How accurately can videokeratographic systems measure surface elevation?

Introduction: Surface topography, as opposed to dioptric topography, defines the corneal surface in simple terms without assumptions. Accordingly, it is important to know how well surface topography can be measured with current videokeratometric machines.

Purpose: The purpose of this paper is to quantify the accuracy with which the TMS-1 Corneal Modeling System can measure the surface topography of calibrated spherical, elliptical, and bicurve surfaces.

Methods: The Computed Anatomy TMS-1 videokeratometer was used to measure three spherical, three elliptical, and two bicurve surfaces with known characteristics. Surface characteristics were either back-calculated from the dioptric files or directly obtained from the TMS-1 elevation file for each of 6400 points (256 points in each of 25 rings). The accuracy with which each method determined the true surface was quantified by calculating the root mean squared error (RMSE) of the 6400 measured surface elevations from the known surface elevation at each sampling point.

Results: (1) For spherical and elliptical surfaces, back-calculation of surface elevation from the dioptric file can be made with RMSE of 5 mu or less. (2) For spheres but not elliptical surfaces the TMS-1 elevation file defines the surface with RMSE 5 mu or less. (3) The surface area measured by placido-based videokeratometers varies with surface curvature. (4) RMSE in measured surface elevation increase as the distance from the videokeratometric axis increases. (5) For bicurves, the dioptric maps are smoothed by the TMS-1 over abrupt transitions and for large transitions never recover. Additionally, our back-calculation methods further smooth abrupt transitions, making the RMSE of the bicurve surface that is back-calculated from the dioptric file larger than the RMSE of the surface generated from the TMS-1 elevation file.

Conclusions: Surface elevations can be back-calculated from dioptric files with RMSE of 5 microns or less for spheres and elliptical surfaces as long as there are no areas of abrupt transition. If areas of abrupt transition exist, the TMS-1 elevation file provides more accurate surface profile data.