Purpose To identify and quantify sources of error in the refractive outcome of cataract surgery. Setting AMO Groningen BV, Groningen, The Netherlands. Methods Means and standard deviations (SDs) of parameters that influence refractive outcomes were taken or derived from the published literature to the extent available. To evaluate their influence on refraction, thick-lens ray tracing that allowed for asphericity was used. The numerical partial derivative of each parameter with respect to spectacle refraction was calculated. The product of the partial derivative and the SD for a parameter equates to its SD, expressed as spectacle diopters, which squared is the variance. The error contribution of a parameter is its variance relative to the sum of the variances of all parameters. Results Preoperative estimation of postoperative intraocular lens (IOL) position, postoperative refraction determination, and preoperative axial length (AL) measurement were the largest contributors of error (35%, 27%, and 17%, respectively), with a mean absolute error (MAE) of 0.6 diopter (D) for an eye of average dimensions. Pupil size variation in the population accounted for 8% of the error, and variability in IOL power, 1%. Conclusions Improvement in refractive outcome requires better methods for predicting the postoperative IOL position. Measuring AL by partial coherence interferometry may be of benefit. Autorefraction increases precision in outcome measurement. Reducing these 3 major error sources with means available today reduces the MAE to 0.4 D. Using IOLs that compensate for the spherical aberration of the cornea would eliminate the influence of pupil size. Further improvement would require measuring the asphericity of the anterior surface and radius of the posterior surface of the cornea.

Sources of error in intraocular lens power calculation.

PURPOSE: The aim of this study was to design and evaluate in the laboratory a new intraocular lens (IOL) intended to provide superior ocular optical quality by reducing spherical aberration. METHODS: Corneal topography measurements were performed on 71 cataract patients using an Orbscan I. The measured corneal surface shapes were used to determine the wavefront aberration of each cornea. A model cornea was then designed to reproduce the measured average spherical aberration. This model cornea was used to design IOLs having a fixed amount of negative spherical aberration that partially compensates for the average positive spherical aberration of the cornea. Theoretical and physical eye models were used to assess the expected improvement in optical quality of an eye implanted with this lens. RESULTS: Measurements of optical quality provided evidence that if this modified prolate IOL was centered within 0.4 mm and tilted less than 7 degrees, it would exceed the optical performance of a conventional spherical IOL. This improvement occurred without an apparent loss in depth of focus. CONCLUSION: A new IOL with a prolate anterior surface, designed to partially compensate for the average spherical aberration of the cornea, is intended to improve the ocular optical quality of pseudophakic patients. [J Refract Surg 2002;18: 683-691] T

A new intraocular lens design to reduce spherical aberration of pseudophakic eyes.

Purpose To evaluate over almost 2 decades the success of a component of cataract surgery that represents a critical step in reducing the incidence of posterior capsule opacification (PCO); namely, the efficacy of cortical cleanup. Setting Center for Research on Ocular Therapeutics and Biodevices, Storm Eye Institute, Department of Ophthalmology, Medical University of South Carolina, Charleston, South Carolina, USA. Methods Accessioned from the early 1980s to 1997, 3320 eyes obtained postmortem with posterior chamber intraocular lenses were analyzed with respect to formation of a postoperative Soemmering’s ring. This anatomic lesion, the precursor of clinical PCO, represents an important and measurable indication of the quality of cortical cleanup. Its formation was documented using Miyake–Apple posterior photographic analysis. Results The quality and thoroughness of cortical cleanup and overall effectiveness in eliminating retained and/or regenerating cortical cells, as measured by scoring of Soemmering’s rings, showed virtually no net change since the early 1980s. The intensity of Soemmering’s ring was higher in the most recent specimens than in those in the early 1980s. Conclusion The results indicate that renewed attention to cortical cleanup in cataract surgery is warranted for significant reduction in incidence or the elimination of PCO. More attention to the hydrodissection (cortical cleaving hydrodissection) step of the procedure is likely a practical, immediately implementable, and inexpensive remedy.

