Precision of flap measurements for laser in situ keratomileusis in 4428 eyes

PURPOSE: To determine the factor(s) that influence the dimensions and predictability of the LASIK corneal flap with the Automated Corneal Shaper (ACS) or the Summit Krumeich Barraquer microkeratome (SKBM). METHODS: We performed a retrospective, comparative interventional case study of 4,428 eyes. Flap dimensions were measured using subtraction ultrasonic pachymetry during LASIK with one of two microkeratomes. RESULTS: Mean preoperative corneal thickness for all eyes was 555 ± 35 μm. Corneal curvature and refractive astigmatism were inversely related to preoperative corneal thickness (P<.001). With an attempted flap thickness of 160 μm, the ACS flap thickness averaged 119.8 ± 22.9 μm; SKBM flaps averaged 160.9 ± 24.1 μm (P<.001). The coefficient of variation for central pachymetry compared to flap thickness was 6.4% vs. 22.1%. Flap thickness at enhancement was 10 to 17 μm thicker than at primary surgery. An increase in flap thickness was associated with thicker preoperative pachymetry (P<.001) and younger age for both instruments (P<.001) whereas increasing flap thickness was related to flatter preoperative mean keratometry for the ACS (P<.001) and steeper mean keratometry for the SKBM (P=.005). Less preoperative hyperopia or more myopia was related to an increase in flap thickness only for the SKBM (P<.001). CONCLUSIONS: Flap thickness varies significantly depending on the microkeratome used. Factors that influence flap thickness are primarily corneal thickness, patient age, preoperative keratometry, preoperative refraction including astigmatism, and corneal diameter. By understanding the factors that affect flap thickness, one can select a microkeratome system to allow maximum refractive correction while minimizing the risk of ectasia.