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 nor mal and thus the corneal topography (CT) system reference axis. Anteri or corneal surface coordinates (x, y, and z) were computer generated t o simulate spherical and PRK-ablated corneal surfaces. Power maps were reconstructed by using surface tangents relative to various assumed C T reference axes, the original and displaced vertex normals. The simul ation process eliminated focus, alignment, and surface-reconstruction errors. A simulated 5.00-D myopic PRK decentered I mm from the vertex produced a new corneal vertex 0.13 mm from the original position. Axia l power-map generation by using this new reference axis produced a sph erical keratectomy with astigmatic untreated periphery. This computer simulation demonstrates that reference axis changes are significant fo r even modest PRK decentration and can explain apparent steepening in untreated corneal areas. CT users should be au are that pre- and posto perative CT power maps may not be directly comparable.