INFLUENCE OF ABLATION PLUME DYNAMICS ON THE FORMATION OF CENTRAL ISLANDS IN EXCIMER-LASER PHOTOREFRACTIVE KERATECTOMY

Purpose: The aim of this study was to understand the dynamics of ablat ion products during excimer laser photorefractive keratectomy, and the ir influence on the formation of central islands. Method: Laser flash photography was used to investigate the dynamics of ablation products during photorefractive keratectomy. The ablation plume over polymethyl methacrylate (PMMA) and porcine cornea targets ablated with different zone diameters was imaged at various times between 10 mu sec and 100 msec after the ablating laser pulse. On PMMA targets, the profiles of the resulting ablation craters were measured. Results: In all cases, t he ablation products formed a ring vortex. The plume velocities on cor nea were found to be approximately twice as fast as on PMMA for all zo ne diameters. For both materials, the ablation plume evolves faster fo r smaller zone diameters. Central islands were observed for zone diame ters between 1 and 7 mm on PMMA substrates. The islands became more pr onounced with increasing zone diameter. Conclusion: A major cause for the formation of central islands was found to be particle redeposition at the center of the ablation zone. Because of the vortex dynamics, r edeposition of particles favorably occurs at the center of the ablatio n zone. Additionally, the dynamics of the ablation plume lead to a con centration of airborne particles over the center of the ablation zone, which also may contribute to the creation of central islands by parti al absorption of the next excimer laser pulse.