of the main meridians is not 90º and 180º (American Academy Ophthalmology, 2005).

elective way to improve visual acuity in these cases. Different surgical procedures have been

In this chapter, we explore ways to correct regular and irregular astigmatism with contact lenses to improve visual function as compared with the visual acuity obtained with a

The cornea is the main refringent surface of the eye. It represents the largest change in the refractive index, and a small change in the corneal radius induces a large effect on power. For this reason, astigmatisms are most frequently produced by the toricity of the anterior

The toricity of the lens surfaces or tilting of the lens can be responsible for an astigmatism, and this is referred to as lenticular astigmatism. However, the magnitude of a lenticular astigmatism is small and frequently in the direction opposite that of a corneal astigmatism (Benjamin W, 1998). The abnormal location of the fovea with respect to the optic axes could be also responsible of astigmatism. Lens and retina-induced astigmatisms are called internal astigmatism. Clinically, the most important astigmatisms are attributable to the cornea

For this reason, astigmatisms are clinically classified based on the perpendicularity of the

The clinical assessment of the cornea with keratometry and corneal topography is described in previous chapters of this book. A keratometer is one of the most commonly used instruments for corneal curvature measurements. Corneal topography has been a powerful advance in corneal assessment and permits full corneal exploration. The main disadvantages of corneal topography systems include errors in alignment, focusing,

Corneal topography is very useful in corneal assessments to classify corneal astigmatisms. Although the different corneal topography devices are available, all of them generate a color-coded topographical map of the corneal curvature (Figure 1). In general, hot colors (red) are used to represent steeper points of the cornea (with high power and low corneal radius), and cold colors (blue) are used to represent the flatter regions of the cornea (with low power and high corneal radius). In a regular astigmatism, the corneal topography is similar to a tie with two perpendicular main meridians (Figure 1-A). In an irregular astigmatism, the

In regular astigmatisms, the meridians having the maximum and minimum refractive power are separated by a 90° angle. In these cases, the main meridians of the cornea are

Regular astigmatisms may be classified as either with-the-rule or against-the-rule astigmatisms. In with-the-rule astigmatisms, the vertical meridian is the steepest. This type is the more common regular astigmatism, especially in children. In against-the-rule astigmatisms, the horizontal meridian is steeper than vertical one, and this is more frequent found in older adults. The term oblique astigmatism is used to describe a regular astigmatism in which the orientation of the main meridians is not 90º and 180º (American Academy

principal meridians of the cornea in regular and irregular corneas.

calibration and soft and hardware data interpretation (Hom M, 2000).

corneal topography does not have the two perpendicular meridians (Figure 1-B).

perpendicular, and the main focus lines must be orthogonal (Figure 1-A).

proposed to correct irregular astigmatism.

standard ophthalmic spectacle lenses correction.

**2. Regular and irregular astigmatism** 

corneal surface (Benjamin W, 1998).

**2.1 Regular astigmatism** 

Ophthalmology, 2005).

surface.

Fig. 1. Corneal topographies in regular and irregular astigmatisms. A: Regular astigmatism of 4.0 diopters with two perpendicular main meridians. B: Irregular astigmatism of 6.5 diopters with non-perpendicular main meridians.

## **2.2 Irregular astigmatism**

In irregular astigmatisms, the meridians having the maximum and minimum refractive power are separated by an angle other than 90°. In these cases, the principal meridians are not perpendicular to one another. Furthermore, an irregular astigmatism is defined when the orientation of the principal meridians or the amount of astigmatism changes from point to point across the eye pupil (American Academy Ophthalmology, 2005). For this reason, irregular astigmatism is often used to describe patients with irregular corneal surfaces (Figure 1-B). Importantly, all eyes have at least a small amount of irregular astigmatism (American Academy Ophthalmology, 2005) when the entire corneal surface is assessed, but this is not relevant from a clinical point of view. Significant irregular astigmatism is uncommon and could be related to scarred cornea, keratoconus and surgical procedures.

## **3. Contact lens**

A contact lens is a thin plastic or glass lens that is fitted over the cornea to correct various vision defects (American Heritage Dictionary, 2011). Contact lenses are an adequate device to correct refractive errors (American Academy Ophthalmology, 2005), and there are 125 millions of contact lenses wearers in the world. Contact lens compensation of an astigmatism requires the correct selection of the contact lens design for each case (Figure 2). Refractive astigmatism is the sum of the corneal astigmatism and the lenticular astigmatism, so astigmatism correction with contact lenses must consider both types of ocular astigmatisms. This consideration is important in cases in which the disparity between the corneal and refractive astigmatism suggests an important amount of lenticular astigmatism. For example, if a rigid gas permeable (RGP) contact lens is fitted (see below) in a case with a

Fig. 2. General guidelines for contact lens fitting in patients with astigmatism (adapted from Key JE, 1998). RGP: Rigid gas permeable.

millions of contact lenses wearers in the world. Contact lens compensation of an astigmatism requires the correct selection of the contact lens design for each case (Figure 2). Refractive astigmatism is the sum of the corneal astigmatism and the lenticular astigmatism, so astigmatism correction with contact lenses must consider both types of ocular astigmatisms. This consideration is important in cases in which the disparity between the corneal and refractive astigmatism suggests an important amount of lenticular astigmatism. For example, if a rigid gas permeable (RGP) contact lens is fitted (see below) in a case with a

Fig. 2. General guidelines for contact lens fitting in patients with astigmatism (adapted from

Key JE, 1998). RGP: Rigid gas permeable.

higher refractive astigmatism than corneal astigmatism, an important amount of residual astigmatism (related to lenticular astigmatism) could affect the visual acuity. In these cases, a toric design of RGP contact lens or a toric soft contact lens could be suitable.

Contact lens designs have been approved with different lens replacement frequencies (i.e., daily, monthly, frequent replacement) and with different types of wear: daily wear (contact lenses are worn during open-eye time) and extended or continuous wear (contact lenses are worn during sleep and time spent awake). When considering a contact lens to correct an astigmatism, the type of contact lens must be chosen (Table 1). To prevent contact lens rotation with patient blinking, different systems are provided, such as adding a prism ballast (adding extra material in the inferior zone of the lens), truncating or removing the bottom of the lens or creating thin zones (on the top or in the bottom of the lens). Soft toric lenses often incorporate either a prism ballast or thin zones (Figure 3), but RGP toric lenses stabilize better with a back toric surface. RGP front toric lenses also need a stabilization system.


Fig. 3. Soft toric contact lens design. A. Dynamic toric stabilization system used in toric soft contact lens design. This design permits the correction of astigmatisms lower than 8.00 D. B. Prismatic stabilization system located in the inferior contact lens area. Courtesy of Hecht Contactlinsen/Conoptica.

## **3.1 Soft contact lenses**

Soft contact lenses are made of a flexible plastic material, which is normally hydrophilic. These lenses are generally more comfortable than rigid contact lens, and the lens diameter is large, extending beyond the sclerocorneal limbus. When a soft lens is placed on the eye, the lens conforms to the anterior corneal shape, and the refractive effect of the tears between the contact lens and the cornea is minimized.

Soft contact lens must be fabricated with different power in the main meridians to correct the astigmatism. The manufacturing process permits several toric contact lenses with different power, but they always use perpendicular principal meridians to correct regular astigmatisms. For this reason, soft contact lenses are not an adequate option to correct irregular astigmatisms.

## **3.2 Rigid gas permeable contact lenses**

An RGP contact lens is constructed of a rigid plastic that transmits oxygen to the cornea. RGP lenses have a diameter lower than the corneal diameter. The refractive effects of contact lenses when they are placed on the eye depend largely on whether those lenses conform to the corneal topography. RGP contact lenses do not conform to the corneal shape, and the contact lens-cornea interface produces a post-lens tear pool with refractive power because they are not parallel surfaces (anterior corneal surface and posterior contact lens radius). The post-lens tear film is called a lacrimal lens, tear lens or fluid lens (Benjamin W, 1998). The power of the tear lens is determined by the difference in curvature between the cornea and the posterior radius of the contact lens.

Because the refractive index of tears is similar to the refractive index of the cornea, the tear lens or the lacrimal lens can neutralize more than 90% of the regular and irregular corneal astigmatism. The tear lens is an additional lens in which the anterior curvature radius is determined by the back RGP lens radius, and the posterior radius coincides with the anterior corneal curvature. Therefore, the difference in the power of the steepest and flattest corneal meridians is neutralized by the tear lens, and this simplifies the contact lens power calculation on astigmatic corneas. Additionally, this power effect must be considered in the RGP contact lens spherical power calculation. For example, an RGP back surface steeper than the corneal curvature (apical clearance) will produce a tear lens with positive power, and a RGP back surface parallel to the corneal curvature (apical alignment) will produce a tear lens with no power (plano-parallel film). For an RGP back surface that is flatter than the corneal curvature (apical bearing), the power will be negative (like a divergent lens).

The refractive effect of the tear lens would be of paramount importance in regular and irregular astigmatism correction with RGP contact lens (Figure 4).

RGP contact lenses could be manufactured with different powers in the principal meridians with two different posterior radii. Clinically, a regular astigmatism lower than 4.00 D can be corrected with the refractive effect of the tear lens fitting a spherical RGP contact lens. However, the lens could be instable or could flex and affect subject comfort or visual acuity in some cases. For such cases and in higher regular astigmatisms, a toric RGP contact lens could be fitted to improve subject comfort and visual acuity. The exact RGP contact lens fitting technique is not the objective of this chapter.

## **4. Regular astigmatism correction with contact lenses**

A regular astigmatism is easy to correct with contact lenses, although some points must be considered. With an astigmatism lower than 1/3 of the sphere refractive error, a spherical

Soft contact lenses are made of a flexible plastic material, which is normally hydrophilic. These lenses are generally more comfortable than rigid contact lens, and the lens diameter is large, extending beyond the sclerocorneal limbus. When a soft lens is placed on the eye, the lens conforms to the anterior corneal shape, and the refractive effect of the tears between the

Soft contact lens must be fabricated with different power in the main meridians to correct the astigmatism. The manufacturing process permits several toric contact lenses with different power, but they always use perpendicular principal meridians to correct regular astigmatisms. For this reason, soft contact lenses are not an adequate option to correct

An RGP contact lens is constructed of a rigid plastic that transmits oxygen to the cornea. RGP lenses have a diameter lower than the corneal diameter. The refractive effects of contact lenses when they are placed on the eye depend largely on whether those lenses conform to the corneal topography. RGP contact lenses do not conform to the corneal shape, and the contact lens-cornea interface produces a post-lens tear pool with refractive power because they are not parallel surfaces (anterior corneal surface and posterior contact lens radius). The post-lens tear film is called a lacrimal lens, tear lens or fluid lens (Benjamin W, 1998). The power of the tear lens is determined by the difference in curvature between the cornea

Because the refractive index of tears is similar to the refractive index of the cornea, the tear lens or the lacrimal lens can neutralize more than 90% of the regular and irregular corneal astigmatism. The tear lens is an additional lens in which the anterior curvature radius is determined by the back RGP lens radius, and the posterior radius coincides with the anterior corneal curvature. Therefore, the difference in the power of the steepest and flattest corneal meridians is neutralized by the tear lens, and this simplifies the contact lens power calculation on astigmatic corneas. Additionally, this power effect must be considered in the RGP contact lens spherical power calculation. For example, an RGP back surface steeper than the corneal curvature (apical clearance) will produce a tear lens with positive power, and a RGP back surface parallel to the corneal curvature (apical alignment) will produce a tear lens with no power (plano-parallel film). For an RGP back surface that is flatter than the

corneal curvature (apical bearing), the power will be negative (like a divergent lens).

irregular astigmatism correction with RGP contact lens (Figure 4).

**4. Regular astigmatism correction with contact lenses** 

fitting technique is not the objective of this chapter.

The refractive effect of the tear lens would be of paramount importance in regular and

RGP contact lenses could be manufactured with different powers in the principal meridians with two different posterior radii. Clinically, a regular astigmatism lower than 4.00 D can be corrected with the refractive effect of the tear lens fitting a spherical RGP contact lens. However, the lens could be instable or could flex and affect subject comfort or visual acuity in some cases. For such cases and in higher regular astigmatisms, a toric RGP contact lens could be fitted to improve subject comfort and visual acuity. The exact RGP contact lens

A regular astigmatism is easy to correct with contact lenses, although some points must be considered. With an astigmatism lower than 1/3 of the sphere refractive error, a spherical

**3.1 Soft contact lenses** 

irregular astigmatisms.

contact lens and the cornea is minimized.

**3.2 Rigid gas permeable contact lenses** 

and the posterior radius of the contact lens.

Fig. 4. A schematic representation of the lacrimal lens or tear lens in RGP contact lens fitting. The tear lens is represented in green (like the exploration with fluorescein in clinical practice). A. In regular astigmatisms, the tears between the lens and the cornea correct the astigmatism. The tear lens power is determined by the difference in curvature between the cornea and base curve of the contact lens, including the spherical and toric power. B. In irregular astigmatisms (with irregular anterior corneal surface), the tear film completes the space between the contact lens and cornea and homogenizes the irregular surface.

contact lens may be the first option, both soft or RGP. With spherical soft contact lens, where visual acuity could be incorrect, a soft toric or spherical RGP lens must be fitted. Contact lens visual acuity depends of the type of lens chosen. In general, visual acuity in patients with astigmatisms will be better with RGP lenses than with soft contact lenses. We present four cases to illustrate regular astigmatism corrections with soft toric, spherical and toric RGP contact lenses and five cases of irregular astigmatism correction with RGP contact

lenses.

## **4.1 Soft contact lens correction**

A 31-year-old female patient was fitted with contact lenses for the first time. The patient wanted to participate in sports while wearing the contact lenses. Visual acuity, refraction and keratometry are shown in Table 2, and the slit lamp examination was within the normal limits. Corneal topography revealed a regular astigmatism of 2.00 D approximately.


Table 2. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

Toric silicon hydrogel contact lenses were proposed because the patient presented a medium to moderate astigmatism. RGP contact lenses were not recommended because the patient wanted to practice sports in which the contact lenses could be lost. The lens parameters are shown in Table 3.


Table 3. The lens parameters. T: total diameter.

This contact lens design (balanced vertical thickness profile and prism ballasting geometry) uses the natural force of the lids to orient and center the lens during and between blinks. In addition, the lens has integrated aspheric optics to reduce the amount of positive spherical aberration of the eye and to help improve retinal image quality in low-light conditions (Young G, 2003). Contact lens examination revealed good movement and tear exchange without complications. The contact lenses showed good centration with the stabilization marks in the correct position (Figure 5).

## **4.2 Rigid gas permeable contact lens correction**

RGP contact lenses could be a useful option to correct regular astigmatisms and provide a high visual acuity. Lens design could be defined with general rules (Figure 2) according to the patient's astigmatism. For low astigmatisms, a spherical RGP may be recommended (Case 2), but with high astigmatisms, a toric RGP lens could be necessary (Case 3). Toric RGP lenses are also necessary to correct lens astigmatism (Case 4).

## **4.2.1 Case 2: Spherical RGP lens to correct low astigmatism**

A 34-year-old female RGP contact lenses wearer (8 - 10 hours per day) refers good tolerance and vision acuity but wants new contact lenses because of the poor condition of her current ones.


Table 4. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

0.10 -4.25 -1.50 x 5º 1.0 7.90 mm @ 180º /

Table 2. Visual acuities before and after contact lens fitting and manual keratometry.

Toric silicon hydrogel contact lenses were proposed because the patient presented a medium to moderate astigmatism. RGP contact lenses were not recommended because the patient wanted to practice sports in which the contact lenses could be lost. The lens

Toric

This contact lens design (balanced vertical thickness profile and prism ballasting geometry) uses the natural force of the lids to orient and center the lens during and between blinks. In addition, the lens has integrated aspheric optics to reduce the amount of positive spherical aberration of the eye and to help improve retinal image quality in low-light conditions (Young G, 2003). Contact lens examination revealed good movement and tear exchange without complications. The contact lenses showed good centration with the stabilization

RGP contact lenses could be a useful option to correct regular astigmatisms and provide a high visual acuity. Lens design could be defined with general rules (Figure 2) according to the patient's astigmatism. For low astigmatisms, a spherical RGP may be recommended (Case 2), but with high astigmatisms, a toric RGP lens could be necessary (Case 3). Toric

A 34-year-old female RGP contact lenses wearer (8 - 10 hours per day) refers good tolerance and vision acuity but wants new contact lenses because of the poor condition of her current

**refraction DCVA Manual** 

0.05 +7.00 (-2.00 x 130°) 0.4 8.06 mm @ 150° /

VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

Table 4. Visual acuities before and after contact lens fitting and manual keratometry.

**refraction DCVA Manual keratometry VA with CL** 

7.50 mm @ 90º 1.0

**Model Material Manufacturer** 

<sup>A</sup>Bausch&Lomb

**keratometry VA with CL** 

7.62 mm @ 60° 0.4

Balafilcon

**Subjective** 

**Radius T Power Design/** 

8.70 mm 14.00 mm -4.25 -1.50x5º Purevision

Table 3. The lens parameters. T: total diameter.

marks in the correct position (Figure 5).

ones.

**Visual acuity** 

**4.2 Rigid gas permeable contact lens correction** 

RGP lenses are also necessary to correct lens astigmatism (Case 4).

**4.2.1 Case 2: Spherical RGP lens to correct low astigmatism** 

**Subjective** 

VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

**Visual acuity** 

parameters are shown in Table 3.

Fig. 5. Summary of Case 1. A. Detail of the corneal topography of 2.00 D regular with-therule astigmatism. B. Soft contact lens with scleral position. C. Detail of the soft contact lens stabilization marks in the correct position (at five, six and seven o'clock).

The refractive parameters are shown in Table 4. This case present a mild amblyopia related with high compound hyperopic astigmatism. Corneal topography (Figure 6-A) shows a mild corneal astigmatism pattern, which is similar to a refractive astigmatism.

An RGP lens with an aspheric design was selected. With this type of geometry, the corneal astigmatism is corrected with the toroidal tear lens that is formed between the contact lens and cornea (Meyler J.G., 1994). Because there is a mild astigmatism (i.e., the difference between the principal meridians of the cornea is small), an aspheric lens that is stable and without excessive movement is possible.

Table 5 shows the parameters of the final lens. The base curve was selected, in agreement with the nomogram provided by manufacturer, and it is slightly steeper than K on the basis of manual keratometry. Figure 6-B shows the fluorescein pattern in which one meridian has a low amount of fluorescein (corresponding to flattest meridian of the cornea) and another meridian has more fluorescein in peripheral area (corresponding to the steepest meridian of the cornea). The patient showed good tolerance.

For mild refractive astigmatisms, which correspond with corneal astigmatisms, RGP aspheric lens provides a suitable correction with good stability and tolerance.


Table 5. Lens parameters. T: total diameter.

Fig. 6. Summary of Case 2. A. Detail of the corneal topography with an astigmatism of about 2.50 D. B. Fluorescein pattern with aspheric RGP lens that showed a central alignment and good edge clearance in the astigmatic axes.

## **4.2.2 Case 3: Toric RGP lens to correct moderate astigmatism**

A 42-year-old female was fitted with contact lenses for the first time. She had worn spectacles since childhood. Visual acuity, refraction and keratometry are shown in Table 6,

Table 5 shows the parameters of the final lens. The base curve was selected, in agreement with the nomogram provided by manufacturer, and it is slightly steeper than K on the basis of manual keratometry. Figure 6-B shows the fluorescein pattern in which one meridian has a low amount of fluorescein (corresponding to flattest meridian of the cornea) and another meridian has more fluorescein in peripheral area (corresponding to the steepest meridian of

For mild refractive astigmatisms, which correspond with corneal astigmatisms, RGP

Fig. 6. Summary of Case 2. A. Detail of the corneal topography with an astigmatism of about 2.50 D. B. Fluorescein pattern with aspheric RGP lens that showed a central alignment and

A 42-year-old female was fitted with contact lenses for the first time. She had worn spectacles since childhood. Visual acuity, refraction and keratometry are shown in Table 6,

**Model Material Manufacturer** 

Hecht Contactlinsen / Conoptica

aspheric lens provides a suitable correction with good stability and tolerance.

8.00 mm 9.60 mm +7.75 BIAS-S Boston ES

the cornea). The patient showed good tolerance.

Table 5. Lens parameters. T: total diameter.

good edge clearance in the astigmatic axes.

**4.2.2 Case 3: Toric RGP lens to correct moderate astigmatism** 

**Radius T Power Design/** 


and the slit lamp examination was within normal limits. Corneal topography (keratograph topography) revealed a regular astigmatism of approximately 2.00 D (Figure 7-A).

Table 6. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

A bitoric RGP contact lens was proposed because the patient presented a moderate astigmatism and a residual astigmatism. With lenses incorporating a toroidal back surface, rotation is generally not a problem due to the stabilizing effect of the toric back surface on the toric cornea (Efron N, 2002). The lens parameters are shown in Table 7. Examination revealed good movement and centering. The central area displayed good alignment with optimal clearance under the peripheral curve and the edge lift permitted tear exchange (Figure 7-B).

Fig. 7. Summary of Case 3. A. Keratograph topography showed a regular astigmatism. B. Fluorescein pattern with bitoric RGP lens showed central alignment and good edge clearance. The CL marks are aligned with the flattest corneal meridian.


Table 7. The lens parameters. T: Total diameter.

## **4.2.3 Case 4: Toric RGP lens to correct lens astigmatism**

Case 4 is a 38-year-old female with congenital and bilateral subluxation of the lens due to Marfan´s syndrome (a genetic disorder of the connective tissue). The refractive data are summarized in Table 8. The patient wore soft toric contact lens with low water content for about 12 hours per day during last 24 years with good tolerance. Due to hypoxic stimuli maintenance during this time, corneal neovascularization can be observed in both eyes (Efron N, 2004) (Figure 8-A). The patient expressed the need to improve the visual acuity (VA) of her current contact lenses to obtain a driver's license.

RGP contact lenses provide better quality of vision in high ametropias and supply more oxygen permeability than conventional soft contact lens (Ichijima H and Cavanagh HD, 2007). For these two reasons, RGP lens was selected. In this case, the cornea is practically spherical (Figure 8-B), and all of the astigmatism is internal.


Table 8. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

To obtain parallelism between the contact lens and the cornea, the internal surface of the contact lens must be spherical (or aspherical), and the correction of the astigmatism must be performed with a toric design on the external surface of the contact lens. Because contact lenses with internal spherical (or aspherical) surfaces tend to rotate constantly over the cornea with blinking, a stabilization system is necessary to maintain the lens in the correct position. A ballast prism is one of the most used systems.

Table 9 shows the parameters of the final lens. The base curve was selected in agreement with the nomogram provided by the manufacturer. The fluorogram showed parallelism


Table 9. The lens parameters. T: Total diameter. The lens included a prism of 1.5∆ to facilitate lens stabilization.

Case 4 is a 38-year-old female with congenital and bilateral subluxation of the lens due to Marfan´s syndrome (a genetic disorder of the connective tissue). The refractive data are summarized in Table 8. The patient wore soft toric contact lens with low water content for about 12 hours per day during last 24 years with good tolerance. Due to hypoxic stimuli maintenance during this time, corneal neovascularization can be observed in both eyes (Efron N, 2004) (Figure 8-A). The patient expressed the need to improve the visual acuity

RGP contact lenses provide better quality of vision in high ametropias and supply more oxygen permeability than conventional soft contact lens (Ichijima H and Cavanagh HD, 2007). For these two reasons, RGP lens was selected. In this case, the cornea is practically

**refraction DCVA Manual keratometry VA with CL** 

7.60 mm @ 105° 0.8

**Model Material Manufacturer** 

Hecht Contactlinsen / Conoptica

XO

**Model Material Manufacturer** 

Hecht Contactlinsen / Conoptica

MAC Boston ES

**Radius T Power Design/** 

7.90 mm 9.60 mm -4.00 D Bitoric/BIAS

**4.2.3 Case 4: Toric RGP lens to correct lens astigmatism** 

(VA) of her current contact lenses to obtain a driver's license.

spherical (Figure 8-B), and all of the astigmatism is internal.

<0.05 -25.00 (-5.00 x 115°) 0.6 7.80 mm @ 15° /

VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

position. A ballast prism is one of the most used systems.

**Radius T Power Design/** 

8.00 mm 9.40 mm -21.00 -5.25 x 90° BIAS VPT Boston

Table 9. The lens parameters. T: Total diameter. The lens included a prism of 1.5∆ to

Table 8. Visual acuities before and after contact lens fitting and manual keratometry.

To obtain parallelism between the contact lens and the cornea, the internal surface of the contact lens must be spherical (or aspherical), and the correction of the astigmatism must be performed with a toric design on the external surface of the contact lens. Because contact lenses with internal spherical (or aspherical) surfaces tend to rotate constantly over the cornea with blinking, a stabilization system is necessary to maintain the lens in the correct

Table 9 shows the parameters of the final lens. The base curve was selected in agreement with the nomogram provided by the manufacturer. The fluorogram showed parallelism

**VA Subjective** 

facilitate lens stabilization.

Table 7. The lens parameters. T: Total diameter.

Fig. 8. Summary of Case 4. A. Slit lamp examination showed the neovascularization induced by low oxygen transmissibility of soft contact lens. B. Orbscan corneal topography with low irregular surface induced by contact lens corneal warpage. C. Fluorescein pattern with toric RGP lens shows the marks to assess the lens position.

between the contact lens and the cornea (Figure 8-C). The movement of the lens was correct, allowing appropriate lacrimal exchange. To obtain a stable position of the lens and to avoid any rotation with blinking, prism ballast had to be increased above the recommended value. In Figure 8-C, the stabilization marks can be seen, which indicates the horizontal meridian for this contact lens type, with a little rotation that was compensated for in the final power of the lens.

In conclusion, this case shows that in high ametropias and hypoxia-related ocular surface complications, RGP fitting permits correct management and an improvement in visual acuity. In the case of an internal astigmatism entirely, the design of the contact lens must be with an external toric surface, using the corresponding stabilization system to obtain a successful result.

## **5. Irregular astigmatism correction with contact lenses**

Irregular astigmatism correction with RGP lenses allows for significant improvement in visual acuity as compared with standard spectacles correction (Martin R and Rodriguez G, 2005; Titiyal JS, 2006).

For this reason, RGP contact lens management is the first option in some corneal pathologies with irregular cornea, such as keratoconus (Rabinowitz Y, 1998). However, other pathologies, such as Herpex keratitis and other conditions, may produce irregular astigmatisms or irregular cornea. Cases are observed after surgical procedures (corneal keratoplastia, corneal refractive surgery complications and others) and corneal trauma.

## **5.1 Irregular astigmatism after corneal disease**

Different corneal pathologies might induce irregular corneal surfaces. The pathologies include corneal dystrophies, such as keratoconus (Case 5) or pellucid marginal degeneration. Corneal infections might also induce permanent irregular corneal surfaces (Case 6).

We present two representative cases of irregular astigmatism after a surgical procedure: 1) a complicated LASIK surgery (Case 7) and 2) corneal keratoplasty (Case 8). A third case (Case 9) involves corneal trauma and describes its management with RGP contact lenses.

## **5.1.1 Case 5: Keratoconus management with RGP contact lens**

A 27-year-old male patient was referred for contact lens fitting. The patient history revealed that the patient had been diagnosed with keratoconus in the right eye five years ago.

Visual acuity, refraction and keratometry are shown in Table 10. Slit lamp examination showed a corneal leucoma in the keratoconus apex with a decreasing corneal thickness (Figure 9-A).


Table 10. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

Corneal topography is not a requirement for fitting keratoconus patients, but it is certainly a good starting point. Corneal topography establishes the position of the cone apex and a basic shape pattern. Keratograph topography revealed an irregular cornea with an astigmatism of 6.80 D approximately. A multicurve design for the keratoconus RGP contact lenses was proposed to improve the visual acuity (Hwang JS, 2010).

The lens parameters are shown in Table 11. The examination revealed good movement and tear exchange. The central area over the cone displayed central apical clearance in the keratoconus apex and a mild peripheral alignment with slightly optimal clearance under the peripheral curve. Although the edge lift is not ideal, tear exchange was present (Figure 9-C).


Table 11. Lens parameters. T: Total diameter.

170 Astigmatism – Optics, Physiology and Management

acuity. In the case of an internal astigmatism entirely, the design of the contact lens must be with an external toric surface, using the corresponding stabilization system to obtain a

Irregular astigmatism correction with RGP lenses allows for significant improvement in visual acuity as compared with standard spectacles correction (Martin R and Rodriguez G,

For this reason, RGP contact lens management is the first option in some corneal pathologies with irregular cornea, such as keratoconus (Rabinowitz Y, 1998). However, other pathologies, such as Herpex keratitis and other conditions, may produce irregular astigmatisms or irregular cornea. Cases are observed after surgical procedures (corneal keratoplastia, corneal refractive surgery complications and others) and corneal trauma.

Different corneal pathologies might induce irregular corneal surfaces. The pathologies include corneal dystrophies, such as keratoconus (Case 5) or pellucid marginal degeneration. Corneal

We present two representative cases of irregular astigmatism after a surgical procedure: 1) a complicated LASIK surgery (Case 7) and 2) corneal keratoplasty (Case 8). A third case (Case

A 27-year-old male patient was referred for contact lens fitting. The patient history revealed

Visual acuity, refraction and keratometry are shown in Table 10. Slit lamp examination showed a corneal leucoma in the keratoconus apex with a decreasing corneal thickness

**refraction DCVA Manual keratometry VA with CL** 

distorted. 0.6

9) involves corneal trauma and describes its management with RGP contact lenses.

that the patient had been diagnosed with keratoconus in the right eye five years ago.

Table 10. Visual acuities before and after contact lens fitting and manual keratometry.

Corneal topography is not a requirement for fitting keratoconus patients, but it is certainly a good starting point. Corneal topography establishes the position of the cone apex and a basic shape pattern. Keratograph topography revealed an irregular cornea with an astigmatism of 6.80 D approximately. A multicurve design for the keratoconus RGP contact

The lens parameters are shown in Table 11. The examination revealed good movement and tear exchange. The central area over the cone displayed central apical clearance in the keratoconus apex and a mild peripheral alignment with slightly optimal clearance under the peripheral curve. Although the edge lift is not ideal, tear exchange was present (Figure 9-C).

Not taken -6.50 -2.50 x 60º 0.2 Not possible. Mires

VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

lenses was proposed to improve the visual acuity (Hwang JS, 2010).

infections might also induce permanent irregular corneal surfaces (Case 6).

**5.1.1 Case 5: Keratoconus management with RGP contact lens** 

**5. Irregular astigmatism correction with contact lenses** 

**5.1 Irregular astigmatism after corneal disease** 

**VA Subjective** 

successful result.

2005; Titiyal JS, 2006).

(Figure 9-A).

The management of keratoconus with RGP contact lenses is generally time-consuming. Specifically designed RGP contact lens with small diameters can be a good alternative in these cases to improve visual acuity.

## **5.1.2 Case 6: Post-herpes keratitis irregular cornea**

We present the case of a 62-year-old male patient who was referred for contact lens fitting. The patient history revealed that the patient had been diagnosed with herpes zoster ophthalmicus (HZO) five years ago. Painful cutaneous lesions appeared on the right side of his face and are associated with severe ocular pain in the right eye.

Corneal scarring following HZO can cause significant vision loss (Catron T, 2008). Most of these patients require photorefractive keratectomy (Kaufman SC, 2008), keratoprosthesis (Todani A, 2009) or penetrating keratoplasty (Birnbaum F, 2010) for visual acuity recovery. RGP contact lens, which can mask significant amounts of irregular astigmatism, can improve visual acuity in some of these patients (Titiyal JS, 2006; Kanpolat A, 1995; Jupiter DG, 2000).

Visual acuity, refraction and keratometry are shown in Table 12. The patient had never worn spectacles or contact lenses. Slit lamp examination showed two corneal scars in the paracentral area, which affects the pupil axis (Figure 10-A). The scars caused an alteration in the corneal curvature along the vertical axis (Figure 10-C). The corneal topography revealed an irregular cornea with an astigmatism of 14 D.


Table 12. Visual acuities before and after contact lens fitting, refraction and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity. CL: Contact Lens.

The fitting of contact lenses in a patient who has corneal scars caused by corneal diseases is generally difficult. In this case, these scars resulted in a high regular astigmatism, so it could be managed like a standard toric RGP contact lens fitting. After two diagnostic contact lenses in the same visit, the definitive contact lens was calculated.

The lens parameters are shown in Table 13. The examination revealed good centering and movement. The fluorescein patterns (Figure 10-E) showed good central alignment with two paracentral clearances in the two scars zones (vertical meridian), mild peripheral alignment with slightly optimal clearance under the peripheral curve and good edge clearance to facilitate tear exchange.


Table 13. Contact lens parameters. T: Total diameter.

Fig. 9. Summary of Case 5. A. Slit lamp examination showed corneal leucoma in the central area that affects the pupil axis. B. Orbscan elevation topography (anterior elevation is the best fitting surface). C. Fluorescein pattern with multicurve RGP lens. D. Keratograph simulated fluorescein pattern. The software permits the positioning of contact lens like the real fitting. E. Distorted image with the Placido disc. F.- Keratograph topography.

Fig. 9. Summary of Case 5. A. Slit lamp examination showed corneal leucoma in the central area that affects the pupil axis. B. Orbscan elevation topography (anterior elevation is the best fitting surface). C. Fluorescein pattern with multicurve RGP lens. D. Keratograph simulated fluorescein pattern. The software permits the positioning of contact lens like the

real fitting. E. Distorted image with the Placido disc. F.- Keratograph topography.

Fig. 10. Summary of Case 6. A. Slit lamp examination (optical section) that showed two corneal scars along the vertical axis. B. One of the scars affected the pupil axis. C. Orbscan elevation topography showed an irregular corneal surface with high astigmatism (anterior elevation best fitting surface). D. Orbscan keratometric map, which shows a high astigmatism with a fairly regular pattern. E. Fluorescein pattern with toric RGP lens showed central alignment and two apical clearances in the scar zone.

## **5.2 Irregular astigmatism after surgical procedure**

Irregular astigmatism can be found after different surgical procedures, especially in corneal procedures, such as refractive surgery or corneal keratoplasty. In corneal refractive surgery with an excimer laser, irregular corneal surfaces can be found due to different reasons, such as corneal wound healing, corneal keratitis, irregular laser ablation, decentered surgery and others. In corneal keratoplasty, the irregular surface is related to the donor button position and stitch pressure.

In these cases, surgical management could be non-indicated, and RGP contact lens fitting could be a good option for visual acuity improvement. We present two different cases fitted with RGP after decentered LASIK and successful corneal keratoplasty.

## **5.2.1 Case 7: Irregular cornea post Refractive Surgery LASIK**

Male, 37-years old, underwent LASIK ten years ago. The previous refraction was -11.00 D in both eyes. Currently, the patient presents myopic regression and has bad vision when it is corrected with ophthalmic lenses (Table 14). Corneal topography (Figure 11-A) shows the decentered myopic ablation pattern that is responsible for the reduced quality of vision.


Table 14. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

Surgical correction was not possible because of the reduced corneal thickness (Figure 11-D); therefore, RGP lens fitting was indicated with the aim to obtain a regular optical surface. Because the cornea presents an oblate shape (flatter centrally than peripherally), a reverse geometry design was selected to achieve parallelism between the contact lens and the cornea (Figure 11-C).

Table 15 shows the parameters of the final lens. The back optic zone radius of the lens was selected to provide corneal alignment between the first peripheral curve and the peripheral cornea to reduce central pooling in the refractive ablation zone and to have an optimal intermediate fit with a poorly defined contact and slightly wide edge clearance (Martin and Rodriguez, 2005). The fluorescein pattern (Figure 11-C) showed moderate pooling at the central ablated area and a mid-peripheral alignment with slightly optimal clearance under the peripheral curve of the contact lens. Although vision acuity with the contact lens was very similar to that obtained with spectacles, the patient reported significant vision improvement.

In this case, a reverse geometry RGP contact lens fitting was effective to correct surgically induced irregular surfaces with improved patient vision and comfortable wear.


Table 15. The lens parameters. T: Total diameter. The peripheral code was 4.0 diopter steeper than the base radius.

