**3. The measurement of astigmatism**

For a toric IOL, the keratometric astigmatism (both axis and magnitude) of the cornea must be accurately measured.

#### **3.1 Anterior corneal curvature**

Traditionally, keratometry and topography take into account only the anterior corneal curvature [35]. However, nomograms predict total corneal astigmatism based on the power and axis of the anterior corneal astigmatism, assuming a fixed ratio between the anterior and posterior curvature [36]. These methods obviously cannot take outliers and irregularities into account (e.g., post-refractive surgery eyes) [35], thus leading to significant postoperative and/or overcorrection. However, if the agreement of measurement of astigmatism between instruments of different kinds is poor (more than 10°), the selection of toric IOLs requires extra care.

#### **3.2 Posterior corneal curvature**

The astigmatism of posterior cornea is generally minus lens of against-the-rule. As mentioned above, ignoring effects of actual posterior corneal curvature may lead to inaccuracies in total astigmatism estimation in some eyes. In a recent study [36], for those eyes who received IOLs with 2 diopters of cylinder or less, a coefficient of adjustment of 0.75 for with-the-rule astigmatism and 1.41 for against-the-rule astigmatism can be applied to the corneal astigmatism power value to calculate a more appropriate IOL cylinder power than that be calculated by using unadjusted anterior corneal curvature measurements.

Since minimizing the residual refractive error is especially critical in toric multifocal IOLs [37], imaging systems that measure posterior corneal curvature, as well as the new algorithm that incorporates the effect of posterior corneal astigmatism, are increasingly being invented. For example, the Scheimpflug imaging systems, slit scanning systems, and OCT systems could measure posterior corneal curvature, besides the anterior curvature. In a comparative study [35] including a Scheimpflug tomography (OCULUS Pentacam), a Placido topographer (Tomey TMS-5 in Placido mode), a swept source/Fourier domain OCT (CASIA SS-1000), an autokeratometer (Haag-Streit Lenstar), and a hybrid topographer (Tomey TMS-5), the OCULUS Pentacam has the disadvantage of high measuring noise on posterior corneal curvature. Meanwhile, the highest precision for planning toric IOL power and axis was achieved by combining the keratometry and OCT data. In a recent study, Lu et al. found that a novel multicolored spot reflection topographer system

the cases, and spectacle independence has been reported in more than 60% of the patients in previous studies [12, 13, 15–23, 25–30], which is significantly increased compared with nontoric monofocal IOLs [31, 32]. A randomized controlled trial (RCT) compared the outcomes of AcrySof toric IOLs with conventional spherical IOLs and observed a UDVA of 20/40 or better in 92.2% of cases undergoing toric IOL implantation, with 63.4% having a UDVA of 20/25 or better. In contrast, only 81.4% of cases undergoing nontoric IOL implantation had a UDVA of 20/40 or

**IOL Material Design Aspheric Spherical**

Plate haptic, 11.0-mm dialect

C-loop haptic with AVH technology, 12.0–12.5-mm dialect

PMMA-modified C-loop haptic, 12.5-mm dialect

C-loop haptic,13.0-mm dialect

"Tri-Fix" modified C haptic integral with optic, 13.0-mm dialect

Biconvex transitional conic toric design offsetshaped haptic

Bag-in-the-lens, 7.5-mm dialect

C/Plate haptic, 12.0–11.0-mm dialect

10.8–11.2-mm dialect

Modified C-loop PMMA haptics, 13.0-mm dialect

C-loop haptic, 11.6-mm dialect

Silicone Plate haptic,

Hydrophilic acrylic with hydrophobic surface

Hydrophilic acrylic

Hydrophobic acrylic with PMMA haptic tips

Hydrophobic acrylic

Hydrophobic acrylic

Hydrophilic acrylic

Hydrophilic acrylic

Hydrophilic acrylic with hydrophobic surface

Silicone with PMMA haptics

Silicone with PMMA haptics

*Summary of commercially available toric IOLs.*

Acri. comfort (Carl Zeiss Meditec) [12]

*Intraocular Lens*

T-flex (Rayner)

AF-1 Toric (Hoya) [7]

AcrySof (Alcon) [14–19]

TECNIS Toric IOL (Abbott Medical Optics)

Precizon toric IOL (OPHTEC) [21, 22]

Morcher 89A, 92S (Morcher GmbH) [23, 24]

LENTIS Tplus (Oculentis) [7]

STAAR (STAAR Surgical Company) [25]

Light-adjustable lens (Calhoun Vision) [26]

Microsil (HumanOptics)

[27]

**Table 1.**

**36**

[20]