Surgical prevention of posterior capsule opacification. Part 1: Progress in eliminating this complication of cataract surgery.

Purpose To emphasize an important aspect of preventing posterior capsule opacification (PCO), the barrier effect established by the optic of a posterior chamber intraocular lens (PC IOL), and present a new classification regarding capsular bag status after extracapsular cataract extraction, including phacoemulsification. Setting Center for Research on Ocular Therapeutics and Biodevices, Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA. Methods This analysis included 150 consecutive eyes obtained postmortem with United States-manufactured PC IOLs including (1) poly(methyl methacrylate), (2) silicone, and (3) hydrophobic acrylic designs that were accessioned in the Center from September 1995 to January 1, 1998. Gross photographs from behind (Miyake–Apple views) were taken and serial histologic sections prepared. Results Microscopic analysis of the 150 eyes showed that the morphologic appearance of the capsular bag could be grouped into 2 categories: (1) those with little or no evidence of retained cortical material and cells, and (2) those with retained cortical material and cells in which a Soemmering’s ring formed. With the latter, when a distinct barricade to cellular migration created by the IOL optic was noted, 2 discrete configurations occurred, depending on the different geometries of the optic components. With a classic biconvex optic with a curved and tapered edge, in many instances some ingrowth of cells proceeded posteriorly around the edge of the IOL optic in the direction of the central axis. With a lens optic that had a squared, truncated, and relatively thick edge, there was often abrupt termination of cells at the peripheral edge of the optic. The posterior capsule subtending the entire optic zone was therefore relatively or totally cell free. Conclusions The barrier effect of the IOL optic appears to be of critical importance in retarding ingrowth of cells, functioning as a second line of defense when cortical cleanup is incomplete. Analysis of PC IOLs obtained postmortem showed that a square, truncated optic edge seemed to provide the maximum impediment to cell growth behind the IOL optic.

Surgical prevention of posterior capsule opacification. Part 3: Intraocular lens optic barrier effect as a second line of defense.

Purpose To report the incidence, management, and prevention of patient reports of glare and streaks around a point source of light or a dark shadow in the temporal field of vision after acrylic intraocular lens (IOL) implantation. Setting A private practice. Methods Cases in which patients complained vigorously of dysphotopsia were catalogued prospectively during the implantation experience in 6668 consecutive eyes having surgery between January 1995 and June 1999. The techniques of topical-intracameral anesthesia, temporal clear corneal incisions, and phacoemulsification were used in all cases. Alternate IOL styles were selected for use from July 1999 to April 2000. Results Fourteen cases (0.2%) were identified. The complaints resolved in 1, were diminished in 1, and were tolerated without change in 7. Five eyes of 4 patients required IOL exchange with capsular bag placement of a poly(methyl methacrylate) (PMMA) or silicone lens for resolution of symptoms. Selecting alternate IOL styles reduced the incidence of dysphotopsia. Conclusions Glare and streaks from a point source of light represent positive photic expressions of dysphotopsia, and temporal dark shadows represent similar negative photic expressions. Both appear to be associated with shiny square-edge optics made of high-refractive-index acrylic polymer. Intraocular lenses of PMMA and silicone with rounded edges, along with square-edge acrylic IOLs with nonreflective edges, appear less likely to cause clinically significant pseudophakic dysphotopsia.

Positive and negative dysphotopsia in patients with acrylic intraocular lenses.

Ovoid intraocular lenses (IOLs) accounted for approximately 35% of the lenses implanted in 1991. Despite their popularity among surgeons, patient observations and complaints suggest that ovoid lenses are associated with undesired optical images postoperatively. A pilot clinical study was carried out to test that hypothesis. A multicentered investigation of 289 cases with well-centered IOLs determined that 45% of 168 patients with ovoid lenses observed unwanted optical phenomena at some time after surgery, whereas 17% of 121 patients with round IOLs had similar complaints. Based upon the preliminary clinical study a laboratory investigation was designed to assess optical performance differences between ovoid and round IOLs. One testing system used scatterometry and a second, ray tracing analysis. Both methods determined that the truncated, thickened edge of ovoid lenses was associated with significant light scattering, possibly accounting for the extraneous light images observed by some patients. The results of the clinical and laboratory investigations suggest that undesired optical images produced by ovoid IOLs are due to lens shape rather than to the reduced dimension of the optic.