Irregular astigmatism can be found after different surgical procedures, especially in corneal procedures, such as refractive surgery or corneal keratoplasty. In corneal refractive surgery with an excimer laser, irregular corneal surfaces can be found due to different reasons, such as corneal wound healing, corneal keratitis, irregular laser ablation, decentered surgery and others. In corneal keratoplasty, the irregular surface is related to the donor button position

In these cases, surgical management could be non-indicated, and RGP contact lens fitting could be a good option for visual acuity improvement. We present two different cases fitted

Male, 37-years old, underwent LASIK ten years ago. The previous refraction was -11.00 D in both eyes. Currently, the patient presents myopic regression and has bad vision when it is corrected with ophthalmic lenses (Table 14). Corneal topography (Figure 11-A) shows the decentered myopic ablation pattern that is responsible for the reduced quality of

**VA Subjective refraction DCVA Manual keratometry VA with CL** 

Surgical correction was not possible because of the reduced corneal thickness (Figure 11-D); therefore, RGP lens fitting was indicated with the aim to obtain a regular optical surface. Because the cornea presents an oblate shape (flatter centrally than peripherally), a reverse geometry design was selected to achieve parallelism between the contact lens and the

Table 15 shows the parameters of the final lens. The back optic zone radius of the lens was selected to provide corneal alignment between the first peripheral curve and the peripheral cornea to reduce central pooling in the refractive ablation zone and to have an optimal intermediate fit with a poorly defined contact and slightly wide edge clearance (Martin and Rodriguez, 2005). The fluorescein pattern (Figure 11-C) showed moderate pooling at the central ablated area and a mid-peripheral alignment with slightly optimal clearance under the peripheral curve of the contact lens. Although vision acuity with the contact lens was very similar to that obtained with spectacles, the patient reported significant vision

In this case, a reverse geometry RGP contact lens fitting was effective to correct surgically

**Radius T Power Design/ Model Material Manufacturer**  9.15 mm 10.60 mm -9.00 D Ortokon Boston ES Hecht Contactlinsen

Table 15. The lens parameters. T: Total diameter. The peripheral code was 4.0 diopter

induced irregular surfaces with improved patient vision and comfortable wear.

Table 14. Visual acuities before and after contact lens fitting and manual keratometry.

9.30 mm @ 5° 0.8

/ Conoptica

with RGP after decentered LASIK and successful corneal keratoplasty.

0.16 -5.00 0.7 9.45 mm @ 95° /

VA: visual acuity, DCVA: Distance corrected visual acuity, CL: Contact Lens.

**5.2.1 Case 7: Irregular cornea post Refractive Surgery LASIK** 

**5.2 Irregular astigmatism after surgical procedure** 

and stitch pressure.

cornea (Figure 11-C).

improvement.

steeper than the base radius.

vision.

Fig. 11. A summary of Case 7. A. Orbscan elevation topography showed the decentered ablation of the excimer laser. B. Contact lens position with optimum centering. C. Fluorescein pattern of the contact lens. D. The Orbscan pachymetric map showed central thinning of the cornea. E. Corneal topography revealed the effect of the myopic excimer laser ablation.

## **5.2.2 Case 8: Post-penetrating keratoplasty irregular cornea**

A 25-year-old male with history of a bilateral keratoconus and a good tolerance of RGP contact lenses had corneal hydrops in the left eye. Penetrating keratoplasty was required to restore corneal transparency (Figure 12-A). Corneal transplant was successfully performed. After surgery many stitches were removed, but at the time of discharge, some stitches have remained (Figure 12-B) and were responsible for 9 diopters of corneal astigmatism (Figure 12-C). With subjective refraction, the patient obtained good visual acuity (Table 16). Due to high astigmatic aniseikonia induced with spectacles, the correction was made using contact lenses.


Table 16. Visual acuities before and after contact lens fitting, refraction and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity. CL: Contact Lens.

The astigmatism present in this patient was fully corneal but had an irregular component due to the surgery. For this reason, RGP contact lenses were selected instead of soft contact lenses. Lenses with toric back surface were selected to obtain parallelism with the cornea and a stable fitting. This lens type presents two different powers, one in each principal meridian. This design allows for the correction of corneal astigmatism that matches up with a refractive astigmatism. An induced astigmatism is caused by the large difference between the internal radius and the refractive index of the lens material. The neutralization of the induced astigmatism requires a toroidal front surface, so a bitoric lens is needed. The final refractive effect is spherical, and this type is called a compensated bitoric lens: the back toric surface corrects the entire refractive cylinder created due to the corneal toricity and the front surface incorporates the correction for the induced astigmatism (Efron, 2002).

Table 17 shows the final parameters of the lens. The base curves were selected in agreement with the nomogram provided by the manufacturer on the basis of manual keratometry and corneal topography. Figure 12-D showed the fitting fluorogram, which shows the general parallelism between the cornea and contact lens with slightly irregular areas and without excessive contact. The lens movement was correct, allowing adequate tear exchange, and the lens position was stable. The stabilization marks (which in this contact lens type indicates the flattest meridian of the lens) match up with flattest meridian of the cornea. The lens provided excellent visual acuity with good subjective tolerance.


Table 17. Contact lens parameters. T: Total diameter.

#### **5.3 Irregular astigmatism after corneal trauma**

A 38-year-old male patient was referred for contact lens fitting. The patient had undergone vitrectomy (retinal detachment) and lens extraction after open globe injuries due to a work accident in right eye (RE) one year ago. Corneal perforation injuries can cause corneal scars and irregular astigmatisms (McMahon TT, 1997). Most of these patients require penetrating keratoplasty for visual acuity recovery. However, different types of RGP contact lens have been proposed for improved visual acuity in impaired post-traumatic scarred corneas (Grunauer-Kloevekorn C, et al 2004; Boghani S, et al 1991; Kok JH, et al, 1991).

The visual acuity, refraction and keratometry are shown in Table 18. The patient had never worn spectacles or contact lenses. Slit lamp examination showed an inferior conjunctival scar secondary to eye surgery and a corneal scar in the central area that affects the pupil axis (Figure 13-A). The pupil was inferior and nasal decentered. Orbscan II topography revealed an irregular cornea with an astigmatism of 9.50 D.

**Visual acuity Subjective refraction DCVA Manual keratometry VA with CL** 

Table 16. Visual acuities before and after contact lens fitting, refraction and manual keratometry.

The astigmatism present in this patient was fully corneal but had an irregular component due to the surgery. For this reason, RGP contact lenses were selected instead of soft contact lenses. Lenses with toric back surface were selected to obtain parallelism with the cornea and a stable fitting. This lens type presents two different powers, one in each principal meridian. This design allows for the correction of corneal astigmatism that matches up with a refractive astigmatism. An induced astigmatism is caused by the large difference between the internal radius and the refractive index of the lens material. The neutralization of the induced astigmatism requires a toroidal front surface, so a bitoric lens is needed. The final refractive effect is spherical, and this type is called a compensated bitoric lens: the back toric surface corrects the entire refractive cylinder created due to the corneal toricity and the front

Table 17 shows the final parameters of the lens. The base curves were selected in agreement with the nomogram provided by the manufacturer on the basis of manual keratometry and corneal topography. Figure 12-D showed the fitting fluorogram, which shows the general parallelism between the cornea and contact lens with slightly irregular areas and without excessive contact. The lens movement was correct, allowing adequate tear exchange, and the lens position was stable. The stabilization marks (which in this contact lens type indicates the flattest meridian of the lens) match up with flattest meridian of the cornea. The lens

**Radius T Power Design/ Model Material Manufacturer** 

7.20 mm 8.70 mm -2.00 D KAKC-N BTC Boston ES Hecht Contactlinsen

A 38-year-old male patient was referred for contact lens fitting. The patient had undergone vitrectomy (retinal detachment) and lens extraction after open globe injuries due to a work accident in right eye (RE) one year ago. Corneal perforation injuries can cause corneal scars and irregular astigmatisms (McMahon TT, 1997). Most of these patients require penetrating keratoplasty for visual acuity recovery. However, different types of RGP contact lens have been proposed for improved visual acuity in impaired post-traumatic scarred corneas

The visual acuity, refraction and keratometry are shown in Table 18. The patient had never worn spectacles or contact lenses. Slit lamp examination showed an inferior conjunctival scar secondary to eye surgery and a corneal scar in the central area that affects the pupil axis (Figure 13-A). The pupil was inferior and nasal decentered. Orbscan II topography revealed

(Grunauer-Kloevekorn C, et al 2004; Boghani S, et al 1991; Kok JH, et al, 1991).

6.60 mm @ 175º 1.5

/ Conoptica

0.16 +1.00 -9.00 x 80° 1.0 8.30 mm @ 85º /

surface incorporates the correction for the induced astigmatism (Efron, 2002).

provided excellent visual acuity with good subjective tolerance.

Table 17. Contact lens parameters. T: Total diameter.

**5.3 Irregular astigmatism after corneal trauma** 

an irregular cornea with an astigmatism of 9.50 D.

7.90 mm /

VA: visual acuity, DCVA: Distance corrected visual acuity. CL: Contact Lens.

Fig. 12. A summary of Case 8. A. Slit lamp examination showed the penetrating keratoplasty. B. Detail of the deeper stitch. C. Orbscan corneal topography with 9 D of a slightly regular astigmatism in the center of the cornea. D. Fluorescein pattern with toric RGP lens showed central alignment.


Table 18. Visual acuities before and after contact lens fitting and manual keratometry. VA: visual acuity, DCVA: Distance corrected visual acuity CL: Contact Lens.

Reverse-geometry RGP contact lenses were proposed to improve visual acuity. An empirical fitting was provided. After three diagnostic contact lenses in two visits, the definitive contact lens was calculated. The lens parameters are shown in Table 19. Examination revealed good centering and movement.

The fluorescein patterns (Figure 13-D) showed central pooling with a small apical clearance in the scar zone, mild peripheral alignment with slightly optimal clearance under the peripheral curve and good even edge clearance to facilitate tear exchange.


Table 19. The lens parameters. φT: Total diameter. ATD: Anterior tangential design. \* The first peripheral curve (FPC) was 7.60 mm (this radius is steeper than the central radius of the optical zone because we used a reverse-geometry contact lens).

The fitting of contact lenses in a patient who has corneal scars caused by perforating corneal injuries is difficult. RGP reverse-geometry contact lens fitting with large diameters can be a good alternative in these cases. This fitting could take less time and require fewer visits than standard or aspheric RGP contact lenses in these patients. Computer-aided fitting was of limited value in cases with irregular corneal surfaces.

## **6. Conclusions**

Contact lens management of patients with astigmatisms could be an option to improve the visual acuity obtained with spectacles, especially in cases of irregular astigmatisms.

Regular astigmatisms could be corrected with soft or RGP toric contact lenses, but irregular astigmatism is better corrected with RGP lenses adapted to the corneal topography. The effect of the tear lens in patients with an astigmatism fitted with an RGP lens permits optimal correction of the regular and irregular astigmatism and an improvement of the visual acuity.

Astigmatic patient management must include contact lens fitting as a treatment option along with spectacles and refractive procedures.

## **7. Acknowledgment**

The authors acknowledge Hecht Contactlinsen / Conoptica for the collaboration to facilitate Figure 3. The authors do not have any conflicts of interest or commercial relationships with any device or product included in this chapter.

Table 18. Visual acuities before and after contact lens fitting and manual keratometry.

Reverse-geometry RGP contact lenses were proposed to improve visual acuity. An empirical fitting was provided. After three diagnostic contact lenses in two visits, the definitive contact lens was calculated. The lens parameters are shown in Table 19. Examination

The fluorescein patterns (Figure 13-D) showed central pooling with a small apical clearance in the scar zone, mild peripheral alignment with slightly optimal clearance under the

**Radius\* T Power Design/ Model Material Manufacturer** 

Table 19. The lens parameters. φT: Total diameter. ATD: Anterior tangential design. \* The first peripheral curve (FPC) was 7.60 mm (this radius is steeper than the central radius of the

The fitting of contact lenses in a patient who has corneal scars caused by perforating corneal injuries is difficult. RGP reverse-geometry contact lens fitting with large diameters can be a good alternative in these cases. This fitting could take less time and require fewer visits than standard or aspheric RGP contact lenses in these patients. Computer-aided fitting was of

Contact lens management of patients with astigmatisms could be an option to improve the

Regular astigmatisms could be corrected with soft or RGP toric contact lenses, but irregular astigmatism is better corrected with RGP lenses adapted to the corneal topography. The effect of the tear lens in patients with an astigmatism fitted with an RGP lens permits optimal correction of the regular and irregular astigmatism and an improvement of the

Astigmatic patient management must include contact lens fitting as a treatment option

The authors acknowledge Hecht Contactlinsen / Conoptica for the collaboration to facilitate Figure 3. The authors do not have any conflicts of interest or commercial relationships with

visual acuity obtained with spectacles, especially in cases of irregular astigmatisms.

taken

peripheral curve and good even edge clearance to facilitate tear exchange.

FPC 7.60 mm 10.20 mm +11.25 D ATD / Reverse

optical zone because we used a reverse-geometry contact lens).

limited value in cases with irregular corneal surfaces.

along with spectacles and refractive procedures.

any device or product included in this chapter.

VA: visual acuity, DCVA: Distance corrected visual acuity CL: Contact Lens.

**refraction DCVA Manual keratometry VA with** 

7.45 mm @ 60º / 7.50 mm @ 150º.

Mires were grossly distorted. 0.8

geometry Boston XO Hecht Contactlinsen

/ Conoptica

**CL** 

**VA Subjective** 

finger at 1 m Not taken Not

revealed good centering and movement.

Counter

8.20 mm

**6. Conclusions** 

visual acuity.

**7. Acknowledgment** 

Fig. 13. A summary of Case 9. A. Slit lamp examination showed corneal scarring in the central area that affect the pupil axis. B. Orbscan elevation topography showed an irregular corneal surface with high astigmatism (the anterior elevation had the best fitting surface). C. Orbscan-simulated fluorescein pattern of 10.20 mm diameter RGP lens with a back posterior radius of 8.20 mm. Several differences with the real fluorescein pattern are shown. D. Fluorescein pattern with reverse-geometry and large-diameter RGP lens.

#### **8. References**


## **Posterior Chamber Toric Implantable Collamer Lenses – Literature Review**

Erik L. Mertens *Medipolis Eye Centre, Antwerp, Belgium* 

## **1. Introduction**

180 Astigmatism – Optics, Physiology and Management

American Academy Ophthalmology (2005). *Basic and Clinical Sciences Course: Clinical optics.* American Academy Ophthalmology, San Francisco, Californa (USA). American Heritage Dictionary (2011). Available from : www.houghtonmifflinbooks.com Benjamin W (1998). *Boris's Clinical refraction*, WB Saunders Company, ISBN 7216-5688-9,

Birnbaum F, Reinhard T (2010). *Penetrating keratoplasty in corneal infections with herpes simplex* 

Boghani S, Cohen EJ, Jones-Marioneaux S. (1991). *Contact lenses after corneal lacerations*.

Efron N (2002). *Contact lens practice*. Butterworth-Heinemann. Oxford, ISBN 7506-4690-X,

Efron N (2004). Contact lens complications. Butterworth-Heinemann. Oxford, ISBN

Grunauer-Kloevekorn C, Habermann A, Wilhelm F, et al (2004). *Contact lens fitting as a* 

Hom M (2006). *Manual of contact lens prescribing and fitting with CD-Rom*, Butterworth-

Hwang JS, Lee JH, Wee WR, Kim MK (2010). *Effects of multicurve RGP contact lens use on* 

Ichijima H, Cavanagh HD (2007). *How Rigid Gas-Permeable lenses supply more oxygen to the cornea than silicone hydroges: a nex model.* Eye & Contact Lens 33(5): 216-223. Jupiter DG, Katz HR (2000). *Management of irregular astigmatism with rigid gas permeable* 

Kanpolat A, Ciftci OU (1995). *The use of rigid gas permeable contact lenses in scarred corneas*.

Kaufman SC, 2008. *Use of photorefractive keratectomy in a patient with a corneal scar secondary to* 

Key JE (1998). *The CLAO Pocket Guide to Contact Lens Fitting*. 2nd ed. CLAO Publications,

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McMahon TT, Devulapally J, Rosheim KM, et al (1997). *Contact lens use after corneal trauma*. J

Meyler JG, Ruston DM (1994). *Rigid Gas Permeable aspheric back surface contact lenses- a review.* 

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New Orleans, USA.

Posterior Chamber Phakic Toric Implantable Collamer Lenses have become increasingly used to correct refractive error associated with astigmatism. These devices are claimed to provide high efficacy in terms of refractive correction. This book chapter is an updated review on the safety and effectiveness and potential complications of the toric implantable collamer lens (Toric ICL) published in peer-review literature.

Toric implantable collamer lens (Toric ICL) from Staar Surgical Inc., Monrovia, CA, is a posterior chamber phakic intraocular lens that has been demonstrated to provide safe, effective, predictable and stable visual and refractive outcomes among various refractive ranges of ammetropia1-4. The present review will focus on the use of Toric ICL in the treatment of myopic astigmatism in normal eyes as well as in eyes with keratoconus, pellucid marginal degeneration, after keratoplasty, and also as a secondary piggyback lens.

## **2. Toric ICL in normal astigmatic eyes**

The clinical outcomes of the U.S. FDA TICL clinical trial5 has been published supporting the efficacy and predictability of the TICL in the treatment of myopic astigmatism up to -4.00 diopters (D). In this study, two hundred ten eyes of 124 patients with pre-operative myopia between 2.38 and 19.5 D (spherical equivalent) and 1 to 4 D of astigmatism were enrolled. They analyzed the uncorrected visual acuity (UCVA), refraction, best spectacle-corrected visual acuity (BSCVA), adverse events, and postoperative complications. At 12 months postoperatively, the proportion of eyes with 20/20 or better UCVA (83.1%) was identical to the proportion of eyes with preoperative 20/20 or better BCVA (83.1%); 76.5% had postoperative BCVA better than or equal to preoperative BCVA. The mean manifest refractive cylinder dropped from 1.93 ± 0.84 at baseline to 0.51 ±0.48 D postoperatively, a 73.6% decrease in astigmatism. Mean spherical equivalent refraction improved from - 9.36±2.66 D preoperatively to 0.05±0.46 D postoperatively. A total of 76.9% of eyes were predicted accurately to within ±0.5D, 97.3% to within ±1.0 D, and 100% to within ±2.0 D of predicted spherical equivalent. Postoperatively, 37.6% of eyes had a BCVA of 20/12.5 or better, compared with a preoperative level of 4.8%. BCVA of 20/20 or better occurred in 96.8% postoperatively, compared with 83.1% preoperatively. Mean improvement in BCVA was 0.88 lines; there were 3 cases (1.6%) that lost ≥2 lines of BCVA, whereas 18.9% of cases improved by ≥2 lines. A total of 76.4% of cases gained ≥1 lines of BCVA, whereas only 7.5% of cases lost the equivalent amount (Fig. 1). Three ICL removals were performed without significant loss of BCVA, and 1 clinically significant-lens opacity was observed. They concluded that their results support the efficacy and predictability of Toric ICL implantation to treat moderate to high myopic astigmatism, without identifying important safety concerns during the follow-up.

Fig. 1. Safety: Changes in Lines of vision (BCVA) before and after 12 months of Toric ICL Implantation

Kamiya et al.6 have also analyzed the outcomes of the Toric ICL being compared with wavefront-guided laser in situ keratomileusis (LASIK) in high myopic astigmatism. They studied 30 eyes (18 patients) having Toric ICL implantation and 24 eyes (17 patients) having wavefront-guided LASIK (Technolas 217z) to correct high myopic astigmatism (spherical equivalent ≤-6.0D; refractive cylinder r≥1.0 D). At 6 months, the mean safety index was 1.28±0.25 in the Toric ICL group and 1.01±0.16 in the LASIK group and the mean efficacy index, 0.87±0.15 and 0.83± 0.23, respectively. All eyes in the Toric ICL group and 71% of eyes in the LASIK group were within ±1.00 D of the targeted spherical equivalent correction. The mean change in manifest refraction from 1 week to 6 months was -0.04±0.24 D in the Toric ICL group and -0.60±0.49 D in the LASIK group. There were no significant complications in the Toric ICL group; 2 eyes (8.3%) in the LASIK group required enhancement ablations. They concluded that Toric ICL implantation was better than wavefront-guided LASIK in eyes with high myopic astigmatism in almost all measures of safety, efficacy, predictability, and stability, suggesting that Toric ICL implantation may become a viable surgical option to treat high myopic astigmatism.

Following a comparison from Kamiya et al.6, Choi et al.7 compared the results between Toric ICL and bioptics (ICL + excimer laser ablation) for the correction of myopic astigmatism. They performed a retrospective evaluation in 29 eyes (20 patients) with Toric ICL implantation and 26 eyes (17 patients) treated with bioptics. For eyes treated with bioptics, corneal ablation was performed at 1.5 to 5 months (mean 2.56 months) after ICL implantation by laser epithelial keratomileusis in 17 eyes, LASIK in 8 eyes, and photorefractive keratectomy in 1 eye. UCVA, BCVA, refraction, adverse events, safety, and

of cases lost the equivalent amount (Fig. 1). Three ICL removals were performed without significant loss of BCVA, and 1 clinically significant-lens opacity was observed. They concluded that their results support the efficacy and predictability of Toric ICL implantation to treat moderate to high myopic astigmatism, without identifying important safety

Fig. 1. Safety: Changes in Lines of vision (BCVA) before and after 12 months of Toric ICL

Kamiya et al.6 have also analyzed the outcomes of the Toric ICL being compared with wavefront-guided laser in situ keratomileusis (LASIK) in high myopic astigmatism. They studied 30 eyes (18 patients) having Toric ICL implantation and 24 eyes (17 patients) having wavefront-guided LASIK (Technolas 217z) to correct high myopic astigmatism (spherical equivalent ≤-6.0D; refractive cylinder r≥1.0 D). At 6 months, the mean safety index was 1.28±0.25 in the Toric ICL group and 1.01±0.16 in the LASIK group and the mean efficacy index, 0.87±0.15 and 0.83± 0.23, respectively. All eyes in the Toric ICL group and 71% of eyes in the LASIK group were within ±1.00 D of the targeted spherical equivalent correction. The mean change in manifest refraction from 1 week to 6 months was -0.04±0.24 D in the Toric ICL group and -0.60±0.49 D in the LASIK group. There were no significant complications in the Toric ICL group; 2 eyes (8.3%) in the LASIK group required enhancement ablations. They concluded that Toric ICL implantation was better than wavefront-guided LASIK in eyes with high myopic astigmatism in almost all measures of safety, efficacy, predictability, and stability, suggesting that Toric ICL implantation may become a viable surgical option to

Following a comparison from Kamiya et al.6, Choi et al.7 compared the results between Toric ICL and bioptics (ICL + excimer laser ablation) for the correction of myopic astigmatism. They performed a retrospective evaluation in 29 eyes (20 patients) with Toric ICL implantation and 26 eyes (17 patients) treated with bioptics. For eyes treated with bioptics, corneal ablation was performed at 1.5 to 5 months (mean 2.56 months) after ICL implantation by laser epithelial keratomileusis in 17 eyes, LASIK in 8 eyes, and photorefractive keratectomy in 1 eye. UCVA, BCVA, refraction, adverse events, safety, and

concerns during the follow-up.

Implantation

treat high myopic astigmatism.

efficacy were assessed preoperatively and 1, 6, and 12 months postoperatively. At 1 month postoperatively, UCVA in the Toric ICL group was significantly higher than in the bioptics group (P=.02). However, the difference in UCVA at 12 months was not significant. At 12 months, mean spherical equivalent refraction was 0.33±0.21 D in the Toric ICL group and 0.29±0.41 D in the bioptics group (P=.07). Mean astigmatic error was higher in the Toric ICL group (-0.42±0.32 D) than in the bioptics group (-0.32±0.38 D) (P=.10). In the bioptics group, the mean refractive cylinder at 12 months decreased from that reported at 6 months because of retreatment performed in two eyes. Safety and efficacy were not statistically different between groups. One eye with a Toric ICL was treated to correct lens decentration and two crystalline lens opacities were observed after bioptics. They concluded that Toric ICL implantation provides reliable visual outcomes similar to bioptics and that the advantages of Toric ICL implantation are a more stable visual outcome and the elimination of laser treatments and their inherent risks.

Bhikoo et al.8 reported their outcomes at 12-months follow-up in 77 eyes with moderate to high myopic astigmatism who underwent Toric ICL implantation. The preoperative mean spherical equivalent ranged from -2.50D to -15.00 D of myopia and from 1.00 D to 7.00 D of astigmatism. At 12 months, mean manifest refractive cylinder decreased 81% from 2.38 D to 0.44 D. Mean manifest refractive cylinder within 1.00 D occurred in 99% (76/77) of eyes, whereas 86% (66/77) was within 0.75 D. 99% (76/77) had postoperative BCVA better than or equal to preoperative values, whereas 78% (60/77) gained up to one line BCVA and 1% (1/77) lost one line BCVA. Uncorrected binocular vision of 6/6 or better occurred in 90% (38/42) of patients compared with binocular BCVA of 6/6 or better in 67% (28/42) preoperatively. One ICL was replaced due to low vaulting and two eyes with astigmatism of 3.25 D and 3.50 D received subsequent LASIK to reduce residual small refractive errors. Indications for ICL were: myopia too high for LASIK (73%), cornea too thin for LASIK (44%) and contact lens intolerance (33%). Night halos were reported in 10% (8/77) of eyes at 12 months, one ICL was removed due to unrecognized preoperative glaucoma and there were no cases of cataract formation, or endophthalmitis. They concluded that the outcome supports the safety, efficacy and predictability of Toric ICLs to treat myopic astigmatism.

In a recent study, Kamiya et al.9 assessed the 1-year clinical outcomes of Toric ICL implantation for moderate to high myopic astigmatism in 56 eyes of 32 consecutive patients, with spherical equivalent errors of -4.00 to -17.25 D and cylindrical errors of -0.75 to -4.00 D. They analyzed UCVA, BCVA, safety index, efficacy index, predictability, stability, adverse events and measured the higher order aberrations (HOAs) and the contrast sensitivity function. LogMAR UCVA and BCVA were -0.11±0.12 and -0.19±0.08 1 year after surgery, respectively. The safety and efficacy indices were 1.17±0.21 and 1.00±0.29 with 91% and 100% of the eyes within 0.5 and 1.0 D, respectively, of the targeted correction. Manifest refraction changes of -0.07±0.27 D occurred from 1 week to 1 year. For a 4-mm pupil, fourthorder aberrations were changed, not significantly, from 0.05±0.02 μm before surgery to 0.06±0.03 μm after surgery (P = 0.38). Similarly, for a 6-mm pupil, fourth-order aberrations were not significantly changed, merely from 0.20 ± 0.08 μm before surgery to 0.23 ± 0.11 μm after surgery (P = 0.15). The area under the log contrast sensitivity function was significantly increased from 1.41 ± 0.15 before surgery to 1.50 ± 0.13 after surgery (P < 0.001). No visionthreatening complications occurred during the observation period. They concluded that in their experience, the Toric ICL performed well in correcting moderate to high myopic astigmatism during a 1-year observation period, suggesting its viability as a surgical option for the treatment of such eyes.

Alfonso et al.10 analyzed their outcomes with the lens in 55 eyes; assessment included UCVA, BCVA, refraction, vault and adverse events 12 months post-surgery. Preoperatively, the mean sphere in the 55 eyes was -4.65± 3.02 D (range -0.50 to -12.50 D) and the mean cylinder, -3.03±0.79 D (range -1.25 to -4.00 D). At 12 months, the mean Snellen decimal UCVA was 0.80±0.20 and the mean BCVA, 0.85±0.18; 62.0% of eyes had a BCVA of 20/20. More than 50.0% of eyes gained 1 or more lines of BCVA. The treatment was highly predictable for spherical equivalent (r2 = 0.99) and astigmatic components J0 (r2 = 0.97) and J45 (r2 = 0.99). Of the eyes, 94.5% were within ± 0.50 D of the attempted spherical equivalent and all were within ±1.00 D. For J0, 94.5% of eyes were within ±0.50 D and for J45, 98.2% of eyes; all eyes were within ± 1.00 D. The efficacy index was 0.95 at 3 months and 1.08 at 1 year. They concluded that the UCVA and BCVA with the Toric ICL were good and highly stable over 12 months, confirming the procedure is safe, predictable, and effective for correction of moderate to high astigmatic. Similarly, they also analyzed the outcomes for eyes with preoperative cylinder values higher than 4.00D11. The study included 15 eyes of 12 patients (9 women). Preoperatively, the mean manifest spherical refraction was -1.98 D±1.32 (range -0.50 to -5.50 D) and the mean refractive cylinder, -4.85±0.83 D (range -6.50 to -4.00 D). At 12 months, the mean refractive cylinder was -0.55±0.52 D (range -1.50 to 0.00 D), with 93.3% of eyes having less than 1.00 D of cylinder. The mean spherical equivalent was - 0.31±0.42 (range -1.00 to 0.75 D), with more than 70% of eyes within ±0.50 D of the target. For the astigmatic components, 93.3% of eyes were within ±1.00 D of J0 (r2 = 0.98) and all eyes were within ±1.00 D of J45 (r2 = 0.98). The mean UCVA was 0.70±0.20 and the mean BCVA, 0.83±0.12, bieng the overall efficacy index 0.90. Postoperatively, all eyes had unchanged BCVA or gained 1 or more lines. They consluded, that the refractive outcomes and improvement in UCVA and BCVA were rapidly achieved and remained fairly consistent throughout the follow-up period, supporting the use of TICL in eyes with high astigmatism.

In a recent paper, Mertens12 assessed the predictability, efficacy, safety and stability in patients who received a Toric ICL to correct moderate to high myopic astigmatism. He studied 43 eyes of 23 patients with a mean spherical refraction of −4.98 ± 3.49 D (range: 0 to −13 D), and a mean cylinder of −2.62 ± 0.97 D (range: −1.00 to −5.00 D). Main outcomes measures evaluated during a 12-month follow-up included UCVA, refraction, BCVA, vault, and adverse events. At 12 months the mean Snellen decimal UCVA was 0.87 ± 0.27 and mean BCVA was 0.94 ± 0.21, with an efficacy index of 1.05. More than 60% of the eyes gained ≥1 line of BCVA (17 eyes, safety index of 1.14). The treatment was highly predictable for spherical equivalent (*r*2 = 0.99) and astigmatic components: J0 (*r*2 = 0.99) and J45 (*r*2 = 0.90) (Fig.2). The mean spherical equivalent dropped from −7.29 ± 3.4 D to −0.17 ± 0.40 D at 12 months. Of the attempted spherical equivalent, 76.7% of the eyes were within ±0.50 D and 97.7% eyes were within ±1.00 D, respectively. For J0 and J45, 97.7% and 83.7% were within ±0.50 D, respectively. He concluded that the outcomes of the study support the safety, efficacy, and predictability of Toric ICL implantation to treat moderate to high myopic astigmatism.

In addition, it should be considered that custom-designed Toric ICL may correct large sphero-cylindrical refractive errors. Mertens et al.13 reported a case of a 40-year-old woman with high astigmatism and thin corneas who underwent bilateral custom-designed Toric ICL implantation. The appropriate Toric ICL power was calculated to be -8.00 +8.00 x 96**°** for

astigmatism during a 1-year observation period, suggesting its viability as a surgical option

Alfonso et al.10 analyzed their outcomes with the lens in 55 eyes; assessment included UCVA, BCVA, refraction, vault and adverse events 12 months post-surgery. Preoperatively, the mean sphere in the 55 eyes was -4.65± 3.02 D (range -0.50 to -12.50 D) and the mean cylinder, -3.03±0.79 D (range -1.25 to -4.00 D). At 12 months, the mean Snellen decimal UCVA was 0.80±0.20 and the mean BCVA, 0.85±0.18; 62.0% of eyes had a BCVA of 20/20. More than 50.0% of eyes gained 1 or more lines of BCVA. The treatment was highly predictable for spherical equivalent (r2 = 0.99) and astigmatic components J0 (r2 = 0.97) and J45 (r2 = 0.99). Of the eyes, 94.5% were within ± 0.50 D of the attempted spherical equivalent and all were within ±1.00 D. For J0, 94.5% of eyes were within ±0.50 D and for J45, 98.2% of eyes; all eyes were within ± 1.00 D. The efficacy index was 0.95 at 3 months and 1.08 at 1 year. They concluded that the UCVA and BCVA with the Toric ICL were good and highly stable over 12 months, confirming the procedure is safe, predictable, and effective for correction of moderate to high astigmatic. Similarly, they also analyzed the outcomes for eyes with preoperative cylinder values higher than 4.00D11. The study included 15 eyes of 12 patients (9 women). Preoperatively, the mean manifest spherical refraction was -1.98 D±1.32 (range -0.50 to -5.50 D) and the mean refractive cylinder, -4.85±0.83 D (range -6.50 to -4.00 D). At 12 months, the mean refractive cylinder was -0.55±0.52 D (range -1.50 to 0.00 D), with 93.3% of eyes having less than 1.00 D of cylinder. The mean spherical equivalent was - 0.31±0.42 (range -1.00 to 0.75 D), with more than 70% of eyes within ±0.50 D of the target. For the astigmatic components, 93.3% of eyes were within ±1.00 D of J0 (r2 = 0.98) and all eyes were within ±1.00 D of J45 (r2 = 0.98). The mean UCVA was 0.70±0.20 and the mean BCVA, 0.83±0.12, bieng the overall efficacy index 0.90. Postoperatively, all eyes had unchanged BCVA or gained 1 or more lines. They consluded, that the refractive outcomes and improvement in UCVA and BCVA were rapidly achieved and remained fairly consistent throughout the follow-up period, supporting the use of TICL in eyes with high

In a recent paper, Mertens12 assessed the predictability, efficacy, safety and stability in patients who received a Toric ICL to correct moderate to high myopic astigmatism. He studied 43 eyes of 23 patients with a mean spherical refraction of −4.98 ± 3.49 D (range: 0 to −13 D), and a mean cylinder of −2.62 ± 0.97 D (range: −1.00 to −5.00 D). Main outcomes measures evaluated during a 12-month follow-up included UCVA, refraction, BCVA, vault, and adverse events. At 12 months the mean Snellen decimal UCVA was 0.87 ± 0.27 and mean BCVA was 0.94 ± 0.21, with an efficacy index of 1.05. More than 60% of the eyes gained ≥1 line of BCVA (17 eyes, safety index of 1.14). The treatment was highly predictable for spherical equivalent (*r*2 = 0.99) and astigmatic components: J0 (*r*2 = 0.99) and J45 (*r*2 = 0.90) (Fig.2). The mean spherical equivalent dropped from −7.29 ± 3.4 D to −0.17 ± 0.40 D at 12 months. Of the attempted spherical equivalent, 76.7% of the eyes were within ±0.50 D and 97.7% eyes were within ±1.00 D, respectively. For J0 and J45, 97.7% and 83.7% were within ±0.50 D, respectively. He concluded that the outcomes of the study support the safety, efficacy, and predictability of Toric ICL implantation to treat moderate to high myopic

In addition, it should be considered that custom-designed Toric ICL may correct large sphero-cylindrical refractive errors. Mertens et al.13 reported a case of a 40-year-old woman with high astigmatism and thin corneas who underwent bilateral custom-designed Toric ICL implantation. The appropriate Toric ICL power was calculated to be -8.00 +8.00 x 96**°** for

for the treatment of such eyes.

astigmatism.

astigmatism.

Fig. 2. Preoperative versus 12 month postoperative best corrected visual acuity (BCVA) after Toric implantable collamer lens implantation. *Clinical Ophthalmology 2011:5 369-375*

the right eye and -8.50 +7.50 x 86**°** for the left eye with an optical zone of 5.5 mm and 6.875 mm at the corneal plane. Their results, at 3 and 6 months postoperatively, showed that UCVA and DCVA of both eyes improved to 20/20 and 20/16, respectively. At 19 months, UCVA was 20/20 and 20/16 in the right and left eyes, respectively, and BCVA had improved to 20/16 and 20/10, respectively. The subjective refraction was stable, with a change of -0.37±0.17 D from preoperative to 19 months postoperatively. Throughout the postoperative period, iridotomies remained patent and the corneas were clear. They concluded that bilateral implantation of the custom-designed Toric ICL successfully corrected the patient's high astigmatism. Preoperative subjective refractive cylinder of -5.25 x 6**°** in the right eye and -5 x 176**°** in the left eye changed to -0.5 x 77 degrees and -0.5 x 115**°**, respectively, after Toric ICL implantation. There was almost no change in corneal astigmatism. This customized approach led to UCVA of 20/20 in the right eye and 20/16 in the left eye, and DCVA of 20/16 in the right eye and 20/10 in the left eye.

Mertens described the importance of preoperative marking of the eye's horizontal axis prior to Toric ICL implantation12-13 (Fig. 3). This axis would be the reference for later alignment of the lens to the target axis (Fig 4). When doing so intra-operatively, the surgeon must pay attention to use the entrance pupil as a reference for axis marking instead of the geometrical center of the cornea, thus avoiding undesired edge glare and induced coma and other secondary aberrations. Postoperatively, the vaulting of the Toric ICL can be easily assessed with the slit lamp (Fig. 5)

Fig. 3. Preoperative marking of the eye's horizontal axis prior to Toric ICL implantation

Fig. 4. Yellow line indicates lens horizontal axis connecting the 2 diamond-shaped marks of the Toric ICL. The arrows indicate that the surgeon aligned the lens at 5° CCW from the horizontal meridian.

