The theoretical vs. measured laser resection for laser in situ keratomileusis

PURPOSE: To measure the excimer laser resection during and after laser in situ keratomileusis (LASIK) and to determine the rate of stromal ablation and factors predicting its measurement. METHODS: A retrospective, comparative, interventional case study of 6010 eyes undergoing LASIK surgery was performed. In vivo ultrasonic pachymetry was performed as a measure of the laser resection (1'MLR). The theoretical laser resection generated by one of three separate excimer lasers (Summit Apex Plus, Alcon LadarVision, VISX) was recorded. The change in central corneal thickness measured at enhancement surgery (2'MLR) and the change in the residual stromal thickness prior to the laser ablation at the primary and enhancement procedure (3'MLR) were recorded as separate measures of the laser resection. Paired sample t test and regression analysis was performed to determine the relationships and to develop a model predictive of laser resection for each laser. RESULTS: For all lasers, 1'MLR produced the highest estimate compared to 3'MLR. Laser-induced stromal desiccation, microkeratome effects, and change in measured flap thickness were the most likely causes for the differences. Laser resection was overestimated in the Alcon theoretical laser resection group, but underestimated in the Summit and VISX groups. The difference between 2'MLR and 3'MLR was due to epithelial hyperplasia, which measured 8.3, 17.8, and 10.8 μm in the Alcon, Summit, and VISX enhancement groups, respectively. Stromal ablation rates were 11.79, 8.26, and 12.71 μm per spherical equivalent diopter laser setting for the Alcon, Summit, and VISX lasers, respectively, when measured with 3'MLR. Multiple pre-and perioperative factors were associated with laser resection. CONCLUSIONS: To accurately predict residual corneal thickness, the expected laser resection for an attempted refractive change must be known. As the laser resection can be laser-specific, the surgeon needs to establish the mean and range of tissue removal for a given attempted correction.