Repeatability of Zone Averages Compared to Single Point Measurements of Maximal Curvature in Keratoconus.

Abstract Purpose To evaluate the repeatability of curvature zone averages centered on the point of maximum curvature (Kmax) compared to that of the single-point Kmax. Design Comparative reliability analysis. Methods Setting : American University of Beirut Medical Center, Beirut, Lebanon. Study Population 65 eyes of 65 adult keratoconus patients. Patients with other ocular disease, history of ocular surgery or trauma, and contact lens wear within 2 weeks of image acquisition were excluded. Observation Procedures Eyes were evaluated with 3 consecutive scans using the Galilei dual Scheimpflug-Placido system. Main Outcome Measures Repeatability of axial and instantaneous Kmax single-points, and zone averages with radii of 0.1mm to 2.0mm, centered on them. Repeatability was assessed by within-subject standard deviations, repeatability limits (r), and intraclass correlation coefficients. Results Axial curvature zone averaging yielded clinically acceptable repeatability only in eyes with Kmax≤50D, for radii of 1.5mm and 2.0mm (r=0.87D and r=0.76, respectively, versus r=0.91 for the single-point axial Kmax). Compared to instantaneous Kmax, clinically acceptable repeatability was achieved with instantaneous zone averages of at least 1.5mm radius in eyes with Kmax≤50D (r=0.99D and r=0.70D, respectively), and 2.0mm radius in eyes with Kmax>50D (r=2.28D and r=0.87D, respectively). For all eyes, the repeatability limit of the location of Kmax was 0.82mm and 0.80mm for axial and instantaneous curvature, respectively. Conclusions Instantaneous curvature zone averages centered on Kmax yielded a greater improvement in repeatability than axial zone averages and reached clinical adequacy with radii of at least 1.5mm, for eyes with Kmax≤50D, and with a 2.0mm radius for eyes with Kmax>50D.