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C. Dorronsoro, D. Cano, S. Barbero, J. Merayo, L. Llorente, S. Marcos; Understanding the Standard Algorithm for Corneal Refractive Surgery Using Laser Ablation of PMMA Surfaces . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2535.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Previous analytical and computational studies show that the Munnerlyn algorithm (the nominal laser ablation profile used in standard myopic corneal refractive procedures) should not induce the increased amounts of corneal asphericity found clinically after LASIK or PRK. Theoretically, prolate corneas should become more prolate, and oblate corneas should become more oblate. The current work further investigates the expected changes in corneal asphericity after laser ablation by stuying the ablation profiles on polymethylmethacrylate (PMMA) surfaces. Methods: A narrow-beam flying spot excimer laser (Chiron Technolas 217-C-LASIK, Bausch&Lomb Surgical) provided with conventional software was used to ablate flat and spherical (radius from 7.5 to 8.6 mm) PMMA surfaces. Spherical corrections from –3 D to –12 D were applied, with optical zones from 5 to 7 mm. The topography of spherical surfaces was tested by videokeratoscopy prior to ablation. The ablation profile on flat surfaces was measured by contact profilometry. The change in asphericity after ablation of spherical surfaces was assessed by digital thickness gauge, and videokeratoscopy (with artificial tear coating). Results: The change in curvature induced by laser ablation on PMMA surfaces matched the observed change in curvature on real corneas, after application of a 3x correcting factor in ablation depth. There are not large discrepancies between the profile measured on flat surfaces and the Munnerlyn equation (0.02 µm RMS fitting errors). However, when the measured profile is computationally applied on aspheric corneas (both prolate and oblate), we found in all cases an increase in corneal asphericity (0.25 on average), contrary to theoretical predictions, but lower than the typical increase in asphericity found clinically (0.63 on average). The profile of ablated PMMA spherical surfaces shows a similar trend. Conclusions: (1) Minor discrepancies from the application of the Munnerlyn ablation algorithm can result in an increase of corneal asphericity (and consequent increase of spherical aberration). (2) The fact that real corneas still show larger increases in asphericity after ablation, compared to PMMA surfaces, is suggestive of additional biomechanical effects. (3) Understanding the ablation profiles on PMMA surfaces and how they compare to theoretical predictions and real outcomes is critical for improving corneal ablation algorithms.
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