Biomechanical and optical behavior of human corneas before and after photorefractive keratectomy.

PURPOSE:To evaluate numerically the biomechanical and optical behavior of human corneas and quantitatively estimate the changes in refractive power and stress caused by photorefractive keratectomy (PRK). SETTING:Athineum Refractive Center, Athens, Greece, and Politecnico di Milano, Milan, Italy. DESIGN:Retrospective comparative interventional cohort study. METHODS:Corneal topographies of 10 human eyes were taken with a scanning-slit corneal topographer (Orbscan II) before and after PRK. Ten patient-specific finite element models were created to estimate the strain and stress fields in the cornea in preoperative and postoperative configurations. The biomechanical response in postoperative eyes was computed by directly modeling the postoperative geometry from the topographer and by reproducing the corneal ablation planned for the PRK with a numerical reprofiling procedure. RESULTS:Postoperative corneas were more compliant than preoperative corneas. In the optical zone, corneal thinning decreased the mechanical stiffness, causing local resteepening and making the central refractive power more sensitive to variations in intraocular pressure (IOP). At physiologic IOP, the postoperative corneas had a mean 7% forward increase in apical displacement and a mean 20% increase in the stress components at the center of the anterior surface over the preoperative condition. CONCLUSION:Patient-specific numerical models of the cornea can provide quantitative information on the changes in refractive power and in the stress field caused by refractive surgery. FINANCIAL DISCLOSURES:No author has a financial or proprietary interest in any material or method mentioned.