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 São Paulo, São 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 μm or thicker (thick-flap group).

RESULTS

The 29 eyes (16 patients) in the ectasia group had thicker flaps (146 μm) than the 24 eyes (20 patients) in the high-PTA group (118 μm) 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 μm versus 43 μm) 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.

FINANCIAL DISCLOSURE: Drs. Santhiago and Smadja are consultants to Ziemer Ophthalmic Systems AG. No other author has a financial or proprietary interest in any material or method mentioned.