Background and Aim: Residual refractive error is one of the most common complications of keratorefractive surgeries including laser assisted in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). This study aimed to determine the incidence rate and risk factors for re - treatment following photorefractive keratectomy (PRK) in patients with myopia and myopic astigmatism.

Materials and Methods: A case control study was performed on the surgical records of all eyes that underwent PRK from May 2009 to May 2012 at Farabi Eye Hospital by one surgeon. During this period, 70 eyes with indication for retreatment (cases) and 158 control eyes were identified. All of the eyes included in the study had refraction data at least 9 months post-operatively. Student t, Man-Whitney U and chi-square tests were used for univariate analysis of presumed associations. All variables with a P value of < 0.2 on univariate tests were entered in a multiple logistic regression to estimate adjusted odds ratios (ORs) and 95% confidence intervals (CIs) for the risk factors of interest.

Results: Pre-op (pre-operative) manifests refraction spherical equivalent ≥-5.00 diopter, intended/nominal optical zone diameter of<6 mm and ocular fixational instability during surgery were associated with an increased risk of retreatment (all P values <0.001) and maintained their significance on multiple logistic regression with strong odd ratios of 6.12, 6.71 and 7.89 respectively. No statistically significant association was found between cases and controls in variables of age, sex, follow-up time, pre-op astigmatism, pre-op keratometry reading, pre-op pachymetry reading, pupil diameter and Kappa angle (all P values>0.05).

Conclusion: Small optical zone, deep ablation and unstable fixation during laser ablation are strong predictors for retreatment after photorefractive keratectomy.

Keywords: Re-treatment, Photorefractive keratectomy, Myopia, Myopic astigmatism