Comparison of Pentacam and Orbscan IIz on posterior curvature topography measurements in keratoconus eyes.

PURPOSE: To compare the measurements of posterior elevation above the best-fit sphere and of pachymetry in established keratoconus patients using 2 different technologies--Scheimpflug imaging with the Pentacam and scanning slit combined with Placido imaging with Orbscan IIz.

DESIGN: Retrospective comparative case series.

PARTICIPANTS: Patients with confirmed keratoconus by biomicroscopy and Placido topography who had corneal topography scans by the Pentacam and Orbscan IIz on their visit from April 2004 to April 2005. These scans were obtained routinely for patient diagnosis, treatment, and progression.

METHODS: The mean difference, standard deviation (SD), and 95% limits of agreement (LOA) were calculated, and Bland-Altman plots were constructed for best-fit sphere radius of curvature, posterior elevation above the best-fit sphere, and pachymetry of the thinnest point.

RESULTS: Thirty-six eyes of 29 patients were analyzed. The average radius for posterior best-fit sphere was 5.97 microm (range, 4.69-6.79) for the Pentacam and 6.00 microm (range, 4.97-6.55) for Orbscan IIz. The mean difference (Pentacam - Orbscan IIz) for the radius generated for the best-fit sphere was -0.03+/-0.22 SD with a 95% confidence interval (CI) of -0.11 to 0.04 and a 95% LOA of -0.46 to 0.40 (P = 0.362). The mean posterior elevation by best-fit sphere fixed to the apex was 34.86 microm (range, 3-120) for the Pentacam and 48.50 microm (range, 11-118) for Orbscan IIz. The mean difference was -13.64+/-26.08 SD (95% CI, -22.46 to -4.81; 95% LOA, -64.75 to 37.48; P = 0.003). The average thinnest point for the Pentacam was 443.6 microm (range, 164-587), and that for Orbscan IIz was 445.9 microm (range, 204-597). The mean difference in the thinnest point for the Pentacam and Orbscan IIz was -2.28+/-35.55 (95% CI, -14.31 to 9.75; 95% LOA, -71.95 to 67.39; P = 0.703).

CONCLUSION: Both the Pentacam and Orbscan IIz determine similar thinnest points but have a measurable difference in posterior elevations above the best-fit sphere, despite similar radii of curvature. This difference may be important in the screening of patients for refractive surgery to avoid surgery on patients with early keratoconus. This study cannot determine if the Pentacam is underestimating the posterior vault or if Orbscan is overestimating this height, and further studies with a standardized test object are needed.