Relative contribution of flap thickness and ablation depth to the percentage of tissue altered in ectasia after laser in situ keratomileusis

PURPOSE: To evaluate the contribution of flap thickness and ablation depth to the percentage of tissue altered (PTA) (flap thickness plus ablation depth divided by central corneal thickness) in determining the risk for ectasia after laser in situ keratomileusis (LASIK). SETTING: University of Sao Paulo, Sao Paulo, Brazil; Cleveland Clinic, Cleveland, Ohio, and Emory Vision at Emory University, Atlanta, Georgia, USA. DESIGN: Retrospective comparative study. METHODS: The study included 1 group of eyes with bilateral normal preoperative topography that developed ectasia after LASIK (ectasia group) and 2 control groups of eyes that did not develop ectasia after LASIK, 1 with 40% or more of tissue altered (high-PTA group) and 1 with flaps 120 mu m or thicker (thick-flap group). RESULTS: The 29 eyes (16 patients) in the ectasia group had thicker flaps (146 mu m) than the 24 eyes (20 patients) in the high-PTA group (118 mu m) and derived a higher portion of PTA from flap thickness. The ectasia group had a higher PTA (45%) than the 100 eyes (50 patients) in the thick-flap group (34%) and greater ablation depths (94 mu m versus 43 mu m) and derived a higher portion of the PTA from ablation depth. CONCLUSIONS: Flap thickness had more impact than ablation depth; however, thicker flaps were insufficient to create ectasia unless coupled with greater ablation depths and thus a high PTA. This percentage was a more significant factor than the variables it comprised. (C) 2015 ASCRS and ESCRS