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 ablation products during excimer laser photorefractive keratectomy, and their 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 μsec and 100 msec after the ablating laser pulse. On PMMA targets, the profiles of the resulting ablation craters were measured. Results: In all cases, the ablation products formed a ring vortex. The plume velocities on cornea were found to be approximately twice as fast as on PMMA for all zone diameters. For both materials, the ablation plume evolves faster for smaller zone diameters. Central islands were observed for zone diameters between 1 and 7 mm on PMMA substrates. The islands became more pronounced 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, redeposition of particles favorably occurs at the center of the ablation zone. Additionally, the dynamics of the ablation plume lead to a concentration of airborne particles over the center of the ablation zone, which also may contribute to the creation of central islands by partial absorption of the next excimer laser pulse.