title: Corneal topographic evaluation of decentration in photorefractive keratectomy: treatment displacement vs intraoperative drift.
abstract: OBJECTIVE
  To evaluate treatment displacement and movement during treatment (drift) after excimer laser photorefractive keratectomy using tangential topographic maps.
  
  
  METHODS
  Forty-eight eyes of 48 patients showing axial decentration of 0.30 mm or more at 1 month posttreatment were reevaluated retrospectively to determine treatment displacement of the center of the photorefractive keratectomy ablation from the center of the pupil. A drift index was calculated to determine the relative degree of movement (drift) during treatment. We subdivided patients into four groups based on the degree of treatment displacement and drift and compared the mean axial decentration and the mean best-corrected logMAR visual acuity among the subgroups.
  
  
  RESULTS
  Mean treatment displacement +/- SD from the center of the entrance pupil was 0.34 +/- 0.21 mm. Thirty-eight eyes (79.2%) had ablations within 0.50 mm from the center of the entrance pupil. We observed downward displacement in 27 eyes (56.2%) and upward displacement in 21 eyes (43.8%). The drift index showed a positive, statistically significant correlation with best-corrected visual acuity (r = .58, P < .0001). Patients with low displacement and low drift had mean logMAR best-corrected visual acuity of 0.91, which was statistically significantly better than patients with high displacement and high drift (r = 0.64; P = .009).
  
  
  CONCLUSIONS
  In patients with gross decentration by axial topography after photorefractive keratectomy, tangential corneal topography is valuable in evaluating and differentiating photorefractive keratectomy treatment displacement from movement during treatment (drift). Patients with high drift index have worse visual outcomes after photorefractive keratectomy than those exhibiting high treatment displacement.
authors.affiliation: Corneal and Refractive Surgery Services, Massachusetts Eye and Ear Infirmary, Boston 02114, USA.
authors.forename: D T
authors.initials: DT
authors.lastname: Azar
authors.forename: P C
authors.initials: PC
authors.lastname: Yeh
mesh_terms: D003315:Cornea / Q000473:pathology* / Q000601:surgery; D019781:Corneal Topography; D006801:Humans; D007431:Intraoperative Complications / Q000473:pathology*; D054018:Lasers, Excimer; D008027:Light; D009216:Myopia / Q000601:surgery; D018903:Photorefractive Keratectomy / Q000009:adverse effects*; D011680:Pupil; D012189:Retrospective Studies; D014792:Visual Acuity