Role of Asphericity in Choice of IOLs for Cataract Surgery

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1 Role of Asphericity in Choice of IOLs for Cataract Surgery Delhi J Ophthalmol 2015; 25 (3): DOI: Aman Khanna, Rebika Dhiman, Rajinder Khanna, Yajuvendra Singh Rathore, Spriha Arun Khanna eye centre E-368 Nirman Vihar, Vikash Marg, New Delhi Dr Rajendra Prasad Centre for Ophthalmic Sciences All India Institute of Medical Sciences, New Delhi, India Address for correspondence Advances in surgical techniques, as well as the invention of new intraocular lens (IOL) materials and designs, have increased expectations from cataract surgery to beyond simply providing good visual acuity. The cornea itself induces some degree of positive spherical aberration which is compensated by the negative spherical aberration of the clear crystalline lens. However, this compensation gradually decreases as the crystalline lens ages and particularly after cataract extraction and intraocular lens implantation. Conventional spherical IOLs add positive spherical aberration to the pre-existing aberrations caused by the cornea, increasing the total spherical aberration of the eye. One key factor contributing to postoperative spherical aberration is IOL design which has undergone dramatic changes to compensate for the positive corneal spherical aberration, with the emergence of aspheric IOL s it has become a challenging task for ophthalmologists to choose the appropriate IOL from a wide variety available. This review article will provide information regarding asphericity and different aspheric IOL s available and application of different strategies while choosing specific IOL. Keywords : aspheric IOLs cataract surgery spherical abberations Aman Khanna MS Khanna eye centre E-368 Nirman Vihar, Vikash Marg, New Delhi dramanrkhanna@gmail.com With recent modifications and improvements in surgical techniques, biometry, and intraocular lens (IOL) technology, cataract surgeons have been capable of consistently achieving highly accurate quantitative refractive results following lens replacement surgery. The modern cataract surgeon, however, is now embarking on the quest for perfect vision beyond a simple 20/20 standard, making cataract surgery a form of refractive surgery. This does not mean getting the patient to 20/10. Rather, it means that the focus is now shifting from visual acuity to other parameters, such as contrast sensitivity and glare disability, in order to achieve the highest possible quality of vision. Cataract surgeons are becoming refractive surgeons, and IOL manufacturers are beginning to incorporate advanced refractive technology towards the same objective.aspheric IOLs are the first new technology IOLs to reflect the refractive shift in cataract surgery. Asphericity Asphericity is a measure of shape of a refractive medium and how it affects bending of light, when light passes through an optical medium, the shape of the optical media affects where the central and peripheral rays eventually focus. Descriptively, shape of refractive surface can be denoted as, spherical, prolate aspheric or oblate aspheric. A sphere is perfectly round a prolate asphere is steeper in the centre and flatter in the periphery whereas, oblate asphere is flatter in the centre and steeper at the periphery. Indirectly asphericity, can be correlated to spherical aberration (SA). Spherical aberration is one of the many higher order aberrations, but found out to be one of the most significant in degrading the quality of vision. It is fourth order aberration measured in microns as root mean square (RMS). When parallel rays of light passes through a spherical medium, the central rays focus more posteriorly, while the peripheral rays focus more anteriorly in an even fashion, making the SA mildly positive, as represented in (Figure 1). In a prolate asphere, the peripheral rays owing to flatter periphery focuses more posteriorly thus more coincident to central rays, as represented in (Figure 2). So by maintaining or inducing prolateness there is reduction in SA, which further increases the quality of the vision and sharpens the focus. Two parts in the eye that exerts asphericity are the human lens and the cornea:- Importance of asphericity Glasser and Campbell had shown that 185

2 Khanna A et al ISSN Figure 1 : Diagram demonstrating multiple focal points caused by spherical aberrations in lenses. spherical aberration (SA) of the crystalline lens changes considerably with age, moving from a negative SA value to a positive one1. Jack Holladay further demonstrated that side effects of myopic LASIK were likely due to the fact that the procedure turned a prolate human eye into an oblate one, with a sphericity or Q-value more akin to that of a frog than of a predator eagle. 