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Precision of three topography instruments in keratoconus subjects
T T McMahon1, R J Anderson, C E Joslin
1University of Illinois at Chicago Department of Ophthalmology & Visual Sciences, College of Medicine, 60612, USA. timomcma@uic.edu
Purpose:
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.
Conclusion:
Results support the notion of a loss in repeatability for all three instruments when corneal irregularity is present, which reduces test-retest reliability.
