Journal of Refractive Surgery

Review 

A New Slant on Toric Intraocular Lens Power Calculation

Giacomo Savini, MD; Kenneth J. Hoffer, MD, FACS; Pietro Ducoli, MD

Abstract

PURPOSE:

The AcrySof Toric intraocular lens (IOL) (Alcon Laboratories, Inc., Fort Worth, TX) is designed to correct corneal astigmatism ranging from 0.67 to 4.11 diopters (D). The authors reviewed the clinical outcomes of this IOL and investigated possible improvements of the online calculator provided by the manufacturer.

METHODS:

Review of published studies.

RESULTS:

The AcrySof Toric IOL can provide good results, although a mean overcorrection or undercorrection relative to the intended correction has been found by some authors. Stability over time has been reported to be excellent. Rotation occurs mainly in the first postoperative month and is greater in eyes with a longer axial length due to the larger capsule size. The online calculator of this IOL may be improved by considering the posterior corneal astigmatism and better calculating the conversion of the IOL cylinder from the IOL plane to the corneal plane, which may be inaccurate for two reasons. First, given the variable distance between the IOL and the cornea in short and long eyes, the fixed ratio (1.46) provided by the manufacturer cannot be used to calculate this conversion. Second, the online calculator does not take into account the effect of varying IOL sphere power.

CONCLUSION:

The AcrySof Toric IOL is a reliable choice to correct corneal astigmatism at the time of cataract surgery. Results will be improved once the online calculator by the manufacturer considers the posterior corneal astigmatism and the variable ratio between the toricity at the IOL and corneal plane.

[J Refract Surg. 2013;29(5):348–354.]

From G.B. Bietti Eye Foundation-IRCCS, Rome, Italy (GS, PD); and Jules Stein Eye Institute, University of California, Los Angeles, and St. Mary’s Eye Center, Santa Monica, California (KJH).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (KJH, GS); analysis and interpretation of data (PD); drafting of the manuscript (GS); critical revision of the manuscript (PD, KJH)

Correspondence: Giacomo Savini, MD, G.B. Bietti Eye Foundation-IRCCS, Via Livenza 3, 00198 Rome, Italy. E-mail: giacomo.savini@alice.it

Received: December 19, 2013
Accepted: March 05, 2013

Abstract

PURPOSE:

The AcrySof Toric intraocular lens (IOL) (Alcon Laboratories, Inc., Fort Worth, TX) is designed to correct corneal astigmatism ranging from 0.67 to 4.11 diopters (D). The authors reviewed the clinical outcomes of this IOL and investigated possible improvements of the online calculator provided by the manufacturer.

METHODS:

Review of published studies.

RESULTS:

The AcrySof Toric IOL can provide good results, although a mean overcorrection or undercorrection relative to the intended correction has been found by some authors. Stability over time has been reported to be excellent. Rotation occurs mainly in the first postoperative month and is greater in eyes with a longer axial length due to the larger capsule size. The online calculator of this IOL may be improved by considering the posterior corneal astigmatism and better calculating the conversion of the IOL cylinder from the IOL plane to the corneal plane, which may be inaccurate for two reasons. First, given the variable distance between the IOL and the cornea in short and long eyes, the fixed ratio (1.46) provided by the manufacturer cannot be used to calculate this conversion. Second, the online calculator does not take into account the effect of varying IOL sphere power.

CONCLUSION:

The AcrySof Toric IOL is a reliable choice to correct corneal astigmatism at the time of cataract surgery. Results will be improved once the online calculator by the manufacturer considers the posterior corneal astigmatism and the variable ratio between the toricity at the IOL and corneal plane.

[J Refract Surg. 2013;29(5):348–354.]

From G.B. Bietti Eye Foundation-IRCCS, Rome, Italy (GS, PD); and Jules Stein Eye Institute, University of California, Los Angeles, and St. Mary’s Eye Center, Santa Monica, California (KJH).