## **3. TICL in eyes with keratoconus**

In keratoconic eyes in which keratorefractive or other alternative refractive procedures were not a good or feasible option, Toric ICL implantation showed promising results. In this case, for example, Coskunseven et al.14 evaluated the results of combined Intacs (Addition

Fig. 3. Preoperative marking of the eye's horizontal axis prior to Toric ICL implantation

Fig. 4. Yellow line indicates lens horizontal axis connecting the 2 diamond-shaped marks of the Toric ICL. The arrows indicate that the surgeon aligned the lens at 5° CCW from the

In keratoconic eyes in which keratorefractive or other alternative refractive procedures were not a good or feasible option, Toric ICL implantation showed promising results. In this case, for example, Coskunseven et al.14 evaluated the results of combined Intacs (Addition

horizontal meridian.

**3. TICL in eyes with keratoconus** 

Fig. 5. Two white arrows: Clearance from the Toric ICL (red arrow) and the crystalline lens (\*) is assessed with the slit-lamp. An optical section focused on the implant will allow the surgeon to observe the Toric ICL-lens space (vault).

Technology, Fremont, CA) and the TICL implantation in keratoconic patients with extreme myopia and irregular astigmatism. They reported the outcomes in three eyes of two consecutive highly myopic keratoconic patients who had undergone Toric ICL implantation after Intacs procedure. Implantation of the Toric ICLs was performed at intervals between six and 10 months after Intacs procedure. They did not encounter intraoperative or postoperative complications. An improvement in UCVA and DCVA was found after Intacs and TICL procedures in all eyes. All eyes were within 1 D of emmetropia, whereas the mean manifest refractive spherical equivalent refraction decreased from -18.50±2.61 D (range, - 16.75 to -21.50 D) to 0.42 D (range, 0 to -0.75 D). The mean difference between preoperative and last follow-up UCVA and BCVA was a gain of 6.67±1.15 lines (ranging from six to eight lines) and 4.33± 2.52 lines (ranging fromof two to seven lines), respectively. They concluded that combined Intacs and Toric ICL implantation in a two-step procedure is an effective method for correcting keratoconic patients with extreme myopia.

Kamiya et al.15 showed two patients in whom Toric ICL have been effective for the correction of high myopic astigmatism with stable keratoconus. Both patients had a history of contact lens intolerance, and refraction and corneal topography were stable for 3 to 4 years. Preoperatively, the manifest refraction was -10.00 -6.00 x 100**°** in case 1 and -8.00 -2.75 x 100**°** in case 2. Postoperatively, the manifest refraction was +0.50 -1.00 x 90**°** in case 1 and -

0.25 -1.25 x 100**°** in case 2. UCVA and BCVA were markedly improved after implantation in both patients without progressive sign of keratoconus during 1-year follow-up. They concluded that Toric ICL implantation may be an alternative for the correction of high myopic astigmatism in eyes with stable keratoconus. Recently, the authors have increased the sample size16. In a new study they evaluated 27 eyes of 14 patients with spherical equivalents of -10.11 ± 2.46 D and astigmatism of -3.03±1.58 D who underwent Toric ICL implantation for mild keratoconus. LogMAR UCVA and LogMAR BCVA were -0.09 ± 0.16 and -0.15 ± 0.09 respectively, 6 months after surgery. The safety and efficacy indices were 1.12 ± 0.18 and 1.01 ± 0.25. At 6months, 85% and 96% of the eyes were within ±0.5 and ±1.0 D respectively of the targeted correction. No vision-threatening complications occurred during the observation period. They again concluded that Toric ICL implantation was good in all measures of safety, efficacy, predictability, and stability for the correction of spherical and cylindrical errors in eyes with early keratoconus, suggesting its viability as a surgical option for the treatment of such eyes.

Alfonso et al.17 also implanted the Toric ICL in 30 keratoconic eyes (21 patients) with a mean myopia of -5.38±3.26 D (range -13.50 to -0.63 D) and a mean cylinder of -3.48±1.24 D (range - 1.75 to -6.00 D). At 12 months, 86.7% of the eyes were within ±0.50 D of the attempted refraction and all eyes were within ±1.00 D. For the astigmatic components J0 and J45, 83.3% of eyes and 86.7% of eyes, respectively, were within ±0.50 D. The mean Snellen UCVA was 0.81±0.20 and the mean BCVA, 0.83±0.18; BCVA was 20/40 or better in 29 eyes 96.7% of eyes and 20/25 or better in 22 eyes (73.3%). No eyes lost more than 2 lines of BCVA; 29 eyes (96.7%) maintained or gained 1 or more lines being the efficacy index of 1.07 and the safety index, 1.16. There were no complications or adverse events concluding that Toric ICL implantation is a predictable, effective procedure to correct ametropia in eyes with keratoconus.

## **4. Toric ICL after penetrating keratoplasty**

The use of the Toric ICL after penetrating keratoplasty has been also proposed. Alfonso et al.18, evaluated the efficacy, predictability, and safety of Toric ICL after this technique in 15 eyes that had preoperative myopia ranging from -2.00 to -17.00 D or astigmatism from -1.50 to -7.00 D. Twenty-four months postoperatively, the mean Snellen decimal UCVA was 0.51±0.30. The UCVA was 20/40 or better in 7 eyes (46.6%) and the mean BCVA was 0.79±0.22. The BCVA was 20/40 or better in 12 eyes (80%) and 20/25 in 6 eyes (40%). No eye lost more than 1 line of acuity, 2 eyes gained 1 line, and 5 eyes gained more than 2 lines; 8 eyes were unchanged, being the safety index 1.58. The spherical equivalent was within ±1.00 D in 80% of eyes and within ±0.50 D in 66.6% of eyes, with a mean postoperative value of - 0.95±1.12 D. At 24 months, the mean endothelial cell loss was 8.1%. they concluded that the results found indicate that Toric ICL is a viable treatment for myopia and astigmatism after penetrating keratoplasty in patients for whom glasses, contact lenses, or corneal refractive surgery are contraindicated.

A case report of Akcay et al.19 also adds valuable literature to this application. They describe that the patient's manifest refraction improved from -8.0 -1.75 x 170**°** preoperatively, with an UCVA of 0.15 and a BCVA of 0.4, to +0.75 -0.50 x 130**°** postoperatively, with a UCVA of 0.8 and a BCVA of 1.0. No serious complications or refractive changes occurred during the 1 year follow-up concludind that implantation of a myopic TICL in phakic eyes is an option to correct postkeratoplasty anisometropia and astigmatism.

0.25 -1.25 x 100**°** in case 2. UCVA and BCVA were markedly improved after implantation in both patients without progressive sign of keratoconus during 1-year follow-up. They concluded that Toric ICL implantation may be an alternative for the correction of high myopic astigmatism in eyes with stable keratoconus. Recently, the authors have increased the sample size16. In a new study they evaluated 27 eyes of 14 patients with spherical equivalents of -10.11 ± 2.46 D and astigmatism of -3.03±1.58 D who underwent Toric ICL implantation for mild keratoconus. LogMAR UCVA and LogMAR BCVA were -0.09 ± 0.16 and -0.15 ± 0.09 respectively, 6 months after surgery. The safety and efficacy indices were 1.12 ± 0.18 and 1.01 ± 0.25. At 6months, 85% and 96% of the eyes were within ±0.5 and ±1.0 D respectively of the targeted correction. No vision-threatening complications occurred during the observation period. They again concluded that Toric ICL implantation was good in all measures of safety, efficacy, predictability, and stability for the correction of spherical and cylindrical errors in eyes with early keratoconus, suggesting its viability as a surgical option

Alfonso et al.17 also implanted the Toric ICL in 30 keratoconic eyes (21 patients) with a mean myopia of -5.38±3.26 D (range -13.50 to -0.63 D) and a mean cylinder of -3.48±1.24 D (range - 1.75 to -6.00 D). At 12 months, 86.7% of the eyes were within ±0.50 D of the attempted refraction and all eyes were within ±1.00 D. For the astigmatic components J0 and J45, 83.3% of eyes and 86.7% of eyes, respectively, were within ±0.50 D. The mean Snellen UCVA was 0.81±0.20 and the mean BCVA, 0.83±0.18; BCVA was 20/40 or better in 29 eyes 96.7% of eyes and 20/25 or better in 22 eyes (73.3%). No eyes lost more than 2 lines of BCVA; 29 eyes (96.7%) maintained or gained 1 or more lines being the efficacy index of 1.07 and the safety index, 1.16. There were no complications or adverse events concluding that Toric ICL implantation is a predictable, effective procedure to correct ametropia in eyes with

The use of the Toric ICL after penetrating keratoplasty has been also proposed. Alfonso et al.18, evaluated the efficacy, predictability, and safety of Toric ICL after this technique in 15 eyes that had preoperative myopia ranging from -2.00 to -17.00 D or astigmatism from -1.50 to -7.00 D. Twenty-four months postoperatively, the mean Snellen decimal UCVA was 0.51±0.30. The UCVA was 20/40 or better in 7 eyes (46.6%) and the mean BCVA was 0.79±0.22. The BCVA was 20/40 or better in 12 eyes (80%) and 20/25 in 6 eyes (40%). No eye lost more than 1 line of acuity, 2 eyes gained 1 line, and 5 eyes gained more than 2 lines; 8 eyes were unchanged, being the safety index 1.58. The spherical equivalent was within ±1.00 D in 80% of eyes and within ±0.50 D in 66.6% of eyes, with a mean postoperative value of - 0.95±1.12 D. At 24 months, the mean endothelial cell loss was 8.1%. they concluded that the results found indicate that Toric ICL is a viable treatment for myopia and astigmatism after penetrating keratoplasty in patients for whom glasses, contact lenses, or corneal refractive

A case report of Akcay et al.19 also adds valuable literature to this application. They describe that the patient's manifest refraction improved from -8.0 -1.75 x 170**°** preoperatively, with an UCVA of 0.15 and a BCVA of 0.4, to +0.75 -0.50 x 130**°** postoperatively, with a UCVA of 0.8 and a BCVA of 1.0. No serious complications or refractive changes occurred during the 1 year follow-up concludind that implantation of a myopic TICL in phakic eyes is an option to

for the treatment of such eyes.

surgery are contraindicated.

**4. Toric ICL after penetrating keratoplasty** 

correct postkeratoplasty anisometropia and astigmatism.

keratoconus.

## **5. Toric ICL in eyes with pellucid marginal degeneration**

Kamiya et al.20 have also recently reported a case in which Toric ICL effectively corrected the refractive errors of pellucid marginal degeneration. They described preoperatively that, in the patient's right eye, the manifest refraction was -10.5 -3.5 x 55**°**, the UCVA was 20/1000, and the BCVA was 20/16; in the left eye, the manifest refraction was -11.0 - 6.5 x 130**°** and the UCVA and BCVA were 20/1000 and 20/20, respectively. After bilateral implantation of a TICL, in the right eye, the manifest refraction was +1.50 - 0.75 x 10**°**, the UCVA was 20/16, and the BCVA was 20/12.5; in the left eye, the manifest refraction was +2.5 -3.25 x 125**°** and the UCVA and BCVA were 20/40 and 20/16, respectively. They did not find signs of progressive disease and no vision-threatening complication were observed during the 6-months follow-up. They considered that Toric ICL implantation may be a viable surgical option for the correction of high myopic astigmatism in eyes with pellucid marginal degeneration.

## **6. Toric ICL for secondary piggyback**

The last indication that has been considered for the use of Toric ICL is piggyback implantation. Kojima et al.21 investigated eight pseudophakic eyes of five patients who underwent piggyback insertion of a Toric ICL to correct residual refractive error. The results showed that pre- and 6-month postoperatively logMAR UCVA were 0.759±0.430 and 0.201±0.458, respectively, with all eyes within ±0.50 D of intended spherical equivalent refraction and refractive astigmatism within ±0.50 D in five (62.5%) eyes and ±1.00 D in seven (87.5%) eyes. No eyes lost more than one line of BCVA and pupillary block occurred in one eye on postoperative day 1. They concluded that piggyback insertion of a Toric ICL appears to be effective and predictable in correcting refractive error in pseudophakic eyes.

## **7. Complications and adverse events with Toric ICL**

Sanders et al. 5 reported secondary surgical interventions in 5 eyes (2.4%) in the Toric ICL study cohort. In 3 eyes the ICL was removed, in one case due to PI-related visual symptoms, in the second case due to trace anterior subcapsular opacity and in the last case due to oversizing and induced anisocoria. One eye had the ICL replaced with a smaller diameter ICL and another eye had a repositioning due to surgical misalignment.

In a more recent study, Kamiya K et al9 reported secondary surgical interventions in five eyes (8.9%). These eyes required repositioning of the lens very early post-operatively, ranging from one day to one week due to off-axis alignment. Two eyes required late repositioning due to off-axis secondary to a traumatic event. Finally, three eyes developed asymptomatic subcapsular opacity, none of them requiring an ICL removal because there was no impact on BCVA. Otherwise they reported no cases on pigment dispersion, pupillary block or other vision-threatening complications during their follow-up period.

Reported adverse events related to Toric ICL are those applicable to ICL in addition to early surgical misalignment and rotation of the implant. Careful marking of the eye's axis and attention to marking the target axis are essential to ensure proper surgical alignment of the Toric ICL. Lens rotations may occur if the lens is too short for the eye's anatomy and in these cases an exchange for a longer diameter ICL should resolve the problem. In rare instances, an optimally vaulting lens may be found off-axis post-operatively; Navas et al22 found that repositioning of the lens back to target axis when the vault is optimal yield a satisfactory outcome in their case study.

Most common complications and adverse events reported with the ICL platform in general include: early replacements due to sizing issues (under- or over-sizing), lens repositioning (surgical misalignment or true early rotation, late rotation), development of anterior subcapsular opactity which becomes clinically significant requiring lens removal and cataract surgery, pupillary block and/or angle closure with elevated IOP due to non functioning peripheral iridotomies (PIs), (too small, too peripheral, occluded or narrowed), and to a lesser extent symptoms of glare/halos or lines coming from the peripheral iridotomies or from the smaller optical zone related to the patient's mesopic pupil diameter. Based on the meta-analysis from Chen et al.23 where different ICL lens designs were included (prototype and obsolete versions as well as currently available V4 model) the most common complication was cataract formation. Several factors involved in cataract development discussed in this analysis included age, degree of myopia, low vault, surgical trauma, learning curve, steroid use, lens design, pre-existing opacities, trauma and inflammation. Several other peer-reviewed articles support the relatively low incidence of complications with the ICL.

In conclusion, Toric ICL has been worldwide used for astigmatism correction showing their efficacy, predictability, stability and safety. Toric ICLs are considered an attractive approach, based in large part on the phenomenal acceptance of intraocular lenses for not only the aphakic or cataract patient but also, recently, the refractive patient. The present chapter reviewed the outcomes for normal astigmatic eyes and also those found in different keratoconic eyes, post-penetrating keratoplasty, eyes with pellucid marginal degeneration and also in pseudophakic eyes with the use ICL as secondary piggyback lens. In general terms, the results of Toric ICL implantation from these studies are in agreement confirming its predictability, efficacy, together with safety outcomes, making this option as a highly reliable alternative in the treatment of moderate to high astigmatism. Then, TICLs are safe and effective tools to compensate for different degrees of astigmatism, involving quite low risks.

## **8. References**


repositioning of the lens back to target axis when the vault is optimal yield a satisfactory

Most common complications and adverse events reported with the ICL platform in general include: early replacements due to sizing issues (under- or over-sizing), lens repositioning (surgical misalignment or true early rotation, late rotation), development of anterior subcapsular opactity which becomes clinically significant requiring lens removal and cataract surgery, pupillary block and/or angle closure with elevated IOP due to non functioning peripheral iridotomies (PIs), (too small, too peripheral, occluded or narrowed), and to a lesser extent symptoms of glare/halos or lines coming from the peripheral iridotomies or from the smaller optical zone related to the patient's mesopic pupil diameter. Based on the meta-analysis from Chen et al.23 where different ICL lens designs were included (prototype and obsolete versions as well as currently available V4 model) the most common complication was cataract formation. Several factors involved in cataract development discussed in this analysis included age, degree of myopia, low vault, surgical trauma, learning curve, steroid use, lens design, pre-existing opacities, trauma and inflammation. Several other peer-reviewed articles support the relatively low incidence of complications

In conclusion, Toric ICL has been worldwide used for astigmatism correction showing their efficacy, predictability, stability and safety. Toric ICLs are considered an attractive approach, based in large part on the phenomenal acceptance of intraocular lenses for not only the aphakic or cataract patient but also, recently, the refractive patient. The present chapter reviewed the outcomes for normal astigmatic eyes and also those found in different keratoconic eyes, post-penetrating keratoplasty, eyes with pellucid marginal degeneration and also in pseudophakic eyes with the use ICL as secondary piggyback lens. In general terms, the results of Toric ICL implantation from these studies are in agreement confirming its predictability, efficacy, together with safety outcomes, making this option as a highly reliable alternative in the treatment of moderate to high astigmatism. Then, TICLs are safe and effective tools to compensate for different degrees of astigmatism, involving quite low

[1] Sanders DR, Doney K, Poco M. United States Food and Drug Administration clinical trial

[2] Uusitalo RJ, Aine E, Sen NH, Laatikainen L. Implantable contact lens for high myopia. J

[3] Lackner B, Pieh S, Schmidinger G et al. Long-term results of implantation of phakic posterior chamber intraocular lenses. J Cataract Refract Surg 2004;30:2269-2276. [4] Pesando PM, Ghiringhello MP, Di MG, Fanton G. Posterior chamber phakic intraocular

[5] Sanders DR, Schneider D, Martin R et al. Toric Implantable Collamer Lens for moderate

[6] Kamiya K, Shimizu K, Igarashi A, Komatsu M. Comparison of Collamer toric

to high myopic astigmatism. Ophthalmology 2007;114:54-61.

follow-up. Ophthalmology 2004;111:1683-1692.

Cataract Refract Surg 2002;28:29-36.

of the Implantable Collamer Lens (ICL) for moderate to high myopia: three-year

lens (ICL) for hyperopia: ten-year follow-up. J Cataract Refract Surg 2007;33:1579-

implantable contact lens implantation and wavefront-guided laser in situ

outcome in their case study.

with the ICL.

risks.

**8. References** 

1584.

keratomileusis for high myopic astigmatism. J Cataract Refract Surg. 2008;34:1687- 93


## **Femtosecond Laser-Assisted Astigmatism Correction**

Duna Raoof-Daneshvar and Shahzad I. Mian *University of Michigan, W.K. Kellogg Eye Center, USA* 

## **1. Introduction**

192 Astigmatism – Optics, Physiology and Management

[21] Kojima T, Horai R, Hara S, Nakamura H, Nakamura T, Satoh Y, Ichikawa K. Correction

piggyback toric Implantable Collamer Lens. J Refract Surg. 2010;26:766-9. [22] Navas A, Munoz-Ocampo M, Graue-Hernández E O., Gómez-Bastar A. Spontaneous

[23] Chen LJ, Chang YJ, Kuo JC, Rajagopal R, Azar DT. Metaanalysis of cataract

2008;34:1181-1200

of residual refractive error in pseudophakic eyes with the use of a secondary

rotation of a Toric Implantable Collamer Lens. Case Rep Ophthalmol 2010;1:99-104.

development after phakic intraocular lens surgery. J Cataract Refract Surg

Femtosecond lasers generate ultrashort pulses while utilizing minimal energy and inflicting trivial damage to surrounding tissues. The U.S. Food and Drug Administration (FDA) approved the IntraLase® femtosecond laser (Abbott Inc., Abbott Park, IL) for commercial use in 2000 for lamellar corneal surgery. Both the predictability and accuracy of femtosecond lasers have provided multiple applications of this unique laser in refractive surgery. In this chapter, we summarize the surgical techniques that have been developed for astigmatism correction utilizing the femtosecond laser. Novel methods that may be used to treat astigmatism include femtosecond laser-assisted keratotomy, limbal relaxing incisions, intracorneal ring segments, anterior lamellar keratoplasty, and excimer laser correction (laser in situ keratomileusis, or LASIK). (Table 1) The versatility and distinctive nature of the femtosecond laser have allowed its application in multiple avenues of corneal surgery and show promise in the treatment of astigmatism.


Table 1. Surgical Techniques for Femtosecond Laser-Assisted Astigmatism Correction

## **2. Femtosecond laser history and principles**

The earliest application of near-infrared (1053 nm) lasers in ophthalmology was with the focused neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, which has a pulse duration in the nanosecond (10-9) range and produces photodisruption. Also known as photoionization, this process vaporizes small volumes of tissue with the formation of cavitation gas bubbles consisting of carbon dioxide and water, which ultimately dissipate into the surrounding tissue (Juhasz et al., 1999). Since power is a function of energy per unit time, for a given energy, decreasing the time increases the power. By shortening the pulse duration of the near-infrared laser from the nanosecond to the femtosecond (10-15) range, the zone of collateral tissue damage is significantly reduced. The femtosecond laser is similar to a Nd:YAG laser, but with an ultra-short pulse duration that is capable of producing smaller shock waves and cavitation bubbles (Stern, 1989). Thermal damage to neighboring tissue in the cornea has been measured to be in the order of 1 μm (Lubatschowski et al., 2000). Additionally, the near-infrared femtosecond laser can be focused anywhere within or behind the cornea and is also capable, to a certain extent, of passing through optically hazy media such as an edematous cornea.

The initial application of the femtosecond laser for corneal surgery was developed in the early 1990's in collaboration between the W.K. Kellogg Eye Center and the University of Michigan College of Engineering (Perry & Mourou, 1994). In 1997, The IntraLase® Corporation was founded which developed a femtosecond laser that scanned over the target tissue with a highly precise computer-operated optical delivery system (currently owned by Abbott Laboratories, Abbott Park, IL). This system was approved by the FDA in 2000 and the first commercial laser was introduced to the market in 2001 for creation of laser in situ keratomileusis (LASIK) flaps (Ratkay-Traub et al., 2001). The IntraLase® femtosecond laser system relies on a low-pressure (35 – 50 mm Hg) suction ring to align and stabilize the globe. A flat glass contact lens, which is attached to the laser delivery system, is then used to applanate the cornea within the suction ring. Laser pulses are delivered to make a lamellar corneal cut. The pattern then generates a circle at the edge of the lamellar plane that is successively moved anteriorly toward the applanation lens, making the flap edge. An internal shutter mechanism leaves a hinge of predetermined arc and location, but can be varied in advance by the surgeon (Sugar, 2007).

The femtosecond laser's unique technology has rapidly progressed since its inception. It was initially introduced as a 10-kHz laser, but the current widely-used IntraLase® system fires at a pulse rate of 60-kHz. In the new 150-kHz IntraLase® femtosecond system, with its highprecision computer control, the delivery system can create cuts of a wide variety of geometric depths, shapes, diameters, wound configurations, spot separation, and energy. There are multiple other commercially available femtosecond laser systems at the time of writing: Technolas Perfect Vision 520 FS (Technolas Perfect Vision, Munich, Germany), VisuMax Femtosecond System (Carl Zeiss Meditec, Jena, Germany), and Femto LDV (Ziemer Group, Port, Switzerland) (Table 2, Reggiani-Mello & Krueger, 2011).

#### **3. Astigmatic keratotomy**

In astigmatic keratotomy, incisions can be limbal, arcuate, or transverse and are traditionally performed free-hand or with a mechanical keratome (Poole and Ficker, 2006). Arcuate keratotomy has superior predictability and therefore remains the most popular procedure (Price et al, 2007). However, both free-hand and mechanized astigmatic keratotomy suffer from technical limitations including lack of precision and reproducibility of incision depth and length and presence of skip lesions. The instruments used for astigmatic keratotomy are frontcutting diamond blades and mechanized trephines, which can lead to corneal perforations, irregular astigmatism, undercorrections and worsening of the pre-existing astigmatism (Hoffart et al., 2009; Krachmer et al., 1980). Femtosecond laser technology offers the ability

into the surrounding tissue (Juhasz et al., 1999). Since power is a function of energy per unit time, for a given energy, decreasing the time increases the power. By shortening the pulse duration of the near-infrared laser from the nanosecond to the femtosecond (10-15) range, the zone of collateral tissue damage is significantly reduced. The femtosecond laser is similar to a Nd:YAG laser, but with an ultra-short pulse duration that is capable of producing smaller shock waves and cavitation bubbles (Stern, 1989). Thermal damage to neighboring tissue in the cornea has been measured to be in the order of 1 μm (Lubatschowski et al., 2000). Additionally, the near-infrared femtosecond laser can be focused anywhere within or behind the cornea and is also capable, to a certain extent, of passing through optically hazy

The initial application of the femtosecond laser for corneal surgery was developed in the early 1990's in collaboration between the W.K. Kellogg Eye Center and the University of Michigan College of Engineering (Perry & Mourou, 1994). In 1997, The IntraLase® Corporation was founded which developed a femtosecond laser that scanned over the target tissue with a highly precise computer-operated optical delivery system (currently owned by Abbott Laboratories, Abbott Park, IL). This system was approved by the FDA in 2000 and the first commercial laser was introduced to the market in 2001 for creation of laser in situ keratomileusis (LASIK) flaps (Ratkay-Traub et al., 2001). The IntraLase® femtosecond laser system relies on a low-pressure (35 – 50 mm Hg) suction ring to align and stabilize the globe. A flat glass contact lens, which is attached to the laser delivery system, is then used to applanate the cornea within the suction ring. Laser pulses are delivered to make a lamellar corneal cut. The pattern then generates a circle at the edge of the lamellar plane that is successively moved anteriorly toward the applanation lens, making the flap edge. An internal shutter mechanism leaves a hinge of predetermined arc and location, but can be

The femtosecond laser's unique technology has rapidly progressed since its inception. It was initially introduced as a 10-kHz laser, but the current widely-used IntraLase® system fires at a pulse rate of 60-kHz. In the new 150-kHz IntraLase® femtosecond system, with its highprecision computer control, the delivery system can create cuts of a wide variety of geometric depths, shapes, diameters, wound configurations, spot separation, and energy. There are multiple other commercially available femtosecond laser systems at the time of writing: Technolas Perfect Vision 520 FS (Technolas Perfect Vision, Munich, Germany), VisuMax Femtosecond System (Carl Zeiss Meditec, Jena, Germany), and Femto LDV

In astigmatic keratotomy, incisions can be limbal, arcuate, or transverse and are traditionally performed free-hand or with a mechanical keratome (Poole and Ficker, 2006). Arcuate keratotomy has superior predictability and therefore remains the most popular procedure (Price et al, 2007). However, both free-hand and mechanized astigmatic keratotomy suffer from technical limitations including lack of precision and reproducibility of incision depth and length and presence of skip lesions. The instruments used for astigmatic keratotomy are frontcutting diamond blades and mechanized trephines, which can lead to corneal perforations, irregular astigmatism, undercorrections and worsening of the pre-existing astigmatism (Hoffart et al., 2009; Krachmer et al., 1980). Femtosecond laser technology offers the ability

(Ziemer Group, Port, Switzerland) (Table 2, Reggiani-Mello & Krueger, 2011).

media such as an edematous cornea.

varied in advance by the surgeon (Sugar, 2007).

**3. Astigmatic keratotomy** 


Table 2. Features of Current Femtosecond Laser Devices (Adapted from Reggiani-Mello and Krueger, 2011)

to control the desired shape, length, radius and depth of incisions in astigmatic keratotomy. Axial topographic maps are used to identify the steep meridians and a standardized nomogram is used to generate a surgical plan with paired incisions for each patient. Multiple studies have found femtosecond-assisted laser arcuate keratotomy to have enhanced predictability and a reduced rate of complications (Bahar et al., 2008; Hoffart et al., 2009).

## **3.1 Indications and surgical planning**

This novel technique has been primarily described in treating high astigmatism following penetrating keratoplasty incisions (Buzzonetti et al., 2008; Harissi-Dagher & Azar, 2008; Kiraly et al., 2008; Kumar et al., 2010). Preoperative evaluation should include a comprehensive examination, manual keratometry, pachymetry, and corneal topography. Anterior segment optical coherence tomography (AS-OCT) may also be used to determine incision depth.

Next, treatment parameters must be determined, which are comprised of incision depth, incision arc length, and optical zone diameter. Nomograms have been established to set these parameters, and vary with the amount of astigmatism and the age of the patient (Chu et al., 2005).

## **3.2 Surgical technique**

In the United States, only the IntraLase® and Femto LDV® systems are enabled with software for astigmatic keratotomy at the time of writing (Wu, 2011). This procedure is performed under topical anesthesia. The limbus is initially marked with gentian violet to compensate for cyclotorsion. The patient is placed under the operating microscope and prepared in a similar fashion for laser vision correction. An optical zone marker centered on the pupil is used to mark the zone diameter followed by an axis marker to indicate the planned locations of incisions. The corneal thickness at the optical zone along the planned incision sites is measured with the use of an ultrasound pachymeter. Alternatively, AS-OCT can be used in the preoperative surgical planning, applying the intended treatment diameter to the AS-OCT image and then the caliper tool to determine depth of suggested arcuate incision, at the planned location along the cornea.

After entering the treatment parameters, the suction ring is placed, followed by the applanation cone which is centered on the pupil. The treatment screen shows the locations of the incisions and the suction ring can be used to rotate the eye to ensure proper axis alignment. After the incisions are created and the suction ring and applanation cone are released, a Sinskey hook is immediately used to open the incisions. Postoperative care includes use of topical antibiotics and steroids for several weeks.

## **3.3 Outcomes**

A growing number of studies have investigated femtosecond laser use in astigmatic keratotomy with promising results. The majority of these reports evaluated improvement in astigmatism following penetrating keratoplasty. In the United States, Harissi-Dagher and Azar were the first to report the outcome of femtosecond laser-assisted astigmatic keratotomy. In two patients, distance corrected visual acuity, DCVA, improved from 20/100 to 20/30 and from 20/200 to 20/60, and astigmatism was reduced by 3.6 D (from 8.5 to 4.9 D) and 2.7 D (from 7.0 to 4.3 D), respectively. The paired arcuate incisions were created just inside the graft-host junction within the corneal donor stroma. In this particular study, a depth of 400µm was the maximal allowed by the IntraLase® software which precluded deeper incisions (Harissi-Dagher & Azar, 2008). Concurrently in Germany, Kiraly et al. described the use of the femtosecond laser to perform arcuate incisions to correct high astigmatism in 10 post-keratoplasty patients. DCVA improved in 8 patients, the average corneal astigmatism was reduced by 3 diopters and the average refractive astigmatism by 4 D (Kiraly et al., 2008).

In an Italian study, Buzzonetti et al. used the IntraLase® system on 9 eyes in which paired 70° arc length incisions were performed at 80% of the corneal depth. Mean preoperative DCVA improved from 20/30 to 20/25, while the mean refractive astigmatism decreased by 6.00 D and the mean keratometric value decreased by 4.60 D (Buzzonetti et al., 2008). Kymionis et al. similarly reported a beneficial result in a patient with nonorthogonal postkeratoplasty astigmatism. Using the keratoplasty software on the IntraLase® 30-kHz system, two anterior arcuate incisions (60° arc length, from 180° to 240° and from 320° to 20°) were created at 75% depth of the thinnest measurement of the cornea. The patient's DCVA improved from 20/50 to 20/32 and manifest cylinder was reduced from 4.0 to 0.5 D. Improvement of topographic irregular astigmatism including surface regularity index and surface asymmetry index were noted as well (Kymoinnis et al., 2009). Using the 60-kHz IntraLase®, Kook et al. recently reported similar outcomes in 10 eyes. At 13 months, the mean uncorrected visual acuity and mean topometric astigmatism improved from (logMAR) 1.27 and 9.3 D, to (logMAR) 1.12 and 6.5 D, respectively (Kook et al., 2011).

In a large series of 37 post-keratoplasty eyes with greater than 5 D of regular astigmatism, Kumar et al. showed improvement of UCVA (logMAR 1.08 ± 0.34 to 0.80 ± 0.42), DCVA (logMAR 0.45 ± 0.27 to 0.37 ± 0.27), reduction of absolute cylindrical power (7.46 ± 2.70 to 4.77 ± 3.29), and reduction of the astigmatism vector (2.52 × 122º ± 5.4 to 0.41 × 126º ± 4.0).

prepared in a similar fashion for laser vision correction. An optical zone marker centered on the pupil is used to mark the zone diameter followed by an axis marker to indicate the planned locations of incisions. The corneal thickness at the optical zone along the planned incision sites is measured with the use of an ultrasound pachymeter. Alternatively, AS-OCT can be used in the preoperative surgical planning, applying the intended treatment diameter to the AS-OCT image and then the caliper tool to determine depth of suggested arcuate

After entering the treatment parameters, the suction ring is placed, followed by the applanation cone which is centered on the pupil. The treatment screen shows the locations of the incisions and the suction ring can be used to rotate the eye to ensure proper axis alignment. After the incisions are created and the suction ring and applanation cone are released, a Sinskey hook is immediately used to open the incisions. Postoperative care

A growing number of studies have investigated femtosecond laser use in astigmatic keratotomy with promising results. The majority of these reports evaluated improvement in astigmatism following penetrating keratoplasty. In the United States, Harissi-Dagher and Azar were the first to report the outcome of femtosecond laser-assisted astigmatic keratotomy. In two patients, distance corrected visual acuity, DCVA, improved from 20/100 to 20/30 and from 20/200 to 20/60, and astigmatism was reduced by 3.6 D (from 8.5 to 4.9 D) and 2.7 D (from 7.0 to 4.3 D), respectively. The paired arcuate incisions were created just inside the graft-host junction within the corneal donor stroma. In this particular study, a depth of 400µm was the maximal allowed by the IntraLase® software which precluded deeper incisions (Harissi-Dagher & Azar, 2008). Concurrently in Germany, Kiraly et al. described the use of the femtosecond laser to perform arcuate incisions to correct high astigmatism in 10 post-keratoplasty patients. DCVA improved in 8 patients, the average corneal astigmatism was reduced by 3 diopters and the average refractive astigmatism by 4

In an Italian study, Buzzonetti et al. used the IntraLase® system on 9 eyes in which paired 70° arc length incisions were performed at 80% of the corneal depth. Mean preoperative DCVA improved from 20/30 to 20/25, while the mean refractive astigmatism decreased by 6.00 D and the mean keratometric value decreased by 4.60 D (Buzzonetti et al., 2008). Kymionis et al. similarly reported a beneficial result in a patient with nonorthogonal postkeratoplasty astigmatism. Using the keratoplasty software on the IntraLase® 30-kHz system, two anterior arcuate incisions (60° arc length, from 180° to 240° and from 320° to 20°) were created at 75% depth of the thinnest measurement of the cornea. The patient's DCVA improved from 20/50 to 20/32 and manifest cylinder was reduced from 4.0 to 0.5 D. Improvement of topographic irregular astigmatism including surface regularity index and surface asymmetry index were noted as well (Kymoinnis et al., 2009). Using the 60-kHz IntraLase®, Kook et al. recently reported similar outcomes in 10 eyes. At 13 months, the mean uncorrected visual acuity and mean topometric astigmatism improved from (logMAR) 1.27 and 9.3 D, to (logMAR) 1.12 and 6.5 D, respectively (Kook et al., 2011). In a large series of 37 post-keratoplasty eyes with greater than 5 D of regular astigmatism, Kumar et al. showed improvement of UCVA (logMAR 1.08 ± 0.34 to 0.80 ± 0.42), DCVA (logMAR 0.45 ± 0.27 to 0.37 ± 0.27), reduction of absolute cylindrical power (7.46 ± 2.70 to 4.77 ± 3.29), and reduction of the astigmatism vector (2.52 × 122º ± 5.4 to 0.41 × 126º ± 4.0).

incision, at the planned location along the cornea.

**3.3 Outcomes** 

D (Kiraly et al., 2008).

includes use of topical antibiotics and steroids for several weeks.

This study also showed that the refractive effect of astigmatic keratotomy stabilized at 3 months. For all cases, incision depth was 90% and incisions were placed at 0.5 mm within graft-host junction. Overcorrection was noted initially which led the authors to adjust the incision arc length: 40º to 60º for up to 6 D, 65º to 75º for 6 to 10 D, and 90º for >10 D of astigmatism (Kumar et al., 2010). Similarly, using the Technolas FS® laser system, which allows for deeper incision depth, Nubile et al. treated 12 post-keratoplasty eyes with incision depth of 90% and arc length of 40º to 80º within 1 mm of the graft-host junction. In addition to improved UCVA and DCVA, mean astigmatism was reduced from 7.16±3.07 to 2.23±1.55 D at one month (Nubile et al., 2009).

In a retrospective comparative case series, Bahar et al. compared the outcomes of IntraLase® enabled astigmatic keratotomy and manual astigmatic keratotomy. Twenty eyes underwent manual astigmatic keratotomy using a diamond blade and 20 eyes underwent femtosecond laser assisted keratotomy. Both groups had improvement of UCVA and DCVA but only the femtosecond group achieved a statistically significant improvement. Compared with the manual technique, use of a femtosecond laser showed a trend towards greater improvement of visual acuity and defocus equivalent as well as greater reduction of absolute cylinder. Although manual keratotomy resulted in shift of astigmatism axis, the femtosecond laser brought the mean astigmatic vector closer to neutral (Bahar et al., 2008).

