Comparison of three videokeratoscopes in measurement of toric test surfaces

PURPOSE: We compared the accuracy of the Computed Anatomy TMS-1 (1.41), the EyeSys Laboratories Corneal Analysis System (2.1), and the Visioptic EH- 270 (3.0) videokeratoscopes in measuring toric surfaces. These non- rotationally symmetric aspheric surfaces served as models of corneal astigmatism. METHODS: Precision diamond-turned toric surfaces modeling 0.00 diopter (D) to 7.00 D of astigmatism were fabricated. A three-dimensional contact profiler was developed to calibrate the aspheric surfaces. Videokeratoscopic data taken at 'best focus' were compared to the theoretical shape to quantify device measurement errors. RESULTS: The Computed Anatomy system measurement accuracy shows no statistically significant correlation between measurement error and surface toricity (r²<0.13). Measurement error increased linearly with surface astigmatism for the EyeSys Laboratories system (0.12 D rms error per D of astigmatism, r²>0.96, p<0.001 and the Visioptic system (0.03 D error per D of astimatism, r²=0.88, p<0.001). CONCLUSIONS: This study found systematic performance differences among the three machines. Under ideal alignment conditions, the Computed Anatomy TMS-1 is more accurate at detecting astigmatism. The EyeSys Laboratories Corneal Analysis System apparently underestimates the amount of surface astigmatism because of excessive data smoothing. The Visioptic EH-270 errors are primarily in the central zones and may be due to ring localization errors.