Reliability of a Scheimpflug-based tomographer compared with a Placido-based topographer and specular microscopy in healthy eyes.

Abstract

Purpose: To evaluate the reliability and comparability of a Scheimpflug-based tomographer relative to a Placido-based topographer and specular microscopy in healthy eyes.

Methods: This cross-sectional study included 40 patients (80 eyes). Each eye underwent randomized imaging with a Scheimpflug-based tomographer, a Placido-based topographer, and Tomey EM-4000 specular microscopy. Three acquisitions per device were obtained. For interdevice comparisons, the best-quality scan per eye/device was selected, whereas all three scans were used for intradevice repeatability analyses. Unreliable scans were repeated (up to five attempts) and excluded if acceptable quality was not achieved, resulting in variable denominators. Between-device comparisons were performed using generalized estimating equations with participant-level clustering and robust standard errors and were supplemented by Bland-Altman analysis.

Results: The effective sample size varied by parameter (independent summaries: 59-67 eyes; paired comparisons: 48-51 eyes). In paired-eye analyses, the Scheimpflug-based tomographer measured slightly higher keratometry values than the Placido-based topographer (K1: 43.95 vs. 43.78 D, p=0.003; K2: 44.91 vs 44.73 D, p=0.002), more negative Q-values (p=0.001), smaller photopic pupil diameter (p<0.001), and shorter kappa distance (p<0.001). Mean absolute differences were 0.32 D for K1 and 0.30 D for K2, with high dispersion for angular metrics (kappa angle coefficient of variation: 195%).

Conclusions: The Scheimpflug-based tomographer provides reproducible corneal measurements in healthy eyes. However, systematic differences relative to the Placido-based topographer-particularly for keratometry, asphericity, and pupil and kappa metrics-suggest limited interchangeability. Consistent device use is recommended when these parameters inform clinical decision-making.