In the only known prospective randomized study at this time, Hoffart et al. compared the effectiveness of arcuate keratotomy with a femtosecond laser with incision depth set at 75% depth with mechanized astigmatic keratotomy using the Hanna® keratome (Moria, Doylestown, PA). Two groups of 20 eyes were randomly assigned to each method. Although no statistically significant differences were detected at six months, a wider spread of angle of error and an almost significant difference of mean absolute angle of error suggested a larger misalignment of treatment during mechanized astigmatic keratotomy (Hoffart et al., 2009).

Femtosecond laser assisted astigmatic keratotomy has also been shown to be effective in the management of astigmatism after descemet's stripping endothelial keratoplasty. In a case report, Levinger et al. showed absolute cylinder reduction from 5.75 to 2.75 D, improvement of UCVA from 20/300 to 20/60 and DCVA improvement from 20/100 to 20/40 (Levinger et al., 2009). In another case report, a patient with high astigmatism following Descemet's stripping automated endothelial keratoplasty (DSAEK) underwent femtosecond-assisted astigmatic keratotomy (Yoo et al., 2009). Six months after the procedure, the UCVA remained unchanged while the DCVA decreased from 20/40 to 20/50. The manifest refractive astigmatic error increased from +5.25x163 to +7.50x80 (surgically induced astigmatism was approximately 12.75 D with an overcorrection of about 7.50 D). This report showed that in post-DSAEK patients, adding the DSAEK donor corneal lenticule thickness in the preoperative peripheral corneal thickness measurements can result in full-thickness recipient corneal incisions and overcorrection.

Limited data exists demonstrating the use of femtosecond laser-assisted astigmatic keratotomy in reducing naturally occurring astigmatism. Abbey et al. reported a study in which this technique was performed on a patient with naturally occurring astigmatism of 5.25 D in both eyes. Treatment parameters were based on the modified Lindstrom nomogram for naturally occurring astigmatism and guided by the topographic cylinder axis. Significant improvement of UCVA (counting fingers to 20/50, 20/200 to 20/30) and reduction of manifest cylinder power (2.5 D, 3.0 D) were seen at one year (Abbey et al., 2009). Additional studies must be conducted to verify the efficacy of this technique in natural astigmatism, but the results presented thus far are promising.

## **3.4 Complications**

Several complications have been noted in the published series specific to femtosecond laserassisted astigmatic keratotomy (Table 3). In the study by Nubile et al., two intraoperative microperforations occurred in 1 of the 2 cuts. Both cases presented a slight intraoperative leak but required no specific action other than application of a bandage contact lens. They were self-sealing, and the anterior chambers were maintained with no postoperative sequelae. A mild, transient, inflammatory reaction adjacent to the keratotomies was observed in all patients and resolved within one week. There were no cases of immunologic rejection during the follow-up. The healing and clinical outcomes up to 6 months after surgery were uneventful in all cases (Nubile et al., 2009). In their study of 37 eyes, Kumar et al. reported that 8% of eyes experienced rejection, all of which resolved with topical steroids. Overcorrection occurred initially in 24% of eyes, which required resuturing of the astigmatic keratotomy incisions. After adjusting the treatment parameters in the subsequent eyes, overcorrection decreased to 11%. Two thirds of the eyes that experienced overcorrection were keratoconic, suggesting the ectatic eyes may be at increased risk for overcorrection (Kumar et al., 2010).

To date, there is one case report in the literature in which there was evidence of a largethickness perforation immediately after femtosecond-assisted astigmatic keratotomy (Vaddavalli et al, 2011). The perforation was noted only after the incision was opened with a Sinskey hook with leakage of aqueous from the incision site. An air bubble was also noted in the anterior chamber before the incision was opened. Therefore, the surgeon must watch for air bubble in the anterior chamber which may indicate a full thickness perforation. In this report, the authors successfully treated the perforation with a bandage contact lens, topical steroids, and antibiotics. At 1 month, all incisions had healed well with no signs of infection. Careful peripheral pachymetric measurements can help avoid full-thickness incisions. Early recognition of full-thickness incision with air bubbles in the anterior chamber can help avoid separation of incision and leakage of aqueous. The adhesions in the femtosecond laser incisions help prevent leakage prior to mechanical separation.


Table 3. Complications Associated with Femtosecond Laser-Assisted Astigmatic Keratotomy

## **4. Limbal relaxing incisions**

Limbal relaxing incisions have traditionally been used to correct low degree of astigmatism at the time of cataract surgery. They may be used to correct up to 3.5 D of astigmatism, flattening the steepest meridian of the cornea and eliminating a source of refractive error

Several complications have been noted in the published series specific to femtosecond laserassisted astigmatic keratotomy (Table 3). In the study by Nubile et al., two intraoperative microperforations occurred in 1 of the 2 cuts. Both cases presented a slight intraoperative leak but required no specific action other than application of a bandage contact lens. They were self-sealing, and the anterior chambers were maintained with no postoperative sequelae. A mild, transient, inflammatory reaction adjacent to the keratotomies was observed in all patients and resolved within one week. There were no cases of immunologic rejection during the follow-up. The healing and clinical outcomes up to 6 months after surgery were uneventful in all cases (Nubile et al., 2009). In their study of 37 eyes, Kumar et al. reported that 8% of eyes experienced rejection, all of which resolved with topical steroids. Overcorrection occurred initially in 24% of eyes, which required resuturing of the astigmatic keratotomy incisions. After adjusting the treatment parameters in the subsequent eyes, overcorrection decreased to 11%. Two thirds of the eyes that experienced overcorrection were keratoconic, suggesting the ectatic eyes may be at increased risk for overcorrection

To date, there is one case report in the literature in which there was evidence of a largethickness perforation immediately after femtosecond-assisted astigmatic keratotomy (Vaddavalli et al, 2011). The perforation was noted only after the incision was opened with a Sinskey hook with leakage of aqueous from the incision site. An air bubble was also noted in the anterior chamber before the incision was opened. Therefore, the surgeon must watch for air bubble in the anterior chamber which may indicate a full thickness perforation. In this report, the authors successfully treated the perforation with a bandage contact lens, topical steroids, and antibiotics. At 1 month, all incisions had healed well with no signs of infection. Careful peripheral pachymetric measurements can help avoid full-thickness incisions. Early recognition of full-thickness incision with air bubbles in the anterior chamber can help avoid separation of incision and leakage of aqueous. The adhesions in the femtosecond laser

> **Complications Associated with Femtosecond Laser-Assisted Astigmatic Keratotomy**

 Full-thickness perforation Inflammatory reaction

Limbal relaxing incisions have traditionally been used to correct low degree of astigmatism at the time of cataract surgery. They may be used to correct up to 3.5 D of astigmatism, flattening the steepest meridian of the cornea and eliminating a source of refractive error

Microperforation

 Graft rejection Overcorrection Undercorrection Table 3. Complications Associated with Femtosecond Laser-Assisted Astigmatic Keratotomy

incisions help prevent leakage prior to mechanical separation.

**3.4 Complications** 

(Kumar et al., 2010).

**4. Limbal relaxing incisions** 

(Nichamin, 2006). The results have been limited due to this technique's low predictability and reliability. For instance, an axis misalignment of just 5° results in a 17% reduction in effect (Nichamin, 2006). Inconsistencies in the results of manual limbal relaxing incisions are presumed to be related to imprecision in depth, axis, arc length, and optical zone. Theoretically, the improved accuracy afforded by the femtosecond laser could enhance the reliability of outcomes of laser limbal relaxing incisions. To date, no published studies have reported the use of femtosecond laser to create limbal relaxing incisions. The use of the femtosecond laser in cataract surgery will allow for more accurate placement and predictability of limbal relaxing incisions for astigmatism correction.

## **5. Intracorneal ring segments**

Intracorneal ring segments (Intacs; Addition Technology, Des Plaines, IL or Keraring; Mediphacos, Belo Horizonte, Brazil) have been used for the correction of mild to moderate keratoconus and for correction of low myopia. Intacs are clear, thin, semicircular inserts made of polymethylmethacrylate (PMMA) that are implanted in the deep corneal stroma with the goal of modifying corneal curvature and subsequently generating refractive changes. They shorten the central arc length of the corneal surface which leads to flattening of the cornea. Traditionally, manual dissection is used to create the channels for the intracorneal ring segments. Femtosecond laser technology has been used to create channels for the intracorneal ring segments and has been shown to be comparable to manual dissection (Kouassi et al., 2011; Kubaloglu et al., 2010; Kubaloglu et al., 2011; Pinero et al., 2009; Rabinowitz et al. 2006). (Figure 1)

Fig. 1. Intracorneal ring segment implanted in a patient with keratoconus

## **5.1 Indications**

Intracorneal ring segments are used in the management of astigmatism in multiple corneal ectatic disorders, such as keratoconus, and to reduce corneal steepening and refractive errors in pellucid marginal degeneration and post-LASIK ectasia. Intracorneal ring segments are an alternative option for visual rehabilitation for these patients and may delay or prevent the need for corneal grafting. Additionally, they are useful for patients who exhibit contact lens intolerance and in whom spectacle correction does not provide optimal visual acuity.

#### **5.2 Technique**

This procedure is typically performed under topical anesthesia. First, the corneal thickness is measured by pachymety at the area of implantation. The suction ring of the femtoseond laser system is then placed and centered. The glass lens is applanated to the cornea to fixate the eye and help maintain the precise distance from the laser head to the focal point. An entry cut with the femtosecond laser is created with the aim of allowing access for ring placement in the tunnel. The tunnel is then created at approximately 70–80% of the corneal thickness within 15 seconds. Subsequently, the ring segments are inserted in the created tunnels. To this date, there are no published studies comparing the visual and refractive outcomes for implanting intracorneal ring segments using these different locations (temporal versus relative to the astigmatism axis). Future studies must be done to clarify the role of the corneal incision in the outcomes obtained after intracorneal ring segment implantation.

### **5.3 Outcomes**

In a retrospective case series, 118 eyes of 69 patients with keratoconus underwent Intacs implantation with the assistance of a femtosecond laser. In eyes with an inferior cone, a 0.45 mm Intacs insert was placed inferiorly to lift the cone and a 0.25 mm Intacs insert was placed superiorly to flatten the cornea and decrease baseline keratoconic asymmetric astigmatism. In eyes with central keratoconus, Intacs were inserted in the cornea according to the preoperative spherical equivalent in each eye. Intacs were inserted to 70% corneal depth and were successfully implanted in all eyes using a 15-kHz IntraLase femtosecond laser without intraoperative complications. At the end of the first postoperative year, 81.3% of eyes had improved UCVA and 73.7% had improved DCVA. The mean keratometry decreased from 51.6 D to 47.7 D, and the mean refractive spherical equivalent decreased from -7.6 D to -3.7 D (Ertan et al., 2006a). In a similar study, 9 eyes of 6 patients with pellucid marginal corneal degeneration had implantation of Intacs segments by a femtosecond laser technique. The UCVA improved from pre-operatively to 6 months after Intacs implantation: the mean difference was 3.5±1.6 lines, which was statistically significant. The mean preoperative spherical refraction decreased from -3.86±2.91 D to -2.77±1.43 D and the mean cylindrical refraction from -2.41±2.27 D to -0.94±1.07 D (Ertan et al., 2006).

This group also has published a larger series with 306 keratoconic eyes (Ertan et al., 2008). All eyes underwent femtosecond laser assisted Intacs implantation with similar technique as discussed earlier. At a mean follow-up of 10.4 months, the DCVA improved in 71.6% of eyes and the UCVA improved in 75.7% of eyes. The mean keratometry significantly decreased from 50.7 D to 47.9 D and the mean manifest spherical refraction from -6.04 D to -3.09 D. The mean manifest cylindrical refraction reduced from -4.11 D to -3.82 D, although this was not statistically significant.

In another report, Cockunseven et al. showed similar promising results. Fifty eyes of 32 keratoconic patients had a statistically significant reduction in the spherical equivalent refractive error (mean of -5.62±4.15 D to -2.49±2.68 D) at 12 month follow-up. The UCVA before implantation was 20/40 or worse in 47 eyes, whereas at one year, 14 (28%) of 50 eyes had a UCVA of 20/40 or better. Thirty-nine eyes (68%) experienced a DCVA gain of one to four lines at one year (Coskunseven et al., 2008).

need for corneal grafting. Additionally, they are useful for patients who exhibit contact lens

This procedure is typically performed under topical anesthesia. First, the corneal thickness is measured by pachymety at the area of implantation. The suction ring of the femtoseond laser system is then placed and centered. The glass lens is applanated to the cornea to fixate the eye and help maintain the precise distance from the laser head to the focal point. An entry cut with the femtosecond laser is created with the aim of allowing access for ring placement in the tunnel. The tunnel is then created at approximately 70–80% of the corneal thickness within 15 seconds. Subsequently, the ring segments are inserted in the created tunnels. To this date, there are no published studies comparing the visual and refractive outcomes for implanting intracorneal ring segments using these different locations (temporal versus relative to the astigmatism axis). Future studies must be done to clarify the role of the corneal incision in the

In a retrospective case series, 118 eyes of 69 patients with keratoconus underwent Intacs implantation with the assistance of a femtosecond laser. In eyes with an inferior cone, a 0.45 mm Intacs insert was placed inferiorly to lift the cone and a 0.25 mm Intacs insert was placed superiorly to flatten the cornea and decrease baseline keratoconic asymmetric astigmatism. In eyes with central keratoconus, Intacs were inserted in the cornea according to the preoperative spherical equivalent in each eye. Intacs were inserted to 70% corneal depth and were successfully implanted in all eyes using a 15-kHz IntraLase femtosecond laser without intraoperative complications. At the end of the first postoperative year, 81.3% of eyes had improved UCVA and 73.7% had improved DCVA. The mean keratometry decreased from 51.6 D to 47.7 D, and the mean refractive spherical equivalent decreased from -7.6 D to -3.7 D (Ertan et al., 2006a). In a similar study, 9 eyes of 6 patients with pellucid marginal corneal degeneration had implantation of Intacs segments by a femtosecond laser technique. The UCVA improved from pre-operatively to 6 months after Intacs implantation: the mean difference was 3.5±1.6 lines, which was statistically significant. The mean preoperative spherical refraction decreased from -3.86±2.91 D to -2.77±1.43 D and the mean

This group also has published a larger series with 306 keratoconic eyes (Ertan et al., 2008). All eyes underwent femtosecond laser assisted Intacs implantation with similar technique as discussed earlier. At a mean follow-up of 10.4 months, the DCVA improved in 71.6% of eyes and the UCVA improved in 75.7% of eyes. The mean keratometry significantly decreased from 50.7 D to 47.9 D and the mean manifest spherical refraction from -6.04 D to -3.09 D. The mean manifest cylindrical refraction reduced from -4.11 D to -3.82 D, although this was not

In another report, Cockunseven et al. showed similar promising results. Fifty eyes of 32 keratoconic patients had a statistically significant reduction in the spherical equivalent refractive error (mean of -5.62±4.15 D to -2.49±2.68 D) at 12 month follow-up. The UCVA before implantation was 20/40 or worse in 47 eyes, whereas at one year, 14 (28%) of 50 eyes had a UCVA of 20/40 or better. Thirty-nine eyes (68%) experienced a DCVA gain of one to

intolerance and in whom spectacle correction does not provide optimal visual acuity.

outcomes obtained after intracorneal ring segment implantation.

cylindrical refraction from -2.41±2.27 D to -0.94±1.07 D (Ertan et al., 2006).

**5.2 Technique** 

**5.3 Outcomes** 

statistically significant.

four lines at one year (Coskunseven et al., 2008).

Studies have shown that creation of channels of intracorneal ring segments using femtosecond laser to be comparable to manual dissection. In a prospective randomized trial (Kubaloglu et al., 2010), 100 consecutive eyes with keratoconus were assigned to have tunnel creation with a mechanical device or a femtosecond laser. Kerarings with a 5.0 mm diameter and 160-degree arc length were implanted in all cases. At one year postoperatively, the UCVA improved by 2.4 lines in the mechanical group and 2.0 lines in the femtosecond group and the DCVA by 3.3 lines and 2.7 lines, respectively. There were no statistically significant differences between the 2 groups in visual or refractive results. Moreover, in a study by Rabinowitz et al. comparing the results of femtosecond laser (6-month results) and mechanical (12-month results) tunnel creation for Intacs implantation in 10 eyes, both groups showed significant reduction in average keratometry, spherical equivalent refraction, DCVA and UCVA. Statistical analysis, however, did not reveal any statistically significant differences between the two groups for any single parameter studied. Overall success, defined as contact lens or spectacles tolerance, was 85% in the laser group and 70% in the mechanical group (Rabinowitz et al. 2006). In another comparison by Kubaloglu et al., 96 eyes of 75 patients with keratoconus were retrospectively studied and their results showed that there was no statistically significant difference in any parameter between the group that underwent corneal tunnels with femtosecond laser (26 eyes) and those that underwent mechanical tunnel placement (70 eyes) (Kubaloglu et al., 2011). Additionally, in a retrospective study, Pinero et al. evaluated 146 eyes and demonstrated that intracorneal ring segments implantation using both mechanical and femtosecond laser-assisted procedures provide similar visual and refractive outcomes (Pinero, et al., 2009). Significant differences were found between the 2 groups for eyes implanted with Intacs for primary spherical aberration, coma, and other higher-order aberrations, favoring the femtosecond group (P≤0.01). Similarly, Kouassi et al. compared the two modalities using anterior segment optical coherence tomography in an observational prospective study. Their study demonstrated no statistical significant different in depth predictability (Kouassi, et al., 2011).

#### **5.4 Complications**

Intraoperative complications during intrastormal corneal ring segment implantation are rare. Reports have included segment decentration and inadequate tunnel depth. Ring segment extrusion, corneal neovascularization, mild deposits surrounding ring segments, and focal edema can also occur. In their study of 118 eyes undergoing Intacs placement with femtosecond laser, Ertan et al. found that 15.2% of eyes developed epithelial plugs at the incision site. During the first 6 months postoperatively, a few granulomatous particles were observed around the Intacs segments in 8.5% of eyes, which resolved with steroid drops (Ertan et al., 2006a). Segment extrusion occurred in 3 out of 306 eyes at 6 months postoperatively. Yellow particles around the segment, an epithelial plug at the incision site, and corneal haze around the segment were common observations during follow-up (Ertan et al., 2008).

In the largest survey to date, Coskunseven et al. conducted a retrospective chart review of 531 patients (850 eyes) who underwent Keraring (Mediphacos, Brazil) insertion. Intraoperatively, there were 22 (2.7%) cases of incomplete channel formation. Intraoperative complications included endothelial perforation (0.6%), and incorrect entry of the channel (0.2%). Postoperatively, there were 11 (1.3%) cases of segment migration, two (0.2%) cases of corneal melting and one (0.1%) case of mild infection. The overall complication rate was 5.7% (49 cases out of 850 eyes). To avoid the incidence of endothelial perforation, the authors suggested accurate pachymetry in a 5-mm optical zone at the implantation site. The reference point is set as the point of thinnest pachymetry at the channel locations. Endothelial perforation can be prevented by stopping channel creation as soon as the complication is recognized before the incision (Coskunseven et al., 2011).

In an investigation to analyze the deviation of Intacs implanted in 59 eyes from the pupillary center, Ertan et al. found that the mean horizontal deviation was 788.33µm ± 500.34 with temporal displacement in all eyes. The mean vertical deviation was 370.83 ± 313.17µm and there was an inferior displacement in 28.81% of eyes and superior displacement in 66.10% of eyes. This study showed that during applanation for Intacs by a femtosecond laser, the cornea and pupil are not in their natural position, which leads to decenteration and misalignment of the segments. Therefore, the authors suggested marking the pupillary center on the natural corneal position before the applanation and making the arrangement according to this reference point to prevent decenteration in channel creation with the femtosecond laser (Ertan et al., 2007).

## **6. Anterior lamellar keratoplasty**

Lamellar keratoplasty may be necessary for correction of irregular astigmatism especially in the setting of keratoconus or post LASIK ectasia. Anterior lamellar keratoplasty is a partialthickness corneal transplantation used in eyes with pathology limited to the anterior layers. Advantages of anterior lamellar keratoplasty include less invasive surgery as well as reduced risk of rejection. The major limitations with this procedure are the technical challenges of performing manual dissections and the resulting stromal interface irregularities between the donor and recipient interface. These complications may result in induced irregular astigmatism and loss of best-corrected visual acuity. Recent surgical advancements have led to renewed interest in anterior lamellar keratoplasty for appropriate corneal pathology. The femtosecond laser with its ability to perform precise, preprogrammed corneal dissections at a variety of depths and orientations has been a significant tool in the advancement of new lamellar keratoplasty techniques.

#### **6.1 Indications**

Astigmatism resulting from superficial corneal scars, after trauma, keratitis or corneal epithelial or anterior stromal dystrophies is the major indication for anterior lamellar keratoplasty.

#### **6.2 Technique**

The procedure may be performed under topical anesthesia. To create the donor graft, corneoscleral donor tissue is first mounted on an artificial anterior chamber. A donor graft is created using a 30-kHz IntraLase system with the following settings: donor lenticule thickness, 160 to 270 μm (thickness of the lenticule adjusted in relation to depth of the lesions according to the anterior segment OCT findings); donor lenticule diameter, 7.5 to 8.2 mm, spiral method; 1.9 to 2.9 microjoules spiral energy; 2.3 to 3.0 microjoules side cut energy; 360° side cut, 70° to 80° side cut angle; tangential spot separation, 11 to 12; and radial spot separation, 9 to 11 (Yoo et al., 2008). Depending on the donor tissue quality and edema, up to 20% additional thickness may be added to the donor lenticule to adjust for donor tissue swelling. The range of energy should be adjusted according to the severity of the corneal scar, with higher spiral energy and lower tangent and radial spot separation for denser scars.

A recipient corneal lenticule is next created using similar femtosecond laser settings except that the recipient corneal lenticule is set to be 0.1 mm smaller in diameter than the donor graft diameter. The host corneal button is then removed and replaced with the donor lenticule on the recipient residual corneal stromal bed. The keratectomy incision should be dried with methylcellulose sponges. After approximately 5 minutes (to dehydrate the cornea and improve adhesion), the flap is checked for adhesion by depressing the peripheral host cornea and ensuring that the resulting indentation radiated into the lenticule (similar to checking for flap adhesion after LASIK with the striae test). A bandage contact lens is fitted over the cornea. Patients are then placed on a topical antibiotic and steroid for one week, and steroid drops should be slowly tapered over several months.

## **6.3 Outcomes**

202 Astigmatism – Optics, Physiology and Management

reference point is set as the point of thinnest pachymetry at the channel locations. Endothelial perforation can be prevented by stopping channel creation as soon as the

In an investigation to analyze the deviation of Intacs implanted in 59 eyes from the pupillary center, Ertan et al. found that the mean horizontal deviation was 788.33µm ± 500.34 with temporal displacement in all eyes. The mean vertical deviation was 370.83 ± 313.17µm and there was an inferior displacement in 28.81% of eyes and superior displacement in 66.10% of eyes. This study showed that during applanation for Intacs by a femtosecond laser, the cornea and pupil are not in their natural position, which leads to decenteration and misalignment of the segments. Therefore, the authors suggested marking the pupillary center on the natural corneal position before the applanation and making the arrangement according to this reference point to prevent decenteration in channel creation with the

Lamellar keratoplasty may be necessary for correction of irregular astigmatism especially in the setting of keratoconus or post LASIK ectasia. Anterior lamellar keratoplasty is a partialthickness corneal transplantation used in eyes with pathology limited to the anterior layers. Advantages of anterior lamellar keratoplasty include less invasive surgery as well as reduced risk of rejection. The major limitations with this procedure are the technical challenges of performing manual dissections and the resulting stromal interface irregularities between the donor and recipient interface. These complications may result in induced irregular astigmatism and loss of best-corrected visual acuity. Recent surgical advancements have led to renewed interest in anterior lamellar keratoplasty for appropriate corneal pathology. The femtosecond laser with its ability to perform precise, preprogrammed corneal dissections at a variety of depths and orientations has been a

Astigmatism resulting from superficial corneal scars, after trauma, keratitis or corneal epithelial

The procedure may be performed under topical anesthesia. To create the donor graft, corneoscleral donor tissue is first mounted on an artificial anterior chamber. A donor graft is created using a 30-kHz IntraLase system with the following settings: donor lenticule thickness, 160 to 270 μm (thickness of the lenticule adjusted in relation to depth of the lesions according to the anterior segment OCT findings); donor lenticule diameter, 7.5 to 8.2 mm, spiral method; 1.9 to 2.9 microjoules spiral energy; 2.3 to 3.0 microjoules side cut energy; 360° side cut, 70° to 80° side cut angle; tangential spot separation, 11 to 12; and radial spot separation, 9 to 11 (Yoo et al., 2008). Depending on the donor tissue quality and edema, up to 20% additional thickness may be added to the donor lenticule to adjust for donor tissue swelling. The range of energy should be adjusted according to the severity of the corneal scar, with higher spiral energy and

A recipient corneal lenticule is next created using similar femtosecond laser settings except that the recipient corneal lenticule is set to be 0.1 mm smaller in diameter than the donor

or anterior stromal dystrophies is the major indication for anterior lamellar keratoplasty.

significant tool in the advancement of new lamellar keratoplasty techniques.

lower tangent and radial spot separation for denser scars.

complication is recognized before the incision (Coskunseven et al., 2011).

femtosecond laser (Ertan et al., 2007).

**6. Anterior lamellar keratoplasty** 

**6.1 Indications** 

**6.2 Technique** 

Yoo et al. first described this technique, performed at the Bascom Palmer Eye Institute in 12 eyes. In this study, AS-OCT was used in order to estimate the depth of scar tissue in the recipient cornea. The donor lenticule thickness was adjusted based on the depth of the lesion obtained from the AS-OCT measurements. At 12 months, the mean UCVA was improved in 7 (58.3%) compared with preoperative levels. The DCVA was unchanged or improved in all eyes when compared with the preoperative levels. Preoperatively, DCVA was 20/50 or worse in all eyes (range, HM–20/50), whereas at the last follow-up examination 10 (83%) of 12 eyes had DCVA of 20/50 or better (range, 20/80–20/25). The mean difference between preoperative and postoperative DCVAs was a gain of 3.8 lines (range, unchanged–8 lines). In all patients, both UCVA and DCVA stabilized between 1- and 6-month follow-up examinations. Therefore, the sutureless procedure resulted in the absence of irregular astigmatism and faster visual rehabilitation (Yoo et al., 2008).

To date, the longest term results of femtosecond assisted anterior lamerally keratoplasty in the literature were presented by Shousha and colleagues. Thirteen consecutive patients with anterior corneal pathologies were evaluated over a mean of 31 months post-operatively. The DCVA was significantly improved over preoperative values at the 12-, 18-, 24-, and 36 month visits. DCVA greater than 20/30 was achieved in 54% of patients at the 12-month visit when all 13 patients were available for follow-up, in 50% and 33% of patients at the 18 and 24-month visits, respectively, when 12 patients were available, and in 60% and 50% of patients at the 36- and 48-month visits when 5 and 2 patients were available, respectively. The BSCVA of the eye that completed the 60- and 70-month visits was 20/50. Patients achieved a mean gain of 5 lines of BSCVA at the 6-, 12-, 18-, and 24-month visits, 4 lines at the 36-month visit, 5 lines at the 48-month visit, and 6 lines at the 60- and 72-month visits. At a mean of five weeks postoperatively, 83.3% of patients achieved DCVA within 2 lines of that recorded at the 24-month visit. At the 12-month visit, mean spherical equivalent and refractive astigmatism were −0.4 diopters and 2.2 diopters, respectively, with no significant shift from preoperative values or values recorded in different follow-up visits (Shousha et al., 2011). Additional studies must be performed in order to determine treatment of astigmatism with anterior lamellar keratoplasty.

#### **6.4 Complications**

In the case series described above by Yoo et al., two eyes developed postoperative complications requiring additional surgery. In one eye, there was residual corneal scarring requiring phototherapeutic keratectomy (PTK; 40 μm deep) 10 months after femtosecond laser assisted anterior keratoplasty. The second procedure was performed due to anisometropia using hyperopic photorefractive keratectomy (PRK) over the graft (with attempted correction +1.00+3.00×26) 4 months after femtosecond anterior lamellar keratoplasty. Haze formation was noted during the first three postoperative months and resolved in the following 9 months. Six patients developed dry eye signs and symptoms. All patients were treated with artificial tears and punctal occlusion. An improvement in dryness was found in these patients during the next 3 to 12 month follow-up. No graft rejection, infection, or epithelial ingrowth were noted in this series of patients (Yoo et al., 2008).

Similarly, in the study conducted by Shousha et al, residual corneal scar tissue was noted in 6 of the 11 eyes, despite the fact that PTK was performed intraoperatively on 3 of them. Despite the incomplete removal of scar tissue, those cases gained an average of 6.5 lines of BSCVA at the 6-month visit compared with preoperative BSCVA. Residual deposits were also noted in the 2 eyes. One eye developed an epithelial ingrowth in the interface 4 months postoperatively. Mild interface haze was noted in 3 eyes. One case had a thinned, steep cornea that was noted in the immediate postoperative period. Sequential manifest refraction and topography in the follow-up period showed progressive steepening of the cornea and an increase in refractive and topographic astigmatism, suggesting an ongoing ectatic process. At the last follow-up visit, the average keratometric reading was 50.7 D with 7 D of topographic cylinder. No rejection, failure, or infection was found in this case series, and all cases retained clarity of their grafts to the end of their follow-up period (Shousha et al., 2011).

## **7. Excimer laser correction**

LASIK is a lamellar laser refractive surgery in which excimer laser ablation is done under a partial-thickness lamellar corneal flap. Astigmatism can be managed with excimer laser correction, where the excimer laser is used to reshape the surface of the cornea by removing anterior stromal tissue. A microkeratome was previously used to create a corneal flap with a shift over the last decade to femtosecond laser. The microkeratome used an oscillating blade to cut the flap after immobilization of the cornea with a suction ring. Microkeratomes from several companies cut the lamellar flaps with either superior or nasal hinges, and can cut to depths of 100–200 μm.

Several effective options for laser refractive surgery are available to treat varying degrees of astigmatism. The choices can broadly be divided into lamellar (LASIK) and surface (photorefractive keratectomy, laser epithelial keratomileusis [LASEK], and Epi-LASIK) ablation. Here, we describe the surgical technique for LASIK.

## **7.1 Indications and surgical planning**

The preoperative assessment must include history of stable refraction, refraction, keratotomy, pachymetry, tear production, and complete eye examination.

### **7.2 Technique**

First, the disposable applanation lens attached to the laser aperture is docked into the suction ring centered on the pupil. The suction ring is then locked into the applanation lens. The femtosecond laser is pre-programmed for each procedure with a planned flap diameter, flap thickness, hinge angle, raster energy, and side-cut energy. The flap is then created using a raster pattern, moving back and forth across the diameter of the flap. Initially, a pocket is created to allow the carbon dioxide and water gas bubbles to escape during photodisruption in order to minimize the opaque bubble layer. The suction is released, and the applanation lens and suction ring complex are lifted off the patient's eye.

Next, the patient is positioned at the excimer laser. After the eyelid skin is cleaned and draping is placed using a sterile technique, the flap edge is marked with a 2.0 mm diameter radial keratotomy optical zone marker dipped in gentian violet. A Sinskey hook is used to enter the lamellar interface adjacent to the hinge to allow a blunt spatula to be inserted in the lamellar plane and moved gently back and forth to break residual adhesions to lift the flap. The excimer laser is used to perform the stromal ablation. The flap is subsequently repositioned. Topical steroid and antibiotic are placed in the eye and tapered over the next few weeks.

#### **7.3 Outcomes**

204 Astigmatism – Optics, Physiology and Management

attempted correction +1.00+3.00×26) 4 months after femtosecond anterior lamellar keratoplasty. Haze formation was noted during the first three postoperative months and resolved in the following 9 months. Six patients developed dry eye signs and symptoms. All patients were treated with artificial tears and punctal occlusion. An improvement in dryness was found in these patients during the next 3 to 12 month follow-up. No graft rejection, infection, or epithelial ingrowth were noted in this series of patients (Yoo et al., 2008). Similarly, in the study conducted by Shousha et al, residual corneal scar tissue was noted in 6 of the 11 eyes, despite the fact that PTK was performed intraoperatively on 3 of them. Despite the incomplete removal of scar tissue, those cases gained an average of 6.5 lines of BSCVA at the 6-month visit compared with preoperative BSCVA. Residual deposits were also noted in the 2 eyes. One eye developed an epithelial ingrowth in the interface 4 months postoperatively. Mild interface haze was noted in 3 eyes. One case had a thinned, steep cornea that was noted in the immediate postoperative period. Sequential manifest refraction and topography in the follow-up period showed progressive steepening of the cornea and an increase in refractive and topographic astigmatism, suggesting an ongoing ectatic process. At the last follow-up visit, the average keratometric reading was 50.7 D with 7 D of topographic cylinder. No rejection, failure, or infection was found in this case series, and all cases retained clarity of their grafts to the end of their follow-up period (Shousha et al.,

LASIK is a lamellar laser refractive surgery in which excimer laser ablation is done under a partial-thickness lamellar corneal flap. Astigmatism can be managed with excimer laser correction, where the excimer laser is used to reshape the surface of the cornea by removing anterior stromal tissue. A microkeratome was previously used to create a corneal flap with a shift over the last decade to femtosecond laser. The microkeratome used an oscillating blade to cut the flap after immobilization of the cornea with a suction ring. Microkeratomes from several companies cut the lamellar flaps with either superior or nasal hinges, and can cut to

Several effective options for laser refractive surgery are available to treat varying degrees of astigmatism. The choices can broadly be divided into lamellar (LASIK) and surface (photorefractive keratectomy, laser epithelial keratomileusis [LASEK], and Epi-LASIK)

The preoperative assessment must include history of stable refraction, refraction,

First, the disposable applanation lens attached to the laser aperture is docked into the suction ring centered on the pupil. The suction ring is then locked into the applanation lens. The femtosecond laser is pre-programmed for each procedure with a planned flap diameter, flap thickness, hinge angle, raster energy, and side-cut energy. The flap is then created using a raster pattern, moving back and forth across the diameter of the flap. Initially, a pocket is created to allow the carbon dioxide and water gas bubbles to escape during photodisruption

ablation. Here, we describe the surgical technique for LASIK.

keratotomy, pachymetry, tear production, and complete eye examination.

**7.1 Indications and surgical planning** 

2011).

**7. Excimer laser correction** 

depths of 100–200 μm.

**7.2 Technique** 

LASIK has been successfully used to correct low to moderate astigmatism. In a report by the American Academy of Ophthalmology (Sugar et al., 2001), 160 articles were reviewed by a panel of experts with an objective to describe LASIK for myopia and astigmatism and examine the evidence to evaluate the procedure's efficacy and safety. LASIK was found to be effective and predictable in terms of obtaining very good to excellent uncorrected visual acuity for eye s treated with mild to moderate astigmatism (<2.0 diopters). Arbelaez et al. evaluated the postoperative clinical outcomes in eyes with astigmatism greater than 2.0 diopters that underwent LASIK using a femtosecond laser. At 6 months, 84% of the 50 eyes evaluated achieved 20/20 or better uncorrected distance visual acuity (UDVA) and 40% achieved 20/16 or better UDVA. Forty-four percent of eyes were within ±0.25 diopters of the attempted astigmatic correction, and 78% were within ±0.50 diopters (Arbelaez et al., 2009).

There is a wide collection of published studies that have compared the use of femtosecond laser and mechanical microkeratome in corneal flap creation. In one of the earliest comparative studies that investigated results obtained with the femtosecond laser versus those seen with a mechanical microkeratome (Hansatome Microkeratome; Bausch & Lomb, Rochester, New York and the Carriazzo-Barraquer Microkeratome; Moira, Anthony, France), Stonecipher and Kezirian found that there was better flap thickness predictability, fewer complications and less surgically induced astigmatism in the femtosecond laser eyes (Stonecipher and Kezirian, 2004). Tran et al. conducted a prospective, randomized clinical study, which compared induced aberrations following flap creation with the femtosecond laser and the Hansatome Microkeratome. Their results showed that the simple act of flap creation can change lower and higher-order aberrations and that there was a significant increase in higher-order aberrations seen in the microkeratome eyes but not in the femtosecond laser eyes (Tran et al., 2005). Additionally, in another prospective, contralateral eye study comparing the femtosecond laser and a blade microkeratome, the uncorrected visual acuity and manifest refractive outcomes were better in the femtosecond laser eyes (Durrie and Kezirian, 2005). Of note, the IntraLase Corporation supported the above three studies either directly or through providing financial compensation to the study's authors.

