Structural Analysis of the Cornea Using Scanning-Slit Corneal Topography in Eyes Undergoing Excimer Laser Refractive Surgery

PURPOSE: To review the time course of corneal anteroposterior shift and refractive stability after myopic excimer laser keratorefractive surgery. METHODS: We examined 65 eyes undergoing photorefractive keratectomy (PRK) and 45 eyes undergoing laser in situ keratomileusis (LASIK). Corneal elevation maps and pachymetry were obtained by scanning-slit corneal topography before; 1 week; and 1, 3, 6, and 12 months after surgery. RESULTS: Both PRK and LASIK induced significant forward shifts of the cornea. Corneal forward shift was progressive up to 6 months after PRK, but no progression was seen after LASIK. Progressive thinning and expansion of the cornea were not observed after either procedure. The amount of corneal forward shift showed a significant negative correlation with preoperative corneal thickness (r = -0.586; P < 0.01) and a significant positive correlation with the amount of myopic correction (r = 0.504; P < 0.01). A significant correlation was found between the amount of forward shift and the degree of myopic regression after surgery (r = -0.347; P < 0.05). CONCLUSION: Myopic PRK and LASIK induce significant forward shifts of the cornea, which are not true corneal ectasia. Eyes with thinner corneas and higher myopia requiring greater ablation are more predisposed to anterior protrusion of the cornea. Corneal forward shift was progressive up to 6 months after PRK but not progressive after LASIK. Forward shift of the cornea can be one of the factors responsible for myopic regression after surgery.