Preoperative corneal thickness, ablation depth, and flap thickness are all known to play a role in biomechanical change after a LASIK procedure. But how do these individual factors interact to contribute to ectasia risk? Dr. Marcony Santhiago and colleagues developed a new metric, the percent of anterior tissue depth altered, that accurately describes this relationship: PTA = (FT+AD)/CCT, where PTA = percent tissue altered, FT = flap thickness, AD = ablation depth and CCT = preoperative central corneal thickness. In a retrospective study that included patients with bilateral normal preoperative Placido-based corneal topography who later developed ectasia after LASIK, and patients who had uncomplicated LASIK, PTA more accurately represented the risk of ectasia than any of its individual components. Compared to other variables, PTA had higher prevalence, higher odds ratio, and higher predictive capabilities for ectasia risk than moderate to high Ectasia Risk Score System values, as well as the individual factors from that system, including residual stromal bed, central corneal thickness, spherical equivalent refraction, ablation depth, and age.