In a recent study that evaluated the thickness and side-cut angle of LASIK flaps using Fourier-domain optical coherence tomography (OCT), flap creation for bilateral LASIK was performed using an IntraLase, VisuMax, or Femto LDV femtosecond laser or a microkeratome. The study found that flap morphology differed according to the system used and the 3 femtosecond laser systems appeared to be superior to the microkeratome system (Ahn et al., 2011).

Alternatively, multiple reports have demonstrated no significant difference in visual acuity and corneal aberrations between LASIK with femtosecond laser compared with mechanical microkeratome (Calvo et al., 2010; Patel et al., 2007; Chan et al., 2008). In a randomized, controlled, paired-eye study, Patel et al. evaluated 21 patients (42 eyes) that received LASIK for myopia or myopic astigmatism astigmatism to compare corneal high-order aberrations and visual acuity after LASIK with the flap created by a femtosecond laser to LASIK with the flap created by a mechanical microkeratome. Results showed no difference between the two groups in terms of high-contrast visual acuity, contrast sensitivity, and forward light scatter at 6 months after LASIK (Patel et al., 2007). In a similar prospective, randomized, paired-eye study, Calvo et al. showed the planar configuration of the femtosecond laser flap did not offer any advantage in corneal high-order aberrations or visual acuity through 3 years after LASIK (Calvo et al., 2010).

Most recently, a meta-analysis of seven prospective randomized controlled trials describes a total of 577 eyes with the goal of comparing femtosecond and microkeratome LASIK for myopia (Zhang et al, 2011). At 6 months or more of follow-up, no significant differences were found in the efficacy, accuracy, or safety of the two modalities. In eyes that had undergone femtosecond LASIK, however, the postoperative total aberrations and spherical aberrations were significantly lower. In a larger meta-analysis describing a total of 3,679 eyes, Chen et al. also found no significant differences between the two modalities in regards to visual acuity, final refractive error and astigmatism, or changes in higher order aberrations (Chen at al., 2012). Eyes in which femtosecond laser was utilized in flap creation, on the other hand, had significantly more predictable flap thickness than eyes in which the microkeratome was used. Although these two meta-analyses did not specifically investigate flap creation in astigmatism treatment, they both demonstrated that the use if femtosecond laser was not superior in regards to safety and efficacy when compared to the microkeratome, but it did have the potential advantage of increased predictability and reduced higher order aberrations.

#### **7.4 Complications**

In a study that aimed to describe complications associated with femtosecond laser-assisted flap creation in LASIK surgery, Haft et al. retrospectively evaluated 4772 eyes that underwent LASIK with the IntraLase femtosecond laser. All flaps were made with the 15 and 30-kHz IntraLase femtosecond laser. Forty-four (0.92%) eyes had direct or indirect complications due to flap creation. Thirty-two eyes had indirect complications (diffuse lamellar keratitis (DLK) and transient light sensitivity), 20 (0.42%) eyes developed DLK and 12 (0.25%) eyes had transient light sensitivity syndrome. Twelve (0.25%) eyes had direct femtosecond laser flap-related complications, 8 (0.17%) eyes had premature breakthrough of gas through the epithelium within the flap margins, 3 (0.06%) eyes had incomplete flaps due to suction loss, and 1 (0.02%) eye had irregular flap due to previous corneal scar. In summary, less than 1% of eyes had direct or indirect complications due to femtosecond laser flap creation, and LASIK complications specifically related to the IntraLase femtosecond laser did not cause loss of best spectacle-corrected visual acuity in any eyes (Haft et al., 2009).

In a prospective randomized contralateral eye study, Mian et al. investigated whether corneal sensation and dry-eye signs and symptoms after LASIK with a femtosecond laser are affected by varying hinge position, hinge angle, or flap thickness. Superior and temporal hinge positions, 45-degree and 90-degree hinge angles, and 100 μm and 130 μm corneal flap thicknesses were compared. The study evaluated 190 consecutive eyes (95 patients). Corneal sensation was reduced at all postoperative visits, with improvement over 12 months. There was no difference in corneal sensation between the different hinge positions, angles, or flap thicknesses at any time point. This study also showed that dry-eye syndrome after LASIK with a femtosecond laser was mild and improved after 3 months (Mian et al., 2009).

#### **8. Summary**

206 Astigmatism – Optics, Physiology and Management

Alternatively, multiple reports have demonstrated no significant difference in visual acuity and corneal aberrations between LASIK with femtosecond laser compared with mechanical microkeratome (Calvo et al., 2010; Patel et al., 2007; Chan et al., 2008). In a randomized, controlled, paired-eye study, Patel et al. evaluated 21 patients (42 eyes) that received LASIK for myopia or myopic astigmatism astigmatism to compare corneal high-order aberrations and visual acuity after LASIK with the flap created by a femtosecond laser to LASIK with the flap created by a mechanical microkeratome. Results showed no difference between the two groups in terms of high-contrast visual acuity, contrast sensitivity, and forward light scatter at 6 months after LASIK (Patel et al., 2007). In a similar prospective, randomized, paired-eye study, Calvo et al. showed the planar configuration of the femtosecond laser flap did not offer any advantage in corneal high-order aberrations or visual acuity through 3

Most recently, a meta-analysis of seven prospective randomized controlled trials describes a total of 577 eyes with the goal of comparing femtosecond and microkeratome LASIK for myopia (Zhang et al, 2011). At 6 months or more of follow-up, no significant differences were found in the efficacy, accuracy, or safety of the two modalities. In eyes that had undergone femtosecond LASIK, however, the postoperative total aberrations and spherical aberrations were significantly lower. In a larger meta-analysis describing a total of 3,679 eyes, Chen et al. also found no significant differences between the two modalities in regards to visual acuity, final refractive error and astigmatism, or changes in higher order aberrations (Chen at al., 2012). Eyes in which femtosecond laser was utilized in flap creation, on the other hand, had significantly more predictable flap thickness than eyes in which the microkeratome was used. Although these two meta-analyses did not specifically investigate flap creation in astigmatism treatment, they both demonstrated that the use if femtosecond laser was not superior in regards to safety and efficacy when compared to the microkeratome, but it did have the potential advantage of increased predictability and

In a study that aimed to describe complications associated with femtosecond laser-assisted flap creation in LASIK surgery, Haft et al. retrospectively evaluated 4772 eyes that underwent LASIK with the IntraLase femtosecond laser. All flaps were made with the 15 and 30-kHz IntraLase femtosecond laser. Forty-four (0.92%) eyes had direct or indirect complications due to flap creation. Thirty-two eyes had indirect complications (diffuse lamellar keratitis (DLK) and transient light sensitivity), 20 (0.42%) eyes developed DLK and 12 (0.25%) eyes had transient light sensitivity syndrome. Twelve (0.25%) eyes had direct femtosecond laser flap-related complications, 8 (0.17%) eyes had premature breakthrough of gas through the epithelium within the flap margins, 3 (0.06%) eyes had incomplete flaps due to suction loss, and 1 (0.02%) eye had irregular flap due to previous corneal scar. In summary, less than 1% of eyes had direct or indirect complications due to femtosecond laser flap creation, and LASIK complications specifically related to the IntraLase femtosecond laser did not cause loss of best spectacle-corrected visual acuity in any eyes (Haft et al.,

In a prospective randomized contralateral eye study, Mian et al. investigated whether corneal sensation and dry-eye signs and symptoms after LASIK with a femtosecond laser are affected by varying hinge position, hinge angle, or flap thickness. Superior and temporal

years after LASIK (Calvo et al., 2010).

reduced higher order aberrations.

**7.4 Complications** 

2009).

Indications and techniques for femtosecond laser use for correction of astigmatism are evolving. The functionality of the femtosecond laser as a blade in the cornea has helped improve precision and safety of existing procedures to correct astigmatism. Future clinical trials will further establish the clinical efficacy and optimal technique for use of femtosecond lasers for correction of astigmatism. Although the cost of this technology currently has limited wide-scale use, adaptation of femtosecond lasers for cataract surgery may allow availability and reduction in expenses.

## **9. References**


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*3Greece* 

*4United Kingdom* 

## **Optimized Profiles for Astigmatic Refractive Surgery**

Samuel Arba-Mosquera1,2, Sara Padroni3, Sai Kolli4 and Ioannis M. Aslanides3 *1Grupo de Investigación de Cirugía Refractiva y Calidad de Visión, Instituto de Oftalmobiología Aplicada, University of Valladolid, Valladolid, 2SCHWIND eye-tech-solutions, Kleinostheim, 3Emmetropia Mediterranean Eye Clinic, Heraklion, 4Moorfields Eye Hospital, London, 1Spain 2Germany* 

## **1. Introduction**

210 Astigmatism – Optics, Physiology and Management

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Laser versus mechanical microkeratome laser in situ keraomileusis for myopia: Metanalysis of randomized constolled trials. Journal of Cataract and Refractive For the correction of astigmatism, many different approaches have been tested, with different degrees of success, through the years1. Patient satisfaction in any refractive surgery, wavefront-guided or not, is primarily dependent on successful treatment of the lower order aberrations (LOA) of the eye (sphere and cylinder). Achieving accurate clinical outcomes and reducing the likelihood of a retreatment procedure are major goals of refractive surgery. LASIK has been successfully used for low to moderate myopic astigmatism, whether LASIK is acceptably efficacious, predictable, and safe in correcting higher myopic astigmatism is less documented, especially with regard to the effects of astigmatic corrections in HOA's.

The correction of astigmatism has been approached using several techniques and ablation profiles. There are several reports showing good results for compound myopic astigmatism using photorefractive keratectomy (PRK) and LASIK, but ablation profiles usually cause a hyperopic shift because of a coupling effect in the flattest corneal meridian. A likely mechanism of this coupling effect is probably due to epithelial remodeling and other effects such as smoothing by the LASIK flap. In cases of large preoperative amounts of astigmatism, deviations from the target refractive outcome are usually attributed to "coupling factors." Nevertheless, the investigation of the coupling factor remains a rather difficult task, because it seems to be dependent on various factors. Individual excimer laser systems may have different coupling factors, cutting the flap could alter the initial prescription and different preoperative corneal curvatures (K-reading) may have influence on coupling factor.

#### **2. Induction of aberrations**

While for quasi-spherical corrections the focus has been moved from primary refractive outcomes to effects of the ablation in postoperative high order aberrations (HOA)5,38,28, for

Fig. 1. Representations of the astigmatic error. Top: Topographic astigmatism measured at the corneal vertex withhin the central 3mm disc showing 4D of astigmatism. Middle left: 7mm diameter wavefront irregular astigmatism measured at the pupil centre showing 4.6D regular astigmatism with 0.4D coma and 0.3D spherical aberration. Middle right: 7mm diameter wavefront more regular astigmatism measured at the corneal vertex showing 4.6D regular astigmatism with only 0.2D coma and 0.2D spherical aberration. Bottom left: 4mm diameter refractive irregular astigmatism measured at the pupil centre showing 4.25D regular astigmatism with 0.2D coma and 0.1D spherical aberration. Bottom right: 4mm diameter refractive more rergular astigmatism measured at the corneal vertex showing 4.25D regular astigmatism with 0D coma and 0.1D spherical aberration.

Fig. 1. Representations of the astigmatic error. Top: Topographic astigmatism measured at the corneal vertex withhin the central 3mm disc showing 4D of astigmatism. Middle left: 7mm diameter wavefront irregular astigmatism measured at the pupil centre showing 4.6D regular astigmatism with 0.4D coma and 0.3D spherical aberration. Middle right: 7mm diameter wavefront more regular astigmatism measured at the corneal vertex showing 4.6D regular astigmatism with only 0.2D coma and 0.2D spherical aberration. Bottom left: 4mm diameter refractive irregular astigmatism measured at the pupil centre showing 4.25D regular astigmatism with 0.2D coma and 0.1D spherical aberration. Bottom right: 4mm diameter refractive more rergular astigmatism measured at the corneal vertex showing

4.25D regular astigmatism with 0D coma and 0.1D spherical aberration.

astigmatism the focus mainly remained at the primary refractive outcomes, principally due to the encountered problems as "coupling factors"5 or cyclotorsion errors38, which result in residual astigmatism.

#### **% Residual Astigmatism vs. Torsion error**

Fig. 2. Residual astigmatism magnitude vs. torsion error (assuming the astigmatic error and the correcting cylinder are at the same plane and are equal in dioptric power).

Refractive surgeons have been observing post-operatively a resulting hyperopic refraction on the sphere (hyperopic shift) whenever they applied a negative cylinder onto the cornea. This output sphere was not planned and depends on several factors:


Due to all these reasons, it was an issue for the surgeons to properly include this effect in their nomograms to achieve the intended refraction.

Most of the LASER manufacturers and surgeons used the "Coupling Factor"2 defined as the averaged output sphere per single diopter of pure negative cylinder achieved.

Despite of its empirical nature, this Coupling Factor allows the surgeon to plan the treatment with reasonable success.

The hint for one of the sources of this "coupling effect" was the analysis of a pure negative cylinder case. When a pure negative cylinder is applied, the neutral axis becomes refractive, being deeper at the centre compare to the periphery.

The "Coupling Factor" is a nomogram-like "adjustment" introduced by surgeons to achieve the intended result. With the introduction of Wavefront guided ablation volumes and loss of efficiency compensation factors; these effects should be mainly compensated in the devices by refined algorithms instead of nomogrammed by the surgeon.

The currently available methods allow for the correction of refractive include astigmatism3 defects. One of the unintended effects induced by laser surgery is the induction of aberrations, which causes halos and reduced contrast sensitivity4. The loss of ablation efficiency at non-normal incidence can explain, in part, many of these unwanted effects, such as induction of high order astigmatism of postoperative corneas after myopic surgery.

Considering a loss of efficiency model applied to a simple myopic astigmatism profile, the neutral axis becomes refractive, being less ablated in the periphery as compared to the centre, whereas the refractive axis "shrinks," steepening the curvature and then slightly increasing the myopic power of the axis as well as inducing aberrations. The net effect can be expressed as an unintended myopic ablation (hyperopic shift), and a small undercorrection of the astigmatic component.

Fig. 3. Effect of the hyperopic shift and coupling factor in the ablative refractive correction. Left: attempted correction. Right: simulated correction considering uncompensated reflection, projection, and geometric efficiency losses. Notice that an unintended -0.75D extra myopic spherical correction is induced, and a reduction of 0.5D in the astigmatic correction.

Several models have been proposed to compensate for those effects.

The models by Arba-Mosquera&de-Ortueta5 provide a general method to analyze the loss of ablation efficiency at non-normal incidence in a geometrical way. The model is comprehensive and directly considers curvature, system geometry, applied correction, and astigmatism as model parameters, and indirectly laser beam characteristics and ablative spot properties. The model replaces the direct dependency on the fluence by a direct dependence on the nominal spot volume and on considerations about the area illuminated by the beam, reducing the analysis to pure geometry of impact. Compensation of the loss of ablation efficiency at non-normal incidence can be made at relatively low cost and would directly improve the quality of results.

The proposed models provide results essentially identical to those obtained with the model by Dorronsoro-Cano-Merayo-Marcos5. Additionally, it offers an analytical expression including some parameters that were ignored (or at least not directly addressed) in previous analytical approaches.

Different effects interact; the beam is compressed due to the loss of efficiency, but at the same time expands due to the angular "projection." Losses of ablation efficiency at nonnormal incidence in refractive surgery, may explain up to 45% of the reported increase in

The currently available methods allow for the correction of refractive include astigmatism3 defects. One of the unintended effects induced by laser surgery is the induction of aberrations, which causes halos and reduced contrast sensitivity4. The loss of ablation efficiency at non-normal incidence can explain, in part, many of these unwanted effects, such as induction of high order astigmatism of postoperative corneas after myopic surgery. Considering a loss of efficiency model applied to a simple myopic astigmatism profile, the neutral axis becomes refractive, being less ablated in the periphery as compared to the centre, whereas the refractive axis "shrinks," steepening the curvature and then slightly increasing the myopic power of the axis as well as inducing aberrations. The net effect can be expressed as an unintended myopic ablation (hyperopic shift), and a small

Fig. 3. Effect of the hyperopic shift and coupling factor in the ablative refractive correction. Left: attempted correction. Right: simulated correction considering uncompensated reflection, projection, and geometric efficiency losses. Notice that an unintended -0.75D extra myopic spherical correction is induced, and a reduction of 0.5D in the astigmatic

The models by Arba-Mosquera&de-Ortueta5 provide a general method to analyze the loss of ablation efficiency at non-normal incidence in a geometrical way. The model is comprehensive and directly considers curvature, system geometry, applied correction, and astigmatism as model parameters, and indirectly laser beam characteristics and ablative spot properties. The model replaces the direct dependency on the fluence by a direct dependence on the nominal spot volume and on considerations about the area illuminated by the beam, reducing the analysis to pure geometry of impact. Compensation of the loss of ablation efficiency at non-normal incidence can be made at relatively low cost and would directly

The proposed models provide results essentially identical to those obtained with the model by Dorronsoro-Cano-Merayo-Marcos5. Additionally, it offers an analytical expression including some parameters that were ignored (or at least not directly addressed) in previous analytical

Different effects interact; the beam is compressed due to the loss of efficiency, but at the same time expands due to the angular "projection." Losses of ablation efficiency at nonnormal incidence in refractive surgery, may explain up to 45% of the reported increase in

Several models have been proposed to compensate for those effects.

undercorrection of the astigmatic component.

correction.

approaches.

improve the quality of results.

aberrations. The loss of efficiency is an effect that should be offset in commercial laser systems using sophisticated algorithms that cover most of the possible variables. Parallel to the clinical developments, increasingly capable, reliable, and safer laser systems with better resolution and accuracy are required.

Corneal curvature and applied correction play an important role in the determination of the ablation efficiency and are taken into account for accurate results. However, corneal toricity and applied astigmatism do not have a relevant impact as long as their values correspond to those of normal corneas. Only when toricity or astigmatism exceeds 3 D, their effects on ablation efficiency start to be significant. Surface asphericity showed minor effects.

The loss of efficiency in the ablation and non-normal incidence are responsible for much of the induction of aberrations observed in the treatments as well as many undercorrections observed in astigmatism with major implications for treatment and optical outcome of the procedure. Compensation can be made at relatively low cost and directly affects the quality of results.

Considering this model of loss of efficiency, we have applied it for different LASER peak radiant exposures (FWHM 1mm, Gaussian profile), then we have calculated the "Coupling Factor" according to the averaged output sphere per single diopter of pure negative cylinder achieved.


Table 1. Theoretical coupling factor as a function of the laser fluence.

Today, several approaches to import, visualize, and analyze high detailed diagnostic data of the eye (corneal or ocular wavefront data) are offered. At the same time, several systems are available to link diagnostic systems for measurement of corneal and ocular aberrations6 of the eye to refractive laser platforms. These systems are state-of-the-art with flying spot technology, high repetition rates, fast active eye trackers, and narrow beam profiles. Consequently, these systems offer new and more advanced ablation capabilities, which may potentially suffer from new sources of "coupling" (different Zernike orders5 affecting each other with impact on the result). The improper use of a model that overestimates or underestimates the loss of efficiency will overestimate or underestimate its compensation and will only mask the induction of aberrations under the appearance of other sources of error.

In coming years, the research and development of algorithms will continue on several fronts in the quest for zero aberration. This includes identification of sources for induction of aberrations, development and refinement of models describing the pre-, peri- and postoperative biomechanics of the cornea, development of aberration-free profiles leaving pre-existing aberrations of the eye unchanged, redevelopment of ablation profiles to compensate for symptomatic aberrated eyes in order to achieve an overall postoperative zero level of aberration (corneal or ocular)7. Finally, the optimal surgical technique (LASIK (Laser assisted in-situ Keratomileusis), LASEK (Laser Epithelial Keratomileusis), PRK (Photorefractive Keratectomy), Epi-LASIK ...) to minimize the induction of aberrations to a noise level has not yet been determined8.

## **3. Baseline for refractive profiles**

When a patient is selected for non-customized aspherical treatment, the global aim of the surgeon should be to leave all existing high-order-aberrations (HOA) unchanged because the best-corrected visual acuity, in this patient, has been unaffected by the pre-existing aberrations. Hence, all factors that may induce HOAs, such as biomechanics, need to be taken into account prior to the treatment to ensure that the preoperative HOAs are unchanged after treatment38.

Then, in the treatments, the goals should be:


Even though the condition of stigmatism, that origins "free of aberration" verified for two points (object and image) and for a conicoid under limited conditions, is very sensitive to


Table 2. Level of detail of the treatment approaches considered at the AMARIS system.

compensate for symptomatic aberrated eyes in order to achieve an overall postoperative zero level of aberration (corneal or ocular)7. Finally, the optimal surgical technique (LASIK (Laser assisted in-situ Keratomileusis), LASEK (Laser Epithelial Keratomileusis), PRK (Photorefractive Keratectomy), Epi-LASIK ...) to minimize the induction of aberrations to a

When a patient is selected for non-customized aspherical treatment, the global aim of the surgeon should be to leave all existing high-order-aberrations (HOA) unchanged because the best-corrected visual acuity, in this patient, has been unaffected by the pre-existing aberrations. Hence, all factors that may induce HOAs, such as biomechanics, need to be taken into account prior to the treatment to ensure that the preoperative HOAs are

a. For aspherical treatments: no induced aberrations; a change in asphericity depending

b. For wavefront-guided treatments: change in aberrations according to diagnosis; change in asphericity depending on the corrected defocus and on the C(n,0) coefficients applied. Even though the condition of stigmatism, that origins "free of aberration" verified for two points (object and image) and for a conicoid under limited conditions, is very sensitive to

> **Aberration-free Treatment**

Aspheric ablation profile Yes Yes Yes

Using PresbyMAX

+ Cylinder Yes Yes Yes

laser beam Yes Yes Yes

aberrations (HOA) Preserved Yes Yes

aberrations (biomechanical effect) Yes Yes Yes

Table 2. Level of detail of the treatment approaches considered at the AMARIS system.

**Corneal Wavefront Treatment**

Using PresbyMAX

Yes Yes Yes

**Ocular Wavefront Treatment**

> Using PresbyMAX

noise level has not yet been determined8.

**3. Baseline for refractive profiles** 

Then, in the treatments, the goals should be:

unchanged after treatment38.

on the corrected defocus.

Bi-aspheric ablation profile for the correction of Presbyopia

Simultaneous correction of Sphere

Correction of high order

Compensation by microkeratome usually induced aberrations (biomechanical effect)

Compensation of ablation induced

Compensation of energy loss of the

small deviations and decentrations (a question that usually arises in refractive surgery), the goal of these profiles is not to achieve an stigmatism condition postoperatively, but rather to maintain the original HO wavefront-aberration.

The optical quality in an individual can be maximized for a given wavelength and a given distance by canceling the aberration of his wavefront and optimizing his defocus (for a single distance), but this has direct implications dramatically negative for the optical quality for the rest of wavelengths (greater negative effect the more extreme is the wavelength). However, the optical quality of a person showing a certain degree of aberration of his wavefront decreases compared to the maximum obtainable in the absence of aberration, but it has direct positive implications in the "stability" of the optical quality for a wide range for wavelengths (which covers the spectral sensitivity of the human eye)9 and in the depth of focus, i.e. for a range of distances that can be considered "in-focus" simultaneously. Lastly, moderate levels of wavefront-aberration favor the stability of the image quality for wider visual fields10. In such a way, there are, at least, three criteria (chromatic blur, depth of focus, wide field vision) favoring the target of leaving minor amounts of not clinically relevant aberrations (the proposed "aberration-free" concept).

With simple spherical error, degradation of resolution begins for most people with errors of 0.25 D. A similar measure can be placed on the error due to cylinder axis error.

Optimized patterns for refractive surgery aiming to be neutral for aberrations together with the consideration of other sources of aberrations such as blending zones, eye-tracking, and corneal biomechanics having close-to-ideal ablation profiles should improve the clinical results decreasing the need for nomograms, and reducing the induced aberrations after surgery.

## **4. The astigmatic refraction problem**

Classical ametropias (myopia, hyperopia and astigmatism) are, similarly to aberration errors, differences to a reference surface, and are included in the, more general, wavefront error. However, classical ametropias are used to be described, not in units of length, but in units of optical refractive power.

It is, then, necessary to find a relationship between wavefront error magnitudes and classical ametropias11,12,13,14,15. This relationship is often called "objective wavefront refraction":

The quadratic equivalent of a wave-aberration map can be used as a relationship between wavefront-error magnitudes and classical ametropias. That quadratic is a sphero-cylindrical surface, which approximates the wave aberration map. The idea of approximating an arbitrary surface with a quadratic equivalent is a simple extension of the ophthalmic technique of approximating a sphero-cylindrical surface with an equivalent sphere.


A common way to fit an arbitrarily aberrated wavefront with a quadratic surface is to find the surface that minimizes the sum of the squared deviations between the two surfaces.

The least-square fitting method is the basis of the Zernike wavefront expansion. Since the Zernike expansion employs an orthogonal set of basic functions, the least-square solution is simply given by the second-order Zernike coefficients of the aberrated wavefront, regardless of the values of the other coefficients. These second-order Zernike coefficients can be converted into a sphero-cylindrical prescription in power vector notation of the form [J0, M, J45].

$$J\_o = \frac{-8\sqrt{6}C\_2^{+2}}{PD^2} \tag{1}$$

$$M = \frac{-16\sqrt{3}C\_2^0}{PD^2} \tag{2}$$

$$J\_{45} = \frac{-8\sqrt{6C\_2^{-2}}}{PD^2} \tag{3}$$

where PD is the pupil diameter, M is the spherical equivalent, J0, the cardinal astigmatism and J45 the oblique astigmatism. The components J0, M, and J45 represent the power of a Jackson crossed cylinder with axes at 0 and 90°, the spherical equivalent power, and the power of a Jackson crossed cylinder with axes at 45 and 135°, respectively.

The power-vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of sphere, cylinder, and axis in the minus-cylinder or pluscylinder formats used by clinicians.

$$S = M - \frac{C}{2} \tag{4}$$

$$C = 2\sqrt{J\_0^2 + J\_{45}^2} \tag{5}$$

$$A = \frac{\arctan\left(\frac{J\_{45}}{J\_0}\right)}{2} \tag{6}$$

Objective wavefront refraction from Seidel aberrations at full pupil size

The Seidel sphere adds a value for the primary spherical aberration to improve, in theory, the fit of the wavefront to a sphere and improve accuracy of the spherical equivalent power.

$$M = \frac{-16\sqrt{3}C\_2^0 + 48\sqrt{5}C\_4^0}{PD^2} \tag{7}$$

Objective wavefront refraction from low order Zernike modes at subpupil size

The same low-order Zernike modes can be used to calculate the refraction for any given smaller pupil size, either by refitting the raw wave-aberration data to a smaller diameter, or by mathematically performing the so-called radius transformation18 of the Zernike expansion to a smaller diameter.

The least-square fitting method is the basis of the Zernike wavefront expansion. Since the Zernike expansion employs an orthogonal set of basic functions, the least-square solution is simply given by the second-order Zernike coefficients of the aberrated wavefront, regardless of the values of the other coefficients. These second-order Zernike coefficients can be converted into a sphero-cylindrical prescription in power vector notation of the form [J0, M,

> 0 2 8 6*<sup>C</sup> <sup>J</sup> PD*

16 3*<sup>C</sup> <sup>M</sup> PD*

45 2 8 6*<sup>C</sup> <sup>J</sup> PD*

power of a Jackson crossed cylinder with axes at 45 and 135°, respectively.

cylinder formats used by clinicians.

expansion to a smaller diameter.

where PD is the pupil diameter, M is the spherical equivalent, J0, the cardinal astigmatism and J45 the oblique astigmatism. The components J0, M, and J45 represent the power of a Jackson crossed cylinder with axes at 0 and 90°, the spherical equivalent power, and the

The power-vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of sphere, cylinder, and axis in the minus-cylinder or plus-

2

2 2

arctan

*A*

Objective wavefront refraction from Seidel aberrations at full pupil size

2

The Seidel sphere adds a value for the primary spherical aberration to improve, in theory, the fit of the wavefront to a sphere and improve accuracy of the spherical equivalent power.

> 16 3 48 5 *C C <sup>M</sup> PD*

The same low-order Zernike modes can be used to calculate the refraction for any given smaller pupil size, either by refitting the raw wave-aberration data to a smaller diameter, or by mathematically performing the so-called radius transformation18 of the Zernike

Objective wavefront refraction from low order Zernike modes at subpupil size

45 0

0 0 2 4 2

 

*J J*

2 2

0 2 2

> 2 2

(1)

(2)

(3)

*<sup>C</sup> S M* (4)

0 45 *C JJ* 2 (5)

(6)

(7)

J45].

Objective wavefront refraction from Seidel aberrations at subpupil size

In the same way, Seidel aberrations can be used to calculate the refraction for any subpupil size.

Objective wavefront refraction from paraxial curvature

Curvature is the property of wavefronts that determines how they focus. Thus, another reasonable way to fit an arbitrary wavefront with a quadratic surface is to match the curvature of the two surfaces at some reference point.

A variety of reference points could be selected, but the natural choice is the pupil center. Two surfaces that are tangent at a point and have the same curvature in every meridian are said to osculate. Thus, the surface we seek is the osculating quadric.

Fortunately, a closed-form solution exists for the problem of deriving the power vector parameters of the osculating quadratic from the Zernike coefficients of the wavefront. This solution is obtained by computing the curvature at the origin of the Zernike expansion of the Seidel formulae for defocus and astigmatism. This process effectively collects all r2 terms from the various Zernike modes.

$$J\_0 = \frac{-8\sqrt{6}C\_2^{-2} + 24\sqrt{10}C\_4^{-2} - 48\sqrt{14}C\_6^{-2} + 240\sqrt{2}C\_8^{-2} - 120\sqrt{22}C\_{10}^{-2} + \dots}{PD^2} \tag{8}$$

$$M = \frac{-16\sqrt{3}C\_2^0 + 48\sqrt{5}C\_4^0 - 96\sqrt{7}C\_6^0 + 480C\_8^0 - 240\sqrt{11}C\_{10}^0 + \dots}{PD^2} \tag{9}$$

$$J\_{45} = \frac{-8\sqrt{6}C\_2^{+2} + 24\sqrt{10}C\_4^{+2} - 48\sqrt{14}C\_6^{+2} + 240\sqrt{2}C\_8^{+2} - 120\sqrt{22}C\_{10}^{+2} + \dots}{PD^2} \tag{10}$$

Objective wavefront refraction from wavefront axial refraction

It is also possible to represent the wavefront aberration in optical refractive power, without the need of simplifying it to a quadric surface, and, therefore, providing a higher level of detail. Straightforward approach for the problem is to use the concept of axial refractive error (vergence maps19) (Fig. 4).

The line of sight represents a chief ray; the wavefront aberration is zero at the pupil centre, and perpendicular to the line of sight. Each point of the wavefront propagates perpendicular to the local surface of the wavefront. The axial distance from the pupil centre to the intercept between the propagated local wavefront and the line of sight expressed in dioptres corresponds to the axial refractive error.

$$AR\mathfrak{x}\left(\rho,\theta\right) = \frac{-1}{r} \frac{\partial W\left(\rho,\theta\right)}{\partial \rho} \tag{11}$$

A schematic comparison of the different quadric methods described here for the determination of the objective wavefront refraction for a given pupil size is depicted in Fig. 5.

Automatic Manifest Refraction Balance

These objective methods for calculating the refraction are optically correct but have some practical limitations in clinical practice20,21.

The devices to obtain the wavefront aberration of an eye use to work in the infrared range (IR), which is invisible for the human eye and avoid undesired miotic effects in the pupil

Fig. 4. Representation of the axial refractive error. The line of sight represents a chief ray; the wavefront aberration is zero at the pupil centre, and perpendicular to the line of sight. Each point of the wavefront propagates perpendicular to the local surface of the wavefront. The axial distance from the pupil centre to the intercept between the propagated local wavefront and the line of sight expressed in dioptres corresponds to the axial refractive error17.

Fig. 5. Comparison of the different quadric methods described here for the determination of the objective wavefront refraction for a given pupil size.

Fig. 4. Representation of the axial refractive error. The line of sight represents a chief ray; the wavefront aberration is zero at the pupil centre, and perpendicular to the line of sight. Each point of the wavefront propagates perpendicular to the local surface of the wavefront. The axial distance from the pupil centre to the intercept between the propagated local wavefront

Fig. 5. Comparison of the different quadric methods described here for the determination of

the objective wavefront refraction for a given pupil size.

and the line of sight expressed in dioptres corresponds to the axial refractive error17.

size. The refractive indices of the different optical elements in our visual system depend on the wavelength of the illumination light. In this way, the propagated wavefront (and the corresponding wavefront aberration) ingoing to (or outcoming from) our visual system depends on the wavelength of the illumination light, leading to the so-called chromatic aberration22.

The different methods provide "slightly" different results, depending on how they are compared to the subjective manifest refraction, one or another correlates better with manifest refraction16.

HOAb influence LOAb (refraction) when analysed for smaller diameters: For full pupil (e.g. 6 mm) the eye sees the world through HOAb producing some multifocality but without defocus, for a smaller pupil (e.g. 4 mm), the optical aberration of the eye is the same but the outer ring is blocked, thereby the eye sees the world through the central part of the HOAb, which may produce some defocus or astigmatism (LOAb, refraction).

A variation of the objective wavefront refraction from low-order Zernike modes at a fixed subpupil diameter of 4 mm was chosen as starting point to objectively include the measured subjective manifest refraction in the wave aberration (Fig. 6 to Fig. 9).

Fig. 6. Zernike refraction of a pure Spherical Aberration (at 6 mm) is per definition 0 because Spherical Aberration is a High Order Aberration mode, when analysed for a smaller diameter (4 mm) produces Defocus.

Fig. 7. Zernike refraction of a pure High Order Astigmatism (at 6 mm) is per definition 0 because of High Order Aberration mode, when analysed for a smaller diameter (4 mm) produces Astigmatism.

Fig. 8. Zernike refraction of a pure Coma (at 6 mm) is per definition 0 because Coma is a High Order Aberration mode, when analysed for a smaller diameter (4 mm) produces only tilt. Notice that coma may have "visual effect" if the visual axis changes producing Astigmatism.

Fig. 9. Zernike refraction of a general wavefront aberration analysed at 6 mm and analysed for a smaller diameter (4 mm).

The expected optical impact of high-order aberrations in the refraction is calculated and modified from the input manifest refraction. The same wave aberration is analysed for two different diameters: for the full wavefront area (6 mm in this study) and for a fixed subpupil diameter of 4 mm. The difference in refraction obtained for each of the two diameters corresponds to the manifest refraction associated to the high-order aberrations (Fig. 10).

The condition is to re-obtain the input manifest refraction for the subpupil diameter of 4 mm. This way, the low-order parabolic terms of the modified wave aberration for the full wavefront area can be determined.

Comprehensive astigmatism planning and analysis

**Step 1.** (Common) Calculation of the correction at the corneal plane

We first recalculate the correction components from the spectacle plane to the corneal plane where the ablation will take place:

$$S\_{CP} = \frac{S\_{sp}}{1 - S\_{sp}VD} \tag{12}$$

Where SCP is the spherical component at corneal plane, SSP is the spherical component at spectacle plane and VD the vertex distance from the corneal plane to the spectacle plane.

Fig. 8. Zernike refraction of a pure Coma (at 6 mm) is per definition 0 because Coma is a High Order Aberration mode, when analysed for a smaller diameter (4 mm) produces only

Fig. 9. Zernike refraction of a general wavefront aberration analysed at 6 mm and analysed

The expected optical impact of high-order aberrations in the refraction is calculated and modified from the input manifest refraction. The same wave aberration is analysed for two different diameters: for the full wavefront area (6 mm in this study) and for a fixed subpupil diameter of 4 mm. The difference in refraction obtained for each of the two diameters corresponds to the manifest refraction associated to the high-order aberrations

The condition is to re-obtain the input manifest refraction for the subpupil diameter of 4 mm. This way, the low-order parabolic terms of the modified wave aberration for the full

We first recalculate the correction components from the spectacle plane to the corneal plane

*SP*

(12)

*S VD*

*SP*

1

Where SCP is the spherical component at corneal plane, SSP is the spherical component at spectacle plane and VD the vertex distance from the corneal plane to the spectacle plane.

*<sup>S</sup> <sup>S</sup>*

*CP*

tilt. Notice that coma may have "visual effect" if the visual axis changes producing

Astigmatism.

(Fig. 10).

for a smaller diameter (4 mm).

wavefront area can be determined.

where the ablation will take place:

Comprehensive astigmatism planning and analysis

**Step 1.** (Common) Calculation of the correction at the corneal plane

Fig. 10. Automatic Refraction Balance. Optical impact of the HOAb the refraction is calculated and balanced from input refraction. Notice that the same wavefront aberration is analysed for two different diameters. The difference in the refraction provided at the two different analysis diameters correspond to the manifest refraction provided by the high order aberration.

