Retrieving Shape From Raw Height Data - A Comparison Of Four Different Corneal Topographers

Abstract

Purpose: : For optical modelling, an accurate description of the shape of the anterior corneal surface is indispensable. Aim of this study was to compare the results of fitting Zernike polynomials to raw corneal height data obtained by four different corneal topographers.

Methods: : We assessed the corneas of both eyes of 30 healthy subjects with four corneal topographers: the Atlas 900 (Carl Zeiss Meditec), Galilei (Ziemer), Orbscan IIz (Bausch & Lomb) and Pentacam HR (Oculus). The sequence of the instruments was randomized. Raw height data were fitted by Zernike polynomials. Subsequently, we determined the mean values with standard deviation, the inter-device variability and the test-retest variability of the Zernike coefficients defocus (z₂⁰), astigmatism (z₂^-2 and z₂²), coma (z₃^-1 and z₃¹) and spherical aberration (z₄⁰), for 5.5 mm and 8.0 mm diameter.

Results: : The smallest test-retest variability was obtained with the Pentacam. Two standard deviations of the differences between test and retest were in the order of magnitude of 0.3 μm, 0.4 μm, 0.1 μm and 0.1 μm for defocus, astigmatism terms, coma terms and spherical aberration, respectively. The other devices had considerably larger test-retest variability, in the order of magnitude of 2 or more times as large. For the anterior surface of the right eye at 5.5 mm diameter, the mean values (standard deviation) of the coefficients were -143 (4.76) [z₂⁰], 0.32 (0.75) [z₂^-2], 1.62 (1.59) [z₂²], -0.12 (0.43) [z₃^-1], 0.25 (0.37) [z₃^-1] and -0.96 (0.17) [z₄⁰] μm (measured with the Pentacam). The mean difference (bias) between the topographers was in the order of magnitude of 0.4 μm, 0.2 μm, 0.1 μm and 0.1 μm for defocus, astigmatism terms, coma terms and spherical aberration, respectively. Similar results were obtained for the left eyes and the analysis at 8 mm diameter.

Conclusions: : The mean difference between the four corneal topographers is of the same order of magnitude as the test-retest variability of the Pentacam. The test-retest variability of the other devices is of the same order of magnitude as the variability of the coefficients in our study population. This indicates that, of the investigated devices, the Pentacam is the most suitable device for describing differences in the shape of the cornea between individuals having healthy corneas.

Keywords: topography • aberrations