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D Gatinel, DT Azar, J Malet, T Hoang-Xuan; Theoretical Analysis of the Maximal . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3944.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the ablation depths of customized myopic and hyperopic excimer laser photoablations aimed at changing corneal asphericity after excimer laser refractive surgery. Methods: The initial and final surface corneal surface were modeled by conic sections. The aspheric profile of ablation was defined as the difference between the initial and final corneal profiles for a given magnitude of treatment (D), optical zone diameter (S), and the maximal depth of ablation was calculated from these equations. Using Taylor's series expansion, an equation was derived that allowed the approximation of the central depth of ablation, t0, for various magnitudes of treatment, optical zone diameters, and asphericity for myopia and hyperopia, respectively. In addition to the Munnerlyn term, M, incorporating Munnerlyn's approximation (-D.S2/3, the equation included an asphericity term, A, and a change of asphericity term, d. This formula (t0 = M+A+d) was used to predict the maximal depth of ablation and the limits of customized asphericity treatments in several theoretical situations. Results: When the initial and final asphericity values were identical (no intended change in asphericity ; d=0), the maximal depth of ablation (t0=M+A) changed linearly with the value of initial asphericity. To achieve a more prolate final asphericity after a myopic treatment, the maximal depth (M+A+d) of ablation was increased. To achieve a more oblate final asphericity after a hyperopic treatment, the maximal depth (M+A+d) of ablation was increased. These effects sharply increased with increasing diameters of the optical zone(s). Similarly, in the case of PRK, the differential increase in epithelial thickness in the zone of maximal ablation as compared to zone of minimal ablation resulted in increased oblateness for myopia, and increased prolateness for hyperopia. Conclusion: Aspheric profiles of ablation result in varying central depths of ablation. This may be of clinical importance in planning aspheric profiles of ablation in LASIK procedures to correct spherical aberration without compromising the mechanical integrity of the cornea.
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