A comparison of 2 methods for estimating residual stromal bed thickness before repeat LASIK.
PURPOSE: To compare 2 methods of calculating residual stromal bed (RSB) thickness after repeat LASIK, to determine which method generates more conservative RSB thickness estimates, and to determine any factors related to the discrepancy between these 2 calculation methods. DESIGN: Retrospective nonrandomized comparative trial. PARTICIPANTS: Seventy-nine patients (one eye per patient) undergoing primary LASIK and 2 repeat procedures (second and third LASIK) from 1998 to 2002. METHODS: We compared calculated RSB thickness after second and third LASIK using either original corneal thickness (CT) minus flap thickness and all ablations (original CT method) or pre-enhancement CT minus flap thickness and enhancement ablation (repeat CT method). MAIN OUTCOMES MEASURES: Differences in calculated RSB thickness after second and third LASIK by each method. RESULTS: Calculated RSB thickness averaged 329 microm by the repeat CT method compared with 305 microm by the original CT method after second LASIK (mean difference, 24 microm; P<0.0001) and 320 microm by the repeat CT method compared with 289 microm by the original CT method after third LASIK (mean difference, 31 microm; P<0.0001). After second LASIK, for 54% of eyes the repeat CT method was greater by >or=20 microm, and for 19% of eyes it was greater by >or=40 microm. After third LASIK, for 67% of eyes the repeat CT method was greater by >or=20 microm, and for 26% of eyes it was greater by >or=40 microm. The repeat CT method gave greater RSB thickness values than the original CT method in 73 of 79 eyes (92%) after second LASIK and in 72 of 79 eyes (91%) after third LASIK. The difference between the 2 methods was not associated with age, gender, initial preoperative refractive error, or refractive error before repeat LASIK. CONCLUSION: Using original preoperative CT measurements provides a more conservative and thus safer approach than using CT measurements obtained before repeat LASIK to calculate RSB thickness after repeat LASIK.