Intrasession and intersession repeatability of the Pentacam system on posterior corneal assessment in the normal human eye

Purpose: To study the intrasession and intersession repeatability of the Pentacam system (Oculus Inc.) in measuring the posterior corneal shape. Setting: School of Optometry, Hong Kong Polytechnic University, Hong Kong, China. Methods: The posterior corneal shape in 1 eye (randomly selected) was measured with the Pentacam system. Three consecutive readings were taken for intrasession repeatability analysis. Measurements were repeated 1 to 2 weeks later for intersession repeatability analysis. The anterior best-fit sphere (BFS) and posterior BFS at the 5.0 mm and 8.0 mm zones, as well as the elevation of the posterior cornea at these 2 zones, were compared. Results: The anterior and posterior BFS demonstrated good intrasession repeatability (Cronbach α ≥0.997; intraclass correlation coefficients ≥0.990). No significant difference was found between the 2 visits. The 95% limits of agreement of posterior BFS were 52.3 to -41.7 μm at the 5.0 mm zone and 66.1 to -60.9 μm at the 8.0 mm zone. The elevation was similar between the 2 visits. The 95% limits of agreement were 4.27 to -5.29 μm at the 5.0 mm zone and 5.33 to -5.77 μm at the 8.0 mm zone. Conclusions: Pentacam provided good performance in assessing the posterior cornea. To improve the intersession repeatability, it is suggested that 3 consecutive readings, rather than 1 image, be taken to generate an average BFS for analysis. When comparing the posterior corneal elevation between sessions, an average BFS generated from the first visit should be used for elevation calculation.School of Optometr