2 The average sphericity of the normal human cornea is positive and remains stable throughout life, but the lens SA changes with age. The cornea has a slightly positive spherical aberration of about 0.3 microns. The positive spherical aberration of the cornea is compensated, in a young person s eye, with the negative spherical aberration of the lens (-0.2 microns). Recent studies have found that a slight positive aberration (+0.10µm) is beneficial and it is associated with a better visual acuity. With advancing age, the crystalline suffers a decrease in its negative spherical aberration, which reaches values close to zero at age 40, henceforth it can also become positive. As a result of these changes, the total spherical aberration of the eye becomes more positive as the spherical aberration of the crystalline not only does not compensate the positive aberration of the cornea, but also adds to it, reducing contrast sensitivity. In the young eye, the negative SA of the crystalline lens balances the positive SA of the cornea, resulting in zero or very low total ocular SA. 3 In older eyes, the crystalline lens loses the ability to compensate for corneal SA, total ocular SA becomes increasingly positive, and the resulting aberrations cause blurred vision and reduced contrast sensitivity, affecting functional vision. Functional vision is reduced as the aging crystalline lens loses the ability to compensate for corneal spherical aberration. The aging eye has positive spherical aberration which causes blurred vision and reduces he contrast sensitivity and also the functional vision. We also know that with age, contrast sensitivity decreases, first at the higher spatial frequencies, then at all the spatial frequencies. 4 Multiple studies show peak visual performance occurs at age 19, when the average spherical aberration is 0.0 microns. As the spherical aberration increases with age, contrast sensitivity decreases. The loss of functional vision can decrease quality of life and compromise driving safety even with continued good Snellen acuity and of course, the onset of cataract exacerbates any pre-existing functional vision problems. Traditional spherical IOLs typically add positive Figure 2 : Diagram demonstrating a perfect lens without spherical aberration focusing all incoming rays to a single point on the optic axis SA, keeping total SA similar to that found in the aging natural lens. Some people have argued that an advantage of positive SA in the aging eye is an increased depth of focus. The corollary to that, of course, would be that sharpening distance vision by correcting SA with an aspheric IOL might worsen near and intermediate vision. Certainly, this will be a concern for anyone who wants the patients to be satisfied with their entire visual experience after IOL surgery. However, several recent publications refute this argument. Jack Holladay pointed out that spherical and aspheric lenses do not differ at all in the depth of focus, but only in the clarity of best focus. 5 Additionally, it is said that slightly negative SA may actually have an accommodative effect when the pupil constricts for near tasks, depending on the lens that is used. Nishi et al also showed a significant negative correlation between range of accommodation and SA. 6 In other words, lower SA is correlated with better accommodation. Wang and Koch recently demonstrated that when all aberrations are corrected, eyes with zero SA have the best depth of focus. 7 If SA was not zero, they also found that slightly negative SA, rather than slightly positive SA, provided better depth of focus. In the years since the correction of spherical aberration was introduced in , numerous strategies have been advocated for the implementation of this concept. The compelling need to do something was highlighted by a study that found that the contrast sensitivity of pseudophakic individuals was not significantly different from age-matched phakic patients, but was significantly worse than that of younger phakic subjects. 9 After performing various studies it was found that corneal topography values of 71 cataract patients showed that the average SA of the human cornea was microns. 8 This was subsequently confirmed in several other studies. 10,11 A model cornea based on these measurements was used to design IOLs having a fixed amount of negative SA to compensate for the positive SA of the average human cornea. Types of aspheric lenses In this article the currently available and popular aspheric lenses that are in the market and FDA approved are described. These include Tecnis (Advanced Medical Optics), 186 Del J Ophthalmol 2015;25(3)

3 Asphericity in choice of IOL the SofPort AO (Bausch & Lomb) and the AcrySof IQ (Alcon Laboratories). The three lenses have different strategies for correction of SA. The Tecnis has a negative SA of 0.27 µm. The SofPort AO lens has an effective SA of almost zero (the SofPort AO is designed to correct the SA of the isolated lens, as such when it is inserted into the eye; it contributes small amounts of positive SA due to the converging incident light from the cornea.). The AcrySof IQ has a negative SA of µm. Thus, if one considers that the average corneal SA of the population is µm, then the SofPort AO does not change this value significantly, whereas the Tecnis corrects this SA fully and the AcrySof IQ compensates to a lesser degree. Tecnis Z The Tecnis IOL was designed with a modified prolate anterior surface to compensate for the average corneal spherical aberration found in the adult eye. It introduces -0.27mm of spherical aberration to the eye measured at 6mm optical zone. The clinical investigation of the Tecnis IOL submitted to the US Food and Drug Administration (FDA) demonstrated elimination of mean spherical aberration as well as significant improvement in functional vision when compared with a standard spherical IOL and it was independent of age. 12 A prospective randomized study showed a nearly 78% gain in peak contrast sensitivity with the new lens, with mesopic contrast sensitivity approximately equivalent to photopic contrast sensitivity with a spherical lens13. Early European studies also showed that it could improve visual quality. 