title: Risk factors of regression and undercorrection in photorefractive keratectomy: a case-control study.
abstract: OBJECTIVE
  To determine risk factors of regression and undercorrection following photorefractive keratectomy (PRK) in myopia or myopic astigmatism.
  
  
  METHODS
  A case-control study was designed in which eyes with an indication for re-treatment (RT) were defined as cases; primary criteria for RT indication, as assessed at least 9mo postoperatively, included an uncorrected distance visual acuity (UDVA) of 20/30 or worse and a stable refraction for more than 3mo. Additional considerations included optical quality symptoms and significant higher order aberrations (HOAs). Controls were chosen from the same cohort of operated eyes which had complete post-operative follow up data beyond 9mo and did not need RT. The cohort included patients who had undergone PRK by the Tissue-Saving (TS) ablation profile of Technolas 217z100 excimer laser (Bausch & Lomb, Rochester, NY, USA). Mitomycin C had been used in all of the primary procedures.
  
  
  RESULTS
  We had 70 case eyes and 158 control eyes, and they were comparable in terms of age, sex and follow-up time (P values: 0.58, 1.00 and 0.89, respectively). Pre-operative spherical equivalent of more than -5.00 diopter (D), intended optical zone (OZ) diameter of less than 6.00 mm and ocular fixation instability during laser ablation were associated with RT indications (all P values <0.001). These factors maintained their significance in the multiple logistic regression model with odd ratios of 6.12, 6.71 and 7.89, respectively.
  
  
  CONCLUSIONS
  Higher refractive correction (>-5.00 D), smaller OZ (<6.00 mm) and unstable fixation during laser ablation of PRK for myopia and myopic astigmatism were found to be strong predictors of undercorrection and regression.
authors.affiliation: Eye Research Center, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran 1336616351, Iran.
authors.forename: Seyed-Farzad
authors.initials: SF
authors.lastname: Mohammadi
authors.affiliation: Eye Research Center, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran 1336616351, Iran ; Department of Optometry, School of Rehabilitation, Iran University of Medical Sciences, Tehran 1676666431, Iran.
authors.forename: Payam
authors.initials: P
authors.lastname: Nabovati
authors.affiliation: Department of Optometry, School of Rehabilitation, Iran University of Medical Sciences, Tehran 1676666431, Iran.
authors.forename: Ali
authors.initials: A
authors.lastname: Mirzajani
authors.affiliation: Eye Research Center, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran 1336616351, Iran.
authors.forename: Elham
authors.initials: E
authors.lastname: Ashrafi
authors.affiliation: Eye Research Center, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran 1336616351, Iran.
authors.forename: Banafsheh
authors.initials: B
authors.lastname: Vakilian
keywords: photorefractive keratectomy; re-treatment; regression; undercorrection