**4. Intraocular lens calculation**

Intraocular lens power calculation is based on the measurement of anatomical eye parameters. Regardless of the formula we apply, to calculate the intraocular lens (IOL) in our patients we must know precisely the axial length (AL), keratometry and anterior chamber depth (ACD). Prediction of IOL power in eyes undergoing retinal surgery can sometimes be challenging and certain considerations should be taken into account.

### **4.1 Axial length measurement**

AL in our patients can be quantified using optical or ultrasonic methods. Optical methods are more comfortable because they do not require contact with the patient and are more examiner-independent. However, there are cases where we will turn to ultrasonic methods, especially because of media opacity.

Measuring AL requires proper foveal fixation, this could be an important source and error in patients with retinal pathologies. [26, 27] Newly developed equipment such as the IOL master 700, incorporates an OCT system to ensure a correct measurement aligned with the patient's fovea [27] this is especially important in cases of macular pathology and staphyloma. [28]

#### **4.2 Axial length in vitrectomized eyes**

In vitrectomized patients with no fluid exchange the vitreous is replaced by aqueous humor. This is not a problem with ultrasonic biometry because of the transmission rate of aqueous and vitreous humors are practically the same (1532 m/sec ultrasound velocity). In the same way, the vitreous has an optical refractive index of 1.3346 and the aqueous of 1.3336. This small difference generates a myopic shift of −0.13 diopters (D) in vitrectomized eyes that has little clinical relevance. [29, 30]

Ultrasound biometry measures AL from corneal vortex to internal limiting membrane along the optical axis. Optical systems quantify AL from corneal vortex to retinal pigment epithelium along visual axis. [31] So, macular status as macular edema or submacular fluid can affect the measurement of axial length in ultrasonic biometry. The difference in measurement with respect to the axis confers superiority to the optical biometer, which achieves more accurate measurements as long as the visual fixation of the patient is preserved to look at the laser target. [32]

Elevated myopia or staphyloma are more common in vitrectomized patients. [33] These factors along with poor visual binding are frequent cause of erroneous AL measurements. It is likely that one of the most complex situations to determine AL is the case of high retinal detachment with macula-off, where the patient cannot fix and foveal detachment generates an underestimation of AL. [34]

#### **4.3 Axial length in oil-filled eyes**

Phacoemulsification and silicone oil (SO) removal in a single act could avoid surgical risks and is optimal for patients with cataract formation in a short time after vitrectomy with SO tamponade. Obtaining accurate AL measurements in silicone oil-filled eyes can be difficult.

**41**

**Figure 2.**

*Cataract Surgery in Post-Vitrectomized Eyes DOI: http://dx.doi.org/10.5772/intechopen.95467*

AL measurement becomes a biometric challenge.

silicone oil (**Figure 2**). [37]

tropic refractive defect.

different [42–45].

factor. [37]

Whenever possible, we should quantify the AL in oil-filled eyes with optical biometers (optical interferometry or reflectance) because of the optical laser is not appreciably affected by SO, by its molecular weight or by the interfaces that remain between aqueous humor and silicone in eyes with incomplete filling. [35, 36] In the main menu of our optical biometer we will select the option "vitreous cavity filled with oil" and the refractive index of light will change from 1.33 in vitreous to 1.4 in

However, cataracts generated by silicone oil are often dense and do not allow optical biometrics to be performed. It is estimated that in 4.7–17% of AL measurements, interferometry cannot be performed due to poor visual acuity, corneal opacity or dense cataract among others. [38, 39] Low coherence reflectometry and optical coherence tomography use longer wavelength than interferometry, so we can assume that the proportion of eyes measured with these techniques should be greater. [40] In cases where measurement with optical systems cannot be performed

