Morphologic and Histopathologic Changes in the Rabbit Cornea Produced by Femtosecond Laser–Assisted Multilayer Intrastromal Ablation

Abstract

PURPOSE: To observe morphologic and histopathologic changes in the midperiphery of the rabbit cornea produced by femtosecond laser-assisted multilayer intrastromal ablation, determine whether this method may be used to correct myopia, and study how the cornea heals when the epithelium is not injured. METHODS: The right eyes of 10 New Zealand White rabbits were used for the experiments. A 60-kHz femtosecond laser delivery system was used, and three lamellar layers of laser pulses were focused starting at a corneal depth of 180 microm and ending at 90 microm from the surface, with each successive layer placed 45 microm anterior to the previous layer. In the interface of the applanation contact lens cone, a 6-mm diameter aluminum circle was placed at the center to block the laser, and ablation was limited to the midperiphery of the cornea. The laser settings were spot/line separation, 10 microm; diameter, 8.5 mm; energy for ablating the stroma, 1.3 microJ. Topography examination was used to document changes in corneal power. Light microscopy, transmission electron microscopy (TEM), and confocal microscopy in vivo were applied to observe changes in the cornea. RESULTS: There was significant change in mean corneal power between baseline and postoperative month 3 (n = 8; P = 0.0001), with a decrease from 46.82 D to 44.42 D. There was no haze formation or refractive regression throughout the follow-up. There were no corneal structural abnormalities under light microscopy. Activated keratocytes and necrotic debris were visible under confocal microscopy. Fibroblasts were observed, and no myofibroblasts appeared under TEM. CONCLUSIONS: Multilayer intrastromal ablation by the femtosecond laser with intact epithelium in the midperiphery of the corneal stroma can flatten the cornea without causing haze formation or refractive regression. This procedure allows the cornea to heal differently than when traditional corneal refractive surgery is performed and the epithelium is damaged.