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G. Yoon, I. Cox, S. MacRae; Spherical Aberration Induced by Refractive Surgery . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2092.
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Purpose: Clinical observation has shown that higher order wave aberrations, especially spherical aberration, are increased after laser refractive surgery. The goal is to use a spherical corneal model to determine how spherical aberration is induced in refractive surgery procedures. Methods: We assumed the preoperative cornea was a spherical surface with various radii of curvature and asphericities. The postoperative cornea was defined as the difference between the preoperative cornea and the ablation thickness profile (Munnerlyn formula). A ray tracing program and Zernike polynomial fitting were used to calculate the induced amount of spherical aberration assuming either (1) a fixed ablation depth for all pulses or (2) a varying ablation depth depending on the incidence angle of the laser and the local corneal curvature. Results: Assuming a fixed ablation depth per pulse, positive (negative) spherical aberration is decreased after a myopic (hyperopic) treatment. For a 6mm diameter cornea, the spherical aberration induced by a +/-10D correction ranged between -0.39 and 0.67 microns when the preoperative cornea had a 7.8 mm radius of curvature and a conic constant of 0.7. This result is the opposite of that observed clinically. However, when assuming a variable ablation depth per pulse, positive (negative) spherical aberration increases with a myopic (hyperopic) treatment. The induced spherical aberration ranged between -0.04 and 0.28 microns for the same conditions described above. The difference in ablation depth accounts for approximately one-third of the clinically observed amount of spherical aberration. These increases in spherical aberration became larger as the preoperative corneal radius of curvature and asphericity became steeper and as the amount of correction was increased. Conclusion: This theoretical eye model takes into account differences in ablation efficiency and qualitatively explains how positive (negative) spherical aberration is induced after a myopic (hyperopic) correction. The discrepancy in the induced spherical aberration between the clinical data and theoretical expectations could be explained by other effects such as the biomechanical and healing responses of the cornea. This model can be incorporated into the ablation algorithm to decrease induced amounts of spherical aberration, improving the outcome of both conventional and customized treatments.
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