To investigate the association of a novel metric, percent tissue altered, with the occurrence of ectasia after laser in situ keratomileusis (LASIK) in eyes with normal corneal topography and to compare this metric with other recognized risk factors.

Retrospective case-control study.

The study included 30 eyes from 16 patients with bilateral normal preoperative Placido-based corneal topography that developed ectasia after LASIK (ectasia group) and 174 eyes from 88 consecutive patients with uncomplicated LASIK and at least 3 years of postoperative follow-up. The following metrics were evaluated: age, preoperative central corneal thickness, residual stromal bed, Ectasia Risk Score System scores, and percent tissue altered, derived from [PTA = (FT + AD)/CCT], where FT = flap thickness, AD = ablation depth, and CCT = preoperative central corneal thickness.

In the ectasia group, percent tissue altered ≥40 was the most prevalent factor (97%), followed by age <30 years (63%), residual stromal bed ≤300 μm (57%), and ectasia risk score ≥3 (43%) (P < .001 for all). Percent tissue altered ≥40 had the highest odds ratio (223), followed by residual stromal bed ≤300 μm (74) and ectasia risk score ≥4 (8). Stepwise logistic regression revealed percent tissue altered ≥40 as the single most significant independent variable (P < .0001).

Percent tissue altered at the time of LASIK was significantly associated with the development of ectasia in eyes with normal preoperative topography and was a more robust indicator of risk than all other variables in this patient population.