The replacement of vitreous with silicone oil implies that the propagation of acoustic waves is modified. The speed of sound in a medium is inversely related to the refractive index of the medium. Because silicone has a higher rate than vitreous, it reduces the speed of sound a 36% approximately. The sound velocity declines from 1532 m/sec in the vitreous to 980 m/sec in 1000 centistokes molecular weight silicone oil. [41] This reduction in speed generates a higher axial length measurement. If we do not calibrate our ultrasonic biometer, we will generate a hyperme-

If we use higher molecular weight silicone oil the speed variation would be

If our ultrasonic biometer does not have a speed adjustment for eyes with silicone oil, we can multiply a corrective factor of 0.64 to the vitreous cavity length obtained with a speed of 1532 m/sec. [46] To calculate the axial length we will have to add the rest of the structures (anterior chamber depth, lens thickness and retrosilicone space) to the value obtained from vitreous cavity with the corrective

Another source of error appears when the vitreous cavity is not completely filled with SO. An aqueous space is generated between the oil and the retina, the "retrosilicone space". It is maximum in supine position, decreases when the patient is erect and is minimized in the prone position. [47] And as we have seen before, it should

*Optical biometry and topography (Aladdin Topcon®). Select silicone oil in biometer before measuring AL.*

#### *Cataract Surgery in Post-Vitrectomized Eyes DOI: http://dx.doi.org/10.5772/intechopen.95467*

*Current Cataract Surgical Techniques*

**4. Intraocular lens calculation**

**4.1 Axial length measurement**

macular pathology and staphyloma. [28]

**4.2 Axial length in vitrectomized eyes**

**4.3 Axial length in oil-filled eyes**

silicone oil-filled eyes can be difficult.

taken into account.

to assess the state of the retina and be able to rule out complications such as vitreous

Intraocular lens power calculation is based on the measurement of anatomical eye parameters. Regardless of the formula we apply, to calculate the intraocular lens (IOL) in our patients we must know precisely the axial length (AL), keratometry and anterior chamber depth (ACD). Prediction of IOL power in eyes undergoing retinal surgery can sometimes be challenging and certain considerations should be

AL in our patients can be quantified using optical or ultrasonic methods. Optical methods are more comfortable because they do not require contact with the patient and are more examiner-independent. However, there are cases where we will turn

Measuring AL requires proper foveal fixation, this could be an important source and error in patients with retinal pathologies. [26, 27] Newly developed equipment such as the IOL master 700, incorporates an OCT system to ensure a correct measurement aligned with the patient's fovea [27] this is especially important in cases of

In vitrectomized patients with no fluid exchange the vitreous is replaced by aqueous humor. This is not a problem with ultrasonic biometry because of the transmission rate of aqueous and vitreous humors are practically the same (1532 m/sec ultrasound velocity). In the same way, the vitreous has an optical refractive index of 1.3346 and the aqueous of 1.3336. This small difference generates a myopic shift of −0.13 diopters (D) in vitrectomized eyes that has little clinical relevance. [29, 30] Ultrasound biometry measures AL from corneal vortex to internal limiting membrane along the optical axis. Optical systems quantify AL from corneal vortex to retinal pigment epithelium along visual axis. [31] So, macular status as macular edema or submacular fluid can affect the measurement of axial length in ultrasonic biometry. The difference in measurement with respect to the axis confers superiority to the optical biometer, which achieves more accurate measurements as long as the visual fixation of the patient is preserved to look at the laser target. [32]

Elevated myopia or staphyloma are more common in vitrectomized patients. [33] These factors along with poor visual binding are frequent cause of erroneous AL measurements. It is likely that one of the most complex situations to determine AL is the case of high retinal detachment with macula-off, where the patient cannot

Phacoemulsification and silicone oil (SO) removal in a single act could avoid surgical risks and is optimal for patients with cataract formation in a short time after vitrectomy with SO tamponade. Obtaining accurate AL measurements in

fix and foveal detachment generates an underestimation of AL. [34]

hemorrhage or retinal detachment that require combined surgery.

to ultrasonic methods, especially because of media opacity.

