Central corneal thickness measurements after myopic photorefractive keratectomy using Scheimpflug imaging, scanning-slit topography, and ultrasonic pachymetry

Abstract

Purpose: To compare central corneal thickness (CCT) measurements obtained with a dual Scheimpflug camera system (Galilei), a scanning-slit topographer (Orbscan II), and an ultrasound (US) pachymeter (SP-2000) after photorefractive keratectomy (PRK) for myopia.

Setting: Ophthalmic Research Center, Labbafinejad Medical Center, Tehran, Iran.

Methods: This prospective study enrolled patients who had PRK to correct myopia or myopic astigmatism and a postoperative follow-up of at least 5 months. The CCT was measured in a single session using Scheimpflug imaging, scanning-slit topography, and US pachymetry. Data were analyzed with paired t tests, linear regression, and Bland-Altman plots.

Results: The mean CCT in the 100 eyes (50 patients) was 524.06 microm +/- 38.56 (SD) with Scheimpflug imaging, 505.92 +/- 47.35 microm with scanning-slit topography, and 496.97 +/- 42.74 microm with US pachymetry. The lower to upper 95% limits of agreement (LoA) with US pachymetry were 2.4 to 51.8 microm for Scheimpflug imaging and -22.2 to 40.1 microm for scanning-slit topography. After application of a correction factor (0.95), the mean corrected Scheimpflug CCT measurement was 497.33 +/- 36.59 microm and the lower to upper 95% LoA with US pachymetry were -25.9 to 25.5 microm. The acoustic factor for scanning-slit topography was 0.94.

Conclusions: The CCT measurements in eyes that had PRK were thicker with Scheimpflug imaging than with US pachymetry or scanning-slit topography in the late postoperative period. With application of a correction factor, the Scheimpflug measurements were closer to US pachymetry values and had better agreement than scanning-slit topography.