PMMA Model of Steep Central Islands Induced by Excimer Laser Photorefractive Keratectomy

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Abstract

We designed a polymethyl 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 CIs detected by the TMS-1 device were identified on CAS-2000 at a 0.5 diopter (D) scale resolution level. Shock wave induced deformation and subsequent dynamic alteration of convection forces 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.

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  • Cited by (0)

    The authors wish to acknowledge Dr. H. W. Thompson for assistance with the statistical analysis, Dr. J. Cohen-Sabban for assistance with the confocal microscopy, the Marzac-Monceau Contact Lens laboratory for kindly providing PMMA contact lenses and assistance for the measurement of lens geometry, and the Alcon, Summit Technologies, Aesculap, Coherent, Chiron, Nidek and Lasersight companies for their assistance with the performance of the experimental photoablations.

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