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (KJH, GS); analysis and interpretation of data (PD); drafting of the manuscript (GS); critical revision of the manuscript (PD, KJH)

Correspondence: Giacomo Savini, MD, G.B. Bietti Eye Foundation-IRCCS, Via Livenza 3, 00198 Rome, Italy. E-mail: giacomo.savini@alice.it

Received: December 19, 2013
Accepted: March 05, 2013

Corneal astigmatism requiring surgical correction is frequently seen in patients undergoing cataract surgery. In two large studies involving 4,540 and 7,500 eyes, a corneal astigmatism 1 diopter (D) or greater was found, respectively, in 34 and 32.3% of cases.1,2 The AcrySof Toric intraocular lens (IOL) (Alcon Laboratories, Inc., Fort Worth, TX) was introduced in 2005 for the correction of preexisting corneal astigmatism. In 2011, the aspheric model replaced the initial non-aspheric one. The IOL design is based on the one-piece AcrySof platform. Toricity is added to the posterior surface. The overall haptic length is 13.0 mm and the optic diameter is 6.0 mm. Toricity at the IOL plane ranges from 1 to 6 D, which according to the manufacturer corresponds to a range of 0.67 to 4.50 D (Table 1).

Range of Astigmatic Corrections Available for the AcrySof Toric IOLa

Table 1: Range of Astigmatic Corrections Available for the AcrySof Toric IOL

The purpose of this review is to analyze the clinical performance of this IOL in patients undergoing phacoemulsification/IOL surgery.

Preoperative Assessment of the Intended Astigmatism Treatment

The aim of toric IOLs is to maximally correct corneal astigmatism. The intended change in cylinder, which can be more appropriately defined as the target-induced astigmatism (TIA), as suggested by Alpins,3 is derived by adding (1) the preoperative corneal astigmatism and (2) the surgically induced change in corneal astigmatism (SICA). The resulting cylinder must be compensated for by the corneal plane cylinder equivalent power of the IOL.

Assessment of Preoperative Corneal Astigmatism

Preoperatively, corneal astigmatism can be measured by means of one of the several technologies currently available, such as manual keratometry,4–7 automatic keratometry (including IOLMaster; Carl Zeiss Meditec, Jena, Germany, and Lenstar; Haag-Streit AG, Köniz, Switzerland),7–15 and simulated keratometry (SimK) provided by either Placido-based corneal topography7,13,16 or Scheimpflug imaging (Pentacam; Oculus Optikgeräte GmbH, Wetzlar, Germany).5,10 Currently, there is no clear evidence that one instrument or technology is superior to the others. For example, Chang et al. did not detect any statistically significant difference among manual keratometry, the IOLMaster, Pentacam, and an autokeratometer.5 However, their study has some limitations, such as the small sample size and the fact that the SICA was not evaluated, and hence the results warrant further investigation. In support of the idea that no instrument is better than others in measuring anterior corneal astigmatism, Lee et al. recently concluded that measurements from autokeratometry, corneal topography/ray-tracing aberrometry, Scheimpflug imaging, and scanning-slit topography are comparable to those obtained with manual keratometry and can be used interchangeably with the latter.17

An important issue that has not yet received proper attention is the way astigmatism is calculated. In all published studies on toric IOLs, the preoperative corneal astigmatism was calculated solely on the basis of the anterior corneal curvature. Because the posterior corneal curvature could not be measured until Scheimpflug cameras became commercially available, its contribution to the total corneal astigmatism was usually estimated using the keratometric refractive index (usually 1.3375), which takes into account the negative dioptric power of the posterior corneal surface but leads to a fictitious value (the so-called “keratometric astigmatism”). This approach, which has been adopted over the past 50 years for IOL power calculation, may lead to suboptimal results in some eyes undergoing toric IOL implantation due to the influence of the posterior corneal curvature on the total corneal astigmatism.