#### **Refractive effect for 1 µm of aberration coefficient**

Fig. 11. Refractive effect of 1µm aberration as a function of the scaling factor from the analysis diameter to the considered refractive zone.

**Refractive effect for 1 DEq of aberration coefficient**

Fig. 12. Refractive effect of 1D aberration as a function of the scaling factor from the analysis diameter to the considered refractive zone.

$$C\_{CP} = \frac{S\_{sp} + C\_{sp}}{1 - (S\_{sp} + C\_{sp})VD} - S\_{cp} \tag{13}$$

Where CCP is the cylindrical component at corneal plane, and CSP is the cylindrical component at spectacle plane.

**Step 2.** (Common) Correction of the corneal keratometries to anterior corneal surface curvatures

We measured the best-fit keratometry readings (K-readings) of Maloney index.

The different refractive indices used for the topography and the ablation planning (keratometric refractive index 1.3375 for the topographies, and corneal refractive index 1.376 for the ablations) were taken into account.

$$K\_{\mathcal{A}\text{CS},i} = K\_i \frac{n\_{\text{Correa}} - n\_{\text{Ar}}}{n\_{\text{Topo}} - n\_{\text{Ar}}} \tag{14}$$

Where KACS,i are the meridional corneal curvatures of the anterior corneal surface, Ki are the Maloney K-readings of the cornea, nCornea is the refractive index of the cornea (1.376), nTopo is the refractive index used by the topographer (1.3375), and nAir is the refractive index of the air (1).

$$K\_{\rm{ACS},i} = K\_i \frac{0.376}{0.3375} \tag{15}$$

Thus, a topographical condition e.g. of 41.65 D at 111°, and 41.21 D at 21°, is considered as 46.40 D at 111°, and 45.91 D at 21° anterior corneal surface curvature, due to the different

8 10 refractive indices used by the topographer (keratometric refractive index 1.3375) and the actual refractive index of the cornea (1.376).

**Step 3.** (Common) Expressing the correction at the corneal plane in power vector notation The conventional refractions in terms of sphere, cylinder and axis in minus-cylinder or pluscylinder formats used by clinicians can be easily converted to a sphero-cylindrical prescription in power vector notation of the form [J0, M, J45].

The mathematical formulation is:

224 Astigmatism – Optics, Physiology and Management

**Refractive effect for 1 DEq of aberration coefficient**

2nd order 4th order 6th order 8th order

(13)

(14)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 **Scaling factor (Refractive Zone / Analysis diameter)**

Fig. 12. Refractive effect of 1D aberration as a function of the scaling factor from the analysis

<sup>1</sup> *SP SP CP CP SP SP S C C S S C VD* 

Where CCP is the cylindrical component at corneal plane, and CSP is the cylindrical

**Step 2.** (Common) Correction of the corneal keratometries to anterior corneal surface

The different refractive indices used for the topography and the ablation planning (keratometric refractive index 1.3375 for the topographies, and corneal refractive index 1.376

*n n K K*

Where KACS,i are the meridional corneal curvatures of the anterior corneal surface, Ki are the Maloney K-readings of the cornea, nCornea is the refractive index of the cornea (1.376), nTopo is the refractive index used by the topographer (1.3375), and nAir is the refractive index of the

Thus, a topographical condition e.g. of 41.65 D at 111°, and 41.21 D at 21°, is considered as 46.40 D at 111°, and 45.91 D at 21° anterior corneal surface curvature, due to the different

*Cornea Air*

*Topo Air*

0.376 0.3375

*K K ACS i i* (15)

*n n* 

We measured the best-fit keratometry readings (K-readings) of Maloney index.

,

*ACS i i*

,


diameter to the considered refractive zone.

for the ablations) were taken into account.

component at spectacle plane.

curvatures

air (1).

**Equivalent C[2,m] (D)**

$$M\_{\rm MR} = S\_{\rm CP} + \frac{C\_{\rm CP}}{2} \tag{16}$$

$$J\_{0,MR} = \frac{-C\_{CP}}{2} \cos\left(2A\_{MR}\right) \tag{17}$$

$$J\_{45,MR} = \frac{-C\_{CP}}{2} \sin\left(2A\_{MR}\right) \tag{18}$$

Where M is the spherical equivalent of the manifest refraction at corneal plane, J0 the cardinal astigmatism and J45 the oblique astigmatism. The components J0, M, and J45, respectively, represent the power of a Jackson crossed-cylinder with axes at 0 and 90°, the spherical equivalent power, and the power of a Jackson crossed-cylinder with axes at 45 and 135°.

**Step 4.** (Common) Expressing the corneal curvatures in power vector notation

The conventional corneal curvatures used by clinicians can be easily converted to spherocylindrical corneal curvatures in power vector notation of the form [J0, M, J45]. The mathematical formulation is:

$$M\_K = \frac{K\_1 + K\_2}{2} \tag{19}$$

$$J\_{0,K} = \frac{K\_1 - K\_2}{2} \frac{\cos\left(2A\_1\right) - \cos\left(2A\_2\right)}{2} \tag{20}$$

$$J\_{45,K} = \frac{K\_1 - K\_2}{2} \frac{\sin\left(2A\_1\right) - \sin\left(2A\_2\right)}{2} \tag{21}$$

Where M is the spherical equivalent of the corneal curvatures, J0 the cardinal astigmatism and J45 the oblique astigmatism.

**Step 5.** (Common) Calculation of the internal astigmatism in power vector notation The internal astigmatism is the difference between the manifest and the corneal astigmatism.

The mathematical formulation is:

$$J\_{0,I} = J\_{0,M\mathbb{R}} - J\_{0,K} \tag{22}$$

$$J\_{45,I} = J\_{45,MR} - J\_{45,K} \tag{23}$$

**Step 6.** (Common) Calculation of the internal astigmatism in clinician notation

The power vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of cylinder and axis in minus-cylinder or plus-cylinder formats used by clinicians.

$$C\_I = 2\sqrt{J\_{0,I}^2 + J\_{45,I}^2} \tag{24}$$

$$A\_{l} = \frac{\arctan\left(\frac{J\_{45,l}}{J\_{0,l}}\right)}{2} \tag{25}$$

**Step 7.** (Common) Calculation of the predicted residual manifest refraction in power vector notation

The predicted residual manifest astigmatism is the difference between the planned and the manifest astigmatism.

The mathematical formulation is:

$$J\_{0,RM} = J\_{0,MR} - J\_{0,P} \tag{26}$$

$$J\_{45,RM} = J\_{45,MR} - J\_{4S,P} \tag{27}$$

**Step 8.** (Common) Calculation of the predicted residual manifest refraction in clinician notation

The power vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of cylinder and axis in minus-cylinder or plus-cylinder formats used by clinicians.

$$\Delta S\_{RM,CP} = M\_{RM} - \frac{C\_{RM,CP}}{2} \tag{28}$$

$$C\_{RM,CP} = 2\sqrt{J\_{0,RM}^2 + J\_{4S,RM}^2} \tag{29}$$

$$A\_{RM} = \frac{\arctan\left(\frac{J\_{45,RM}}{J\_{0,RM}}\right)}{2} \tag{30}$$

**Step 9.** (Common) Expressing the predicted residual manifest refraction in clinician notation at spectacle plane

We then recalculate the correction components from the corneal plane to the spectacle plane:

$$\Delta S\_{RM,SP} = \frac{S\_{RM,CP}}{1 + S\_{RM,CP}VD} \tag{31}$$

$$C\_{RM,SP} = \frac{S\_{RM,CP} + C\_{RM,CP}}{1 + \left(S\_{RM,CP} + C\_{RM,CP}\right)VD} - S\_{RM,SP} \tag{32}$$

**Step 10.** (Common) Calculation of the predicted residual corneal astigmatism in power vector notation

The predicted residual corneal astigmatism is the difference between the planned and the corneal astigmatism.

The mathematical formulation is:

226 Astigmatism – Optics, Physiology and Management

The power vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of cylinder and axis in minus-cylinder or plus-cylinder

arctan

*I*

*A*

2 2

2

**Step 7.** (Common) Calculation of the predicted residual manifest refraction in power

The predicted residual manifest astigmatism is the difference between the planned and the

**Step 8.** (Common) Calculation of the predicted residual manifest refraction in clinician

The power vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of cylinder and axis in minus-cylinder or plus-cylinder

, 2

arctan

**Step 9.** (Common) Expressing the predicted residual manifest refraction in clinician

We then recalculate the correction components from the corneal plane to the spectacle plane:

, 1 *RM CP*

*S VD*

*S*

*RM*

,

*RM SP*

*S*

*A*

notation at spectacle plane

*RM CP RM*

,

*RM CP*

*C*

2 2

45, 0,

 

*J J*

*RM RM*

2

,

*RM CP*

45, 0,

 

*J J*

*I I*

0, 45, 2 *C JJ <sup>I</sup> I I* (24)

(25)

0, 0, 0, *RM MR P J J J* (26)

45, 45, 45, *RM MR P J J J* (27)

*S M* (28)

, 0, 45, 2 *C JJ RM CP RM RM* (29)

(30)

(31)

**Step 6.** (Common) Calculation of the internal astigmatism in clinician notation

formats used by clinicians.

vector notation

The mathematical formulation is:

manifest astigmatism.

notation

formats used by clinicians.

$$J\_{0, \mathcal{RK}} = J\_{0, K} - J\_{0, P} \tag{33}$$

$$J\_{45,RK} = J\_{45,K} - J\_{45,P} \tag{34}$$

**Step 11.** (Common) Calculation of the predicted residual corneal astigmatism in clinician notation

The power vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of cylinder and axis in minus-cylinder or plus-cylinder formats used by clinicians.

$$C\_{RK} = 2\sqrt{J\_{0,RK}^2 + J\_{45,RK}^2} \tag{35}$$

$$A\_{RK} = \frac{\arctan\left(\frac{J\_{45,RK}}{J\_{0,RK}}\right)}{2} \tag{36}$$

**Step 12.** (Common) Expressing the predicted residual corneal astigmatism to keratometric astigmatism

The mathematical formulation is:

$$C\_{RT} = C\_{RK} \frac{n\_{Topo} - n\_{Ar}}{n\_{Carna} - n\_{Ar}} \tag{37}$$

**Step 13.** Possible scenarios for planning the astigmatic correction

We have developed 5 methods to combine the information:


To correct the manifest astigmatism represents considering nothing from the topographical astigmatism.

The mathematical formulation is:

$$M\_{\,\,P} = M\_{\,\, ^{\,MR}} \tag{38}$$

$$J\_{0,P} = J\_{0,MR} \tag{39}$$

$$J\_{45,P} = J\_{45,MR} \tag{40}$$

*b. Plan to correct the corneal astigmatism* 

To correct the corneal astigmatism represents considering only the topographical astigmatism.

The mathematical formulation is:

$$M\_P = M\_K \tag{41}$$

$$J\_{0,P} = J\_{0,K} \tag{42}$$

$$J\_{45,P} = J\_{45,K} \tag{43}$$

#### *c. Plan to minimize the residual global astigmatism magnitude*

To correct a combination of manifest and corneal astigmatism, minimizing the residual global astigmatism magnitude represents in mathematical formulation:

$$C\_{Global} = \sqrt{C\_{RM}^2 + C\_{RK}^2} \tag{44}$$

$$C\_{Global} = 2\sqrt{J\_{0,RM}^2 + J\_{45,RM}^2 + J\_{0,RK}^2 + J\_{45,RK}^2} \tag{45}$$

$$C\_{Global} = \mathcal{Q}\sqrt{\left(J\_{0,MR} - J\_{0,P}\right)^2 + \left(J\_{45,MR} - J\_{45,P}\right)^2 + \left(J\_{0,K} - J\_{0,P}\right)^2 + \left(J\_{45,K} - J\_{45,P}\right)^2} \tag{46}$$

We should find which plan minimizes the global cylinder: The mathematical formulation is:

$$M\_P = M\_{\text{MR}} \tag{47}$$

$$J\_{0,P} = \frac{J\_{0,MR} + J\_{0,K}}{2} \tag{48}$$

$$J\_{45,P} = \frac{J\_{45,MR} + J\_{45,K}}{2} \tag{49}$$

To correct the manifest astigmatism represents considering nothing from the topographical

To correct the corneal astigmatism represents considering only the topographical

To correct a combination of manifest and corneal astigmatism, minimizing the residual

2 2 22

 2 22 2 0, 0, 45, 45, 0, 0, 45, 45, 2 *C J J J J JJ J J Global MR P MR P K P K P* (46)

0, 0,

45, 45,

*J J*

*MR K*

*J J*

*MR K*

0, 2

45, 2

*P*

*P*

*J*

*J*

*MP MMR* (38)

0, 0, *<sup>P</sup> MR J J* (39)

45, 45, *<sup>P</sup> MR J J* (40)

*MP MK* (41)

0, 0, *<sup>P</sup> <sup>K</sup> J J* (42)

45, 45, *<sup>P</sup> <sup>K</sup> J J* (43)

2 2 *C CC Global RM RK* (44)

*MP MMR* (47)

(48)

(49)

0, 45, 0, 45, 2 *C J J JJ Global RM RM RK RK* (45)

astigmatism.

astigmatism.

The mathematical formulation is:

The mathematical formulation is:

The mathematical formulation is:

*b. Plan to correct the corneal astigmatism* 

*c. Plan to minimize the residual global astigmatism magnitude* 

We should find which plan minimizes the global cylinder:

global astigmatism magnitude represents in mathematical formulation:

*d. Plan to minimize the risk of overcorrecting any of the astigmatisms* 

To correct a combination of manifest and corneal astigmatism, minimizing the magnitude of the corrected astigmatism, as much as possible from topography and manifest astigmatism without overcorrecting any of them, represents in mathematical formulation:

$$M\_P = M\_{\text{MR}} \tag{50}$$

$$J\_{0,P} = \begin{cases} J\_{0,MR} < 0, J\_{0,K} < 0 \Longrightarrow \max\left(J\_{0,MR}, J\_{0,K}\right) \\ J\_{0,MR} > 0, J\_{0,K} > 0 \Longrightarrow \min\left(J\_{0,MR}, J\_{0,K}\right) \\ \text{otherwise} \Longrightarrow 0 \end{cases} \tag{51}$$

$$J\_{45,P} = \begin{cases} J\_{45,MR} < 0, J\_{45,K} < 0 \Longrightarrow \max\left(J\_{45,MR}, J\_{45,K}\right) \\ J\_{45,MR} > 0, J\_{45,K} > 0 \Longrightarrow \min\left(J\_{45,MR}, J\_{45,K}\right) \\ \text{otherwise} \Longrightarrow 0 \end{cases} \tag{52}$$

#### *e. Plan to priorize with-the-rule corneal astigmatism*

To correct a combination of manifest and corneal astigmatism, priorizing with-the-rule corneal astigmatism, represents in mathematical formulation:

$$M\_P = M\_{\text{MR}} \tag{53}$$

$$J\_{0,P} = \begin{cases} J\_{0,MR} < 0, J\_{0,K} < 0 \Longrightarrow \max\left(J\_{0,MR}, J\_{0,K}\right) \\ J\_{0,MR} > 0, J\_{0,K} > 0 \Longrightarrow \min\left(J\_{0,MR}, J\_{0,K}\right) \\ \text{otherwise} \Longrightarrow 0 \end{cases} \tag{54}$$

$$J\_{45,P} = \frac{J\_{45,MR} + J\_{45,K}}{2} \tag{55}$$

**Step 14.** (Common) Expressing the ablation plan in clinician notation

The power vector notation is a cross-cylinder convention that is easily transposed into conventional refractions in terms of cylinder and axis in minus-cylinder or plus-cylinder formats used by clinicians.

$$S\_{P,CP} = M\_p - \frac{C\_{P,CP}}{2} \tag{56}$$

$$C\_{P,CP} = \mathcal{2}\sqrt{J\_{0,P}^2 + J\_{45,P}^2} \tag{57}$$

$$\mathcal{A}\_{\boldsymbol{\rho}} = \frac{\arctan\left(\frac{\boldsymbol{J}\_{45,P}}{\boldsymbol{J}\_{0,P}}\right)}{2} \tag{58}$$

**Step 15.** (Common) Expressing the ablation plan in clinician notation at spectacle plane We then recalculate the correction components from the corneal plane to the spectacle plane:

$$S\_{P,SP} = \frac{S\_{P,CP}}{1 + S\_{P,CP}VD} \tag{59}$$

$$C\_{P,SP} = \frac{S\_{P,CP} + C\_{P,CP}}{1 + \left(S\_{P,CP} + C\_{P,CP}\right)VD} - S\_{P,SP} \tag{60}$$

The idea of corneal vs. manifest astigmatism is not new.

The difference is that the decision used to be a "all-in/no-go" decision, either full manifest correction or full corneal astigmatism correction.

We have developed 5 methods to combine the information, from which 2 are the "most novel and interesting ones":


What would you do if a patient shows -1.50x170 corneal astigmatism but -1.50x10 manifest? There are quite a number of parameters to consider:


for instance, the patient is -3.50 -1.50x10 @ 12, and Maloney indices are 43.25x80 and 41.75x170.

At first sight, we are a an easy case with low astigmatisms.

Actually, the patient is -3.36 -1.36x10 @ corneal plane (-1.67 D manifest astigmatism), and Maloney are 48.18x80 and 46.51x170 (-1.67 D corneal astigmatism).

Planning the 5 scenarios:

230 Astigmatism – Optics, Physiology and Management

arctan

2 *<sup>P</sup>*

**Step 15.** (Common) Expressing the ablation plan in clinician notation at spectacle plane We then recalculate the correction components from the corneal plane to the spectacle

*A*

,

*S*

0. Plan to correct the manifest astigmatism (nothing from topography) 1. Plan to correct the corneal astigmatism (all from topography)

**manifest astigmatism without overcorrecting any of them)** 


Maloney are 48.18x80 and 46.51x170 (-1.67 D corneal astigmatism).

At first sight, we are a an easy case with low astigmatisms.

The idea of corneal vs. manifest astigmatism is not new.

correction or full corneal astigmatism correction.

There are quite a number of parameters to consider:

novel and interesting ones":

**topographical astigmatism)** 

rule corneal astigmatism



41.75x170.


plane:

2 2

45, 0,

 

*J J*

,

*P CP*

, 1 *P CP*

 , , , , , , 1

The difference is that the decision used to be a "all-in/no-go" decision, either full manifest

We have developed 5 methods to combine the information, from which 2 are the "most

2. **Plan to correct a combination of manifest and corneal astigmatism, minimizing the residual global astigmatism magnitude (half the way between manifest and** 

3. **Plan to correct a combination of manifest and corneal astigmatism, minimizing the magnitude of the corrected astigmatism (as much as possible from topography and** 

4. Plan to correct a combination of manifest and corneal astigmatism, priorizing with-the-

What would you do if a patient shows -1.50x170 corneal astigmatism but -1.50x10 manifest?

for instance, the patient is -3.50 -1.50x10 @ 12, and Maloney indices are 43.25x80 and

Actually, the patient is -3.36 -1.36x10 @ corneal plane (-1.67 D manifest astigmatism), and

*S C C S S C VD* 

*P CP P CP P SP P SP P CP P CP*

*S*

*S VD*

*P P*

, 0, 45, 2 *C JJ <sup>P</sup> CP P P* (57)

(58)

(59)

(60)


Postoperative predicted refractions would be:


We propose 5 justified scenarios:


## **5. Centration of refractive profiles**

Not to forget the fact that astigmatism (especially high ones) has its main origin in the anterior corneal surface, and topographically is usually found located 2-fold symmetrically from the normal corneal vertex (CV) and not at the pupil centre. Controversy remains regarding where to centre corneal refractive procedures to maximize the visual outcomes. A misplaced refractive ablation might result in undercorrection and other undesirable side effects. The coaxial light reflex seems to lie nearer to the corneal intercept of the visual axis than the pupil centre (PC) and is, thus, recommended that the corneal coaxial light reflex be centered during refractive surgery. Boxer Wachler et al.23 identified the coaxial light reflex and used it as the centre of the ablation. De Ortueta and Arba Mosquera24 used the corneal vertex (CV) measured by videokeratoscopy25 as the morphologic reference to centre corneal refractive procedures.

Mainly, two different centration references that can be detected easily and measured with currently available technologies. PC may be the most extensively used centration method for several reasons. First, the pupil boundaries are the standard references observed by the eye-tracking devices. Moreover, the entrance pupil can be well represented by a circular or oval aperture, and these are the most common ablation areas. Centering on the pupil offers the opportunity to minimize the optical zone size. Because in LASIK there is a limited ablation area of about 9.25 mm (flap cap), the maximum allowable optical zone will be about 7.75 mm. Because laser ablation is a destructive tissue technique, and the amount of tissue removed is directly related to the ablation area diameter,26 the ablation diameter, maximum ablation depth, and ablation volume should be minimized. The planned optical zone should be the same size or slightly larger as the functional entrance pupil for the patients' requirements.

The pupil centre considered for a patient who fixates properly defines the line-of-sight, which is the reference axis recommended by the OSA for representing the wavefront aberration27.

The main HOA effects (main parts of coma and spherical aberrations) arise from edge effects, i.e., strong local curvature changes from the optical zone to the transition zone and from the transition zone to the untreated cornea. It then is necessary to emphasize the use of a large optical zone (6.50 millimeter or more) to cover the scotopic pupil size, and a large and smooth transition zone.

Nevertheless, because the pupil centre is unstable, a morphologic reference is more advisable28,29,30. It is well known that the pupil centre shifts with changes in the pupil size47, moreover, because the entrance pupil we see is a virtual image of the real one.

The CV in different modalities is the other major choice as the centration reference. In perfectly acquired topography, if the human optical system were truly coaxial, the corneal vertex would represent the corneal intercept of the optical axis. Despite the fact that the human optical system is not truly coaxial, the cornea is the main refractive surface. Thus, the corneal vertex represents a stable preferable morphologic reference. However, there are several ways to determine the corneal vertex: the most extensively used one is to determine the coaxial corneal light reflex (1st Purkinje image). Nevertheless, as de Ortueta and Arba Mosquera24 pointed out, there is a problem using the coaxial light reflex because surgeons differ; for instance, the coaxial light reflex will be seen differently depending on surgeon eye dominance, surgeon eye balance, or the stereopsis angle of the microscope. For example, the LadarVision platform (Alcon) uses a coaxial photograph as reference to determine the coaxial light reflex31, which is independent of the surgeons' focus. Ablations can be centered using the pupillary offset, the distance between the pupil centre and the normal CV.

from the normal corneal vertex (CV) and not at the pupil centre. Controversy remains regarding where to centre corneal refractive procedures to maximize the visual outcomes. A misplaced refractive ablation might result in undercorrection and other undesirable side effects. The coaxial light reflex seems to lie nearer to the corneal intercept of the visual axis than the pupil centre (PC) and is, thus, recommended that the corneal coaxial light reflex be centered during refractive surgery. Boxer Wachler et al.23 identified the coaxial light reflex and used it as the centre of the ablation. De Ortueta and Arba Mosquera24 used the corneal vertex (CV) measured by videokeratoscopy25 as the morphologic reference to centre corneal

Mainly, two different centration references that can be detected easily and measured with currently available technologies. PC may be the most extensively used centration method for several reasons. First, the pupil boundaries are the standard references observed by the eye-tracking devices. Moreover, the entrance pupil can be well represented by a circular or oval aperture, and these are the most common ablation areas. Centering on the pupil offers the opportunity to minimize the optical zone size. Because in LASIK there is a limited ablation area of about 9.25 mm (flap cap), the maximum allowable optical zone will be about 7.75 mm. Because laser ablation is a destructive tissue technique, and the amount of tissue removed is directly related to the ablation area diameter,26 the ablation diameter, maximum ablation depth, and ablation volume should be minimized. The planned optical zone should be the same size or slightly larger as the functional entrance pupil for the patients'

The pupil centre considered for a patient who fixates properly defines the line-of-sight, which is the reference axis recommended by the OSA for representing the wavefront

The main HOA effects (main parts of coma and spherical aberrations) arise from edge effects, i.e., strong local curvature changes from the optical zone to the transition zone and from the transition zone to the untreated cornea. It then is necessary to emphasize the use of a large optical zone (6.50 millimeter or more) to cover the scotopic pupil size, and a large

Nevertheless, because the pupil centre is unstable, a morphologic reference is more advisable28,29,30. It is well known that the pupil centre shifts with changes in the pupil size47,

The CV in different modalities is the other major choice as the centration reference. In perfectly acquired topography, if the human optical system were truly coaxial, the corneal vertex would represent the corneal intercept of the optical axis. Despite the fact that the human optical system is not truly coaxial, the cornea is the main refractive surface. Thus, the corneal vertex represents a stable preferable morphologic reference. However, there are several ways to determine the corneal vertex: the most extensively used one is to determine the coaxial corneal light reflex (1st Purkinje image). Nevertheless, as de Ortueta and Arba Mosquera24 pointed out, there is a problem using the coaxial light reflex because surgeons differ; for instance, the coaxial light reflex will be seen differently depending on surgeon eye dominance, surgeon eye balance, or the stereopsis angle of the microscope. For example, the LadarVision platform (Alcon) uses a coaxial photograph as reference to determine the coaxial light reflex31, which is independent of the surgeons' focus. Ablations can be centered

moreover, because the entrance pupil we see is a virtual image of the real one.

using the pupillary offset, the distance between the pupil centre and the normal CV.

refractive procedures.

requirements.

aberration27.

and smooth transition zone.

If an optical zone equivalent to the maximum pupil size (scotopic pupil size or dim mesopic) is applied on the corneal vertex, due to the offset, the ablation will not cover the full pupil area and it will be cut across it. As the pupil aperture represents the only area capable of collecting light, then the full pupil should be cover and an "oversized" OZ centered on the vertex shall be selected as:

$$\text{MOZ} > \text{Popil}\_{\text{Soo}} + 2\left(\left\| \text{OffSet} \right\| + \left\| AETAcc \right\|\right) \tag{61}$$

However, centering in the pupil with a right selected OZ is not an easy task. We know that the pupil centre shifts versus pupil size changes; moreover as the pupil we see (entrance pupil) is a virtual image of the real one.

Fig. 13. **Left:** The black cross indicates the pupil centre and the black circle the maximum pupil boundaries, whereas the orange cross represents the corneal apex. Pay attention that if we apply on the corneal apex an optical zone equivalent to the maximum pupil size (scotopic pupil size or dim mesopic) (blue circle), due to the offset, the ablation will not cover the full pupil area and it will be cut across it. As the pupil aperture represents the only area capable of collecting light, then the full pupil should be cover and an "oversized" OZ centred on the apex shall be selected (green circle). **Right:** Only centring in the scotopic pupil (orange circle and cross) offers the opportunity to minimise the Optical Zone size (OZ), but under the laser pupil size is likely in a photopic state rather than dim mesopic one. Therefore, centring in the laser pupil an optical zone equivalent to the maximum pupil size (scotopic pupil size or dim mesopic) will induce edge effects.

Considering this, for aspherical, or, in general, non-wavefront-guided treatments, in which the minimum patient data set (sphere, cylinder, and axis values) from the diagnosis is used, it is assumed that the patient's optical system is aberration-free or that those aberrations are not clinically relevant (otherwise a wavefront-guided treatment would have been planned). For those reasons, the most appropriate centering reference is the corneal vertex; modifying the corneal asphericity with an ablation profile neutral for aberrations, including loss of efficiency compensations.

Fig. 14. Comparison of topographical findings after centration at the pupil centre and corneal vertex, respectively. Notice the more symmetric topography after CV centration.

For wavefront-guided treatments, change in aberrations according to diagnosis measurements, a more comprehensive data set from the patient diagnosis is used, including the aberrations, because the aberrations maps are described for a reference system in the centre of the entrance pupil. The most appropriate centering reference is the entrance pupil as measured in the diagnosis27.

Providing different centering references for different types of treatments is not ideal, because it is difficult to standardize the procedures. Nevertheless, ray tracing indicates that the optical axis is the ideal centering reference. Because this is difficult to standardize and considering that, the anterior corneal surface is the main refractive element of the human eye, the CV, defined as the point of maximum elevation, will be the closest reference. It shall be, however, noticed that on the less prevalent oblate corneas the point of maximum curvature (corneal apex) might be off centre and not represented by the corneal vertex.

However, it would be interesting to refer the corneal and/or ocular wavefront measurements to the optical axis or the CV. This can be done easily for corneal wavefront analysis, because there is no limitation imposed by the pupil boundaries. However, it is not as easy for ocular wavefront analysis, because the portion of the cornea above the entrance pupil alone is responsible for the foveal vision. Moreover, in patients with corneal problems such as keratoconus/keratectasia, post-LASIK (pupil-centered), corneal warpage induced by contact lens wearing and other diseases causing irregularity on anterior corneal surface, the corneal vertex and the corneal apex may shift. In those cases, pupil centre is probably more stable. Moreover, since most laser systems are designed to perform multiple procedures besides LASIK, it is more beneficial that excimer laser systems have the flexibility to choose different centration strategies.

Due to the smaller angle kappa associated with myopes compared with hyperopes32,33, centration issues are less apparent. However, angle kappa in myopes may be sufficiently large to show differences in results, because it is always desirable to achieve as much standardization as possible and not to treat the myopes using one reference, whereas the hyperopes use a different one.

The use of large optical zones may be responsible for the lack of difference in postoperative visual outcomes using two different centrations. However, hyperopic LASIK provides smaller functional optical zones and, for this reason, special caution shall be paid to these patients34.

Previous studies have reported that based on theoretical calculations with 7.0-mm pupils even for customized refractive surgery, that are much more sensitive to centration errors, it appears unlikely that optical quality would be degraded if the lateral alignment error did not exceed 0.45 mm37. In 90% of eyes, even accuracy of 0.8 mm or better would have been sufficient to achieve the goal37.

A pupillary offset of 0.25 millimeters seems to be sufficiently large to be responsible for differences in ocular aberrations28, however, not large enough to correlate this difference in ocular aberrations with functional vision.

Centering on the pupil offers the opportunity to minimize the optical zone size, whereas centering in the CV offers the opportunity to use a stable morphologic axis and to maintain the corneal morphology after treatment.

## **6. Eye-tracking**

234 Astigmatism – Optics, Physiology and Management

Fig. 14. Comparison of topographical findings after centration at the pupil centre and corneal vertex, respectively. Notice the more symmetric topography after CV centration.

as measured in the diagnosis27.

different centration strategies.

For wavefront-guided treatments, change in aberrations according to diagnosis measurements, a more comprehensive data set from the patient diagnosis is used, including the aberrations, because the aberrations maps are described for a reference system in the centre of the entrance pupil. The most appropriate centering reference is the entrance pupil

Providing different centering references for different types of treatments is not ideal, because it is difficult to standardize the procedures. Nevertheless, ray tracing indicates that the optical axis is the ideal centering reference. Because this is difficult to standardize and considering that, the anterior corneal surface is the main refractive element of the human eye, the CV, defined as the point of maximum elevation, will be the closest reference. It shall be, however, noticed that on the less prevalent oblate corneas the point of maximum curvature (corneal apex) might be off centre and not represented by the corneal vertex. However, it would be interesting to refer the corneal and/or ocular wavefront measurements to the optical axis or the CV. This can be done easily for corneal wavefront analysis, because there is no limitation imposed by the pupil boundaries. However, it is not as easy for ocular wavefront analysis, because the portion of the cornea above the entrance pupil alone is responsible for the foveal vision. Moreover, in patients with corneal problems such as keratoconus/keratectasia, post-LASIK (pupil-centered), corneal warpage induced by contact lens wearing and other diseases causing irregularity on anterior corneal surface, the corneal vertex and the corneal apex may shift. In those cases, pupil centre is probably more stable. Moreover, since most laser systems are designed to perform multiple procedures besides LASIK, it is more beneficial that excimer laser systems have the flexibility to choose

Due to the smaller angle kappa associated with myopes compared with hyperopes32,33, centration issues are less apparent. However, angle kappa in myopes may be sufficiently The Cyclotorsion Problem

The analysis of cyclotorsion movements have been made since the middle of the 20th century. Several papers demonstrate some dynamic compensatory movement to keep the image at the retina aligned to a natural orientation, whereas some suggestions have been made on significant cyclotorsion occurring under monocular viewing conditions35.

Measuring rotation when the patient is upright36 to when the refractive treatments are performed with the patient supine may lead to ocular cyclotorsion, resulting in mismatching of the applied versus the intended profiles37,38. Recently, some equipment can facilitate measurement of and potential compensation for static cyclotorsion occurring when the patient moves from upright to the supine position during the procedure39, quantifying the cyclorotation occurring between wavefront measurement and laser refractive surgery40 and compensating for it41,42,43.

Further measuring and compensating ocular cyclotorsion during refractive treatments with the patient supine may reduce optical noise of the applied versus the intended profiles44,45,46.

It usually happens that the pupil size and centre differ for the treatment compared to that during diagnosis.47 Then, excluding cyclotorsion, there is already a lateral displacement that mismatches the ablation profile. Further, cyclotorsion occurring around any position other than the ablation centre results in additional lateral displacement combined with cyclotorsion.48

Many studies, in the last times have worked out in an excellent way, the methodologies and implications of ocular cyclotorsion, but due to inherent technical problems, not many papers pay attention to the repeatability and reproducibility of the measurements.

Arba Mosquera et al.38 obtained an average cyclotorsional error of 4.39°, which agrees with the observations of Ciccio et al.,49 who reported 4°. However, a non-negligible percentage of eyes may suffer cyclotorsions exceeding 10 degrees. These patients would be expected to have at least 35% residual cylinder.

Without eye registration technologies,50,51 considering that maximum cyclotorsion measured from the shift from the upright to the supine position does not exceed ±14°,49 explains why "classical" spherocylindrical corrections in refractive surgery succeed without major cyclotorsional considerations. The limited amount of astigmatism especially that can be corrected effectively for this cyclotorsional error may explain partly some unsuccessful results reported in refractive surgery.

Considering that the average cyclotorsion resulting from the shift from the upright to the supine position is about ±4°,49 without an aid other than manual orientation, confirms why spherocylindrical corrections in laser refractive surgery have succeeded.

With currently available eye registration technologies, which provide an accuracy of about ±1.5°, opens a new era in corneal laser refractive surgery, because patients may be treated for a wider range of refractive problems with enhanced success ratios. However, this requires a higher resolution than technically achievable with currently available systems.52,53

Bueeler and co-authors54 determined conditions and tolerances for cyclotorsional accuracy. Their OT criterion represents an optical benefit condition, and their results for the tolerance limits (29° for 3-mm pupils and 21° for 7-mm pupils) did not differ greatly from the optical benefit result for astigmatism by Arba Mosquera et al.,38 confirming that astigmatism is the major component to be considered.

Cyclotorsional errors result in residual aberrations and with increasing cyclotorsional error there is a greater potential for inducing aberrations. Eyes having over 10° of calculated cyclotorsion, predict approximately a 35% residual astigmatic error. Because astigmatic error is generally the highest magnitude vectorial aberration, patients with higher levels of astigmatism are at higher risk of problems due to cyclotorsional error.

Ocular cyclotorsion during laser refractive surgery may lead to significant decrease in the refractive outcomes due to inadequate correction or induction of astigmatism and higher order aberrations1. During normal activities, human eyes can undergo significant torsional movements of up to 15 degrees of the resting position depending on the motion and orientation of the patient's head and body2. In particular, there can be a significant degree of cyclotorsion, particularly with monocular viewing conditions, between the seated and supine positions ranging from 0- 16 in published studies1-5. This type of cyclotorsion that occurs when the patient moves from the upright to the supine position is known as static cyclotorsion and can lead to significant unwanted outcomes during refractive laser ablations of astigmatic eyes. Theoretical analyses show that a 4 misalignment can lead to a 14% under-treatment of astigmatism, 6 to 20% under-correction and 16 to a 50% undercorrection1.

Cyclotorsion control may be of 2 types: i) *dynamic* cyclotorsion controls that allows compensation for torsional eye movements during the laser treatments and ii) *static*  cyclotorsion control that allows compensation for torsional differences in eye positions between the patient being in an upright (during diagnosis) and supine position (during surgery). Currently, new installed excimer lasers have the ability to compensate for cyclotorsion, but most of the excimer lasers in use do not have such ability.

