A model to explain the difference between changes in refraction and central ocular surface power after laser in situ keratomileusis

PURPOSE: After refractive surgery, changes in central ocular surface power are usually less than actual changes in refraction. The aim of this paper w as to examine the theoretical changes in the radius of the epithelial-strom al interface and corneal stromal refractive index which could take place to account for some of the empirical findings. METHODS: The parameters of an aspheric human eye model featuring gradient i ndex optics was modified using data from a clinical study evaluating effect s of laser in situ keratomileusis (LASIK) for correction of moderate to hig h myopia. Computations were performed to determine: 1) theoretical postoper ative radius of the epithelial-stromal interface and hence distribution of epithelial thickness, and 2) refractive index of the stroma when the epithe lium was of fixed uniform thickness. RESULTS: Within the central 2-mm diameter refractive zone of the cornea aft er LASIK for myopia, either of two factors could account for the difference between changes in central ocular surface power and the actual change in o cular refraction: 1) a steepening of the epithelial-stromal interface resul ting from a 400% centrifugal increase in epithelial thickness, or 2) a redu ction in stromal refractive index from an average of 1.376 to 1.364. CONCLUSION: The difference between the observed changes in refraction and c entral ocular surface power could be explained by reduction in stromal refr active index and this could be secondary to a 6.5% increase in water conten t of the stroma during the postoperative period. The epithelial-stromal int erface after LASIK is unlikely to steepen by the extent predicted by our mo del. This surface is not the source of the difference between the change in refraction and change in central ocular surface power.