The corneal tomographic analysis was performed using AS-OCT (CASIA SS-1000; Tomey Corporation, Inc., Nagoya, Japan) with an infrared wavelength of 1310 nm and a scanning speed of 30,000 axial scans per second. An alignment function, which automatically moves the head unit to the measurement position based on corneal reflex, and an automatic focusing function were installed into the system. Therefore, the measurement was performed along the vertex normal, and the images were centered on the corneal vertex. The analysis program identified and digitized the anterior and posterior corneal surfaces, and the reference axis of the measurement was aligned with the vertex normal. Tomographic and pachymetric maps were calculated from 16 radial cross-sectional images through the central 10-mm diameter of the corneas obtained over 0.34 seconds. Each cross-sectional image consisted of 512 telecentric A-scans. Swept-source OCT measurements have been shown to have adequate repeatability in normal and keratoconic eyes.
17 The best-fit spheres (BFS) were computed for the anterior and posterior corneal surfaces, and differences between fitted surfaces and real data were plotted as elevation maps. Tomographic maps of corneal thickness also were displayed. Minimum corneal thickness (T
min) was defined as the pachymetric value at the thinnest location. The distance from the thinnest point to the corneal vertex (D
min) was calculated. Calculations of BFS, T
min, and D
min were performed inside the central 9-mm diameter of the cornea. In the axial power maps, corneal power was calculated by a circular approximation that fixed the center of curvature at the corneal vertex based on the data of each localized corneal shape, and the maximal power in the central 8-mm diameter of corneas was defined as the maximal K value (K
max). All calculations used the same calibration files, ensuring all the measurements were performed using identical software and hardware.