**40**

Whenever possible, we should quantify the AL in oil-filled eyes with optical biometers (optical interferometry or reflectance) because of the optical laser is not appreciably affected by SO, by its molecular weight or by the interfaces that remain between aqueous humor and silicone in eyes with incomplete filling. [35, 36] In the main menu of our optical biometer we will select the option "vitreous cavity filled with oil" and the refractive index of light will change from 1.33 in vitreous to 1.4 in silicone oil (**Figure 2**). [37]

However, cataracts generated by silicone oil are often dense and do not allow optical biometrics to be performed. It is estimated that in 4.7–17% of AL measurements, interferometry cannot be performed due to poor visual acuity, corneal opacity or dense cataract among others. [38, 39] Low coherence reflectometry and optical coherence tomography use longer wavelength than interferometry, so we can assume that the proportion of eyes measured with these techniques should be greater. [40] In cases where measurement with optical systems cannot be performed AL measurement becomes a biometric challenge.

The replacement of vitreous with silicone oil implies that the propagation of acoustic waves is modified. The speed of sound in a medium is inversely related to the refractive index of the medium. Because silicone has a higher rate than vitreous, it reduces the speed of sound a 36% approximately. The sound velocity declines from 1532 m/sec in the vitreous to 980 m/sec in 1000 centistokes molecular weight silicone oil. [41] This reduction in speed generates a higher axial length measurement. If we do not calibrate our ultrasonic biometer, we will generate a hypermetropic refractive defect.

If we use higher molecular weight silicone oil the speed variation would be different [42–45].

If our ultrasonic biometer does not have a speed adjustment for eyes with silicone oil, we can multiply a corrective factor of 0.64 to the vitreous cavity length obtained with a speed of 1532 m/sec. [46] To calculate the axial length we will have to add the rest of the structures (anterior chamber depth, lens thickness and retrosilicone space) to the value obtained from vitreous cavity with the corrective factor. [37]

Another source of error appears when the vitreous cavity is not completely filled with SO. An aqueous space is generated between the oil and the retina, the "retrosilicone space". It is maximum in supine position, decreases when the patient is erect and is minimized in the prone position. [47] And as we have seen before, it should

#### **Figure 2.**

*Optical biometry and topography (Aladdin Topcon®). Select silicone oil in biometer before measuring AL.*

be taken into account for IOL calculation. If we do not consider it, leads to a shorter and erroneous measurement of the AL in A-mode biometry.

Abu El Einen et al. [43] found better refractive results in oil-filled eyes explored by inmersion B-guided than in contact A-mode biometry. Although both are echographic techniques, immersion ultrasound prevents us from possible compression of the scanning probe on the cornea and mode B helps us to locate fovea, specially in patients with staphyloma or fluid interfaces. [48]

In addition to slower sound speed, SO absorbs sound, leading to poor penetration with low-quality echoes. [49] This significant sound attenuation generates poor identification of the retinal spike by contact A-mode biometry. [50] In these cases biometry may be unsatisfactory and other methods as we mention below have been proposed. Vitreoretinal surgeons should know that the appearance of cataract occurs after 3 months in 100% of the eyes with SO. [51, 52] Therefore, a useful strategy would be to perform a pre-vitrectomy biometry in all cases with macula on in which there is a possibility of fluid exchange by SO. [53] In these cases, we should take into account that the placement of a scleral buckle during surgery will also modify the axial length of the patient. [54, 55]

Another option is the two-step surgery with the removal of cataract and silicone oil in a first step and the placement of an implant in a second time if the retina remains stable. [56, 57]

El-Baha et al. perform more complex techniques with intraoperative biometry after remove SO with a sterilized ultrasonic biometer probe. [58] Elbendary et al. make an intraoperative calculation with a portable retinoscope. [59] These techniques consume more intraoperative time and require more specific devices that are not available in all centers, including a large stock of IOL powers.