Some of the studies reviewed underscored the importance of considering the posterior corneal surface when planning astigmatism correction with a toric IOL. In a sample of pseudophakic eyes with spherical IOLs, Teus et al. found that the corneal astigmatism calculated with the keratometric index (mean value: 0.91 ± 0.67 D) was higher than the refractive astigmatism (mean value: 0.64 ± 0.72 D).18 This means that the posterior corneal astigmatism partially offsets the anterior one. Ho et al. showed that, in eyes with more than 1 D of astigmatism, 5.9% had either a magnitude of keratometric astigmatism that differed from the total astigmatism (as measured by Pentacam) by greater than 0.5 D or keratometric astigmatism that differed from the total astigmatism axis by greater than 10°.19 Koch et al. found that 4.9% of eyes had a vector difference of more than 0.5 D when comparing astigmatism as measured by standard keratometry and total corneal power.20 Moreover, the difference in the location of the steep meridian between the keratometric astigmatism and the total corneal astigmatism is higher than 10° in 17.2% to 23.7% of eyes.19,20

It is logical to expect that preoperative measurement of total (rather than keratometric) astigmatism would lead to more accurate results with toric IOLs. Further studies are needed to demonstrate this.

Surgically Induced Change in Corneal Astigmatism

The original online AcrySof calculator suggested a SICA default value of 0.50 D, which had to be added to the preoperative keratometric astigmatism to calculate the TIA (ie, the vector of the intended change in cylinder). Adding the manufacturer-suggested SICA has been a common practice followed by a several surgeons.7,10,11,21

However, significant discrepancies are present among the SICA values reported in studies that have assessed this important variable. Using a 2.2-mm posterior limbal incision, for example, Ernest and Potvin6 reported a mean SICA of 0.25 ± 0.13 D in 38 eyes, whereas Hoffmann et al.14 reported a minimal SICA (−0.07 ± 0.42 D) in a sample of 40 eyes. Using a 3.0-mm limbal incision on the steep meridian, Bauer et al. obtained a larger mean SICA, ranging between 0.61 ± 0.43 for the T4 IOL and 1.24 ± 0.51 for the T5 IOL.12 Goggin et al. used a 2.2-mm clear corneal incision and observed a mean SICA of 0.81 ± 0.54 D in 38 eyes.8 Hence, we recommend that surgeons calculate the SICA according to the type (width and location) of incision they perform, rather than relying on the mean value suggested by the manufacturer. By doing so, they can achieve a more accurate estimation of the TIA. Interestingly, the default value (0.5 D) has disappeared from the online calculator of the most recent aspheric AcrySof Toric IOL ( http://www.acrysoftoriccalculator.com/aspheric/Calculator.aspx, accessed September 30, 2012).

Postoperatively, calculating the SICA is also a mandatory step when assessing the outcome of toric IOL implantation. Unfortunately, many of the studies considered did not report the SICA entered into the online calculator, nor did they calculate the SICA of the studied sample.4,16,22–24

A slightly different approach is used by Noel Alpins, MD, FACS, who suggests adding the corneal flattening (rather than the SICA) to the preoperative corneal astigmatism ( http://www.assort.com/ASSORT-toric-calculator.asp, accessed September 30, 2012). The rationale behind this choice is related to the fact that the SICA is not usually perpendicular to the corneal incision, whereas flattening measures the postoperative effect of the incision at the preoperative intended meridian, rather than comparing the preoperative and postoperative astigmatism at different meridians, which is the total SICA. As a consequence, the flattening effect of the incision is usually lower than the SICA (whereas the remainder is the torque that rotates the prevailing astigmatism, rather than reducing it).

