The predictability of corneal flap thickness and tissue laser ablation in laser in situ keratomileusis

Objective: To evaluate the relationship between predicted flap thickness and actual flap thickness and between predicted tissue ablation and actual tissue ablation. Design: Prospective, nonrandomized comparative (self-controlled) trial. Participants: A total of 60 patients (102 eyes) who underwent laser in situ keratomileusis (LASIK). Main Outcome Measures: Subtraction pachymetry was used to determine actual corneal flap thickness and corneal tissue ablation depth. Other measurements included flap diameter and keratometry readings. Results: Actual flap thickness was significantly different (P < 0.0001) from predicted flap thickness. Fifteen eyes had a predicted flap thickness of 160 <mu>m and a mean actual flap of 105 mum (standard deviation [SDI, +/-24.3 mum; range, 48-141 mum). Sixty-four had a predicted flap of 180 mum with an actual flap mean of 125 mum (SD, +/-18.5 mum; range, 82-155 mum). Seventeen eyes had a predicted flap of 200 mum, with an actual flap mean of 144 mum (SD, +/-19.3 mum; range, 108-187 mum). In addition, we found that significantly more tissue (P < 0.0001) was ablated than predicted. Linear regression of the observed ablation on predicted ablation yielded the following relationship: actual ablation = 14.5 + 1.5 (predicted ablation). Neither flap diameter nor flap thickness were found to increase with respect to steeper corneal curvatures. Conclusions: Actual corneal flap thickness was consistently less than predicted regardless of the depth plate used; actual tissue ablation was consistently greater than predicted tissue ablation for the laser used in this study. (C) 2000 by the American Academy of Ophthalmology.