#### **4.4 Silicone oil refractive effect**

In some patients, SO is not removed and is left inside the eye indefinitely. This is the case of eyes with recurrent bleeding or multiple retinal re-detachments among others. In this situation, if we want to extract the cataract we must take into account the refractive effect of SO when calculating IOL. SO acts as a negative lens because of its lower refractive index compared to vitreous. We must add +2 to +3 D to the calculated IOL to compensate for this effect, always in flat-convex lenses with the flat face toward the vitreous cavity. [60]

#### **4.5 Changes in other eye parameters after vitrectomy**

The anterior segment morphology has a crucial role for the refractive results after surgery. Moreover, calculation of effective lens position (ELP) in vitrectomized eyes is influenced by factors inherent to vitreous surgery.

The most uncertain factor in biometry after phaco-vitrectomy is postoperative ACD. Modification of ACD is controversial and there is no consensus on whether it increases or decreases. Mijnsbrugge et al. [61] reported a more posterior position of the IOL in the phacovitrectomy group compared to single phacoemulsification group, attributed to loss of vitreous support. Gülkilik, Neudorfer and Li [62–64] described no significant change in ACD postoperatively in phacovitrectomy group. And Hamoudi and Huang [65, 66] found an earlier position of IOL secondary to capsular fibrosis.

The influence of gas tamponade on refractive outcomes has also been studied, a myopic shift appears related to anterior lens displacement and shallower aqueous depth due to buoyancy and surface tension of the gas. [67] Even when the gas has already completely disappeared, it seems that the IOL could be fixed in a more anterior position. [67]

**43**

*Cataract Surgery in Post-Vitrectomized Eyes DOI: http://dx.doi.org/10.5772/intechopen.95467*

calculation in vitrectomized patients.

Lee et al. [68]

suggest.

the biometric formula.

combined procedures.

**4.6 Lens calculation formulas in vitrectomized eyes**

In recent years, the development of new biometric formulas to calculate the power of IOL to be implanted to our patients has allowed the minimization of post-surgical refractive surprises. New biometric calculation formulas use a variety of strategies, such as the inclusion of more predictive ELP values, the use of ray

There is currently no consensus on the most accurate method for biometric

Lamson et al. [26] observed in a retrospective study that refractive outcomes using eight biometric formulas (Holladay 1, SRK/T, Barrett, Hill-radial basis function, Ladas and Holladay 2) were more variable and more hyperopic than in non-vitrectomized populations. The Holladay 2 formula obtained the highest percentages of postoperative refraction with predicted errors between ±0.50 D and ± 1 D. However, we should point out that the study was retrospective and analyzed a reduced sample of patients. In addition, there were important uncontrolled variables such as the implanted IOL model, which was not the same in all participants of the study. This hyperopic shift in vitrectomized eyes also was reported by

Recently, another retrospective study published by Tan et al. [69] evaluated the refractive results obtained in cataract surgery in vitrectomized eyes by applying next-generation formulas (Barett Universal II, EVO, Kane, and Ladas super formula) against traditional formulas (Haigis, Hoffer Q, Holladay 1, and SRK/T) with Wang-Koch axial length adjustment if required. Before the lens constants were optimized, hyperopic outcomes were noted for all formulas, except for the Kane formula, which revealed no statistically significant bias. However, lens constant

optimization enabled optimal and comparable results for all formulas.

slightly myopic refractive target for the IOL to be implanted (−0.5 D).

without prior cataract as it saves costs and risks of a second intervention. Unlike surgery in previously vitrectomized patients, where the tendency was to a hypocorrection after phacoemulsification (see "LENS CALCULATION FORMULAS IN VITRECTOMIZED EYES"). Phacoemulsification performed concurrently with vitrectomy seems to be associated with myopic shift in the refractive outcome [26, 34, 61, 70] Tranos et al. [70] found that postoperative refractive deviation greater than 0.5 D was associated with shallower ACD and increased macular thickness. Shiraki and Schweitzer [71, 72] related the myopic shift in combined phaco-vitrectomy with the gas tamponade commonly used in cases of retinal detachment. On the other hand, Vandergeest et al. [73] found no tendency toward a myopic shift and they got an elevated percentage of refractive accuracy in

should be used with Wang-Koch's correction.

