Unexpected corneal flattening after laser in situ keratomileusis

PURPOSE: To perform a statistical study of the variation (flattening) from the surgical radius (sculpted in the corneal stroma) to the final radius of the first surface of the cornea after laser in situ keratomileusis (LASIK) for correction of myopia. METHODS: The study included 387 eyes with myopia or myopic astigmatism that underwent LASIK using the Nidek EC-5000 excimer laser. Mean age was 34 years (range, 19 to 75 yr). Mean myopia was -5.19 D (range, 0 to -19.00 D) and mean astigmatism was -1.22 D (range, 0 to -5.00 D). The flattening coefficient was defined as: (f) = (R-post - R-8)/R-8 where R-post was the corneal radius surgery (topography performed 1 month after surgery) and R-8 was the surgical radius sculpted in the stroma. Mean values and standard deviations of the flattening coefficient were calculated for the 387 eyes for four myopic ranges (0 to -3.00 D, -3.00 to -6.00 D, -6.00 to -9.00 D, and more than -9.00 D), and for three astigmatic ranges (0, -0.25 to -2.00 D, and more than -2.00 D). The correlations between the flattening coefficient and several preoperative ocular variables were obtained. RESULTS: A linear combination of quasi-independent ocular variables (age, anterior corneal radius, corneal thickness, sphere and cylinder) was found in order to maximize the correlation with the flattening for every range. Values for the correlations between 0.4 to 0.5 were obtained. CONCLUSION: The flattening phenomenon was characterized by a flattening coefficient (f). According to the results, flattening may be reliably predicted in certain ranges. This knowledge could be relevant for the planning of surgery.