title: Effects of ablation diameter, depth, and edge contour on the outcome of photorefractive keratectomy.
abstract: OBJECTIVE
  To investigate the effects of the ablation diameter, depth, and edge contour on the outcome of excimer laser photorefractive keratectomy (PRK).
  
  
  METHODS
  A prospective study was conducted in which 60 patients (60 eyes) were randomly allocated to 5.00-mm, 6.00-mm, or 5.00 to 6.00-mm multizone treatment groups. All eyes underwent a -6.00 diopter (D) correction using a Summit Omnimed excimer laser.
  
  
  RESULTS
  In eyes treated with 6.00-mm diameter zones, the initial hyperopic shift was reduced, with significant differences at 1 and 4 weeks (p < 0.01). At 6 and 12 months, the refractive changes were closer to the intended correction with 6.00-mm diameters. The predictability of PRK was improved with 6.00-mm zones, with a significant reduction in variance of the refractive changes, at all stages postoperatively (p < 0.05 to p < 0.001). Objective measurements of haze were significantly less at 1, 3, and 6 months with 6.00-mm ablations (p < 0.05). There were no differences between the 5.00-mm and the 5.00- to 6.00-mm multizone groups. Computerized measurements of "night" halo were significantly smaller in the 6.00-mm treatment group at 1 week and 1 month (p < 0.05). At 12 months, two patients treated with 5.00-mm zones and three with the 5.00- to 6.00-mm multizone complained of severe night vision disturbances. No 6.00-mm eyes were similarly affected.
  
  
  CONCLUSIONS
  Treatment with a 6.00-mm spherical ablation diameter produced less initial overcorrection, improved predictability, and was associated with a reduction in postoperative halos and night vision disturbances. Creating a superficial blend zone with a 5.00- to 6.00-mm multizone treatment had no beneficial effect on the outcome.
authors.affiliation: Department of Ophthalmology, United Medical School, St. Thomas' Hospital, London.
authors.forename: D P
authors.initials: DP
authors.lastname: O'Brart
authors.forename: M C
authors.initials: MC
authors.lastname: Corbett
authors.forename: S
authors.initials: S
authors.lastname: Verma
authors.forename: G
authors.initials: G
authors.lastname: Heacock
authors.forename: K M
authors.initials: KM
authors.lastname: Oliver
authors.forename: C P
authors.initials: CP
authors.lastname: Lohmann
authors.forename: M G
authors.initials: MG
authors.lastname: Kerr Muir
authors.forename: J
authors.initials: J
authors.lastname: Marshall
mesh_terms: D000328:Adult; D003315:Cornea / Q000503:physiopathology; D003319:Corneal Stroma / Q000528:radiation effects; D004311:Double-Blind Method; D006801:Humans; D054018:Lasers, Excimer; D008027:Light; D008875:Middle Aged; D009216:Myopia / Q000503:physiopathology / Q000601:surgery*; D009755:Night Blindness / Q000209:etiology; D018903:Photorefractive Keratectomy / Q000379:methods*; D011183:Postoperative Complications; D011184:Postoperative Period; D011446:Prospective Studies; D012029:Refraction, Ocular; D012542:Scattering, Radiation; D016896:Treatment Outcome