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 micrometer and a mean actual flap of 105 micrometer (standard deviation [SD], +/-24. 3 micrometer range, 48-141 micrometer). Sixty-four had a predicted flap of 180 micrometer with an actual flap mean of 125 micrometer (SD, +/-18.5 micrometer range, 82-155 micrometer). Seventeen eyes had a predicted flap of 200 micrometer, with an actual flap mean of 144 micrometer (SD, +/-19.3 micrometer range, 108-187 micrometer). 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.