title: Response of the posterior corneal surface to laser in situ keratomileusis for myopia.
abstract: OBJECTIVE
  To describe the response of the posterior corneal surface in laser in situ keratomileusis (LASIK) and determine whether residual stromal bed thickness or treatment magnitude is predictive of the posterior corneal surface elevation after uneventful LASIK.
  
  
  METHODS
  A private hospital-based refractive surgery practice, Hong Kong SAR, China.
  
  
  METHODS
  Orbscan I (Bausch & Lomb) videokeratography examinations were performed on 1124 patients before and 6 months after LASIK for myopia (mean -6.81 diopters [D] +/- 2.52 [SD]; range -0.88 to -14.50 D). The best-fit sphere (BFS) over the central 9.0 mm region of the posterior corneal surface before and after treatment was compared. The location and magnitude of the 1.0 mm diameter region of highest elevation above the BFS for the central 4.0 mm diameter zone were calculated before and after treatment and compared using a paired t test. Stepwise regression was used to model the best predictors of the posterior radius of the BFS and the central elevation of the corneal surface above the BFS before and after treatment.
  
  
  RESULTS
  The mean radius of curvature of the posterior surface BFS decreased 0.10 mm after LASIK, from 6.31 to 6.21 mm (P<.001). Elevation above this BFS was increased 10 microm within a 1.00 mm diameter region of interest, and this was correlated with postoperative corneal thickness, inferotemporal decentration of the highest point, residual myopia, and steeper central posterior radius of curvature.
  
  
  CONCLUSIONS
  No eye was diagnosed with corneal ectasia at the time of the 6-month postoperative visit. After LASIK, there was a decreased radius of curvature for the BFS of the posterior corneal surface, with the highest elevation point located paracentrally. These findings are similar to the anterior corneal surface changes observed in corneal ectasia after LASIK but smaller in magnitude.
authors.affiliation: College of Optometry, Ohio State University, Columbus, Ohio, USA. twa.1@oau.edu
authors.forename: Michael D
authors.initials: MD
authors.lastname: Twa
authors.forename: Cynthia
authors.initials: C
authors.lastname: Roberts
authors.forename: Ashraf M
authors.initials: AM
authors.lastname: Mahmoud
authors.forename: John S
authors.initials: JS
authors.lastname: Chang
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001251:Astigmatism / Q000601:surgery; D001696:Biomechanical Phenomena; D003315:Cornea / Q000502:physiology*; D003319:Corneal Stroma / Q000502:physiology; D019781:Corneal Topography; D006723:Hong Kong / Q000208:ethnology; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008875:Middle Aged; D009216:Myopia / Q000208:ethnology / Q000503:physiopathology / Q000601:surgery*; D012029:Refraction, Ocular / Q000502:physiology; D012189:Retrospective Studies; D014792:Visual Acuity / Q000502:physiology