The Aberration-Free IOL:

Transcription

1 The Aberration-Free IOL: Advanced Optical Performance Independent of Patient Profile Griffith E. Altmann, M.S., M.B.A.; Keith H. Edwards, BSc FCOptom Dip CLP FAAO, Bausch & Lomb Some of these results were presented at the 2004 Symposium on Cataract, IOL and Refractive Surgery, San Diego, Calif., May Griffith Altmann is an optical engineer for Bausch & Lomb, Rochester, N.Y. Keith Edwards, is a senior clinical research professional at Bausch & Lomb Rochester N.Y. October 2004

2 Overview Wavefront-based methods for measuring visual performance have led to the design and marketing of aspheric intraocular lenses that induce negative spherical aberration, including the Tecnis Z9000 and Z9001 (Advanced Medical Optics) and the Acrysof SN60WF (Alcon Laboratories). As developed originally for the Z9000, this technology uses a prolate anterior surface of the IOL to compensate for the positive spherical aberrations of an average cornea. Basic and clinical studies have shown this can improve contrast sensitivity. However, common levels of IOL decentration or tilt in vivo induce optical 1, 2, 3, 4, aberrations that can compromise visual performance. A new aberration-free IOL technology, with prolate anterior and posterior lens surfaces and no inherent spherical aberration, is being tested by Bausch & Lomb. Optical simulation testing and computer modeling indicate that, even when decentered 1 mm, this new SofPort Advanced Optics (SofPort AO) design for an IOL performs better optically than does a well-centered conventional IOL. Because the SofPort AO design does not introduce spherical or monochromatic higher-order aberrations (HOAs), it also avoids degradation of the retinal image in the presence of mild to moderate decentration and moderate tilt. Computer modeling show this gives the SofPort AO lens design better optical performance overall in a wide range of situations than a negative-spherical-aberration aspheric IOL. Consequently, the SofPort AO technology offers the combination of predictability and optimal optics for all pseudophakic patients, in an easy-to-use, familiar IOL type that is designed to eliminate post-operative surprises. Background In recent years, the adoption of ocular wavefront measurement in clinical ophthalmology has moved the signposts of good vision beyond 20/20 acuity. Studies of wavefront-based LASIK demonstrate that limiting or reducing higher-order aberrations (HOAs) gives patients better contrast sensitivity, and can correct the light-scatter within the ocular system from aberrations such as 5, 6, 7 coma. This improves the quality of the patient s vision, particularly at night. Conventional intraocular lenses also add HOAs and spherical aberration to the visual system, limiting the quality of vision possible with pseudophakia. 4, 8, 9, 10, 11, 12, 13 This knowledge has led to increasing speculation in the IOL world about using wavefront-based methods to improve post-operative contrast sensitivity with customized or wavefront-adjusted IOLs. 14, 15 These new IOLs would need to correct the higherorder, non-axisymmetrical aberrations in the patient s visual system as well as the spherocylindrical refractive error. consider the basic tenet of medicine primum non nocere. before choosing a new IOL design for their patients. Basic optical principles suggest that decentration or tilt of an aspheric IOL with negative spherical aberration might result in a significantly lower optical transfer function than equally decentered standard lens implants, particularly at higher spatial frequencies. 1, 2, 3, 4, 16 This is in part because of induced second- and third-order aberrations, such as astigmatism and coma. Recent reports on negativespherical-aberration aspheric IOLs, detailed below, have confirmed that this optical degradation can occur with this type of IOL design. Aspheric Designs Artal et al. reported in 2002 that the HOAs of the cornea are compensated for in young adults with negative spherical aberration in the rest of the ocular system. 17 Their wavefront analysis in 17 people showed that younger subjects had more corneal HOAs than total ocular aberrations, while the reverse was true for the elderly. They suggested that negative spherical aberration of internal ocular surfaces declines with age. The researchers postulated that the best retinal image for an elderly pseudophake would come from an IOL that took this imbalance in corneal and internal HOAs into account. This year, Amano et al. confirmed an increase in positive spherical aberration in the visual system with age. 18 In 75 eyes of 75 patients, the researchers found that the corneal spherical aberration did not increase with age, but the total ocular spherical aberration did. They attributed the increase to non-corneal, internal changes in the eye, which also increased the patients ocular coma. The Tecnis lens features a prolate anterior surface optimized to neutralize the average spherical aberration found in the eyes of the 71 subjects in the design study. 1 Several studies of this aspheric IOL have confirmed the expected increase in contrast sensitivity, particularly in mesopic conditions, although this has not always been clinically or statistically significant. 1, 2, 3, 4 As a result, other negative-spherical-aberration aspheric IOLs are on the horizon for clinical use. However, ophthalmologists who want to offer their cataract patients wavefront-optimized postoperative vision also must

