Dependability of Pachymetry Measurements after Myopic Advanced Surface Ablation Using Scanning-Slit Topography and Specular Microscopy

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To assess the repeatability, intersession and interobserver reproducibility, and agreement of central corneal thickness (CCT) measurements obtained by scanning-slit topography (SST) and noncontact specular microscopy (NCSM) after advanced surface ablation (ASA). To analyze repeatability, one examiner measured 63 post-myopic ASA eyes five times successively using both techniques randomly. To calculate interobserver reproducibility a second examiner obtained another CCT measurement in a random fashion. To study intersession reproducibility, the first operator obtained CCT measurements from another 24 eyes during two sessions 1 week apart. With regard to intrasession repeatability, SST and NCSM within-subject standard deviation (S(w)) and intraclass correlation coefficient (ICC) were 7.35 and 3.81 μm, and 0.97 and 0.99, respectively. For interobserver reproducibility, SST measurement variability showed correlation with CCT magnitude (r(s) = -0.38; P = 0.002), whereas NCSM did not. NCSM S(w) and ICC were 3.83 μm and 0.99, respectively. For intersession reproducibility, no difference in CCT measurements was found for any technique; S(w) and ICC estimates for SST and NCSM were 12.2 and 8.37 μm, and 0.94 and 0.95, respectively. We found a tendency for the difference (mean SST-NCSM = 13.39 μm) to increase in thicker corneas (r(s) = 0.45, P = 0.001). Both noncontact pachymetry techniques provided highly repeatable and quite reproducible CCT measurements in post-ASA patients having no clinically significant corneal haze, except for SST interobserver reproducibility, which decreased in thinner corneas. However, the techniques were not interchangeable. The estimates provided should help clinicians differentiate real CCT change from noncontact pachymetry measurement variability after ASA.