Corneal topographers have made it possible to accurately map corneal shape. We have been applying this technology to develop a model of the post-PRK cornea. Two inovations were required for enhancing the robustness of the model:

First, it was found that in order to obtain sufficient information about the curvature of peripheral cornea it was necessary to use multiple views. The subject gazed in four directions (up, down, right, left) in addition to straight-ahead gaze.

Second, a PRK analytic shape descripter was developed, with a general and a specific aspect. The general aspect is a Taylor series expansion (similar to Zernike polynomials) with up to 60 coefficents, used for specifying smooth corneal shapes. The following is an idea of how the general desciptor works:

General descriptor = C0 * + C1 * + C2 * + C3 * + C4 * + . . .

The specific aspect is an analytic function for the annular zone covering the PRK transition zone where radial curvature is elevated. The curvature specified by this analytic function smoothly joins the base corneal shape (the first derivative of curvature is continuous). The transition zone function has up to 12 parameters to allow sufficient generality.

Specific aspect (radial and in one dimension):