3 Effects of Decentration Perfect centration of an IOL is rare, and the degree of centration can change over time as the capsule contracts after cataract surgery. In patients without weak zonules or another predisposing condition, the reasons for decentration include in-out of the bag placement, incongruency between bag diameter and overall diameter of lens, large capsulorhexis, asymmetrical capsular coverage, lens placement in sulcus, capsular fibrosis, capsular phimosis and radial bag tears. Decentration of a lens with either positive or negative spherical aberration induces defocus, astigmatism and coma, the magnitude of which depends on the magnitude of the inherent spherical aberration, according to 3, 19, 20 theoretical studies of conventional IOLs. IOL decentration appears to occur more often in silicone IOLs than with those made of other materials. In a 2003 survey of cataract surgeons, silicone IOLs were the most likely material to require explantation, accounting for 27% of all explants. 21 Among 3-piece silicone IOLs, dislocation/decentration was blamed for 34% of the explants of this IOL type. The optical effects of decentration of an IOL can increase when the lens has an aspheric design with negative spherical aberration. In detailing the design of the first such IOL, Holladay et al. noted that the lens s optical performance degraded in the presence of tilt and decentration, particularly the latter. 1 The modulation transfer function (MTF) values for the 5 D to 20 D powers of the IOL fell below those for a conventional IOL at 0.5 mm of decentration. The cutoff was above 0.5 mm for the 30 D Tecnis. With tilt, an angle of greater than 7 degrees reduced the MTF to below that of a conventional IOL. Even at lower levels of decentration, though, there was a measurable increase in total ocular aberrations with the negativespherical-aberration aspheric design, German surgeon Martin Baumeister, MD, has reported. 16 In a bilateral study of 10 patients, he and colleagues compared decentration and resulting postoperative HOAs between the spherical AR40e (Advanced Medical Optics) and the prolate Tecnis. The mean Tecnis decentration 3 months after surgery was 0.3 +/- 0.2 mm, equivalent to the degree of decentration seen in the AR40e. However, only in the Tecnis did the extent of decentration correlate with greater higher-order root mean square HOA values, Dr. Baumeister reported. This was due to an increase in thirdorder aberrations (i.e., coma, which cannot be corrected with spectacles 22 ). In one of the 10 eyes in which it was implanted, the aspheric IOL was decentered by 1 mm. Mean tilt was 2.2 +/- 1.2 degrees, 25% lower than the mean for the AR40e. More recently, Koch and Wang completed a computer simulation confirming that coma would be the greatest aberration induced by a decentered aspheric IOL. 23 They concluded that, with perfect centration, only 10% to 44% of eyes implanted with a wavefront-guided or negative-spherical-aberration aspheric IOL would measure in the lowest 10th percentile of corneal HOAs. For a 4 mm pupil, the decentration would have to be less than 0.34 mm to reduce total HOAs below the level of corneal HOAs in 50% of the eyes, they calculated. It is important to recognize that studies of HOAs from IOL decentration generally are computer simulated with an idealized model eye, in which the visual axis is centered in the pupil. In vivo, however, the various components of the visual system rarely are aligned either with each other or with the actual visual axis, reducing the quality of the retinal image. 24 This would suggest that, in clinical use, the optical performance of a negativespherical-aberration aspheric IOL might be impacted by smaller levels of clinically observed decentration than simulations would indicate. Indeed, even a lens perfectly centered within the capsular bag may be substantially decentered with respect to the visual axis. The mean difference between the location of the achromatic axis and the center of the pupil in young adults is 0.37 mm, with a standard deviation of 0.24mm, according to Rynders et al. 25 In addition, the visual axis can shift as a result of changes in pupil size and shape under varying light levels. A Different Approach: SofPort TM Advanced Optics A conventional IOL adds positive spherical aberration to that which already exists in the cornea. Aspheric IOLs currently in use impart negative spherical aberration, ideally neutralizing the cornea s average level of positive aberration. Rather than trying to correct for this positive aberration, the SofPort Advanced Optics (AO) method for reducing HOAs in the pseudophakic eye avoids introducing any spherical aberration at all. SofPort AO lenses have prolate anterior and posterior surfaces and no inherent spherical aberration. The SofPort AO technology also does not affect the other structural or mechanical characteristics of the B&L lenses to which it is being added, so implanting a SofPort AO-IOL will require no new techniques or equipment for surgeons. The effectiveness of the SofPort AO approach was recently demonstrated using the same experimental model that was originally used to validate and design the Tecnis. 1 In these ray-tracing experiments, a SofPort AO version of Bausch & Lomb s LI61U silicone IOL was tested against a conventional LI61U SE and an aspheric lens with negative spherical 26, 27, 28 aberration. The simulations predicted, at a wide variety of pupil sizes and IOL positions, that the SofPort AO design would produce better vision for patients than an IOL with negative spherical aberrations

4 should. At 0.38 mm of decentration or higher, with a pupil smaller than 5 mm, the SofPort AO s modulation transfer function (MTF) and thus the quality of the retinal image exceeded that of the IOL with negative spherical aberration at all spatial frequencies. In vivo, this should correlate with better contrast sensitivity in low light conditions despite the IOL s decentration. Spatial Frequency at the retina Decentration at 3 mm Decentration at 4 mm Conclusion Bausch & Lomb s method for designing apsheric intraocular lenses overcomes the vision-limiting features not only of negativespherical-aberration aspheric IOLs, but also of conventionally designed IOLs. SofPort AO lenses feature both anterior and posterior prolate surfaces and have no inherent spherical aberration. They require no special implantation procedures beyond those already used with B&L IOLs. Computer modeling has demonstrated the SofPort AO design can prevent the degradation of image quality that occurs when a conventional IOL or a negative spherical aberration aspheric IOL decenters. If these optical advantages are confirmed in upcoming clinical tests, the SofPort Advanced Optics method of IOL design could improve ocular wavefront results for all pseudophakic patients, without risking post-operative visual surprises arising from IOL decentration and tilt. Further analysis via Monte Carlo simulations showed the existing negative spherical aberration aspheric IOL s average performance equaled that of the SofPort AO lens only at a pupil size of 5 mm. In addition, the analysis demonstrated that the SofPort AO design also represents an improvement over conventional IOLs. This is because it does not introduce the spherical aberration that is responsible for increased total HOAs induced by conventional IOLs. Analysis showed that the SofPort AO lens would produce a better quality retinal image than the conventional IOL in all cases, even when it was decentered 1 mm and the LI61U SE was perfectly centered. This is because the pseudophakic eye with a SofPort AO lens has less spherical aberration, and because its decentration does not induce the asymmetrical HOAs seen when a conventional IOL decenters.

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