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.
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