Reliability of Noncontact Pachymetry after Laser In Situ Keratomileusis

Abstract

To assess the repeatability and reproducibility of central corneal thickness (CCT) measurements obtained by combined scanning-slit/Placido-disc topography (Orbscan II; Bausch & Lomb, Rochester, NY) after laser in situ keratomileusis (LASIK) and to compare the results with another noncontact method, specular microscopy (SP-2000P; Topcon, Tokyo, Japan). To analyze intrasession repeatability, one examiner measured 22 postmyopic LASIK eyes 10 times successively in the shortest time possible, using both devices randomly. To study intersession reproducibility, the same operator obtained measurements from another 50 eyes with stable refraction in two consecutive visits at the same time of the day between 6 and 9 months after myopic LASIK. Any association between residual stromal bed thickness and measurement variability was recorded and evaluated. For intrasession repeatability, Orbscan II and Topcon SP-2000P CCT measurements showed a repeatability of 20.2 (4.3%) and 12.8 (2.6%) microm, respectively. Both devices yielded excellent intraclass correlation coefficients (ICC; 0.98 and 0.99, respectively). For intersession reproducibility, no difference in CCT measurements was found. The coefficient of intersession reproducibility were 6.7% (29.5 microm) for Orbscan II and 4.3% (19.5 microm) for Topcon SP-2000P CCT measurements. The ICCs (0.95 and 0.96, respectively) indicated good intersession reliability. Repeatability and reproducibility with both devices were unrelated to stromal bed thickness. Both noncontact pachymeters provide repeatable CCT measurements in transparent postmyopic LASIK corneas after the early postoperative period. Intersession variations in CCT of more than 29 microm with the Orbscan II and 19 microm with the Topcon SP-2000P may reflect true corneal change. These estimates should help investigators and clinicians differentiate actual CCT modification from measurement variability.