Theoretical Effects on Corneal Wavefront Analysis of Corneal Asphericity Change after Excimer Laser Hyperopic Surgery.

Abstract

Abstract: : Purpose:To determine the theoretical relationships between the changes in corneal paraxial power, asphericity, and the corneal wavefront (CWF) expressed with Zernike polynomial expansion after conventional and customized hyperopic excimer laser correction. Methods:The initial corneal profile (CP) was modeled as a conic section of apical radius of curvature R₁ and asphericity Q₁. The theoretical value of the postoperative apical radius of curvature R₂ was computed using a paraxial formula. We computed the postoperative asphericity Q₂ of the CP within the optical zone after the delivery of a Munnerlyn–based hyperopic profile of ablation using conic section fitting and minimization of the squared residuals, for different defocus, initial apical radius of curvature, and asphericity values. Taylor’s series expansions were also used to approximate the change in asphericity. Using scalar products, we also computed the coefficients of the rotationally symmetrical wavefront aberrations (C₂⁰, C₄⁰ and C₆⁰) that were expressed as a function of both its apical radius and asphericity. This allowed approximation of the variations of the CWF after both conventional and customized hyperopic treatments aimed at controlling the postoperative corneal asphericity. Results:Conical least–square fitting allowed the approximation of the postoperative CP as a conic section of apical radius R₂. After a Munnerlyn–based hyperopic treatment, the sign of the asphericity of the CP remains theoretically unchanged, but its value decreased for initially oblate and increased for initially prolate corneas, respectively. A similar trend was obtained with the approximation obtained by the Taylor series expansion. The variation of the apical radius and/or of the asphericity of the CP resulted in alterations of both Zernike coefficients, C₂⁰ and C₄⁰. The former was essentially dependent on the variation of the apical radius, and the latter essentially dependent on the variation of both apical radius and asphericity. Conclusions:Conventional and customized hyperopic profiles of ablation alter the postoperative corneal asphericity and the values of the Zernike coefficients of the CWF. The results of this study may be useful to interpret the postoperative variations of the CP and their consequences on wavefront expansion after both conventional and customized hyperopic profiles of ablation.

Keywords: refractive surgery: optical quality • hyperopia • laser