Methods to Align the IOL at the Intended Axis

Aligning a toric IOL at the intended axis is essential to obtain the desired refraction. There are several methods to achieve alignment. Usually a three-step procedure is followed. First, the horizontal axis (0° to 180°) of the eye is marked preoperatively with the patient sitting upright to correct for cyclotorsion. This is usually done using a reference marker or a slit lamp with a rotating slit. Next, intraoperatively, the desired alignment axis for the toric IOL is marked with an angular graduation instrument. Finally, the toric IOL is implanted and rotated until the IOL markings are aligned with the alignment marks. This procedure has lead to a mean error in AcrySof Toric IOL placement of 5°.7

A more sophisticated technique for aligning toric IOLs relies on the Surgery Guidance SG3000 system (Sensomotoric Instruments, Teltow, Germany), which combines a Reference Unit and a Surgery Pilot. The Reference Unit is a noncontact device that is used outside the operating room to take the preoperative reference image. It acquires a digital image of the eye, where the limbal vessels, scleral vessels, and iris are shown in detail. Simultaneously, the unit performs keratometry using the optical reflections of 12 light-emitting diodes arranged in a 1.9-mm diameter ring. The keratometry results (including the steep and flat meridians of corneal astigmatism) and the position and diameter of the limbus and pupil are shown in an overlay on the digital image. The Surgery Pilot consists of a microscope camera adapter connected to a personal computer. The preoperative image is loaded into the computer, and the rotation angle between the preoperative image and the microscope image is automatically detected (based on the limbal and scleral vessels and on iris characteristics) and overlaid on the camera image. Intraoperatively, the eye tracker provides a real-time update of all overlaid features relative to the camera image.7

Other systems are currently available on the market, such as the Z-Align video system, which is part of the Callisto Eye digital surgical data system (Carl Zeiss Meditec), and the Orange system (WaveTec Vision, Aliso Viejo, CA), but no clinical studies have been performed to assess their efficacy for toric IOL implantation.

Methods to Measure the IOL Position after Surgery

When calculating the refractive result of toric IOL implantation, a mandatory step is to measure the IOL position after surgery and any possible rotation, which will lessen the astigmatic correction. The simplest method is to note the IOL position using slit orientation referenced to the degree scale on the slit lamp.5,8,9,12 Alternatively, a slit lamp with reticle with 5° markings can be used.4 Some surgeons have advocated relying on objective rather than subjective methods. One example is wavefront analysis with the Nidek OPD-Scan refractive power/corneal analyzer system (Nidek, Fremont, CA), which can calculate the lenticular astigmatism (and thus the IOL orientation) by subtracting whole eye aberrations from corneal aberrations.25 A possible drawback of this instrument is the lack of information about the posterior corneal surface astigmatism, whose magnitude and axis are not calculated and are included in the lenticular astigmatism.

More sophisticated methods include using a digital retroillumination image of the IOL and superimposing this image on an angular grid included in specific software such as Adobe Photoshop (Adobe Systems, San Jose, CA).14,26

Methods to Calculate Astigmatic Changes Following Surgery

Astigmatic changes following implantation of toric IOLs must be evaluated by means of vector analysis. Two approaches have gained popularity for this purpose, namely, vectorial analysis according to Alpins,3,27 which has been used by some authors,7,8,14,16,28 and power vectors J0 and J45, as described by Thibos and Horner,29 which have been used by others.21–24,30,31 As the main advantage, the former method allows surgeons to assess several parameters (such as flattening, torque, angle of error, and magnitude of error) involved in the correction of astigmatism but requires specific software. On the contrary, the latter, which can be easily performed on a spreadsheet, makes it possible to calculate any difference between preoperative and postoperative astigmatism but does not provide the above-mentioned parameters.

Many studies do not analyze the vectorial changes and just report the postoperative mean arithmetic refractive cylinder along with corrected and uncorrected best visual acuity.10,13,32 However, this approach does not allow us to fully understand the refractive changes following the implantation of toric IOLs (eg, overcorrection or undercorrection of astigmatism).

Outcomes Using the Alcon Web-Based Toric IOL Calculator

The AcrySof Toric IOL was first tested in a randomized prospective multicenter U.S. Food and Drug Administration study involving 494 patients and 11 investigators (Available at: http://www.accessdata.fda.gov/cdrh_docs/pdf/P930014S015b.pdf, accessed August 14, 2012). The study found a significant reduction in the refractive cylinder, an improvement in uncorrected visual acuity, and good rotational stability at 6 months. Other studies have since reported excellent refractive results with this lens.4,6–8,10,12,13,16,21,23–25,28 The AcrySof Toric IOL has been shown to provide a more predictable and effective correction of astigmatism compared to peripheral corneal relaxing incisions or opposite clear cornea incisions.22,30Table 2 shows the mean preoperative and postoperative astigmatism.

