Topographic determination of corneal asphericity and its lack of effect on the refractive outcome of radial keratotomy.

PURPOSE

The normal human cornea flattens peripherally. The amount of flattening, or asphericity, has traditionally been calculated from multiple keratometric measurements. We devised a mathematical technique for determining asphericity from computed corneal topography. We then determined whether asphericity affects the refractive outcome of radial keratotomy.

METHODS

One eye each of 41 patients who underwent four- or eight-incision radial keratotomy and preoperative computed corneal topography was identified retrospectively and analyzed. The asphericity, P, of each cornea was calculated by fitting Baker's equation (y2 = 2r0x-Px2) to each meridian of the topographic map. For each patient, we calculated the difference between the refractive outcome in diopters for radial keratotomy and the prediction of a quadratic least-squares best-fit model involving optical zone size and age.

RESULTS

Aspericity could be calculated from the topographic maps in all 41 patients and ranged from 0.33 to 1.28, with mean +/- S.D. of 0.82 +/- 0.21. Aphericity varied among the meridians of a cornea, with an average standard deviation among meridians of 0.17. No statistical correlation was found between calculated asphericity and refractive outcome.

CONCLUSIONS

Corneal asphericity can be calculated from corneal topographic maps. Asphericity is not constant in the different meridians of a normal cornea. Corneal asphericity is not useful in predicting the refractive outcome of radial keratotomy.