**4.7 Considerations in combinated phaco-vitrectomy**

As general recommendations to calculate IOL in vitrectomized eyes we

The optimization of the constant in clinical practice or, if not possible, choose a

The presence of silicone oil in the vitreous cavity does not change the choice of

Phaco-vitrectomy is mandatory in cases of retinal surgery with prior cataract. In addition, a large proportion of patients undergoing vitrectomy will develop cataract in the following years. Therefore, phaco-vitrectomy is a common procedure even

Traditional formulas in miopic patients with axial length more than 26 mm

tracing, or artificial intelligence to achieve optimal post-surgical results.

*Current Cataract Surgical Techniques*

be taken into account for IOL calculation. If we do not consider it, leads to a shorter

Abu El Einen et al. [43] found better refractive results in oil-filled eyes explored by inmersion B-guided than in contact A-mode biometry. Although both are echographic techniques, immersion ultrasound prevents us from possible compression of the scanning probe on the cornea and mode B helps us to locate fovea, specially in

In addition to slower sound speed, SO absorbs sound, leading to poor penetration with low-quality echoes. [49] This significant sound attenuation generates poor identification of the retinal spike by contact A-mode biometry. [50] In these cases biometry may be unsatisfactory and other methods as we mention below have been proposed. Vitreoretinal surgeons should know that the appearance of cataract occurs after 3 months in 100% of the eyes with SO. [51, 52] Therefore, a useful strategy would be to perform a pre-vitrectomy biometry in all cases with macula on in which there is a possibility of fluid exchange by SO. [53] In these cases, we should take into account that the placement of a scleral buckle during surgery will also

Another option is the two-step surgery with the removal of cataract and silicone

El-Baha et al. perform more complex techniques with intraoperative biometry after remove SO with a sterilized ultrasonic biometer probe. [58] Elbendary et al. make an intraoperative calculation with a portable retinoscope. [59] These techniques consume more intraoperative time and require more specific devices that are

In some patients, SO is not removed and is left inside the eye indefinitely. This is the case of eyes with recurrent bleeding or multiple retinal re-detachments among others. In this situation, if we want to extract the cataract we must take into account the refractive effect of SO when calculating IOL. SO acts as a negative lens because of its lower refractive index compared to vitreous. We must add +2 to +3 D to the calculated IOL to compensate for this effect, always in flat-convex lenses with the

The anterior segment morphology has a crucial role for the refractive results after surgery. Moreover, calculation of effective lens position (ELP) in vitrecto-

The most uncertain factor in biometry after phaco-vitrectomy is postoperative ACD. Modification of ACD is controversial and there is no consensus on whether it increases or decreases. Mijnsbrugge et al. [61] reported a more posterior position of the IOL in the phacovitrectomy group compared to single phacoemulsification group, attributed to loss of vitreous support. Gülkilik, Neudorfer and Li [62–64] described no significant change in ACD postoperatively in phacovitrectomy group. And Hamoudi and Huang [65, 66] found an earlier position of IOL secondary to capsular fibrosis. The influence of gas tamponade on refractive outcomes has also been studied, a myopic shift appears related to anterior lens displacement and shallower aqueous depth due to buoyancy and surface tension of the gas. [67] Even when the gas has already completely disappeared, it seems that the IOL could be fixed in a more

oil in a first step and the placement of an implant in a second time if the retina

not available in all centers, including a large stock of IOL powers.

and erroneous measurement of the AL in A-mode biometry.

patients with staphyloma or fluid interfaces. [48]

modify the axial length of the patient. [54, 55]

remains stable. [56, 57]

**4.4 Silicone oil refractive effect**

flat face toward the vitreous cavity. [60]

**4.5 Changes in other eye parameters after vitrectomy**

mized eyes is influenced by factors inherent to vitreous surgery.

**42**

anterior position. [67]
