title: Correction factor in Orbscan II in the assessment of corneal pachymetry.
abstract: OBJECTIVE
  Comparison of corneal pachymetry assessment with ultrasound and Orbscan II using acoustic factor and subtraction methods.
  
  
  METHODS
  Ultrasound and Orbscan pachymetry for all patients with LASIK performed between July 2002 and May 2003 were retrospectively analyzed. Comparison between the 2 preoperative measurements was analyzed.
  
  
  RESULTS
  Using a custom acoustic factor of 0.93, there was no significant difference between the overall ultrasound and Orbscan pachymetry (P = 0.696). However, there was underestimation in thick corneas and overestimation in thin corneas. Using the subtraction method as the correcting technique, the magnitude of over- and underestimation was reduced.
  
  
  CONCLUSIONS
  The current correction method by means of acoustic factor may result in error in assessing extremes of corneal thickness. Alternative approaches such as the subtraction method can be considered to achieve more accurate results.
authors.affiliation: Department of Ophthalmology and Visual Sciences, The Chinese University of Hong Kong, University Eye Center, Hong Kong Eye Hospital, Hong Kong. artcheng@netvigator.com
authors.forename: Arthur C K
authors.initials: AC
authors.lastname: Cheng
authors.forename: Emily
authors.initials: E
authors.lastname: Tang
authors.forename: Shaheeda
authors.initials: S
authors.lastname: Mohamed
authors.forename: Dennis S C
authors.initials: DS
authors.lastname: Lam
mesh_terms: D000328:Adult; D000886:Anthropometry; D002138:Calibration; D003315:Cornea / Q000000981:diagnostic imaging / Q000473:pathology*; D019781:Corneal Topography / Q000379:methods*; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000601:surgery*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D014463:Ultrasonography