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D. Cano, S. Barbero, S. Marcos; Computer Simulations of Corneal Shape and Corneal Aberrations after Laser Correction: Do They Match Real Outcomes? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2090.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: 1) To compare predicted changes of corneal elevation and corneal wave aberrations after myopic LASIK to real outcomes. 2) To investigate predicted post-op aberrations after different laser ablation algorithms. Methods: Corneal elevation maps were obtained by videokeratoscopy on a group of 14 patients (age: 28.9 ± 5.4 years;pre-op spherical error: 26.8 ± 2.9 D) before and after (>1 month) standard myopic LASIK. Post-op corneas were simulated by computationally subtracting from pre-op corneas the amount of tissue predicted by the Munnerlyn equation (and its parabolic approximation). Biconic and customized algorithms were also simulated. The applied spherical and cylindrical corrections, and optical zone diameters (4.4 -7 mm) were used in the simulations. Real pre-op total aberrations, measured by laser ray tracing, were used in the customized algorithm simulation. Corneal aberrations were obtained by computer ray tracing through the corneal surfaces. Results: Real and simulated post-op corneas show similar apex curvatures, but there is a large discrepancy between real and simulated post-op asphericity and spherical aberration (measured within the optical zone). Mean pre-op corneal asphericity was negative (-0.14 ± 0.14) and increased to positive values after myopic LASIK surgery (1.08 ± 1.25). Spherical aberration increased from 0.23± 0.11 µm to 0.47± 0.15 µm in real eyes. However, the predicted mean post-op corneal asphericity from the Munnerlyn algorithm was –0.19 ± 0.203, and the predicted post-op mean spherical aberration 0.14 ± 0.10 µm. Thus, the increase of corneal asphericity (and spherical aberration) is not inherent to the Munnerlyn algorithm. A parabollic approximation of the Munnerlyn equation did predict increased post-op asphericity (0.31 ± 0.40) and spherical aberration (0.30 ± 0.12 µm). A biconic algorithm could preserve the pre-op corneal spherical aberration (0.23 ± 0.11 µm), and a customized algorithm produced corneal aberrations that match expectations (74% for spherical aberration and 97% for 3rd & higher order variance) of total aberration correction. Conclusions: 1) The standard Munnerlyn algorithm should not induce the increase of corneal asphericity and spherical aberration after standard myopic LASIK found experimentally, although a parabolic approximation of this algorithm produces outcomes more similar to of real post-op corneas. (2) The biconic and customized ablation algorithms predict a better control of final aberrations. (3) To understand the causes for the discrepancy between predictions and real outcomes is essential for improving corneal refractive surgery procedures.
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