Reported Outcomes for Astigmatism Correction Using the AcrySof Toric IOL

Table 2: Reported Outcomes for Astigmatism Correction Using the AcrySof Toric IOL

It is not yet clear whether the AcrySof Toric IOL results in an overcorrection or undercorrection of presurgical astigmatism. Although Visser et al. did not observe any overcorrection or undercorrection, on average, in a sample of 35 eyes,28 other authors have reported a slight undercorrection of the astigmatism. In a sample of 27 eyes (21 patients) that underwent implantation of the toric models T3 to T7, Alió et al. calculated (using the Alpins vectorial method) a mean undercorrection of approximately 0.30 D relative to the intended correction.16 They attributed this under-correction to a slight misalignment of treatment. Similarly, in a sample of 38 eyes (29 patients) Goggin et al. calculated (using the same vectorial method) a mean undercorrection of approximately 0.40 D.8 They related the undercorrection to three possible causes: the nonzero astigmatic targets imposed by the steps between IOL cylinder powers provided by the manufacturer, the fairly predictable corneal SIA, and the inaccuracy of the corneal plane cylinder equivalent power of the IOL. On the the other hand, Hoffmann et al. found that overcorrection and undercorrection were evenly distributed, notwithstanding slight attempted undercorrection.14 Unfortunately, many of the studies reviewed did not include an investigation of overcorrection or undercorrection.13,30

Other interesting results have been reported. The AcrySof T3 Toric IOL is useful in eyes with low (< 1.50 D) corneal astigmatism compared to the spherical AcrySof because it allows a lower postoperative refractive astigmatism to be achieved.6,31 Ernest and Potvin reported that the axis of the preoperative corneal astigmatism had no effect on the postoperative refractive astigmatism, but this result should be considered with caution because they did not report the refractive results with and without the SICA.6

Possible Improvements of the Alcon Web-Based Toric IOL Calculator

An IOL with a given cylindrical power will correct a variable amount of cylinder at the corneal plane. This variability mainly depends on the distance between the two lenses (ie, the cornea and IOL).33 Goggin et al. pointed out that the Alcon web-based toric IOL calculator does not take into consideration the distance between the corneal and IOL planes when calculating the corneal plane cylinder equivalent power of the IOL.34 In fact, the manufacturer gives a single corneal plane cylinder for each IOL cylinder power; this value is “based on the average pseudophakic eye” and depends on a fixed ratio (1.46) between the cylinder power in the IOL plane and the cylinder power in the corneal plane. For example, the T9 model is labeled with a corneal plane cylinder of 4.11 D for both an IOL with a spherical power of 6 D and an IOL with a spherical power of 30 D. Because short hyperopic eyes and long myopic eyes usually have shallow and deep anterior chambers, respectively, with resulting differences in the distance between the corneal and IOL plane, using a fixed 1.46 ratio can lead to inaccurate outcomes in eyes that are far from the average model.

Goggin et al. described an improved method to calculate the corneal plane cylinder equivalent power of the IOL by means of a thick lens vertex power formula, which contains the data of anterior chamber depth and corneal pachymetry. They found that the SIA could be better predicted using this approach rather than the Alcon online calculator.34 However, some inconsistencies in their method have been reported by several authors.35–37 A better solution had been previously described by Fam and Lim,38 who based their calculation on a thin-lens formula for IOL power calculation, the Holladay 1 formula.39 They suggested calculating the IOL power for the steep and flat meridians separately: the difference between the two values is the required IOL toricity for that eye, on condition that the anterior chamber depth is also separately calculated using the mean corneal power.38 Similarly, Hoffmann et al. calculated the required IOL cylinder as the difference between the IOL power required for the steepest and flattest corneal meridians, although they did not maintain a constant anterior chamber depth.14

