[Comparison of corneal wound healing of photorefractive keratectomy and laser in situ keratomileusis in rabbits].

OBJECTIVE: To compare effects of photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) on the cornea and corneal wound healing and to investigate the possible mechanism of corneal haze and myopic regression histopathologically. METHODS: Twenty-four New Zealand white rabbits were allocated randomly to correct -4.00 and -8.00 diopters and were operated with PRK on right eyes and LASIK on left eyes. At 10 days and 1, 3, and 6 months, corneal haze was observed, refraction was evaluated, and 3 rabbits were randomly selected for each time point to be enucleated, and corneas to be bisected. One half of each cornea was evaluated using transmission electron microscopy, and the other half was evaluated using either light microscopy or immunohistochemical staining for collagen type III, IV, fibronectin (FN) and transforming growth factor-beta(1) (TGF-beta(1)). RESULTS: Different degrees of corneal haze and myopic regression were observed after PRK: the higher the desired myopic correction, the heavier the haze. Corneal wound healing response was greater and lasted longer after PRK than after LASIK. Various pathological findings such as epithelial hyperplasia, basal membrane reforming and extracellular matrix deposits were found in the ablation zone and repair mechanisms were still active at 6 months after PRK. Whereas after LASIK, the interface between the flap and stromal bed was transparent except for growing epithelial plugs and lightly proliferating stroma coinciding with the flap margins. After both PRK and LASIK, all corneal cell types were consistently positive for TGF-beta(1) antibody during the corneal wound healing time. TGF-beta(1) antibody positivity decreased as the wound healing approached completion. The histopathological changes of corneal haze and myopic regression are as follows: epithelial hyperplasia, basal membrane immaturity, anterior stromal keratocyte increase and activity, new collagen III production and irregular arrangement thereof, and FN deposition in the extracellular matrix under the epithelium. CONCLUSIONS: Compared with PRK, LASIK ensures refractive stability with quick wound healing and minimal tissue proliferation resulting in a more promising corneal refractive surgery to correct myopia, especially more severe cases of myopia. The corneal wound healing especially the stromal healing is the key to the cause of haze and myopic regression. TGF-beta(1) may be involved in scar formation during wound healing by stimulating expression of collagen III and FN and is believed to be an important regulating factor.