Computer simulation of centration effects on corneal-topography analysis of excimer laser photorefractive keratectomy ablations.

Excimer laser photorefractive keratectomy (PRK) procedures photoablate the central optical zone, potentially altering the corneal vertex normal and thus the corneal topography (CT) system reference axis. Anterior corneal surface coordinates (x, y, and z) were computer generated to simulate spherical and PRK-ablated corneal surfaces. Power maps were reconstructed by using surface tangents relative to various assumed CT reference axes, the original and displaced vertex normals. The simulation process eliminated focus, alignment, and surface-reconstruction errors. A simulated 5.00-D myopic PRK decentered 1 mm from the vertex produced a new corneal vertex 0.13 mm from the original position. Axial power-map generation by using this new reference axis produced a spherical keratectomy with astigmatic untreated periphery. This computer simulation demonstrates that reference axis changes are significant for even modest PRK decentration and can explain apparent steepening in untreated corneal areas. CT users should be aware that pre- and postoperative CT power maps may not be directly comparable.