title: Assessing the accuracy of Orbscan II post-LASIK: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures.
abstract: OBJECTIVE
  To establish a relationship between the forward protrusion of the cornea (keratectasia), following successful LASIK, and anterior chamber depth.
  
  
  METHODS
  One hundred and fifteen eyes of 59 patients who had undergone Zyoptix LASIK and 39 eyes of 21 patients who had undergone PlanoScan LASIK at the Eye Institute, Centre for Refractive Surgery, Remuera, Auckland, New Zealand, were included in the study. The results of Orbscan II acquisitions, taken before and 8.3 weeks +/- 4.0 (mean +/- standard deviation) following the procedure, were analysed to determine the amount of forward protrusion of the central 2 mm diameter of the posterior surface. The apparent keratectasia was then related to the Orbscan II anterior chamber depth (ACD) measurement in a linear mixed model analysis that included 'right or left eye', 'sex', 'type of LASIK procedure', 'change in pachymetry', 'postoperative pachymetry', 'change in anterior corneal curvature', 'postoperative anterior corneal curvature', 'planned ablation depth' and 'time until follow-up' as factors and covariates.
  
  
  RESULTS
  The mean +/- SD amount of keratectasia determined by Orbscan II was 22.8 +/- 26.8 microm for the Zyoptix group and 16.9 +/- 24.6 microm for the PlanoScan group. The reduction in depth of the anterior chamber was 14.7 +/- 79.3 microm for the Zyoptix group and 18.2 +/- 68.8 microm for the PlanoScan group. The mixed model analysis demonstrated strong relationships with postoperative pachymetry and change in anterior corneal curvature (P < 0.001 and P < 0.001, respectively). Furthermore, the statistical model showed a marked significant difference in the Orbscan II assessment of keratectasia and change in ACD (P < 0.001).
  
  
  CONCLUSIONS
  Using the Orbscan II device, the forward protrusion of the posterior corneal surface appears to coincide with a paradoxical reduction in depth of the anterior chamber. Although the keratectasia found in this study is accompanied by thin postoperative corneas and larger central pachymetry change, the contradictory decrease in anterior chamber depth creates uncertainty of measurement, and of ectasia as a mechanism of regression.
authors.affiliation: Department of Ophthalmology, University of Auckland, Auckland, New Zealand.
authors.forename: Gerard
authors.initials: G
authors.lastname: Cairns
authors.forename: Susan E
authors.initials: SE
authors.lastname: Ormonde
authors.forename: Trevor
authors.initials: T
authors.lastname: Gray
authors.forename: O Bruce
authors.initials: OB
authors.lastname: Hadden
authors.forename: Tony
authors.initials: T
authors.lastname: Morris
authors.forename: Peter
authors.initials: P
authors.lastname: Ring
authors.forename: Charles N J
authors.initials: CN
authors.lastname: McGhee
mesh_terms: D000328:Adult; D000867:Anterior Chamber / Q000473:pathology*; D003315:Cornea / Q000473:pathology*; D019781:Corneal Topography / Q000379:methods; D004108:Dilatation, Pathologic / Q000175:diagnosis; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008297:Male; D008875:Middle Aged; D011182:Postoperative Care; D015203:Reproducibility of Results; D012189:Retrospective Studies