Corneal ablation with short pulse mid-IR laser radiation

Photospallation is proposed as the mechanism behind recent animal studies involving corneal ablation by nanosecond-pulse mid-IR laser beams. Following a brief summary of earlier work directed to refractive procedures in the mid-IR, a preliminary analysis is performed based on simple ID models of thermoelastic expansion developed previously. The results of the analysis indicate that front surface spallation is consistent with the striking tissue ablation characteristics observed in the most recent work with short pulse mid-IR radiation, including very small ablation rates and submicron thermal damage zones. This is because spallation is a mechanical - rather than a thermal - process, allowing tissue to be removed in thin layers at fluences far lower than those used in the earlier corneal studies with mid-IR beams, resulting in minimal heating. We conclude that the existing theoretical basis supports the use of nanosecond pulses as an effective approach to achieving controlled ablation in the presence of very high absorption. We further suggest that such domain of operation may in fact be preferred over shorter pulses, both from a practical standpoint and to mitigate against potential damage from shock waves. Additional validation of the precise nature of corneal ablation with mid-IR nanosecond pulses was obtained from recent ablation rate experiments conducted in gel models, which resulted in submicron ablation rates of magnitudes very similar to those achieved with excimer. A brief summary of these preliminary results is given.