PMMA model of steep central islands induced by excimer laser photorefractive keratectomy

We designed a poly-methyl methacrylate (PMMA) model of refractive steep central islands (CIs) induced by PRK. A standardized photorefractive ablation procedure was performed using seven different excimer lasers on test PMMA specimens including 70 flat plates and 98 convex contact lenses. The resulting surface was analyzed by high-resolution confocal microscopy and computerized videokeratoscopy using both TMS-1 and CAS-2000 systems. A total of 50 (54.9%) CIs were observed using computerized videokeratoscopy. The rate of occurrence of CIS was significantly reduced by shock wave absorption (P = 0.0001), aspiration of fumes (P = 0.0044), and smaller diameter ablation (P = 0.0296). The diameter of the CIs was significantly increased for broad-beam mode ablation (P = 0.016) and for larger ablation zones, (P = 0.042). The refractive power of CIs was significantly increased in the absence of a shock wave absorption system (P = 0.001). Only 20 (40%) of the Cls detected by the TMS-1 device were identified on GAS-2000 at a 0.5 diopter (D) scale resolution level. Shock wave induced deformation and subsequent dynamic alteration of convection for-ces applied to emitted particles may be the primary mechanism underlying the formation of CIs after PRK, regardless of any biological response of the ablated tissue. Reported rates and characteristics of CIs may largely depend upon the specific design of videokeratoscopes. (C) 1997 by Elsevier Science Inc. All rights reserved.