Effects of laser repetition rate on corneal tissue ablation for 193-nm excimer laser light.

BACKGROUND AND OBJECTIVE: The goal of the present work is to assess whether bovine corneal ablations generated at laser repetition rates of up to 400 Hz are comparable to ablations performed at rates consistent with current clinical laser systems. STUDY DESIGN/MATERIALS AND METHODS: A combination of experiments was used to assess a comprehensive range of ablation parameters, including ablation plume dynamics via imaging and transmission, corneal ablation profiles via scanning interferometry, and high-resolution electron microscopy of collagen structure following ablation. RESULTS: Using white-light interferometry analysis, no statistical difference was found between corneal ablation profiles created at 60 and 400 Hz, with an average rate of 0.94 microm/pulse at 60 Hz versus 0.92 microm/pulse at 400 Hz. In addition, based on plume imaging and transmission studies, the bulk ablation plume was found to dissipate on a time-scale less than the pulse-to-pulse separation for a laser repetition rate up to about 400 Hz. A persistent, diffuse gas-phase component of the ablation products was observed and concluded to be comparable at both repetition rates. Finally, SEM and TEM analysis revealed no signs of differential thermal tissue damage, including collagen fibril analysis, for laser repetition rates up to 400 Hz. CONCLUSIONS: In summary, investigation of the relative effects of excimer laser repetition rate on the overall corneal ablation metrics revealed no measurable difference under conditions typical of clinical refractive procedures. This study suggests that increases in ArF laser repetition rates for clinical applications (up to approximately 400 Hz) appear feasible.