Calculation of the static cyclotorsion is based on comparisons of the corneal wavefront image obtained from the Keratron-Scout videokeratoscope [Optikon 2000 S.p.A, Italy] from

eyes may suffer cyclotorsions exceeding 10 degrees. These patients would be expected to

Without eye registration technologies,50,51 considering that maximum cyclotorsion measured from the shift from the upright to the supine position does not exceed ±14°,49 explains why "classical" spherocylindrical corrections in refractive surgery succeed without major cyclotorsional considerations. The limited amount of astigmatism especially that can be corrected effectively for this cyclotorsional error may explain partly some unsuccessful

Considering that the average cyclotorsion resulting from the shift from the upright to the supine position is about ±4°,49 without an aid other than manual orientation, confirms why

With currently available eye registration technologies, which provide an accuracy of about ±1.5°, opens a new era in corneal laser refractive surgery, because patients may be treated for a wider range of refractive problems with enhanced success ratios. However, this requires a higher resolution than technically achievable with currently available

Bueeler and co-authors54 determined conditions and tolerances for cyclotorsional accuracy. Their OT criterion represents an optical benefit condition, and their results for the tolerance limits (29° for 3-mm pupils and 21° for 7-mm pupils) did not differ greatly from the optical benefit result for astigmatism by Arba Mosquera et al.,38 confirming that astigmatism is the

Cyclotorsional errors result in residual aberrations and with increasing cyclotorsional error there is a greater potential for inducing aberrations. Eyes having over 10° of calculated cyclotorsion, predict approximately a 35% residual astigmatic error. Because astigmatic error is generally the highest magnitude vectorial aberration, patients with higher levels of

Ocular cyclotorsion during laser refractive surgery may lead to significant decrease in the refractive outcomes due to inadequate correction or induction of astigmatism and higher order aberrations1. During normal activities, human eyes can undergo significant torsional movements of up to 15 degrees of the resting position depending on the motion and orientation of the patient's head and body2. In particular, there can be a significant degree of cyclotorsion, particularly with monocular viewing conditions, between the seated and supine positions ranging from 0- 16 in published studies1-5. This type of cyclotorsion that occurs when the patient moves from the upright to the supine position is known as static cyclotorsion and can lead to significant unwanted outcomes during refractive laser ablations of astigmatic eyes. Theoretical analyses show that a 4 misalignment can lead to a 14% under-treatment of astigmatism, 6 to 20% under-correction and 16 to a 50% under-

Cyclotorsion control may be of 2 types: i) *dynamic* cyclotorsion controls that allows compensation for torsional eye movements during the laser treatments and ii) *static*  cyclotorsion control that allows compensation for torsional differences in eye positions between the patient being in an upright (during diagnosis) and supine position (during surgery). Currently, new installed excimer lasers have the ability to compensate for

Calculation of the static cyclotorsion is based on comparisons of the corneal wavefront image obtained from the Keratron-Scout videokeratoscope [Optikon 2000 S.p.A, Italy] from

cyclotorsion, but most of the excimer lasers in use do not have such ability.

spherocylindrical corrections in laser refractive surgery have succeeded.

astigmatism are at higher risk of problems due to cyclotorsional error.

have at least 35% residual cylinder.

results reported in refractive surgery.

major component to be considered.

systems.52,53

correction1.

the patient in the upright position and the image taken from the SCHWIND AMARIS laser camera with the patient in the supine position. The laser computer algorithm searches for important landmarks starting at the borderline of the pupil and moving outwards until the image is completely scanned or the number of the prerequisite important points is reached. The software algorithm scans both the iris and the sclera. Mostly the rainbow shape of the iris with the vessels in the sclera provides enough information to register the cyclotorsion and no preoperative marking of the eye is necessary. In the case of a photopic pupil size, the iris delivers more reliable data. However, if the pupil is of scotopic size and the iris is reduced to a thin ring, the structures at the sclera can be detected and used to improve the robustness of the search. Before the treatment starts the advanced cyclotorsion control algorithm of the laser compares the 2 images, superimposes the important landmarks and calculates the angle of rotation. The laser software automatically corrects for the dynamic cyclotorsion. However, the surgeon has the possibility to ask for static cyclotorsion compensation or not, with a range of compensation of +/- 15°. Accuracy of cyclotorsion compensation is increased by the fact that algorithm used by the SCHWIND AMARIS does not rotate the complete volume of ablation but rather compensates each pulse individually for the cyclotorsion.

Fig. 15. **SCC compensation at the AMARIS.** 

The amount of static cyclotorsion that occurs in individuals has ranged from 0- 16 in published studies1-5. In our experience with AMARIS, we observed a low to moderate amount of static cyclotorsion ranging from 0.3- 10 with a mean value of 3.9. Theoretical analyses would suggest that such an average amount of static cyclotorsion would account for a 14% under-correction of astigmatism increasing significantly with larger angles of static cyclotorsion. The static cyclotorsion module available on SCHWIND AMARIS platform produces a significant improvement in both the refractive outcome and full treatment of astigmatism. Thus we can conclude that the software is able to accurately lock on to eye position and compensate for the static cyclotorsion. This significant improvement in astigmatic and refractive outcomes in the SCC group is translated into improved safety. Noteworthy the magnitude and distribution of uncompensated cyclotorsion in former patients treated without SCC is similar to the magnitude and distribution of compensated cyclotorsion in the SCC. The importance of compensation of even small amounts of cyclotorsion would be expected to be even more important in wavefront guided treatments where it has been calculated that to achieve the diffraction limit in 95% of measured normal eyes with a 7.0 mm pupil, alignment of wavefront guided treatment would require a torsional precision of 1 degree or better11.

Not all lasers have specific software and/or hardware to actively compensate for positional cyclotorsion, and some achieve excellent results through alternative approaches. For example, Wavelight lasers achieve excellent outcomes for treatment of astigmatism. This is most likely due to the use of a lighting system which provides an "artificial horizon" which the patient sees when in the supine position under the laser.

The good thing of the SCC with CW is that the same image for topographical analysis is used for CW analysis and for SCC as well (as opposed to OW in which the H-S image is used for OW and another image, simultaneous or not, is used for SCC). The corneal wavefront image and the iris and sclera images are the same, so no mapping is needed. The Keratron keratoscope obtains information about the iris and sclera.

Uncompensated cyclotorsion errors in the SCC group can be attributed to: resolution and accuracy of the diagnosis image, resolution and accuracy of the laser image, possible misalignment of the scanner to the ET camera, possible misalignment of the manifest astigmatism to the topography, etc…

Ocular cyclotorsion during laser refractive surgery may lead to significant decrease in the refractive outcomes due to inadequate correction or induction of astigmatism and higher order aberrations, if astigmatism and higher order aberrations are present AND ONLY IF astigmatism and higher order aberrations are attempted to be corrected.

## **7. Other concerns**

Tissue saving concerns

The real impact of tissue saving algorithms in customized treatments is still discussed in a controversial way. The problem of minimizing the amount of tissue is that it must be done in such a way that:


The amount of static cyclotorsion that occurs in individuals has ranged from 0- 16 in published studies1-5. In our experience with AMARIS, we observed a low to moderate amount of static cyclotorsion ranging from 0.3- 10 with a mean value of 3.9. Theoretical analyses would suggest that such an average amount of static cyclotorsion would account for a 14% under-correction of astigmatism increasing significantly with larger angles of static cyclotorsion. The static cyclotorsion module available on SCHWIND AMARIS platform produces a significant improvement in both the refractive outcome and full treatment of astigmatism. Thus we can conclude that the software is able to accurately lock on to eye position and compensate for the static cyclotorsion. This significant improvement in astigmatic and refractive outcomes in the SCC group is translated into improved safety. Noteworthy the magnitude and distribution of uncompensated cyclotorsion in former patients treated without SCC is similar to the magnitude and distribution of compensated cyclotorsion in the SCC. The importance of compensation of even small amounts of cyclotorsion would be expected to be even more important in wavefront guided treatments where it has been calculated that to achieve the diffraction limit in 95% of measured normal eyes with a 7.0 mm pupil, alignment of wavefront guided treatment would require a

Not all lasers have specific software and/or hardware to actively compensate for positional cyclotorsion, and some achieve excellent results through alternative approaches. For example, Wavelight lasers achieve excellent outcomes for treatment of astigmatism. This is most likely due to the use of a lighting system which provides an "artificial horizon" which

The good thing of the SCC with CW is that the same image for topographical analysis is used for CW analysis and for SCC as well (as opposed to OW in which the H-S image is used for OW and another image, simultaneous or not, is used for SCC). The corneal wavefront image and the iris and sclera images are the same, so no mapping is needed. The

Uncompensated cyclotorsion errors in the SCC group can be attributed to: resolution and accuracy of the diagnosis image, resolution and accuracy of the laser image, possible misalignment of the scanner to the ET camera, possible misalignment of the manifest

Ocular cyclotorsion during laser refractive surgery may lead to significant decrease in the refractive outcomes due to inadequate correction or induction of astigmatism and higher order aberrations, if astigmatism and higher order aberrations are present AND ONLY IF

The real impact of tissue saving algorithms in customized treatments is still discussed in a controversial way. The problem of minimizing the amount of tissue is that it must be done

torsional precision of 1 degree or better11.

astigmatism to the topography, etc…

**7. Other concerns**  Tissue saving concerns

in such a way that:

the patient sees when in the supine position under the laser.

Keratron keratoscope obtains information about the iris and sclera.

astigmatism and higher order aberrations are attempted to be corrected.

a. does not compromise the refractive correction55,56,57,58,59

b. does not compromise the visual performance

c. is safe, reliable and reproducible

**Distribution of the compensated cyclotorsion**

#### **Distribution of the uncompensated cyclotorsion error**

Fig. 16. Comparison of compensated (top) and uncompensated (bottom) torsional errors. Notice the similarities of the distribution of the compensated torsion when using SCC and of the uncompensated torsion when not using SCC. Notice as well, the much tighter distribution around smaller residual torsional errors for the uncompensated torsion when using SCC.

In general, for the same amount of equivalent defocus, the optical blur produced by higher order aberrations increases with increasing radial order and decreases with increasing angular frequencies. With this basis, a simple approach for classification of the clinical relevance of single aberration terms (metric for dioptric equivalence) can be proposed. It is important to bear in mind that 1 diopter of cardinal astigmatism (at 0° for example) does not necessarily have the same effect as 1 diopter of oblique astigmatism (at 45° for example). Despite this, other studies have proved this assumption as reasonable60.

According to this classification, Zernike terms can be considered not clinically relevant if their associated optical blur is lower than < 0.25 D, Zernike terms that might be considered clinically relevant correspond to optical blur values between 0.25 D and 0.50 D, and Zernike terms considered clinically relevant have associated optical blur values larger than 0.50 D.

There are different proposed approaches for minimizing tissue ablation in refractive surgery:

In the multizonal treatments61, the minimization is based on the concept of progressive decreasing corrections in different optical zones. The problem comes from the induced aberrations (especially spherical aberration).

In the treatments planned with smaller optical zone62 combined with bigger transition zones, the minimization is a variation of the multizone concept. The problem comes, as well, from the induced aberrations (especially spherical aberration).

In the treatments planned with smaller optical zone for the cylindrical component63 (in general for the most powerful correction axis), the minimization is based upon the concept of the maximal depth being based on the lowest meridional refraction and the selected optical zone, and the effective optical zone of the highest meridional refraction is reduced to match the same maximal depth. The problem comes from the induced aberrations (especially high order astigmatism).

In the boost slider method, minimization is produced by linear modulation of the ablated volume. The problem comes from induced changes in refraction produced by modulation.

In the Z-clip method64, minimization consists of defining a "saturation depth" for the ablated volume, all points planned to ablate deeper than the saturation value are ablated only by an amount equal to the saturation value. The problem is that this "saturation limit" may occur anywhere in the ablation volume, compromising the refraction when they occur close to the ablation centre, and affecting the induction of aberrations in a complicated way.

In the Z-shift method64, minimization consists of defining a "threshold value" for the ablated volume, no points planned to ablate less than the threshold value are ablated, and the rest of the points are ablated by an amount equal to the original planned ablation minus the threshold value. The problem comes from the fact that this "threshold value" may occur anywhere in the ablation volume, compromising the refraction when they occur close to the ablation centre, and the functional optical zone when occurring at the periphery.

Other minimization approaches65 consist of simplifying the profile by selecting a subset of Zernike terms that minimizes the necessary ablation depth of ablation volume but respecting the Zernike terms considered as clinically relevant.

For each combination subset of Zernike terms, the low order terms are recalculated in a way that it does not compromise the refractive correction. Considering that the Zernike terms are either planned to be corrected or left, it does not compromise the visual performance because all left (not planned to correct) terms are below clinical relevance. The proposed approaches are safe, reliable and reproducible due to the objective foundation upon which they are based. In the same way, the selected optical zone will be used for the correction.

In general, for the same amount of equivalent defocus, the optical blur produced by higher order aberrations increases with increasing radial order and decreases with increasing angular frequencies. With this basis, a simple approach for classification of the clinical relevance of single aberration terms (metric for dioptric equivalence) can be proposed. It is important to bear in mind that 1 diopter of cardinal astigmatism (at 0° for example) does not necessarily have the same effect as 1 diopter of oblique astigmatism (at 45° for example).

According to this classification, Zernike terms can be considered not clinically relevant if their associated optical blur is lower than < 0.25 D, Zernike terms that might be considered clinically relevant correspond to optical blur values between 0.25 D and 0.50 D, and Zernike terms considered clinically relevant have associated optical blur values larger than 0.50 D. There are different proposed approaches for minimizing tissue ablation in refractive

In the multizonal treatments61, the minimization is based on the concept of progressive decreasing corrections in different optical zones. The problem comes from the induced

In the treatments planned with smaller optical zone62 combined with bigger transition zones, the minimization is a variation of the multizone concept. The problem comes, as well,

In the treatments planned with smaller optical zone for the cylindrical component63 (in general for the most powerful correction axis), the minimization is based upon the concept of the maximal depth being based on the lowest meridional refraction and the selected optical zone, and the effective optical zone of the highest meridional refraction is reduced to match the same maximal depth. The problem comes from the induced aberrations

In the boost slider method, minimization is produced by linear modulation of the ablated volume. The problem comes from induced changes in refraction produced by modulation. In the Z-clip method64, minimization consists of defining a "saturation depth" for the ablated volume, all points planned to ablate deeper than the saturation value are ablated only by an amount equal to the saturation value. The problem is that this "saturation limit" may occur anywhere in the ablation volume, compromising the refraction when they occur close to the ablation centre, and affecting the induction of aberrations in a complicated way. In the Z-shift method64, minimization consists of defining a "threshold value" for the ablated volume, no points planned to ablate less than the threshold value are ablated, and the rest of the points are ablated by an amount equal to the original planned ablation minus the threshold value. The problem comes from the fact that this "threshold value" may occur anywhere in the ablation volume, compromising the refraction when they occur close to the

ablation centre, and the functional optical zone when occurring at the periphery.

respecting the Zernike terms considered as clinically relevant.

Other minimization approaches65 consist of simplifying the profile by selecting a subset of Zernike terms that minimizes the necessary ablation depth of ablation volume but

For each combination subset of Zernike terms, the low order terms are recalculated in a way that it does not compromise the refractive correction. Considering that the Zernike terms are either planned to be corrected or left, it does not compromise the visual performance because all left (not planned to correct) terms are below clinical relevance. The proposed approaches are safe, reliable and reproducible due to the objective foundation upon which they are based. In the same way, the selected optical zone will be used for the correction.

Despite this, other studies have proved this assumption as reasonable60.

aberrations (especially spherical aberration).

(especially high order astigmatism).

from the induced aberrations (especially spherical aberration).

surgery:

It is important to remark; the selection of the Zernike terms to be included in the correction is not trivial. Only Zernike terms considered not clinically relevant or minor clinically relevant can be excluded from the correction, but they must not be necessarily excluded. Actually, single Zernike terms considered not clinically relevant will only be disabled when they represent an extra tissue for the ablation, and will be enabled when they help to save tissue for the ablation.

In this way, particular cases are represented by the full wavefront correction, by disabling all not clinically relevant terms, or by disabling all high order terms.

The selection process is completely automatically driven by a computer, ensuring systematic results, and minimization of the amount of tissue to be ablated, simplifying the foreseeable problems of manually selecting the adequate set of terms.

Fig. 17. Optimised Aberration Modes Selection. Based on the wavefront aberration map, the software is able to recommend the best possible aberration modes selection to minimise tissue and time, without compromising the visual quality. Notice that the wavefront aberration is analysed by the software showing the original ablation for a full wavefront correction and the suggested set of aberration modes to be corrected. Notice the difference in required tissue, but notice as well that the most representative characteristics of the wavefront map are still presented in the minimised tissue selection.

A critic to this methodology is the fact that it does not target a diffraction limited optical system. That means it reduces the ablated tissue at the cost of accepting a "trade-off" in the optical quality. However, there are, at least, three criteria (chromatic blur, depth of focus, wide field vision) favoring the target of leaving minor amounts of not clinically relevant aberrations. There are, as well, no foreseeable risks derived from the proposed minimization functions because they propose ablation profiles simpler than the full wavefront corrections. Some drawbacks and potential improvements may be hypothesized:

There may be a sort of "edge" problem considering the case that a Zernike term with DEq of 0.49 D can be enabled or disabled, due to its expected minor clinical relevance, whereas a Zernike term with DEq of 0.51 D shall be corrected.

It is controversial, as well, whether the clinical relevance of every Zernike term can be considered independently. The visual effect of an aberration does not only depend on it but also in the other possible aberration present; e.g. a sum of small, and previously considered clinically irrelevant aberration, could suppose a clear loss of overall optical quality.

A possible improvement comes from the fact that current selection strategy is in an "ON/OFF" fashion for each Zernike term, better corrections and higher amounts of tissue saving could be obtained by using a correcting factor F[n,m] (range 0 to 1) for each Zernike correcting a wavefront of the form:

$$\operatorname{Abl}\left(\boldsymbol{\rho}, \boldsymbol{\theta}\right) = \sum\_{n=0}^{n} \sum\_{m=-n}^{+n} \boldsymbol{F}\_{n}^{m} \mathbf{C}\_{n}^{m} \mathbf{Z}\_{n}^{m} \left(\boldsymbol{\rho}, \boldsymbol{\theta}\right) \tag{62}$$

However, this would correspond to a much higher computation cost.

Another possible improvement would be to consider possible aberration couplings, at least, between Zernike modes of the same angular frequency as a new evaluation parameter.

New algorithms and ablation strategies for efficiently performing laser corneal refractive surgery in a customized form minimizing the amount of ablated tissue without compromising the visual quality are being developed. The availability of such profiles, potentially maximizing visual performance without increasing the factors of risk, would be of great value for the refractive surgery community and ultimately for the health and safety of the patients.

#### **8. References**


A critic to this methodology is the fact that it does not target a diffraction limited optical system. That means it reduces the ablated tissue at the cost of accepting a "trade-off" in the optical quality. However, there are, at least, three criteria (chromatic blur, depth of focus, wide field vision) favoring the target of leaving minor amounts of not clinically relevant aberrations. There are, as well, no foreseeable risks derived from the proposed minimization functions because they propose ablation profiles simpler than the full wavefront corrections.

There may be a sort of "edge" problem considering the case that a Zernike term with DEq of 0.49 D can be enabled or disabled, due to its expected minor clinical relevance, whereas a

It is controversial, as well, whether the clinical relevance of every Zernike term can be considered independently. The visual effect of an aberration does not only depend on it but also in the other possible aberration present; e.g. a sum of small, and previously considered

A possible improvement comes from the fact that current selection strategy is in an "ON/OFF" fashion for each Zernike term, better corrections and higher amounts of tissue saving could be obtained by using a correcting factor F[n,m] (range 0 to 1) for each Zernike

 

(62)

, , *<sup>n</sup> mmm n nn*

0

*n mn*

*FCZ*

Another possible improvement would be to consider possible aberration couplings, at least, between Zernike modes of the same angular frequency as a new evaluation parameter. New algorithms and ablation strategies for efficiently performing laser corneal refractive surgery in a customized form minimizing the amount of ablated tissue without compromising the visual quality are being developed. The availability of such profiles, potentially maximizing visual performance without increasing the factors of risk, would be of great value for the refractive surgery community and ultimately for the health and safety

[1] Chayet AS, Montes M, Gómez L, Rodríguez X, Robledo N, MacRae S. Bitoric laser in situ

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Some drawbacks and potential improvements may be hypothesized:

Zernike term with DEq of 0.51 D shall be corrected.

*Abl* 

However, this would correspond to a much higher computation cost.

correcting a wavefront of the form:

*Ophthalmol.* 2001;108:303-8.

*Ophthalmol.* 2001;108:303-8

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**8. References** 

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S1041-5


## **Measurement and Topography Guided Treatment of Irregular Astigmatism**

Joaquim Murta and Andreia Martins Rosa *Hospitais da Universidade de Coimbra, Portugal* 

## **1. Introduction**

246 Astigmatism – Optics, Physiology and Management

[62] Kermani O, Schmiedt K, Oberheide U, Gerten G. Early results of nidek customized

[63] Kezirian GM. A Closer Look at the Options for LASIK Surgery. *Review of Ophthalmology*;

[64] Goes FJ. Customized topographic repair with the new platform: ZEiSS MEL80/New

[65] Arba Mosquera S, Merayo-Lloves J, de Ortueta D. Tissue-Saving Zernike terms selection in customised treatments for refractive surgery. *J Optom*; 2009; in press

19: S190-S194.

2003; 10: 12.

International *(2007)*

aspheric transition zones (CATz) in laser in situ keratomileusis. *J Refract Surg*; 2003;

CRS Master TOSCA II (Chapter 18, pp. 179-193) in *Mastering the Techniques of Customised LASIK* edited by Ashok Garg and Emanuel Rosen, Jaypee Medical

> Corneal astigmatism occurs when one corneal meridian has a different refractive power from the orthogonal meridian. In regular astigmatism, the two meridians are at 90º from each other, such as a sphere having a cylinder superimposed on its surface. In irregular astigmatism the two meridians are not at 90º from each other or the cornea curvature is not axially symmetric. Irregular astigmatism can be imagined as a sphere, with or without a cylinder on its surface, and several other different shapes superimposed on it. In irregular astigmatism the same meridian has different degrees of curvature, making it impossible for a spherocylindrical lens to correct such irregularity. (1)

> The diagnosis of irregular astigmatism can be suspected when there is loss of spectacle corrected visual acuity but preservation of vision through pinhole or while wearing rigid gas-permeable contact lenses. Other clues for the presence of irregular astigmatism are difficulty in determining the axis of astigmatism during manifest refraction, a significant amount of astigmatism at automated refraction not accepted by the patient and achieving the same visual acuity despite correction of the cylinder in different axis. Patients complain of bad quality of vision resulting from glare, halos, distortion of image and monocular diplopia. The diagnosis can be confirmed with a topographic examination.

## **2. Etiology of irregular astigmatism**

Causes of irregular astigmatism include ectatic disorders, nonectatic disorders, refractive surgery and corneal transplantation. (2) (3)

## **2.1 Ectatic disorders**

Noninflammatory ectatic disorders include keratoconus, pellucid marginal degeneration (PMD), keratoglobus and posterior keratoconus. (2) (3)

KERATOCONUS is the most common ectatic disorder and it is an important cause of irregular astigmatism. It is a non-inflammatory, progressive, ectatic and thinning disease of the cornea, usually bilateral, although asymmetric, with onset at puberty. It manifests as a protusion with paracentral inferior thinning, it may be surrounded by an iron line (Fleisher's ring), it can contain scars and fine posterior stress lines (Vogt striae). Later signs of keratoconus include Munson's sign and Rizutti's phenomenon. (2) (4)

Fig. 1. Keratoglobus. Notice the generalized peripheral thinning.

PELLUCID MARGINAL DEGENERATION is also bilateral, age at onset from 20 to 50 years old, most commonly found in males, with an inferior peripheral band of thinning (usually from 4-8 o'clock with 1 to 2 mm width) and protusion superior to the thinned area. It may have striae, but less frequently than keratoconus. (2) (4)

KERATOGLOBUS is a rare bilateral disorder that presents at birth with a generalized corneal protusion and limbus-to-limbus peripheral thinning, causing the cornea to assume a globular profile. Keratometry measurements can often be as high as 60-70 D. Vogt's striae, sub-epithelial scarring, Fleischer's ring, lipid deposition and corneal vascularisation are rarely found. (2) (4) (Figure 1).

POSTERIOR KERATOCONUS is a very rare corneal disorder, usually unilateral and nonprogressive that is present at birth. There is only an excavation in the posterior and paracentral cornea, but scarring is common. (2)

### **2.2 Nonectatic disorders**

CONTACT LENS WARPAGE refers to the modification of corneal topography associated with all types of contact lens wear. It may manifest clinically with decreased best spectacle corrected visual acuity and irregular mires on keratometry or only on topographic examination in patients seeking refractive surgery. It may resemble keratoconus or pellucid marginal degeneration, or it may be an irregularity with no specific pattern. (3) The most important factor indicating the required time for refractive stabilization after contact lens removal is the amount of time the patient has worn contact lenses. (5) A simple way to memorize when to discontinue contact lens wear before refractive surgery is 1 week for each 5 years of use of soft contact lenses and 2 weeks for rigid lenses.

DRY EYE can cause any pattern of irregular astigmatism or it may make impossible to obtain an adequate topography. Dry eye decreases the smoothness of the epithelium, creates focal irregularities on topography and may appear similar to keratoconus and pellucid marginal degeneration. (3) These irregularities improve with artificial tear instillation prior to the examination. Wavefront aberrations are a measure of irregular astigmatism and have been shown to decrease 2 to 3 times with tear instillation. Sequential aberrometry is an useful objective method to evaluate sequential changes of visual performance related to tear-film dynamics. (6)

PELLUCID MARGINAL DEGENERATION is also bilateral, age at onset from 20 to 50 years old, most commonly found in males, with an inferior peripheral band of thinning (usually from 4-8 o'clock with 1 to 2 mm width) and protusion superior to the thinned area. It may

KERATOGLOBUS is a rare bilateral disorder that presents at birth with a generalized corneal protusion and limbus-to-limbus peripheral thinning, causing the cornea to assume a globular profile. Keratometry measurements can often be as high as 60-70 D. Vogt's striae, sub-epithelial scarring, Fleischer's ring, lipid deposition and corneal vascularisation are

POSTERIOR KERATOCONUS is a very rare corneal disorder, usually unilateral and nonprogressive that is present at birth. There is only an excavation in the posterior and

CONTACT LENS WARPAGE refers to the modification of corneal topography associated with all types of contact lens wear. It may manifest clinically with decreased best spectacle corrected visual acuity and irregular mires on keratometry or only on topographic examination in patients seeking refractive surgery. It may resemble keratoconus or pellucid marginal degeneration, or it may be an irregularity with no specific pattern. (3) The most important factor indicating the required time for refractive stabilization after contact lens removal is the amount of time the patient has worn contact lenses. (5) A simple way to memorize when to discontinue contact lens wear before refractive surgery is 1 week for each

DRY EYE can cause any pattern of irregular astigmatism or it may make impossible to obtain an adequate topography. Dry eye decreases the smoothness of the epithelium, creates focal irregularities on topography and may appear similar to keratoconus and pellucid marginal degeneration. (3) These irregularities improve with artificial tear instillation prior to the examination. Wavefront aberrations are a measure of irregular astigmatism and have been shown to decrease 2 to 3 times with tear instillation. Sequential aberrometry is an useful objective method to evaluate sequential changes of visual performance related to

Fig. 1. Keratoglobus. Notice the generalized peripheral thinning.

5 years of use of soft contact lenses and 2 weeks for rigid lenses.

have striae, but less frequently than keratoconus. (2) (4)

paracentral cornea, but scarring is common. (2)

rarely found. (2) (4) (Figure 1).

**2.2 Nonectatic disorders** 

tear-film dynamics. (6)

PTERYGIUM is a commonly occurring ocular surface disease, characterized by epithelial overgrowth of the cornea, usually bilateral. There is also an underlying breakdown of Bowman's layer and its size is significantly correlated with the magnitude of spherical power, asymmetry, regular and irregular astigmatism. (7) (8) (9) Pterygium removal surgery improves these changes, but regular astigmatism and higher order irregularities may remain. (10) Pterygia are usually located in the nasal interpalpebral cornea, where they induce local flattening and with-the-rule astigmatism. However, as the flattening is asymmetric, irregular astigmatism can also be induced (Figure 2). Changes in the tear film, as local pooling at the head of the pterygium, also induce irregular astigmatism. (3) The development of pterygium-like lesions in axes other than the horizontal (pseudo-pterygium) are secondary to traumatic, inflammatory or vascular conditions.

Fig. 2. Pterygium causing nasal flattening of the cornea and significant irregular astigmatism in the 3 and 5 mm zones (4.2D and 10.6D).

INFECTIOUS DISEASES (keratitis) cause corneal scarring and local flattening, resulting in irregular astigmatism. It is important in these cases to differentiate the contribution of the irregular astigmatism from that of opacity in the decrease of visual acuity; a pinhole and a gas permeable contact lens will improve the irregular astigmatism but not the opacity caused by the keratitis.

IMMUNE-MEDIATED DISEASES include Mooren´s ulcer, atopic keratoconjunctivitis, peripheral ulcerative keratitis, ocular mucous membrane pemphigoid and Stevens-Johnson syndrome. (2) (4) These diseases can cause severe corneal melting and scarring, with resulting irregular astigmatism.

CORNEAL DYSTROPHIES affecting all corneal levels can cause irregular astigmatism. It is important to differentiate the relative contribution of the irregular astigmatism from that of the opacity itself. Corneal dystrophies can be divided according to the level of the cornea involved into 1) Epithelial and Subepithelial Dystrophies (epithelial basement membrane dystrophy, epithelial recurrent erosion dystrophy, subepithelial mucinous corneal dystrophy, Meesmann corneal dystrophy, Lisch dystrophy and gelatinous drop-like corneal dystrophy); 2) Bowman´s layer (Reis-Bucklers, Grayson-Wilbrandt and Thiel-Behnke corneal dystrophies); 3) Stroma (transforming growth factor beta-induced –TGFFBI- corneal dystrophies - granular dystrophy and lattice dystrophy; Macular dystrophy; Schnyder corneal dystrophy; Congenital stromal dystrophy; Fleck dystrophy; Posterior amorphous dystrophy; Central cloudy dystrophy of François and Pre-Descemet corneal dystrophy) and 4) Endothelial (Fuchs endothelial dystrophy, Posterior polymorphous dystrophy, Congenital hereditary endothelial dystrophy 1 and 2 and X-linked endothelial corneal dystrophy). (11)

CORNEAL TRAUMA is an important cause of irregular astigmatism due to scar formation and consequent local variation of the refractive properties of the cornea. Irregular astigmatism occurs relative to the type of trauma as well as with the surgical technique used in the primary repair. (12) It is important, once again, to evaluate the relative contribution of the opacity versus the optical effect of the scar tissue, before attempting laser correction of the irregular astigmatism.

## **2.3 Following refractive surgery**

AFTER RADIAL KERATOTOMY. Healing of the RK incisions is very slow and unpredictable, often incomplete even years after surgery. (13) Healing of these incisions involves irregular fibrous tissue and epithelial plugs, leading to an asymmetric central flattening. Visual distortion and glare are more marked in patients having more than 8 incisions, incisions located inside the 3 mm central zone and hypertrophic scarring. (13) (Figure 3) There is sometimes continuous hyperopic shift that also reduces visual acuity. (13)

AFTER LASIK. Irregular astigmatism can occur due to problems with the laser ablation pattern, after both myopic and hyperopic treatments, or flap related complications (Figure 4). Laser induced problems include decentered ablations, either from misalignment, involuntary eye movement or eye tracker malfunction and central islands. (14) Central islands are steep areas inside the treatment zone that can result from poor laser calibration, improper laser dynamics, central blockage of the laser treatment by laser plume, central corneal water accumulation and individual healing responses. (14) A small optical zone can also cause symptoms of irregular astigmatism, because when the pupil dilates light rays are focused differentially according to the curvature of the area they go through. Flap related complications inducing irregular astigmatism include partial flaps, buttonhole flaps, flap striae, diffuse lamellar keratitis and epithelial ingrowth. (3) Dry eye is also a cause of irregular astigmatism and it should always be considered in pre and post refractive surgery patients.

AFTER PRK. The causes of irregular astigmatism are the same as the ones mentioned for LASIK. PRK eliminates flap related complications, but stromal incursions during

4) Endothelial (Fuchs endothelial dystrophy, Posterior polymorphous dystrophy, Congenital hereditary endothelial dystrophy 1 and 2 and X-linked endothelial corneal

CORNEAL TRAUMA is an important cause of irregular astigmatism due to scar formation and consequent local variation of the refractive properties of the cornea. Irregular astigmatism occurs relative to the type of trauma as well as with the surgical technique used in the primary repair. (12) It is important, once again, to evaluate the relative contribution of the opacity versus the optical effect of the scar tissue, before attempting laser correction of

AFTER RADIAL KERATOTOMY. Healing of the RK incisions is very slow and unpredictable, often incomplete even years after surgery. (13) Healing of these incisions involves irregular fibrous tissue and epithelial plugs, leading to an asymmetric central flattening. Visual distortion and glare are more marked in patients having more than 8 incisions, incisions located inside the 3 mm central zone and hypertrophic scarring. (13) (Figure 3) There is sometimes continuous hyperopic shift that also reduces visual acuity. (13)

Fig. 3. A-Radial keratotomy incisions inducing 6.0 D of topographic astigmatism and irregularity (5.9 D in the 3 mm zone and 9.1 D in the 5mm zone) at Orbscan (B). C- It is

AFTER LASIK. Irregular astigmatism can occur due to problems with the laser ablation pattern, after both myopic and hyperopic treatments, or flap related complications (Figure 4). Laser induced problems include decentered ablations, either from misalignment, involuntary eye movement or eye tracker malfunction and central islands. (14) Central islands are steep areas inside the treatment zone that can result from poor laser calibration, improper laser dynamics, central blockage of the laser treatment by laser plume, central corneal water accumulation and individual healing responses. (14) A small optical zone can also cause symptoms of irregular astigmatism, because when the pupil dilates light rays are focused differentially according to the curvature of the area they go through. Flap related complications inducing irregular astigmatism include partial flaps, buttonhole flaps, flap striae, diffuse lamellar keratitis and epithelial ingrowth. (3) Dry eye is also a cause of irregular astigmatism and it should always be considered in pre and post refractive surgery patients. AFTER PRK. The causes of irregular astigmatism are the same as the ones mentioned for LASIK. PRK eliminates flap related complications, but stromal incursions during

difficult to obtain data over the radial incisions (Topolyzer).

dystrophy). (11)

the irregular astigmatism.

**2.3 Following refractive surgery** 

mechanical epithelial removal, corneal haze and scarring and irregular surface healing can lead to irregular astigmatism. (3)

Fig. 4. Epithelial cells in the flap interface causing significant distortion and irregular astigmatism.

POSTOPERATIVE CORNEAL ECTASIA represents the most severe form of irregular astigmatism after corneal refractive surgery. The true incidence remains underdetermined ranging from 0.04% (15) to 0.6% (16) after LASIK. Preoperative weak corneas, thin residual stromal beds (depending on pre-operative refraction, pre-operative corneal thickness, flap thickness and tissue removed by the excimer laser), trauma and forme fruste or lactent keratoconus can cause post surgical ectasia. (Figure 5).

Fig. 5. Keratoectasia after LASIK performed in 1998 to correct -7.0D OS. Notice the high anterior and posterior float values (0.041 and 0.119), inferior steepening with K values of 49D and thinning at the pachymetry map. Pentacam Belin-Ambrosio Display highlights the ectatic zone by showing the difference between this zone and the sphere that most resembles the normal part of the cornea.

## **2.4 Following penetrating keratoplasty**

PRE-EXISTING RECIPIENT DISEASE. Regional thinning, vascularisation, keratoconus and aphakic patients tend to have more irregular astigmatism. (17) (3)

TREPHINATION TECHNIQUE. Tilt, eccentric trephination, poor quality blades, damaged corneal blocks, asymmetric pressure from lid speculums and scleral rings can all cause irregular astigmatism. (17) Although penetrating keratoplasty (PKP) generally results in clear corneal grafts, the procedure is frequently complicated by refractive imperfections and wound-healing problems. (18) Femtosecond laser corneal surgery has been increasing in popularity and has the potential to overcome many of the problems of manual or automated trephines or microkeratomes.

SUTURE PLACEMENT is crucial to obtain a good refractive outcome following corneal transplantation. The second cardinal suture is the most important in keratoplasty. It determines lateral wound apposition, donor/recipient edge alignment and corneal astigmatism. Sutures must purchase the same amount of tissue in donor and recipient beds, which means having the same length and depth. Sutures can be tied with a variety of techniques, but all require meticulous attention to appropriate suture tension with avoidance of loose or tight knots. (12)

SUTURE REMOVAL. Suture manipulation is a very important factor in the astigmatic outcome. Astigmatism is reduced through suture removal at the steep meridian, as indicated by keratometry or topography. Usually, this meridian is at least 3 D steeper for suture removal. Suture removal in the interrupted suture technique can start at the 4th postoperative month, although care must be taken with older patients and if intense steroid regimen is maintained, as healing is delayed in these cases. The effect of removing an individual interrupted suture is unpredictable and the change in astigmatism may last for several months, making it advisable to wait at least 1 month before removing further sutures. (3) The time elapsed between surgery and suture removal plays an important role. As time goes by, the effect of suture removal lessens, although dramatis changes in astigmatism may occur even 2 or more years after surgery. (19) It might be preferable to leave sutures in place indefinitely once a good outcome as been reached.

