Repeatability of the ABC Parameters of the ABCD Keratoconus Grading System With Varying Zonal Averaging Diameters as Evaluated on Dual Scheimpflug/Placido and OCT/Placido Platforms.

Abstract

Purpose: To evaluate the repeatability of the ABC parameters of the ABCD progression display system, initially developed on the Pentacam HR tomography system (Oculus Optikgeräte GmbH), varying zonal averaging diameters, on two non-native devices.

Methods: Patients with keratoconus underwent three consecutive scans using a dual Scheimpflug/Placido analyzer and an optical coherence tomography (OCT)/Placido platform. Repeatability was assessed for the ABC parameters thinnest pachymetry (C), anterior (A) and posterior (B) keratometry derived as zonal averages centered on the thinnest point, both axial and local anterior radius of curvature (A), posterior radius of curvature (B), and thinnest pachymetry (C). Zonal averaging diameters ranging from 1 to 8 mm were evaluated. Repeatability was assessed by within-subject standard deviation, repeatability limits (r), and coefficient of variation (CoV).

Results: Fifty-four and 77 eyes in the dual Scheimpflug and OCT groups, respectively, were evaluated. For zonal averaging diameters of 1, 2, 3, 4, 5, 6, 7, and 8 mm, the anterior axial curvatures for the dual Scheimpflug/Placido system demonstrated repeatability limits of 1.47, 1.07, 0.80, 0.64, 0.49, 0.40, 0.40, and 0.44 diopters (D), respectively, whereas the posterior axial curvature values were 0.49, 0.25, 0.16, 0.16, 0.17, 0.19, 0.20, and 0.23 D, respectively. For the OCT/Placido group, the 1 to 8 anterior axial curvature values were 1.18, 0.96, 0.65, 0.56, 0.50, 0.45, 0.41, and 0.38 D, respectively, whereas the posterior values were 0.64, 0.48, 0.27, 0.19, 0.16, 0.14, 0.12, and 0.11 D, respectively. Axial curvature measurements were slightly more repeatable than local measurements with the dual Scheimpflug device. For diameters of 3 mm or greater, both technologies demonstrated comparable repeatability for posterior curvatures. Repeatability was better in eyes with maximum curvature (Kmax) values of 50.00 D or less than with Kmax values of greater than 50.00 D. In eyes with Kmax values of 50.00 D or less, anterior axial keratometric zonal averaging centered on the thinnest point showed clinically acceptable repeatability at 1 mm for both the dual Scheimpflug (r = 0.74) and OCT (r = 1.06) groups, and at 3 mm in eyes with Kmax values of greater than 50.00 D for both the dual Scheimpflug (r = 1.01) and OCT (r = 0.72) groups.

Conclusions: Anterior axial keratometric zonal averaging centered on the thinnest corneal pachymetry demonstrates clinically acceptable repeatability at an optimal diameter of 2 mm overall and at 3 mm in eyes with Kmax values of greater than 50.00 D for both devices. A comparable study would help determine whether the established Pentacam's ABCD 3-mm diameter is indeed optimal when using this technology.