title: Role of flap thickness in laser in situ keratomileusis enhancement for refractive undercorrection.
abstract: OBJECTIVE
  To determine which preoperative and/or perioperative factors determine the need for an enhancement for refractive undercorrection in laser in situ keratomileusis (LASIK) for spherical or spherocylindrical myopia.
  
  
  METHODS
  Clinical refractive surgery outpatient facility.
  
  
  METHODS
  In this nested case-control study within a retrospective cohort, 3850 of the 9777 eyes operated on between January 1996 and August 2005 who met the selection criteria were studied. Only patients whose refractive goal was plano were included. Control eyes were those whose latest spherical equivalent (SE) refraction was within +/-0.37 diopters (D) of emmetropia. Cases were defined as enhancements for undercorrection with an SE refraction worse than or equal to -0.50 D performed less than 6 months following the primary surgery.
  
  
  RESULTS
  Factors associated with enhancement included increasing patient age (odds ratio [OR] = 1.048, P<.001), decreasing follow-up time (OR = 0.994, P<.001), increasing minus laser sphere (OR = 0.700, P<.001) and cylinder settings (OR = 0.718, P<.001), female sex (OR = 1.112, P = .046), and corneal toricity (OR = 1.237, P = .012). The Summit laser had a significant risk for enhancement (OR = 1.726, P<.001) compared with the Visx laser, whereas the Wavelight Allegretto had a lowered risk (OR = 0.630, P = .049). Enhancement risk with the Autonomous was no different from that with the Visx (OR = 1.120, P = .342). Increasing flap thickness (OR = 1.009, P<.001) was more strongly associated with enhancement risk than residual stromal bed thickness.
  
  
  CONCLUSIONS
  After controlling for patient age, time to follow-up or surgery, corneal cylinder, sphere and cylinder laser settings, laser manufacturer, patient sex, and corneal thickness, increasing flap thickness was found to be a more important predictor of enhancement for refractive undercorrection than residual stromal thickness.
authors.affiliation: Gordon Binder and Weiss Vision Institute, San Diego, California, USA. gflanagan@ocularsciences.com
authors.forename: Gerald W
authors.initials: GW
authors.lastname: Flanagan
authors.forename: Perry S
authors.initials: PS
authors.lastname: Binder
mesh_terms: D000328:Adult; D000367:Age Factors; D001251:Astigmatism / Q000601:surgery; D016022:Case-Control Studies; D003319:Corneal Stroma / Q000473:pathology*; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008297:Male; D009216:Myopia / Q000601:surgery*; D012029:Refraction, Ocular / Q000502:physiology; D012086:Reoperation; D012189:Retrospective Studies; D012307:Risk Factors; D013524:Surgical Flaps / Q000473:pathology*; D013997:Time Factors