14,15 Acrysof IQ IOL - The lens has an aspheric posterior optic design with a thinner center. It induces microns of SA, compared to the microns induced by the Tecnis lens. Some studies have shown that Navy aviators with excellent visual abilities have small amounts of SA, so theoritically, leaving a small amount of residual SA might be a good thing. However, Steve Schallhorn, who conducted the pilot studies, continues to believe that striving for zero SA remains the most effective target. In his aviator studies, those subjects with SA closer to zero had better mesopic contrast acuity than their fellow pilots with higher SA. 16 Doug Koch recently reported that even though optimal ocular and IOL SA varies widely among eyes, most emmetropic eyes achieved the best image quality with a 6.0-mm pupil when total ocular SA is between to 0.00 microns. 7 In a recent study McCulley and colleagues showed that the Acrysof IQ aspheric lens reduces the positive ocular spherical aberration observed in pseudophakic and elderly eyes, especial ly at larger pupillary diameters (6 mm), with no notable increase in coma. 17 With a 6.0-mm pupil, total SA post-implantation was very close to predicted levels, at 0.09 ± 0.04 microns, compared to 0.43 ± 0.12 microns for patients implanted with Acrysof spherical IOLs(p<0.0001). Sofport AO This IOL has been specifically designed with zero spherical aberration so that it will not contribute to any preexisting higher-order aberrations. As the AO lens has no relationship to the average or actual SA in the eye, it may be less dependent on centration. Nichamin and colleagues found that the optical performance of a model eye was not affected by decentration of the AO, even when the lens was decentered by as much as 1.00 mm. 18 Tolerance levels for the Tecnis aspheric lens require that it be decentered less than 0.4 mm and tilted less than 7 degrees in order to provide optical performance superior to that of a spherical lens. Newer studies have shown that the above values applied to monochromatic light only. In a more real-world situation where polychromatic light is present, the above values nearly double, with about 0.8 mm of decentration and more than 10 degrees of tilt being tolerated19. A number of published studies over the past decade or more have shown that with a continuous curvilinear capsulorrhexis and inthe-bag IOL placement, modern cataract surgery is typically well within such tolerance limits. 20,21 In fact, the optical advantages of aspheric IOL technology have become fairly well accepted although some controversy remains in the areas of functional benefit as it relates to pupil size, IOL decentration, depth of focus and customization22. Some studies 23,24 have shown little or no benefit of aspheric IOLs with smaller pupils, while one laboratory study18 showed that the SofPort AO provides better optical quality than either a negatively aspheric or a spherical IOL under conditions of significant decentration. Strategies while implanting aspherics With the passing years and increasing technology, the question arising for ophthalmologist is customisation of aspheric IOL. Dr. Devgan in his study How to choose an aspheric lens has described an algorithm - Devgan s decision tree 25 (Figure 3). Dr. Beiko in his research of factors having an impact on choice of IOL s in cataract surgery came up with various stratagies for choosing an aspheric IOL. 26 Strategy: Same Aspheric for all Patients This approach certainly has lot of evidence in support. The literature on the AMO Tecnis IOL contains numerous peer-reviewed, prospective, randomized papers that found spherical aberration was reduced or eliminated when a modified, prolate anterior surface IOL is compared to a standard spherical IOL. Many of these studies have found that the functional vision is superior with the Tecnis IOL when contrast sensitivity testing is performed Similarly, studies have also found superior vision with the Acrysof IQ compared to standard lenses. 30,31 The Sofport AO has been shown to provide superior visual function to both an aspheric lens and a standard lens when laboratory conditions of decentration are studied. 18 Piers and coauthors 32 utilized an adaptive optics simulator to assess letter acuity and contrast sensitivity for two different values of spherical aberration. The first condition was the average amount of spherical aberration measured in pseudophakic patients with spherical IOLs. The second condition represented the complete correction of the individual s spherical aberration (Z[4,0]¼0). The researchers found an average improvement in visual acuity associated with the correction of spherical aberration of 10 and 38% measured in white and green light, respectively. Similarly, average contrast-sensitivity measurements improved 32% and 57% in white and green light. When spherical aberration was corrected, visual performance was as good as or better than for the normal 187

