title: [An analysis of measurement error in Orbscan topography system].
abstract: OBJECTIVE
  To discuss the measurement error and the influence factor of Orbscan Topography to provide more accurate data for clinical application.
  
  
  METHODS
  Orbscan Topography System and A-scan ultrasonography were used to measure the corneal thickness and anterior chamber depths pre-operatively and post-operatively, including 200 cases (400 eyes) undergoing laser in situ keratomileusis (Lasik) and 50 cases (100 eyes) undergoing photorefractive keratectomy (PRK). The curvature of posterior corneal surface was measured by Orbscan topography before and after the operations, and the result of measurement was compared.
  
  
  RESULTS
  Pre-operatively, the value of corneal thickness was (18.83 - 24.29) microm lower measured by Orbscan topography than that by A-ultrasonography, and post-operatively the value was (56.32 - 139.52) micro m lower than that by A-ultrasonography. The postoperative value was obviously higher than the pre-operative one. The value of central anterior chamber depth was (0.16 - 0.25) mm lower measured by Orbscan system than that by A-ultrasonography pre-operatively and post-operatively. There was no clinical significance. The difference in the comparison of posterior corneal surface curvature before and after Lasik was -0.60 D measured by Orbscan System. The difference in the comparison of posterior corneal surface curvature before and after PRK was -0.51 D in the lower diopter group and -1.12 D in medium diopter group.
  
  
  CONCLUSIONS
  It is necessary to improve the accuracy of Orbscan Topography System. The Haze and the change of anterior corneal surface curvature after Lasik and PRK are the main causes of measurement error in Orbscan Topography System. Clinically, it is an un-negligible important problem in the measurement of Orbscan system.
authors.affiliation: Ophthalmic Center, the Second Affiliated Hospital, Chongqing University of Medical Sciences, Chongqing 400010, China. kuanrenyk@yahoo.com.cn
authors.forename: Zhi-yu
authors.initials: ZY
authors.lastname: Du
authors.forename: Da-yong
authors.initials: DY
authors.lastname: Zhang
authors.forename: Qing
authors.initials: Q
authors.lastname: Zheng
authors.forename: Yao
authors.initials: Y
authors.lastname: Chen
authors.forename: Hong
authors.initials: H
authors.lastname: Guo
authors.forename: Hong-min
authors.initials: HM
authors.lastname: Yin
mesh_terms: D000293:Adolescent; D000328:Adult; D000867:Anterior Chamber / Q000473:pathology; D003315:Cornea / Q000473:pathology* / Q000601:surgery; D019781:Corneal Topography / Q000379:methods / Q000592:standards*; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer; D008297:Male; D008875:Middle Aged; D018903:Photorefractive Keratectomy; D015203:Reproducibility of Results