Eye models for the prediction of contrast vision in patients with new intraocular lens designs.

Theoretical calculations of the polychromatic modulation transfer function (MTF) and wave-front aberration were performed with physiological eye models. These eye models have an amount of spherical aberration that is representative of a normal population of pseudophakic eyes implanted with two different types of intraocular lens (IOL) made from high-refractive-index silicone. These theoretical calculations were compared with the measured contrast sensitivity function (CSF) under mesopic lighting conditions and with wave-front aberration (obtained with a Hartmann-Shack wave-front sensor) collected from 37 patients bilaterally implanted with the same types of lens. The relationships between the ocular wave-front aberration and the MTF predicted by the eye models and the CSF and the ocular wave-front aberration measured in eyes implanted with IOLs were investigated. The predicted improvements in MTF and wave-front aberration correlated well with the improvements measured in practice. Physiological eye models are therefore useful tools for IOL design.

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

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.

Determining the imaging quality of 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.

Prediction of intraocular lens power using the lens haptic plane concept.

PURPOSE: To test algorithms for the preoperative estimation of the lens haptic plane (LHP) and to assess these in terms of predictability of postoperative refraction. SETTING: S:t Erik's Eye Hospital, Stockholm, Sweden. METHODS: Preoperative axial length, anterior chamber depth (ACD), and cataractous lens thickness were measured in consecutive cases scheduled for phacoemulsification and posterior chamber intraocular lens (IOL) implantation. The algorithms tested used ACD and cataractous lens thickness to estimate LHP. The exact geometry of the IOL was used to calculate postoperative ACD from LHP. Thick lens theory and paraxial ray tracing were used to calculate predicted postoperative refraction. The calculated value was compared with actual refraction 3 to 6 months postoperatively. RESULTS: Mean absolute average error in predicted refraction was 0.38 diopters (D), with 78% of eyes within 0.50 D and 97% within 1.00 D for the best algorithm. CONCLUSIONS: The predictability in postoperative refraction found by applying the LHP concept and paraxial ray tracing was excellent. However, the small sample, with its unusually slight variation in axial lengths, did not allow statistically significant differences between different postoperative refraction prediction methods to be demonstrated.

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

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 10.00 through 30.00 diopter (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 0.5% of the dioptric power, and interlaboratory reproducibility, when following a normalized procedure for correction and conversion, was less than 1.0% of the dioptric power. CONCLUSION: Tolerance limits of +/0 0.30 D in the range 0 to 15.00 D, +/- 0.40 D for 15.50 to 25.00 D, and +/- 0.50 D for 25.50 to 30.00 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 1.0% to the total error in postoperative refractive prediction.

Standardized methods for assessing the imaging quality of intraocular lenses.

The relative merits of three standardized methods for assessing the imaging quality of intraocular lenses are discussed based on theoretical modulation-transfer-function calculations. The standards are ANSI Z80.7 1984 from the American National Standards Institute, now superseded by ANSI Z80.7 1994, and the proposed ISO 11979-2 from the International Organization for Standardization. They entail different test configurations and approval limits, respectively: 60% resolution efficiency in air, 70% resolution efficiency in aqueous humor, and 0.43 modulation at 100 line pairs/mm in a model eye. The ISO working group found that the latter corresponds to 60% resolution efficiency in air in a ring test among eight laboratories on a sample of 39 poly(methyl) methacrylate lenses and four silicone lenses spanning the power (in aqueous humor) range of 10-30 D. In both ANSI Z80.7 1994 and ISO 11979-2, a 60% resolution efficiency in air remains an optional approval limit. It is concluded that the ISO configuration is preferred, because it puts the intraocular lens into the context of the optics of the eye. Note that the ISO standard is tentative and is currently being voted on.