Precision of three topography instruments in keratoconus subjects

Purpose. To determine the test-retest reliability of three popular and comm ercially available Placido-ring videokeratography instruments in subjects w ith keratoconus. Methods. Nine subjects (16 eyes) with keratoconus of varyi ng degrees of severity had up to four images per eye generated, in random o rder, from the EyeSys Model II, Dicon CT 200 and the Keratron Corneal Analy zer. Test-retest analyses for the images were sampled at four locations: 1. 5 mm nasal, inferior, temporal, and superior from center. The average stand ard deviation of all points was used to determine the short-term variabilit y 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, 0.7 9 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 instrum ents when corneal irregularity is present, which reduces test-retest reliab ility.