Accuracy of Orbscan II slit-scanning elevation topography

Abstract

Purpose: To establish the accuracy of Orbscan II (Orbtek Inc.) slit-scanning elevation
topography in analyzing the anterior surface of complex test objects.

Setting: Discipline of Ophthalmology, University of Auckland, Faculty of Medical
and Health Sciences, Auckland, New Zealand.

Methods: Six test objects were created from 2 materials: standard calibration poly(methyl methacrylate) (PMMA) (Orbtek Inc.) and a research PMMA material. The test objects were produced with spherical (radii of curvature 6.00 mm, 7.67 mm, and 8.88 mm), aspherical (apical radius 7.67, eccentricity 0.5, Q _0.25), and toroidal (7.67/7.92 mm radii of curvature) surfaces. The accuracy of the test surfaces was established by Form Talysurf Analysis. A single calibrated Orbscan II device was used to obtain 20 separate anterior elevation maps of each test object. The data obtained from Orbscan II, at 0.2 mm intervals along the chosen meridian, were directly compared with the Talysurf values for each test surface.

Results: Orbscan II measurements of all test objects were statistically significantly
different from the Talysurf values (P _ .001). The test objects produced from standard calibration material were more accurately measured by Orbscan II than the objects produced from the research material. Data obtained by Orbscan II from the central 3.5 mm of all test objects were more accurate than peripheral data when compared with the Talysurf values (P _ .001).

Conclusions: Orbscan II anterior surface elevation measurements differed significantly
from Form Talysurf Analysis of complex test surfaces. However, the magnitude of the errors in the measurement of standard test objects was small, less than 0.20 _m centrally and 0.70 _m peripherally. Clinically, if similar accuracy of measurement is confirmed in the human eye, anterior surface elevation maps can be considered accurate representations of corneal shape.