Repeatability and Agreement of Three Scheimpflug-Based Imaging Systems for Measuring Anterior Segment Parameters in Keratoconus

PURPOSE. To assess the repeatability and agreement of three rotating Scheimpflug cameras, Pentacam, Galilei, and Sirius, in measuring the mean keratometry (Km), thinnest corneal thickness (TCT), anterior chamber depth (ACD), and mean posterior keratometry (pKm) in keratoconus patients in a prospective study. METHODS. Fifty-five eyes of 55 patients with keratoconus underwent three consecutive scans on each machine, performed by a single operator. Within-subject standard deviation (Sw), test-retest repeatability (TRT), and coefficient of variation (COV) for assessing repeatability and Bland-Altman plots for the agreement between the mean measurements of each machine were examined. RESULTS. The Sw of Km and pKm measurements with Pentacam (0.23 and 0.10 diopters [D], respectively) were significantly lower (better) than those of Galilei (0.60 and 0.17) and Sirius (0.23 and 0.36). The Sw of TCT measurements with Sirius (8.88 mu m) was significantly lower than that of Galilei (11.64 mu m). The COV ranged between 0.5 for the Km measurements of Pentacam and 2.8 for the TCT measurements of Galilei. Significant proportional bias in agreement was detected for the pKm measurements with all the three device pairs and for the ACD measurements between Pentacam and Galilei and between Galilei and Sirius. CONCLUSIONS. Though Pentacam, Galilei, and Sirius showed repeatable measurements for Km, TCT, ACD, and pKm, repeatabilities with Pentacam and Sirius were better than those with Galilei. There were significant differences in the measurements between the three devices; hence they cannot be used interchangeably for anterior segment measurements in keratoconus patients.