title: Precision of three topography instruments in keratoconus subjects.
abstract: OBJECTIVE
  To determine the test-retest reliability of three popular and commercially available Placido-ring videokeratography instruments in subjects with keratoconus.
  
  
  METHODS
  Nine subjects (16 eyes) with keratoconus of varying degrees of severity had up to four images per eye generated, in random order, from the EyeSys Model II, Dicon CT 200 and the Keratron Corneal Analyzer. Test-retest analyses for the images were sampled at four locations: 1.5 mm nasal, inferior, temporal, and superior from center. The average standard deviation of all points was used to determine the short-term variability of the measurements.
  
  
  RESULTS
  The short-term variability (in diopters) of the Dicon, EyeSys, and Keratron was 0.61 to 3.31 D, 0.94 to 1.51 D, and 0.58 to 2.85 D, respectively, for axial distance maps and 1.07 to 6.82 D, 0.79 to 1.77 D, and 1.23 to 3.03 D for tangential curvature maps.
  
  
  CONCLUSIONS
  Results support the notion of a loss in repeatability for all three instruments when corneal irregularity is present, which reduces test-retest reliability.
authors.affiliation: University of Illinois at Chicago Department of Ophthalmology & Visual Sciences, College of Medicine, 60612, USA. timomcma@uic.edu
authors.forename: T T
authors.initials: TT
authors.lastname: McMahon
authors.forename: R J
authors.initials: RJ
authors.lastname: Anderson
authors.forename: C E
authors.initials: CE
authors.lastname: Joslin
authors.forename: G A
authors.initials: GA
authors.lastname: Rosas
mesh_terms: D003315:Cornea / Q000473:pathology*; D019781:Corneal Topography / Q000295:instrumentation / Q000592:standards*; D006801:Humans; D007640:Keratoconus / Q000175:diagnosis*; D015203:Reproducibility of Results