RECURRENCE OF ECTATIC DISEASE. Keratoconus may recur 7 to 40 years after penetrating keratoplasty, with a mean latency of 17 years. (20) (21) (22) Possible explanations include incomplete removal of the ectatic host cornea when 4-6 mm diameter trephinations are performed, subclinical ectatic disease in the donor cornea, production by host epithelium of degradative enzymes that can weaken the donor cornea and infiltration of the graft by host keratocytes that produce abnormal collagen and lead to recurrence of disease. (22)

## **3. Evaluation of irregular astigmatism**

## **3.1 Orbscan**

Irregular astigmatism may have specific features at topography or have an undefined pattern. Specific features include keratoconus, pellucid marginal degeneration, decentered ablation, decentered steep and central island.

## **3.1.1 Keratoconus**

Suspicious patterns include inferior elevation, inferior thinning in the area of maximal protusion and inferior steepening. (23) Keratoconus may present an asymmetric bow tie pattern, where there is a significant difference in the width of the lobes or a significant difference (> 1D) in the dioptric power at 1.5 mm from the centre. (3) It is also suspicious when the two principal meridians are not perpendicular to each other (Figure 6).

TREPHINATION TECHNIQUE. Tilt, eccentric trephination, poor quality blades, damaged corneal blocks, asymmetric pressure from lid speculums and scleral rings can all cause irregular astigmatism. (17) Although penetrating keratoplasty (PKP) generally results in clear corneal grafts, the procedure is frequently complicated by refractive imperfections and wound-healing problems. (18) Femtosecond laser corneal surgery has been increasing in popularity and has the potential to overcome many of the problems of manual or automated

SUTURE PLACEMENT is crucial to obtain a good refractive outcome following corneal transplantation. The second cardinal suture is the most important in keratoplasty. It determines lateral wound apposition, donor/recipient edge alignment and corneal astigmatism. Sutures must purchase the same amount of tissue in donor and recipient beds, which means having the same length and depth. Sutures can be tied with a variety of techniques, but all require meticulous attention to appropriate suture tension with

SUTURE REMOVAL. Suture manipulation is a very important factor in the astigmatic outcome. Astigmatism is reduced through suture removal at the steep meridian, as indicated by keratometry or topography. Usually, this meridian is at least 3 D steeper for suture removal. Suture removal in the interrupted suture technique can start at the 4th postoperative month, although care must be taken with older patients and if intense steroid regimen is maintained, as healing is delayed in these cases. The effect of removing an individual interrupted suture is unpredictable and the change in astigmatism may last for several months, making it advisable to wait at least 1 month before removing further sutures. (3) The time elapsed between surgery and suture removal plays an important role. As time goes by, the effect of suture removal lessens, although dramatis changes in astigmatism may occur even 2 or more years after surgery. (19) It might be preferable to

RECURRENCE OF ECTATIC DISEASE. Keratoconus may recur 7 to 40 years after penetrating keratoplasty, with a mean latency of 17 years. (20) (21) (22) Possible explanations include incomplete removal of the ectatic host cornea when 4-6 mm diameter trephinations are performed, subclinical ectatic disease in the donor cornea, production by host epithelium of degradative enzymes that can weaken the donor cornea and infiltration of the graft by host keratocytes that produce abnormal collagen and lead to recurrence of

Irregular astigmatism may have specific features at topography or have an undefined pattern. Specific features include keratoconus, pellucid marginal degeneration, decentered

Suspicious patterns include inferior elevation, inferior thinning in the area of maximal protusion and inferior steepening. (23) Keratoconus may present an asymmetric bow tie pattern, where there is a significant difference in the width of the lobes or a significant difference (> 1D) in the dioptric power at 1.5 mm from the centre. (3) It is also suspicious

when the two principal meridians are not perpendicular to each other (Figure 6).

leave sutures in place indefinitely once a good outcome as been reached.

trephines or microkeratomes.

disease. (22)

**3.1 Orbscan** 

**3.1.1 Keratoconus** 

avoidance of loose or tight knots. (12)

**3. Evaluation of irregular astigmatism** 

ablation, decentered steep and central island.

Fig. 6. Keratoconus. The irregularity in the 3 and 5mm zone is 5.2 and 5.6 (higher than 1.5D and 2.0D respectively), anterior float is 0.057 (>0.025) and posterior float is 0.108 (>0.04). Pachymetry is also abnormal with central thickness of 400 µm and thinnest point = 386 µm.

The value for the posterior elevation difference from best fit sphere (posterior float) > 0.04 mm and an anterior float value > 0.025 mm are suspicious of keratoconus. (23) A posterior value > 0.05 mm is usually accompanied by other signs of ectasia. Other clues for the presence of keratoconus are irregularity at the 3 mm zone > ± 1.5 D, irregularity at the 5 mm zone > ± 2.0 D, the thinnest part of the cornea being > 30 µm thinner than the centre, the thinnest part of the cornea being more than 2.5 mm away from the centre and the peripheral cornea not being at least 20 µm thicker than the centre. (23)

## **3.1.2 Pellucid marginal degeneration**

There is a band of stromal thinning 1-2 mm wide occurring 1-2 mm central to the inferior limbus. In contrast to keratoconus, protusion occurs superior to the area of thinning. (23) (2)There is central against the rule astigmatism with a classic "kissing doves" or crab claw pattern inferiorly. (2) (Figure 7). However, Lee et al (24) have discussed that a "clawshaped" pattern is not diagnostic for pellucid marginal degeneration and that such patterns may also be found in keratoconus. Slit-lamp signs and pachymetry maps must be considered in conjunction with corneal topography for a reliable diagnosis.

Fig. 7. Pellucid marginal degeneration, with crab claw pattern and inferior thinning.

### **3.1.3 Decentered ablation and decentered steep**

Sagittal or axial curvature maps are poor indicators of the location of previous corneal treatments due to the difference between the curvature map´s reference axis, the line of sight and the corneal apex. (25) Elevation maps should be used instead. Elevation maps show the misalignment of the centre of ablation from optical centre.

#### **3.1.4 Central island**

A central island is a central area of relatively less flattening that measures >1.0 mm in size and >1.0 diopter (D) in power and does not extend to the periphery.

#### **3.2 Pentacam**

The patterns described previously for Orbscan can also be seen with Pentacam. However, there are further criteria that can help recognizing initial ectasia.

Elevation maps are very useful to detect initial ectasia. A central or paracentral islands pattern with positive elevation values > 10 µm for the anterior surface or > 15 µm for the posterior surface are suspicious of keratoconus. There is usually displacement of the thinnest region in the pachymetry map towards the island. (25)

The Belin/ Ambrosio Enhanced Ectasia Display maintains the principle of the best fit sphere but instead of using a "normal" sphere it uses a reference surface that more closely resembles the patient´s own normal portion of the cornea. To do this, a 4 mm optical zone centered on the thinnest part of the cornea is excluded from the calculation of the reference shape. The effect is minor in normal eyes but enhances the abnormal portion of the cornea in ectasia patients. The difference maps display the relative change in elevation from the baseline elevation map to the exclusion map. Changes between 6 and 12 µm for the front surface and 10 to 20 µm for the back surface are suspicious. Values greater than 12 and 20 µm for the anterior and posterior surfaces are typically seen in patients with known keratoconus. (25)

#### **3.3 Wavefront aberrometry**

Wavefront sensing is a tecnhique of measuring the complete refractive status, including irregular astigmatism, of an optical system. (26) A wavefront aberration is defined as the deviation of the wavefront that originates from the measured optical system from reference wavefront that comes from an ideal optical system. The unit for wavefront aberrations is microns or fractions of wavelengths and it is expressed as the root mean square or RMS. (26) The shape of the wavefront can be described by Zernike polynomials, which are a combination of trigonometric functions. Zernike polynomials can be grouped into lower order or higher order aberrations (HOA). HOA include third order and advancing higher Zernike modes. High levels of HOA have a detrimental effect on retinal image quality that is pupil size dependent. (27) In normal eyes, the predominant ocular aberrations are the second order errors, which include three terms: defocus and regular astigmatism in the two directions. The third order has four terms: coma (horizontal and vertical) and trefoil (horizontal and vertical) and the fourth order has tetrafoil, secondary astigmatism and spherical aberration. Spectacles can correct for only the second order aberrations and not the HOA that represent irregular astigmatism. (26)

In keratoconus there is a prominent increase of vertical coma due to a corneal component. (28) In addition, trefoil, tetrafoil and secondary astigmatism are higher in keratoconic eyes. (26) The direction of the vertical coma (negative sign) is the opposite of normal eyes, that is, a prominent vertical coma with an inferior slow pattern, attributed to an inferior shift of the cone´s apex. (27) However, vertical coma may be higher in the lesser involved eye of patients diagnosed with keratoconus, suggesting that this is the earliest manifestation of keratoconus. (27) Gobbe et al (29) demonstrated that the corneal derived wavefront error of vertical coma is the best detector to differentiate between suspected keratoconus and normal corneas. Trefoil aberration in keratoconus is also the reverse of that of normal eyes. (26)

In pellucid marginal degeneration the mean axes of the coma are the reverse of normal eyes, but the magnitude of the coma is less than in keratoconic eyes. The mean axes of the trefoil and the sign of sperical aberration are opposite to that of keratoconus. (26)

Refractive surgeries tend to increase the total HOA and induce a shift from mainly comalike aberrations pre op to sperical like aberration post op. (26)

HOA can also have some advantageous effects. For example, coma-like aberrations contribute to an apparent accomodation in pseudophakic eyes. (30) So, although it is important to reduce the HOA for better optical quality of the image, the depth of field might be reduced. (26) Also, the reduction of total spherical aberration after aspheric IOL implantation may degrade distance-corrected near and intermediate visual acuity. (31)

## **3.4 Allegro topolyzer**

254 Astigmatism – Optics, Physiology and Management

Sagittal or axial curvature maps are poor indicators of the location of previous corneal treatments due to the difference between the curvature map´s reference axis, the line of sight and the corneal apex. (25) Elevation maps should be used instead. Elevation maps show the

A central island is a central area of relatively less flattening that measures >1.0 mm in size

The patterns described previously for Orbscan can also be seen with Pentacam. However,

Elevation maps are very useful to detect initial ectasia. A central or paracentral islands pattern with positive elevation values > 10 µm for the anterior surface or > 15 µm for the posterior surface are suspicious of keratoconus. There is usually displacement of the

The Belin/ Ambrosio Enhanced Ectasia Display maintains the principle of the best fit sphere but instead of using a "normal" sphere it uses a reference surface that more closely resembles the patient´s own normal portion of the cornea. To do this, a 4 mm optical zone centered on the thinnest part of the cornea is excluded from the calculation of the reference shape. The effect is minor in normal eyes but enhances the abnormal portion of the cornea in ectasia patients. The difference maps display the relative change in elevation from the baseline elevation map to the exclusion map. Changes between 6 and 12 µm for the front surface and 10 to 20 µm for the back surface are suspicious. Values greater than 12 and 20 µm for the anterior

Wavefront sensing is a tecnhique of measuring the complete refractive status, including irregular astigmatism, of an optical system. (26) A wavefront aberration is defined as the deviation of the wavefront that originates from the measured optical system from reference wavefront that comes from an ideal optical system. The unit for wavefront aberrations is microns or fractions of wavelengths and it is expressed as the root mean square or RMS. (26) The shape of the wavefront can be described by Zernike polynomials, which are a combination of trigonometric functions. Zernike polynomials can be grouped into lower order or higher order aberrations (HOA). HOA include third order and advancing higher Zernike modes. High levels of HOA have a detrimental effect on retinal image quality that is pupil size dependent. (27) In normal eyes, the predominant ocular aberrations are the second order errors, which include three terms: defocus and regular astigmatism in the two directions. The third order has four terms: coma (horizontal and vertical) and trefoil (horizontal and vertical) and the fourth order has tetrafoil, secondary astigmatism and spherical aberration. Spectacles can correct for only the second order aberrations and not the

In keratoconus there is a prominent increase of vertical coma due to a corneal component. (28) In addition, trefoil, tetrafoil and secondary astigmatism are higher in keratoconic eyes. (26) The direction of the vertical coma (negative sign) is the opposite of normal eyes, that is,

and posterior surfaces are typically seen in patients with known keratoconus. (25)

**3.1.3 Decentered ablation and decentered steep** 

**3.1.4 Central island** 

**3.3 Wavefront aberrometry** 

HOA that represent irregular astigmatism. (26)

**3.2 Pentacam** 

misalignment of the centre of ablation from optical centre.

and >1.0 diopter (D) in power and does not extend to the periphery.

there are further criteria that can help recognizing initial ectasia.

thinnest region in the pachymetry map towards the island. (25)

The ALLEGRO Topolyzer (WaveLight Laser Technologie AG, Germany) is a combination of placido based topography system and an integrated kerato-meter. The patterns described previously can also be seen with it and there are several useful parameters and indices that can help with the diagnosis of irregular astigmatism.

## **3.4.1 Fourier analysis**

The Topolyzer performs a Fourier analysis on the topographic image, allowing the study of the resulting individual waves:

#### **Decentration**

Decentration measures the tilt between the optical axis of the videokeratoscope and the optical vertex of the cornea. In a normal cornea it is < 0.45 mm for sagittal curvature and 1.88 for tangential curvature. Figure 8.

#### **Regular astigmatism**

In a normal cornea, regular astigmatism is represented as a cross. Keratoconus is often associated with a rotation of the astigmatic axis from the centre to the periphery, resulting in a spiral pattern. Figure 9.

#### **Irregularities**

The Irregularities field only contains wave components that cannot be corrected by means of a sphere, cylinder or prism. In a normal cornea the mean of all irregularities is less than 0.03 mm for sagittal curvature and 0.141 for tangential curvature. Figure 10.

## **3.4.2 Zernike analysis**

The Topolyzer performs a Zernike analysis on measured height data. It calculates for each Zernike polynomial a coefficient which describes the contribution of that polynomial to the height data. The Zernike coefficients can be viewed as "Z separate" or "Z vectors" modes. The relative contribution of each Zernike polynomial (tilt, astigmatism, focus, trefoil, coma, spherical aberration, etc) is displayed in numerical values. Abnormal values will appear in red. In keratoconus, for example, the coma will often be increased. In addition, the Topolyzer calculates an aberration coefficient from the Zernike coefficients. Values exceeding 1.0 indicate that there are atypical wave components.

Fig. 8. Decentered PRK myopic ablation Orbscan (top left) and Topolyzer (top right). Decentration value was 0.50 mm. The T-CAT ablation profile (bottom left) and the post op Orbscan (bottom right) showing a more regular cornea.

Fig. 9. Fourier analysis of a keratoconus patient, displaying the typical spiral pattern.

Fig. 10. The mean of all irregularities in this sagittal curvature map of a post radial keratotomy patient is 0.194 (normal value below 0.03 mm).

The Zernike 2D Display Mode represents Zernike polynomials in 2 dimensions and might be a better way to recognize initial keratoconus. It represents more accurately the apex of the cone, which may not be correctly depicted by the sagittal curvature map, as discussed previously. The height of the cone quantifies the degree of keratoconus. The higher the value the more advanced the keratoconus is.

## **3.4.3 Indices**

256 Astigmatism – Optics, Physiology and Management

spherical aberration, etc) is displayed in numerical values. Abnormal values will appear in red. In keratoconus, for example, the coma will often be increased. In addition, the Topolyzer calculates an aberration coefficient from the Zernike coefficients. Values

Fig. 8. Decentered PRK myopic ablation Orbscan (top left) and Topolyzer (top right). Decentration value was 0.50 mm. The T-CAT ablation profile (bottom left) and the post op

Fig. 9. Fourier analysis of a keratoconus patient, displaying the typical spiral pattern.

Orbscan (bottom right) showing a more regular cornea.

exceeding 1.0 indicate that there are atypical wave components.

Indices are calculated from curvature, height, Fourier and Zernike analysis data. Borderline values are displayed in yellow and abnormal values in red.

ISV – the Index of surface variance gives the deviation of individual corneal radii from the mean value. Elevated in all types of irregularities (scars, keratoconus, etc). Abnormal ≥ **37**.

IVA – Index of vertical asymmetry compares the symmetry of corneal radii from the superior to the inferior cornea. Elevated in keratoconus and pellucid marginal degeneration. Abnormal ≥ 0.28.

KI – Keratoconus index. Elevated especially in keratoconus. Abnormal >1.07

CKI – Center keratoconus index. Elevated especially in central keratoconus. Abnormal ≥ **1.03**.

RMin – The smallest radius of curvature in the field of measurement. Elevated in keratoconus. Abnormal < 6.71.

IHA – Index of height asymmetry. Gives the degree of symmetry of height data with respect to the horizontal meridian as axis of reflection (superior versus inferior). Sometimes more sensitive than the IVA. Abnormal ≥ 19.

IHD – Index of height decentration. Gives the degree of decentration in vertical direction. Elevated in keratoconus. Abnormal ≥ 0.014.

ABR – Aberration coefficient. If there are no abnormal corneal aberrations, aberration coefficient is 0.0, otherwise becomes 1.0 or greater, depending on the degree of aberration. Abnormal ≥ **1**.

KKS – Keratoconus stage. This index follows the Amsler classification.

## **4. Wavelight allegretto wave topography-guided ablation treatment**

## **4.1 Principle of topography guided treatments**

Topography guided treatments can be performed with several acquisition systems linked to an excimer laser. Some examples are the iVIS suite, the VISX system, the CRS-Master software combined with the MEL 80 laser, the CATz algorithm combined with the Nidek CXIII excimer laser and the Schwind system.

We will focus on Allegro T-CAT system, the one we use.

Topography-guided treatments (T-CAT) can be planned from both the ALLEGRO Oculyzer and the ALLEGRO Topolyzer and are indicated for eyes with severe irregularities and corneal disorders. The Allegro Oculyzer is a Scheimpflug imaging system similar to the Pentacam. The Topolyzer is a Placido based system with 11 rings that generate 22,000 measuring points and an integrated keratometer.

T-CAT treatments are based on the principle of reshaping a patient's irregular cornea to the best fit asphere, thereby removing the excess tissue in order to transform an irregular cornea into a symmetric regular cornea. (3) It also allows the correction of the refractive error, but one has to take into account the change in refraction induced by the correction of the irregularities.

## **4.2 Indications and decision tree**

T-CAT software allows the treatment of corneal scars, small optical zones, decentrations, forme fruste keratoconus and other corneal irregularities. The approved range of treatment for myopia is -14D, for hyperopia +6 and for astigmatism ± 6D.

The correction of irregular astigmatism can be done by either one of 2 customized approaches: wavefront guided or topography guided treatments. The recommended decision tree is displayed in Figure 11.

Fig. 11. Recommended decision tree regarding the choice between topography guided, wavefront guided, wavefront optimized (the usual ablation profile) and Q factor optimized treatments.

If BCVA is bellow 20/20, if there are mesopic symptoms and an irregular topography, a wavefront measurement should be performed. If measurements are reproducible and valid, a wavefront guided treatment can be done. If not, a T-CAT treatment should be preferred. Wavefront measurements are difficult to obtain in irregular corneas, such as in scars, PRK haze, corneal incisions (RK, penetrating keratoplasty) and in the presence of lens opacity. Even if ocular wavefront can be captured several times, the aberration maps often cannot be relied on for treatment planning because they differ markedly from one another and there is no way to know which, if any, is correct. (32) Another problem is that wavefront guided treatments assume that it is possible to correct all the aberrations of the eye on the cornea, so that the postoperative cornea could compensate for all the internal aberrations. In other words, that the location of the aberration does not matter. But the location of the aberration does make a difference. For example, treating non-anterior cylinder (lenticular astigmatism) on cornea gives an unsatisfactory result, with more cylinder left untreated. (3) The resulting cornea can be irregular, since it is compensating for irregularities that are not its own. Vision can decrease over time, because lens irregularities, for example, change over time. The treatment itself creates new aberrations that modify the preoperative aberration map, due to epithelial hyperplasia, stromal remodeling and the LASIK flap. There are also variations in ocular aberrations with age and accommodation. Having said this, when a patient's corneal aberrations correlate with wavefront aberrations, either a wavefront or a topography-guided approach can be used. The major limitation of T-CAT is that it may need a second procedure to address the refractive error.

T-CAT software has been associated with corneal cross linking for the stabilization of progressive ectasia. (33) The Athens protocol (34) (35) involves performing a T-CAT treatment with a reduction in the amount of sphere and cylinder correction (up to 70 percent of the cylinder error and up to 70 percent of the spherical error in order not to remove more than 50 microns of stroma) and corneal cross-linking on the same day. To minimize tissue ablation, the effective optical zone is decreased to 5.5 mm. This approach intends to stop the progression of the disease at the same time it reduces irregular astigmatism by reshaping the cornea. The results are promising and open a new field of applications for topography guided treatments.

#### **4.3 Surgical plan**

258 Astigmatism – Optics, Physiology and Management

Topography guided treatments can be performed with several acquisition systems linked to an excimer laser. Some examples are the iVIS suite, the VISX system, the CRS-Master software combined with the MEL 80 laser, the CATz algorithm combined with the Nidek

Topography-guided treatments (T-CAT) can be planned from both the ALLEGRO Oculyzer and the ALLEGRO Topolyzer and are indicated for eyes with severe irregularities and corneal disorders. The Allegro Oculyzer is a Scheimpflug imaging system similar to the Pentacam. The Topolyzer is a Placido based system with 11 rings that generate 22,000

T-CAT treatments are based on the principle of reshaping a patient's irregular cornea to the best fit asphere, thereby removing the excess tissue in order to transform an irregular cornea into a symmetric regular cornea. (3) It also allows the correction of the refractive error, but one has to take into account the change in refraction induced by the correction of the irregularities.

T-CAT software allows the treatment of corneal scars, small optical zones, decentrations, forme fruste keratoconus and other corneal irregularities. The approved range of treatment

The correction of irregular astigmatism can be done by either one of 2 customized approaches: wavefront guided or topography guided treatments. The recommended

Fig. 11. Recommended decision tree regarding the choice between topography guided, wavefront guided, wavefront optimized (the usual ablation profile) and Q factor optimized

**4. Wavelight allegretto wave topography-guided ablation treatment** 

**4.1 Principle of topography guided treatments** 

We will focus on Allegro T-CAT system, the one we use.

for myopia is -14D, for hyperopia +6 and for astigmatism ± 6D.

CXIII excimer laser and the Schwind system.

measuring points and an integrated keratometer.

**4.2 Indications and decision tree** 

decision tree is displayed in Figure 11.

treatments.

Before advancing to treatment it is useful to check several issues.

### **4.3.1 Manifest refraction**

A manifest refraction as accurate as possible is very important, because T-CAT can incorporate the refractive error treatment.

## **4.3.2 Pachymetry**

Pachymetry will be needed during the planning of the surgery.

#### **4.3.3 Evaluation of exam quality**

Pupil should always be correctly identified by the Topolyzer on the camera image.

Topographic maps should be similar to each other. The best way to check this is in the display "Compare examinations". Maps that are substantially different from others should not be exported to the Wavelight laser. Up to 8 maps will be averaged by the system and the percentage of the data contained in the chosen optical zone (usually a 6.5mm) is displayed on the last column (Figure 12). Maps with less than 90% of data are excluded automatically. Although the asphericity value can be modified, this adjustment has a poor predictability. (36)


Fig. 12. Left – Mean sagittal topography of the acquisitions displayed on the right. There is decentration of a myopic ablation, performed ten years ago. Right- The software averages up to 8 acquisitions. Examinations containing less than 90% of data in the optical zone are rejected (in this case the examination marked in red was eliminated).

## **4.3.4 Modification of treatment**

The next screen is the actual ablation profile. Despite being possible to turn the tilt on, it is recommended to turn it off because in this mode the software attempts to restore the morphologic axis while sparing the most amount of tissue.

This screen displays the clinical refraction, which is better to leave unfilled, the Topolyzer refraction and the modified refraction. As mentioned previously, the correction of the irregularities will induce a shift in refraction, therefore, if the manifest refraction is entered without taking into account the ablation profile a resulting refractive error is obtained. Patients need to understand that a second refractive procedure may be necessary and that the primary goal of this treatment is to improve the corrected visual acuity. Despite this, it is possible to minimize the resulting refractive error. (37)

percentage of the data contained in the chosen optical zone (usually a 6.5mm) is displayed on the last column (Figure 12). Maps with less than 90% of data are excluded automatically. Although the asphericity value can be modified, this adjustment has a poor predictability.

Fig. 12. Left – Mean sagittal topography of the acquisitions displayed on the right. There is decentration of a myopic ablation, performed ten years ago. Right- The software averages up to 8 acquisitions. Examinations containing less than 90% of data in the optical zone are

The next screen is the actual ablation profile. Despite being possible to turn the tilt on, it is recommended to turn it off because in this mode the software attempts to restore the

This screen displays the clinical refraction, which is better to leave unfilled, the Topolyzer refraction and the modified refraction. As mentioned previously, the correction of the irregularities will induce a shift in refraction, therefore, if the manifest refraction is entered without taking into account the ablation profile a resulting refractive error is obtained. Patients need to understand that a second refractive procedure may be necessary and that the primary goal of this treatment is to improve the corrected visual acuity. Despite this, it is

rejected (in this case the examination marked in red was eliminated).

morphologic axis while sparing the most amount of tissue.

possible to minimize the resulting refractive error. (37)

**4.3.4 Modification of treatment** 

(36)

Fig. 13.1

Fig. 13.2

#### Fig. 13.3

Fig. 13. The first screen shows the ablation profile with no values introduced in the "Clinical" and "Modified" boxes. This shows the necessary ablation to regularize the cornea. The ablation will induce spherical aberration (Zernike coefficient C12 of 1.09) which will have to be compensated by changing the sphere in the "Modified" field. In the second screen the sphere has been modified to -0.7, which makes the C4 component similar to the C12 (C4=1.07 and C12=1.09). The third screen displays the final ablation profile. Because the patient manifest refraction is -2.0D, sphere is modified to -2.5 (-0.7 plus -2.0, but leaving the patient slightly myopic due to age). Cylinder has been reduced because the topographic cylinder is much higher than the refractive.

First, no refraction should be entered in the "modified refraction" field (Figure 13). This shows the ablation profile that regularizes the cornea. The ablation depths can be known by positioning the mouse in the desired area and clicking the left button.

Second, identify the induced change in sphere and the amount of treatment needed to compensate for it. Enlargement of an optical zone, for example, will ablate more tissue in the periphery than in the centre, resembling a hyperopic correction. As a general rule, 15 µm of ablation difference are equivalent to 1D. If 45 µm are ablated in the periphery and 15 µm in the centre, this will induce approximately 2D of myopia (30 µm = 2D, tissue was removed in the periphery, making the central cornea more steep). In our later cases we have compensated the spherical aberration (C12) induced by the treatment with an adjustment of the sphere (C4) to equilibrate the C4 and C12 components. The amount of sphere is changed until the C4 and C12 components are similar (Figure 13, first and second screen).

It is usually better not to change too much the topographic cylinder value and axis and care must be taken when the patient's manifest refraction is not consistent with the measurement obtained by the Allegro Topolyzer. (37) Choosing the manifest refraction will probably result in a persistent irregular cornea, whereas choosing the Topolyzer refraction will probably result in a regular cornea and improvement of corrected visual acuity, but at the expense of reduced uncorrected visual acuity, which might be perceived by the patient as a bad result. The topographic cylinder should be selected in most cases, even if different from the manifest refraction, because there are irregularities (such as coma) that can be perceived by the patient as cylinder on manifest refraction.

Finally, integrate the manifest refraction (sphere) to calculate the final treatment.

## **4.4 Surgery**

262 Astigmatism – Optics, Physiology and Management

Fig. 13.3

"Clinical" and "Modified" boxes. This shows the necessary ablation to regularize the cornea. The ablation will induce spherical aberration (Zernike coefficient C12 of 1.09) which will have to be compensated by changing the sphere in the "Modified" field. In the second screen the sphere has been modified to -0.7, which makes the C4 component similar to the C12 (C4=1.07 and C12=1.09). The third screen displays the final ablation profile. Because the patient manifest refraction is -2.0D, sphere is modified to -2.5 (-0.7 plus -2.0, but leaving the patient slightly myopic due to age). Cylinder has been reduced because the topographic

First, no refraction should be entered in the "modified refraction" field (Figure 13). This shows the ablation profile that regularizes the cornea. The ablation depths can be known by

Second, identify the induced change in sphere and the amount of treatment needed to compensate for it. Enlargement of an optical zone, for example, will ablate more tissue in the periphery than in the centre, resembling a hyperopic correction. As a general rule, 15 µm of ablation difference are equivalent to 1D. If 45 µm are ablated in the periphery and 15 µm in the centre, this will induce approximately 2D of myopia (30 µm = 2D, tissue was removed in the periphery, making the central cornea more steep). In our later cases we have compensated the spherical aberration (C12) induced by the treatment with an adjustment of the sphere (C4) to equilibrate the C4 and C12 components. The amount of sphere is changed

It is usually better not to change too much the topographic cylinder value and axis and care must be taken when the patient's manifest refraction is not consistent with the measurement

until the C4 and C12 components are similar (Figure 13, first and second screen).

positioning the mouse in the desired area and clicking the left button.

Fig. 13. The first screen shows the ablation profile with no values introduced in the

cylinder is much higher than the refractive.

Either a PRK or LASIK can be done. We prefer PRK with MMC 0.02% 15 seconds because it spares tissue and retreatments are easier. It avoids performing a deeper flap in post LASIK complications patients and flap related complications in post RK patients (namely flap fragmentation). The ablation depth is usually < 80 µm and haze is very rare with this approach.

In post penetrating keratoplasty patients it is important to wait for refractive stabilization. We usually wait for 6 months after removal of all stitches and perform PRK with MMC 0.02% 15 seconds. PRK avoids the burden of the pressure on the transplant with LASIK and possible flap related complications due to scar tissue. The post operative medication is the same used in routine PRK, although a close follow up is needed with these patients.

## **5. Clinical pearls and conclusions**

Topography guided treatments are indicated in irregular corneas with poor visual acuity and mesopic symptoms, as in corneal scars, decentrations, small optical zones and post penetrating keratoplasty.

The main objective of the procedure is to improve the corrected visual acuity and the mesopic symptoms. The correction of the refractive error may have to be done in a second procedure.

Verify correct pupil identification by the software, choose good quality maps and check the ablation profile with no refraction. Analyze the change in refraction induced by this profile and neutralize it. Add the manifest refraction to the treatment plan.

In the presence of scars, RK or previous flap related complications prefer PRK. In other cases PRK and LASIK present good results.

## **6. References**


[5] *Predicting time to refractive stability after discontinuation of rigid contact lens wear before* 

[6] *Changes in ocular aberrations after instillation of artificial tears in dry-eye patients.* Montes-

[7] *Quantitative analysis of regular and irregular astigmatism induced by pterygium.* 

[8] *Effects of pterygium on corneal spherical power and astigmatism.* Tomidokoro A, Miyata K,

[9] *Effect of pterygium surgery on corneal topography: a prospective study.* Bahar I, Loya N, Weinberger D, Avisar R. 2, Mar 2004 Mar, Cornea, Vol. 23, pp. 113-7. [10] *Effect of pterygium excision on induced corneal topographic abnormalities.* Stern GA, Lin A. 1,

[11] *ICD3D Classification of the Corneal Dystrophies.* The Cornea Society. 10, Dec 2008, Cornea,

[12] Macsai, Marian S, [ed.]. *Ophthalmic Microsurgical Suturing Techniques.* s.l. : Springer,

[13] Basic and Clinical Science Course. *Refractive Surgery.* s.l. : American academy of

[14] Dimitri Azar, Brian Boxer Wachler, Eric D. Donnenfeld, William J. Dupps JR, Jerome C.

[16] *Risk factors and prognosis for corneal ectasia after LASIK.* Randleman JB, Russell B, Ward

[17] *Progressive keratectasia after laser in situ keratomileusis.* Rad AS, Jabbarvand M, Sai N. 5,

[18] *An analysis and interpretation of refractive errors after penetrating keratoplasty.* EM, Perlman.

[19] *Femtosecond-laser–assisted Descemet's stripping endothelial keratoplasty.* Cheng YY, Pels E,

[20] *Changes in keratometric astigmatism after suture removal more than one year after penetrating* 

[21] *Histologic evidence of recurrent keratoconus seven years after keratoplasty.* Kremer I,

*keratoplasty.* Mader TH, Yuan R, Lynn MJ, Stulting RD, Wilson LA, Waring GO 3rd.

Eagle RC, Rapuano CJ, Laibson PR. 4, Apr 1995, Am J Ophthalmol, Vol. 119,

Nuijts RM. 1, Jan 2007, J Cataract Refract Surg, Vol. 33, pp. 152-5.

[15] http://one.aao.org/lms/courses/managing\_lasik\_complications/index.htmv.

Ramos-Esteban, Parag A. Majmudar, Sonia H. Yoo,. LASIK and PRK: Managing Complications. *American Academy of Ophthalmology Web site.* [Online] 2009 2009.

MA, Thompson KP, Stulting RD. 2, Feb 2003, Ophthalmology, Vol. 110, pp. 267-

Cataract and Refractive Surgery, 2004, Vol. 30, pp. 2290-2294.

Vol. 30, pp. 1649-1652.

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107, pp. 1568-71.

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1, Jan 1988, Ophthalmology, Vol. 88, pp. 39-45.

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*refractive surgery.* Tsai PS, Dowidar A, Naseri A, McLeod SD. s.l. : Journal of

Mico R, Araceli Caliz A, Alio JL. 2004, Journal of Cataract and Refractive Surgery,

Tomidokoro A, Oshika T, Amano S, Eguchi K, Eguchi S. 4, Jul 1999, Cornea, Vol.

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[38] *Topography-guided photorefractive keratectomy with Wavelight Allegretto Wave Eye-Q excimer laser system.* Quadrado M., Rosa A., Vasconcelos H., Tavares C., Murta J. s.l. : XXVIII Congress of the ESCRS, 2010.

## **Toric Intraocular Lenses in Cataract Surgery**

Nienke Visser, Noël J.C. Bauer and Rudy M.M.A. Nuijts

*University Eye Clinic Maastricht, The Netherlands* 

## **1. Introduction**

266 Astigmatism – Optics, Physiology and Management

[38] *Topography-guided photorefractive keratectomy with Wavelight Allegretto Wave Eye-Q excimer* 

XXVIII Congress of the ESCRS, 2010.

*laser system.* Quadrado M., Rosa A., Vasconcelos H., Tavares C., Murta J. s.l. :

In modern cataract surgery, spectacle freedom is becoming more and more important. Emmetropia can be achieved for patients with myopic or hyperopic refractive errors by selecting the appropriate spherical lens power. However, approximately 20% of patients who undergo cataract surgery have 1.25 diopters (D) of corneal astigmatism or more. (Ferrer-Blasco, Montes-Mico et al. 2009; Hoffmann and Hutz 2010) Not correcting the astigmatism component at the time of cataract surgery will fail to achieve spectacle independence.

In patients with substantial amounts of corneal astigmatism several options exist to correct astigmatism during or after cataract surgery. Limbal relaxing incisions or opposite clear corneal incisions may be performed to reduce astigmatism during cataract surgery. After cataract surgery, laser refractive surgery may be used to correct residual refractive errors, including cylinder errors. However, corneal incision procedures are relatively unpredictable and laser refractive surgery may be associated with complications such as dry eyes, wound healing problems and infections. (Bayramlar, Daglioglu et al. 2003; de Oliveira, Solari et al. 2006; Kato, Toda et al. 2008; Thomas, Brunstetter et al. 2008) Toric IOLs now provide the opportunity to correct corneal astigmatism, offering patients with pre-existing astigmatism optimal distance vision without the use of spectacles or contact lenses with a cylindrical correction. Furthermore, the recent introduction of multifocal toric IOLs offers patient with pre-existent corneal astigmatism the opportunity not only to achieve spectacle independence for distance vision, but also for near and intermediate visual acuities.

## **2. Toric intraocular lenses**

The first toric IOL was presented by Shimizu et al. in 1994. (Shimizu, Misawa et al. 1994) This was a non-foldable three-piece toric IOL made from poly-methyl methacrylate (PMMA). It consisted of an oval optic with loop haptics and was available in cylinder powers of 2.00 D or 3.00 D. Postoperatively, about 20% of the IOLs rotated 30 degrees or more and almost 50% of IOLs rotated more than 10 degrees. Rotational stability is a crucial factor in the safety and efficacy of toric IOLs, since as little as 10 degrees of axis misalignment reduces the efficacy of the astigmatic correction by 33%. Misalignment of more than 30 degrees may even induce astigmatism. (Shimizu, Misawa et al. 1994) Since 1994, many advancements have been made in toric IOL technology, including improvements in IOL material and design and refinements in surgical technique. These advances have led to an improved postoperative rotational stability and excellent visual outcomes using currently available toric IOls. Table 1 provides an overview of the characteristics of the currently available toric IOLs.


^ = Same IOL model under different name;

\* = Highest cylinder powers are custom made;