4 Khanna A et al ISSN diameter; 2. Application of a software package such as VOL- CT to transform the topography elevation data into preoperative corneal Zernike coefficients, with special attention to Z[4,0] fourth-order spherical aberration at the 6mm optical zone; 3. Application of an IOL calculation formula, such as the Holladay 2, to determine correct IOL power for the desired postoperative spherical equivalent; 4. Determination of desired postoperative total ocular spherical aberration and selection of IOLtype. Figure 3 : Diagram representing Dr Devgan s Decision Tree for the selection of aspheric lenses spherical aberration case for defocus as large as + -1D. Therefore, these researchers concluded that completely correcting ocular spherical aberration improves spatial vision in the best-focus position without compromising the subjective tolerance to defocus. On the other hand, it has alternatively been suggested that providing Z[4,0]¼ þ 0.1mm of postoperative spherical aberration represents a better choice. 33 This line of reasoning originated from a study 34 demonstrating that 35 young subjects with uncorrected visual acuity of 20/15 or better had a mean total spherical aberration of Z [4,0]¼þ0.110_0.077mm. Recently, Beiko et al 35 presented data from a series of 696 eyes confirming the mean corneal spherical aberration of þ 0.27 mm used in the design of the Tecnis IOL. In this study it was concluded individuals should be measured to determine their unique value when considering correction of this aberration. In addition, they noted that keratometry and the corneal Q value do not correlate well with spherical aberration, and that therefore corneal spherical aberration must be measured directly with a topographer. The main criticism of the one lens for all patients strategy is that it is akin to using a single-powered IOL in all cases of aphakia, irrespective of the parameters of the eye being implanted. This is a good first step, but experience shows that selecting the appropriate power of IOL provides superior uncorrected vision. Similarly, it should not be a huge leap of faith to accept that selecting an appropriate aspheric correction should provide the best functional vision. One method of proceeding with customized selection of aspheric IOLs involves the following protocol 36 : 1. Preoperative testing to include corneal topography as well as axial length determination, anterior chamber depth, phakic lens thickness and corneal white-to white Strategy: Target Aspheric Correction Although the average corneal SA for the population is µm, the standard deviation is large and approaches 0.10 µm or one-third of the value. The implication of this is that there is a wide spread in the population of the value of the SA and it cannot be assumed that the individual patient undergoing surgery has the average value. In fact, there is even further evidence of a racial difference in this value; Asian eyes have been measured to have an average corneal SA of µm. 37 Reports have been presented that support this concept as being workable; measurement of the pre op corneal asphericity was followed by implantation of an aspheric lens with a specific target value in mind. Postoperatively, the SA was measured with a wavefront analyzer and found to be near the predicted value. 38 Unfortunately, since each lens design comes in only one SA power, this targeting approach has limited success. If one is to use a single lens design, then the following strategy has merit. Strategy: Match Corneal SA and Refraction for Optimization of Vision It has been known for a while that different optical aberrations interact to affect visual performance. However, the nature of this interaction has been elusive. The first indication of an interaction was reported at ARVO in It was found that patients following corneal refractive surgery preferred 0.25 D of myopia for each µm of SA. More recently, attempts have been made to determine the optimal amount of SA in an IOL. Using data from 154 eyes, Wang and Douglas Koch simulated implantation of lenses with different amounts of SA and with different amounts of defocus. They found that the maximum image quality depended on an interaction between the residual SA and the defocus. For plano defocus, a SA of µm was found to be ideal; for myopia of -0.5 D, a SA of µm; and for hyperopia of +0.5 D, a SA of -0.2 µm was found to give best image quality Conclusion As technology continues to advance and our patients broaden their demands for optimal visual quality after surgery, the customized selection of an IOL not only with respect to spherical aberration but also toricity, multifocality, and color (selective light filtering) is becoming more important. This has become a large but exciting challenge for 188 Del J Ophthalmol 2015;25(3)

5 Asphericity in choice of IOL both surgeons and the ophthalmic industry. Aspheric IOLs can potentially provide superior optical quality, especially in low light and low contrast situations. Given the relatively small differences between aspheric lenses not to mention between standard and aspheric lenses deciding which one to implant in a given patient can be a challenge. However, many surgeons are using the specific asphericity of each lens as a way to tailor the choice to the individual s optical system. But few questions still remain to be answered- Do the benefits outweigh the extra cost and time? What is the optimal desired amount of spherical aberration to be corrected? But with the modern cataract surgeons, now embarking on the quest for perfect vision we can say one thing for sure that the Aspheric IOL are here to stay. Financial & competing interest disclosure The authors do not have any competing interests in any product/ procedure mentioned in this study. 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