Excimer laser ablation rate and corneal hydration.

During excimer laser photorefractive keratectomy, dehydration of the cornea begins as soon as the epithelium is removed. Corneal hydration might affect the excimer laser ablation rate, which could affect the accuracy of correction. We studied the effect of corneal hydration on the excimer laser ablation rate in bovine eyes. To control hydration, bovine corneoscleral rims were equilibrated in dextran solutions of varying concentrations. One button trephined from each rim underwent laser ablation. Hydrated tissue ablation rates (amount of collagen, ground substance, and water removed per pulse) and dry component ablation rates (amount of collagen and ground substance removed per pulse) were calculated from mass removed. The hydrated tissue ablation rate at physiologic hydration was 0.40 micron/pulse. As corneal hydration increased, the hydrated tissue ablation rate increased by 5.6 micrograms/cm2/pulse per increase in unit corneal hydration (simple linear regression analysis, P = .0001). The dry component ablation rate decreased linearly by 0.82 microgram/cm2/pulse per unit increase in corneal hydration (simple linear regression analysis, P = .0001). Both clinical data and theoretical arguments imply that dry component ablation rate determines refractive outcome after photorefractive keratectomy. Since the dry component ablation rate increases as the cornea dries, significant dehydration of the cornea before ablation might lead to relative overcorrections of myopia. Surgeons should use a technique that minimizes changes in hydration to maximize the predictability of excimer laser photorefractive keratectomy.