Undesired light images associated with ovoid intraocular lenses

Purpose: To validate the proposed optical requirements of a draft international standard for intraocular lenses (IOLs). Setting: Eight optical testing laboratories in the United States, Germany; Japan, and The Netherlands. Methods: The testing laboratories performed modulation transfer function (MTF) tests on various IOLs using a model eye and visual resolution tests in air. Each laboratory performed duplicate measurements on a set of 43 lenses that was circulated among the testing laboratories. Results: The interlaboratory tests showed that the MTF measurements using a model eye had better repeatability and reproducibility than the more common industry practice of resolution testing in air with parallel light and the United States Air Force three-bar target. However, the two methods correlated well. The commonly applied criterion that an IOL resolve in air at least 60% of the Rayleigh cutoff spatial frequency corresponded to a minimum requirement of 0.43 MTF units at 100 mm−1 in a model eye. Conclusions: Either criterion may be applied in accordance with a proposed international standard for IOLs. The model eye method can be applied over a broader range of dioptric powers and is relevant for materials that interact with aqueous. Both tests appear to have a greater ability to detect unwanted surface aberrations than resolution testing of IOLs in a water cell using parallel light, a method described in the current American National Standards Institute standard.

Determining the imaging quality of intraocular lenses

Purpose To model the feasibility of an accommodating intraocular lens (IOL) that allows near vision by means of an anterior translation within the capsular bag Setting University of Waterloo, Waterloo, Ontario, Canada Methods Model eyes were constructed and analyzed based on experimental data using Code V, a computerized optical design tool The potential near vision of IOLs of different powers was calculated as they were moved anteriorly within the capsular bag The conditions under which a spherical lens performs well and when an aspheric design should be considered were determined Results Accommodation (the dioptric change from the far to the near point) varies linearly with lens movement and is sensitive to corneal and IOL powers Simple equations were derived and accurately predicted induced accommodation Retinal image quality varies significantly with pupil size and IOL power However, image quality is minimally affected by the amount of induced accomodation Conclusions From an optical standpoint accommodation with adequate image quality can be achieved by anterior movement of a spherical IOL within the capsular bag

Optical analysis of an accommodating intraocular lens.

Purpose: To describe a testing program conducted by a standards group as a guide for setting international tolerances for intraocular lens (IOL) dioptric power Setting: Multicenter study Methods: Seven biconvex, poly(methyl methacrylate) IOLs ranging in power from 1000 through 3000 diopters (D) were circulated among nine participating laboratories experienced in IOL optical measurements Each laboratory performed repeated optical tests to determine dioptric power These results were analyzed for repeatability and reproducibility in accordance with methods specified by the International Organization for Standardization Results: Intralaboratory repeatability was less than 05% of the dioptric power, and interlaboratory reproducibility, when following a normalized procedure for correction and conversion, was less than 10% of the dioptric power Conclusion: Tolerance limits of ±030 D in the range 0 to 1500 D, ±040 D for 1550 to 2500 D, and ±050 D for 2550 to 3000 D have been proposed as an international standard for IOLs The contribution of IOL power error within the limits of the standard are estimated to contribute less than 10% to the total error in postoperative refractive prediction

Accuracy in determining intraocular lens dioptric power assessed by interlaboratory tests.

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Undesired light images associated with ovoid intraocular lenses

Determining the imaging quality of intraocular lenses

Optical analysis of an accommodating intraocular lens.

Accuracy in determining intraocular lens dioptric power assessed by interlaboratory tests.