Location of the major corneal reference points.

Abstract

The two principal applications of corneal topography are those pertaining to corneal shape, such as contract lens posterior surface design, and those pertaining to corneal power, such as predicting power changes after refractive surgery. Corneal topography measurements for these applications have advantages when related to two major reference points, the corneal apex for shape and the corneal sighting center for power. Instantaneous corneal power, toricity, and axis were measured by videokeratography in 20 right eyes using the standard instrument alignment and alignments at each of the 2 major reference points. In comparing the central corneal measurements by videokeratography using the standard alignment to alignment at the corneal sighting center, there were minimal differences of less than 0.50 D for central power, toricity, and axis. In comparing the results for regular alignment to apex alignment there were significant differences for power (greater than 0.50 D) in 5 eyes, toricity in 3 eyes, and axis shifts of greater than 10 degrees in 5 eyes. In comparing the results for the corneal sighting center to apex alignments there were significant differences for power in 4 eyes, toricity in 2 eyes, and axis in 5 eyes. Changes of greater than 1.00 D occurred in the corneal periphery. In comparing standard videokeratograph alignment to alignment at the corneal sighting center, there were statistically insignificant differences in videokeratography results for corneal power, toricity, and axis. Significantly larger differences were found in comparing standard alignment and alignment at the corneal apex, especially for measurements in the corneal periphery.