Minimizing thermal damage in corneal ablation with short pulse mid-infrared lasers

Photospallation is proposed as the primary mechanism behind our recent anim al studies involving corneal ablation by nanosecond-pulse mid-IR laser beam s. Following a brief summary of earlier work directed to refractive procedu res in the mid-IR, a preliminary analysis is presented, based on simple one -dimensional models of thermoelastic expansion developed previously. The re sults of the analysis indicate that front sur face spallation is consistent with the striking tissue ablation characteristics observed in our recent i n vivo work with short pulse beams, including very small ablation rates and submicron thermal damage zones. This is attributed to the fact that spalla tion is a mechanical-rather than a thermal-mechanism, which allows tissue t o be removed in small layers at fluences far lower than those used in the e arlier corneal studies with mid-IR beams, typically under 200 mJ/cm(2), res ulting in minimal heating of tissue. Unlike prior work in the area of photo spallation, we also suggest that the existing theoretical basis supports th e 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 be preferred over shorter pulses, both from a practical standpoint and to mitigate against potential damage from shock wa ves. (C) 1999 Society of Photo-Optical Instrumentation Engineers. [S1083-36 68(99)00204-X].