Accuracy of videokeratography for instantaneous radius in keratoconus

ABSTRACT Purpose. Present videokeratographs provide corneal radius of curvature measurements in terms of axial (sagittal) radius and, for some instruments, instantaneous (tangential) radius. The EyeSys videokeratograph allows conversion from axial to instantaneous radius based on an algorithm, which assumes a basic corneal shape of elliptical form. Method. We tested the accuracy of the EyeSys instantaneous radius algorithm for 29 keratoconic corneas using the criterion that instantaneous radius of the cone apex could be determined accurately by aligning the cone apex with the optic axis of the videokeratoscope. Results. There was a close relation between the instantaneous values obtained from the regular and apex alignments for the 17 eyes with cone apices below 55 D (95% limits of agreement -0.39 to 1.49 D), but not for the 12 eyes above 55 D (95% limits of agreement -4.87 to 6.61 D). The mean distance of the cone position from the center of the map was 1.16 mm for the instantaneous map and 1.59 mm for the axial map. Conclusion. Assuming the criterion for accuracy to be the corneal radii found when the videokeratograph was aligned with the cone apex, the instantaneous map for the EyeSys videokeratograph at regular alignment provided reasonably valid measurements for corneas up to 55 D, but large errors existed for some corneas of higher power.