Factors Predictive of LASIK Flap Thickness With the Hansatome Zero Compression Microkeratome

PURPOSE: To determine the explanatory power of preoperative variables and corneal flap thickness in laser in situ keratomileusis (LASIK) using the Hansatome zero compression microkeratome (Bausch & Lomb, Rochester, NY).

METHODS: A prospective, nonrandomized, comparative interventional case study was performed on 250 eyes of 129 consecutive patients who underwent LASIK surgery using the Hansatome zero compression microkeratome. A 160-µm or 180-µm microkeratome head and an 8.5- or 9.5-mm suction ring were used in the procedures. Preoperative measurements included refraction, spherical equivalent, keratometry, intraocular pressure, corneal white-to-white, anterior chamber depth, and corneal eccentricity. Corneal thickness was measured intraoperatively using ultrasonic pachymetry before and after flap creation, and the difference was taken as flap thickness. Flap diameter was measured with a corneal gauge. Data were analyzed using simple, multiple, step-wise linear and non-linear regression analyses and two-tailed t tests.

RESULTS: The mean fl ap thickness was 12417 µm with the nominal 160-µm head and 14220 µm with the nominal 180-µm head. One third (33%) of the total variation in fl ap thickness could be accounted for by three preoperative variables: average corneal thickness, spherical equivalent refraction, and choice of 160- or 180-µm microkeratome head. A simple correlation of 0.114 was noted between corneal eccentricity and fl ap thickness, but this variable did not add signifi cant explanatory power on multiple regression analysis. Linear regression analysis allowed determination of a fl ap thickness nomogram with a standard error of the estimate of 16.9 µm and a 95% confi dence interval of 33.1.

CONCLUSIONS: Corneal thickness is the most systematic predictor of corneal fl ap thickness using the Hansatome microkeratome. Because three preoperative variables account for only 33% of the range in fl ap thickness, future studies should focus on variations in blade extension and corneal biomechanical factors, which may also play an important role in determining fl ap thickness. [J Refract Surg. 2005;21:253-259.]