[13]

**power (D)**

Y 10.0 to +32.0

Y 10.0 to +35.0

Y +6.0 to +30.0

Y +6.0 to +34.0

Y +5.0 to +34.0

Y +1.0 to +34.0

N +10.0 to +30.0 D

Y 10.0 to +35.0

N +9.5 to +28.5

Y +17.0 to +24.0

N 10.0 to +35.0

**Cylinder power (D) at IOL plane**

> 1.0–12.0 (0.50 steps)

> 1.0–11.0 (0.25 steps)

1.5–6.0 (0.75 steps)

1.0–6.0 (0.75 steps)

1.5–6 (0.5–1.0 steps)

1.0–10.0 (0.5 steps)

> 0.5–8.0 (0.25 steps)

0.25–12.0 (0.75–1.0 steps)

2.0 or 3.5 2.8

0.75–2.0 3.0

1.0–15.0 (1.0 steps) **Incision size (mm)**

<2.0

<2.0

2.0

2.2

2.2

2.2

2.5

2.6

3.4

could provide high repeatable measurements in (both anterior and posterior) corneal power and astigmatism [38].

**4.3 Intraoperative wavefront aberrometry**

*DOI: http://dx.doi.org/10.5772/intechopen.90153*

significant role in achieving optimal outcomes.

**5. Surgery techniques**

*Toric Intraocular Lenses*

**5.1 Marking techniques**

based methods.

Intraoperative wavefront aberrometry is increasingly being used to estimate the toric IOL power and axis of placement based on the aphakic refraction, especially in

0.43 0.33 D with Optiwave Refractive Analysis (ORA; WaveTec Vision Systems Inc., CA, USA) in post laser-assisted in situ keratomileusis (LASIK) cases undergoing toric IOL implantation, which were more accurate than those obtained by the

Many issues, such as accurate marking technique, clear corneal incisions, intraoperative alignment of the toric IOL, capsulorhexis, and IOL centration, play a

Preoperative reference and axis marking techniques could be broadly categorized as manual methods, image-guided systems, and intraoperative aberrometry-

The three-step manual technique is at present most commonly used [47], which is fairly accurate [48]. The first step is preoperative marking of the reference axis, which is commonly placed in the horizontal 3'o and 9'o clock positions. The second step is intraoperative alignment of the reference mark. The marking may be performed with a skin marking pen or needle. The patient should be sitting erect in a straight-ahead gaze while marking the reference axis. A change in patient position from sitting to supine may induce significant cyclotorsion; studies reported up to 28° of cyclotorsion in 68% of cases [49]. The manual marking methods have been

Image-guided systems and intraoperative aberrometry have advantages compared with manual marking. The image-guided system based on the concept of landmarks to place the axis marks [50], which could be iris crypts, nevi, brush fields, etc. The systems capture a preoperative reference image and calculated the location of these marks and their distance in degrees from the target IOL axis. Then the system generated a final plan which provides simple angular directions from

There are a few image-guided systems at present such as CALLISTO Eye and Z Align (Carl Zeiss Meditec, Jena, Germany), VERION (Alcon, Fort Worth, Texas), TrueGuide (TrueVision 3D Surgical System, Santa Barbara, Calif), Osher Toric Alignment System (OTAS, Haag-Streit, Koeniz, Switzerland), and iTrace System (Tracey Technologies, Houston, Tx). Besides alignment, image-guided systems also contribute to planning the incisions, capsulorhexis size, and optimal IOL centration.

Intraoperative IOL positioning is the key procedure to sustain rotation stability. During IOL alignment, the IOL should be left about 3–5° anticlockwise of the final desired lens position, followed by complete OVD removal and hydration of the wounds. Most open-loop IOLs can be rotated only clockwise, and a complete rerotation will be needed if the IOL rotates further clockwise of the target axis.

post-refractive surgery cases. A recent study reported only a mean error of

standard SRK/T formula and the online ASCRS calculator.

limited by smudging of the dye, irregular, and broad marks.

each reference mark to the planned axis of IOL placement.

**5.2 Intraoperative toric IOL alignment**

**39**

### **3.3 Surgically induced astigmatism**

Besides naturally occurring astigmatism, the surgically induced astigmatism (SIA) is also an important factor for the appropriate option of a toric IOL. The SIA could be influenced by position and length of incisions [39]. Meanwhile, to achieve minimum residual refractive astigmatism for specific patients, the incisions could be determined by the magnitude and axis of preoperative keratometric astigmatism [4]. The application of femtosecond laser-assisted cataract surgery (FLACS) could minimize SIA.