However, there is no demonstration to date that any of these alternative approaches lead to more accurate results than the standard online calculator provided by the manufacturer. Interestingly, the online toric calculators of other manufacturers (eg, Abbott, Carl Zeiss Meditec) do not omit the anterior chamber depth influence on the corneal plane cylinder equivalent power of the IOL. Similarly, the Assort calculator developed by Noel Alpins, MD, FACS ( http://www.assort.com/ASSORT-toric-calculator.asp, accessed September 30th, 2012) takes into account the predicted IOL position to convert the cylinder at the IOL plane to the cylinder the corneal plane.

Another interesting issue that has received little to no attention is the variation of the IOL corneal plane equivalent cylinder power depending on the sphere power of the IOL. Even with constant anterior chamber depth and constant IOL plane cylinder power, a different corneal plane equivalent cylinder power is achieved by two IOLs with different sphere power due to the different vergence of rays.

Rotational Stability

Several studies have reported an excellent rotational stability of the AcrySof Toric IOL. Chang showed that such stability was higher in comparison to a plate-haptic silicone IOL and attributed this result to the ability of the hydrophobic acrylic material to adhere to the posterior capsule.9 Similar results were later reported by other authors.40 The mean IOL rotation of AcrySof Toric IOLs during the first 6 months was observed to range between 0.9° and 5.06°.4,12,16,21,41,42 A rotation greater than 10° occurred in between 0% and 6.7% of eyes according to several studies.4,9,12,13 One of the cited authors subsequently reported that a rotation greater than 15° had been observed in 3 of 263 cases.43 Rotation occurs mainly in the first postoperative month and is greater in eyes with a longer axial length due to the larger capsule size.42

In the few cases requiring IOL repositioning, Chang suggested reinflating the capsular bag with balanced salt solution through a 30-gauge cannula via a paracentesis site, without using any ophthalmic viscosurgical device, and then rotating the IOL into the proper alignment using the same cannula tip.43

Conclusion

The AcrySof Toric IOL enables predictable and stable correction of preexisting corneal astigmatism. Future studies should address some important issues. First, the influence of posterior corneal astigmatism on the clinical outcome of toric IOL implantation: does taking it into consideration really improve our results? Second, the amount of SICA related to the implantation of the AcrySof Toric IOL and which factors influence it. Finally, the accuracy of the online calculator provided by the manufacturer and how it may be improved by taking into account the distance between the IOL and the cornea.

References

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Range of Astigmatic Corrections Available for the AcrySof Toric IOLa

AcrySof ModelCylinder Power at IOL Plane (D)Cylinder Power at Corneal Plane (D)
T21.000.67
T31.501.03
T42.251.55
T53.002.06
T63.752.57
T74.503.08
T85.253.60
T96.004.11

Reported Outcomes for Astigmatism Correction Using the AcrySof Toric IOL

StudyNo. of EyesPreoperative Cylinder (Mean ± SD)Postoperative Refractive Cylinder (Mean ± SD)aP
Ruíz-Mesa et al.24322.46 ± 0.99 D (refractive)0.53 ± 0.30 DNA
Cervantes-Coste et al.10194.00 ± 1.10 D (keratometric)0.55 ± 0.60 D< .0001
Alio et al.16272.87 ± 0.78 D (refractive)0.95 ± 0.40 D< .01
Ahmed et al.132341.70 ± 0.40 D (refractive)0.40 ± 0.40 D< .001
Carey et al.25512.29 ± 0.89 D (refractive)0.81 ± 0.59 DNA
Mendicute et al.22302.34 ± 1.28 D (refractive)0.72 ± 0.43 D< .10
Koshi et al.21301.97 ± 0.58 D (keratometric)0.80 ± 0.39 DNA
Hoffmann et al.14433.49 ± 1.31 D (refractive)0.67 ± 0.39 DNA
Mingo-Botín et al.30201.89 ± 0.57 D (refractive)0.61 ± 0.41 D.000

10.3928/1081597X-20130415-06

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