Validity of percentage tissue altered as a screening formula for post laser-assisted keratomileusis ectasia in Indian eyes.

Purpose: To calculate a modified percentage tissue altered (mPTA) in post laser-assisted in-situ keratomileusis (LASIK) eyes and to validate its role as an independent factor to evaluate ectasia in the Indian population.

Methods: A total of 333 consecutive eyes with normal preoperative corneal topography by combined placido and scheimpflug imaging-based topography system (SIRIUS) who underwent LASIK using a microkeratome between 2011 and 2014 at a tertiary level teaching hospital in south India, were retrospectively analyzed. Preoperatively patient's refraction, flap thickness (FT), ablation depth (AD), residual stromal bed (RSB), and thinnest corneal thickness (TCT) were recorded.