Corneal Asphericity Change after Excimer Laser Hyperopic Surgery: Theoretical Effects on Corneal Profiles and Corresponding Zernike Expansions (original) (raw)
2004, Investigative Ophthalmology & Visual Science
PURPOSE. To determine the theoretical relationships between the changes in corneal paraxial power, asphericity, and the corresponding Zernike polynomial expansion after conventional and customized excimer laser correction of hyperopia. METHODS. The initial corneal profile was modeled as a conic section of apical radius of curvature R 1 and asphericity Q 1. The theoretical value of the postoperative apical radius of curvature R 2 was computed by using a paraxial formula from the value of R 1 and hyperopic defocus D. The postoperative asphericity Q 2 of the corneal surface was computed within the optical zone of diameter S after the delivery of a Munnerlyn-based profile of ablation for hyperopia using conic section-fitting and minimization of the squared residuals. These calculations were repeated for different values of defocus, initial apical radius of curvature, and asphericity. Taylor series expansions were also used to provide an approximation aimed at predicting change in asphericity. The coefficients of a Zernike polynomial expansion of the rotationally symmetrical corneal profile (defocus C 2 0 , spherical aberration C 4 0 , secondary spherical aberration C 6 0) were also computed, by using scalar products applied to the considered corneal profile modeled as a conic section and were expressed as a function of both its apical radius and asphericity. This allowed approximation of the variations of the Zernike polynomial expansion of the corneal profiles by subtracting the postoperative coefficient weighting a particular aberration from that of the preoperative one in different theoretical situations, after both conventional and customized hyperopia treatments aimed at controlling the postoperative corneal asphericity and delivered over a normalized pupil diameter. RESULTS. Conical least-squares fitting was unambiguous, allowing approximation of the postoperative corneal profile as a conic section of apical radius R 2. After a Munnerlyn-based hyperopia treatment, the sign of the asphericity of this profile remains theoretically unchanged, but its value decreased for initially oblate and increased for initially prolate corneas, respectively. A similar trend was noted with the approximation obtained by the Taylor series expansion. The alteration of the METHODS Effect of Conventional (Spherical) Hyperopia Ablation on Postoperative Corneal Asphericity and Corresponding Zernike Polynomial Expansions Determination of Corneal Asphericity after Conventional (Noncustomized) Hyperopia Ablation. Current excimer laser ablations for treating hyperopia rely on the work of Munner-From the
Loading Preview
Sorry, preview is currently unavailable. You can download the paper by clicking the button above.