title: The effect of globe fixation on ablation zone centration in photorefractive keratectomy.
abstract: OBJECTIVE
  For optimal vision, the ablation zone in photorefractive keratectomy should be centered over the entrance pupil. During ablation, the globe can be immobilized by the surgeon, with a suction ring around the corneoscleral limbus. Alternatively, the globe can be immobilized by patient fixation on a target, unassisted by the surgeon. We investigated which method results in better centration of the ablation zone over the entrance pupil, by using an objective, mathematical method to determine the ablation zone center.
  
  
  METHODS
  Forty-eight eyes from 48 patients who underwent photorefractive keratectomy by the two techniques were studied retrospectively. The centers of the ablation zones were objectively determined by a weighted center of mass algorithm applied to the preoperative minus postoperative difference maps. The validity of the objective method was confirmed by comparison to subjective estimates of ablation zone centers made by independent human observers.
  
  
  RESULTS
  The 19 eyes treated by surgeon fixation had an average decentration of the ablation zone of 0.63 +/- 0.31 mm (range, 0.01 to 1.00 mm), and the 29 eyes treated by patient fixation had an average decentration of 0.41 +/- 0.23 mm (range, 0.11 to 1.18 mm) (P = .027).
  
  
  CONCLUSIONS
  The center of the ablation zone can be determined mathematically from the topographic map, to avoid observer bias. In this study, unassisted patient fixation during photorefractive keratectomy produced more accurate centration of the ablation zone than did surgeon fixation and has the potential for maximizing the quality of vision postoperatively.
authors.affiliation: Jules Stein Eye Institute, Los Angeles, CA 90024-7003, USA.
authors.forename: J
authors.initials: J
authors.lastname: Terrell
authors.forename: S J
authors.initials: SJ
authors.lastname: Bechara
authors.forename: A
authors.initials: A
authors.lastname: Nesburn
authors.forename: G O
authors.initials: GO
authors.lastname: Waring
authors.forename: J
authors.initials: J
authors.lastname: Macy
authors.forename: R K
authors.initials: RK
authors.lastname: Maloney
mesh_terms: D000465:Algorithms; D003315:Cornea / Q000601:surgery*; D005123:Eye / Q000033:anatomy & histology; D005403:Fixation, Ocular; D005500:Follow-Up Studies; D006801:Humans; D007091:Image Processing, Computer-Assisted; D053685:Laser Therapy / Q000379:methods*; D009216:Myopia / Q000601:surgery*; D011680:Pupil / Q000502:physiology; D015203:Reproducibility of Results; D012189:Retrospective Studies