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To calculate the thinnest point percentage tissue altered (TP-PTA) in post-laser assisted in situ keratomileusis (LASIK) eyes and to validate its role as an independent factor to evaluate ectasia in Indian population.
333 consecutive eyes who underwent LASIK using a microkeratome with normal pre-operative corneal topography by combined Placido and Scheimpflug Imaging based topography system (SIRIUS) between 2011 and 2014 at a tertiary level teaching hospital in south India were retrospectively analysed. Pre-operative data collected included patient’s refraction, flap thickness (FT), ablation depth (AD), residual stromal bed (RSB) and thinnest corneal thickness (TCT). TP-PTA was calculated as (FT+AD)/TCT. TP-PTA was grouped into <0.4 (low risk), 0.4-0.45 (moderate risk) and >0.45 (high risk). All patients were called for follow up and underwent a topography to look for ectasia.
60.1%, 29.1% and 10.8% patients had TP-PTA of <0.4, 0.4-0.45 and >0.45 respectively. However, after a minimum follow up of 2 years, none of the patients had any sign of ectasia.
Careful selection of patients is mandatory before proceeding for LASIK. Role of TP-PTA >0.4 as an independent risk factor for post-LASIK ectasia is questionable in Indian eyes, with TCT , RSB, degree of myopia, AD and normal corneal tomography being more important. A modified formula needs to be investigated.