Comparison of the effects of absorption coefficient and pulse duration of 2.12-mum and 2.79-mum radiation on laser ablation of tissue

Erbium and holmium lasers are attractive for minimally invasive surgical applications as they operate at wavelengths where tissues exhibit strong absorption due to their water content and because these wavelengths are transmittable through optical fibers. In this study, the basic physical mechanisms underlying tissue ablation and the laser-induced tissue effects using pulsed Er:YSGG (2.79 mum) and Ho:YAG (2.12 mum) laser radiation are presented and compared, Q-switched (tau=40 ns, E 50 mJ) and free-running (tau=250 and 400 mus) Er:YSGG (E=100 mJ) and Ho:YAG (E 1 J) laser energy was delivered in water via a 400-mum fiber. The dimension and lifetime of the expanding and collapsing bubbles and the laser-induced pressure in water after each laser pulse were measured with fast-flash videography and time-resolved pressure measurements. Depending on the absorption coefficient, pulse energy, and pulse duration, three different regimes were distinguished: evaporation, tensile-stress-induced cavitation, and explosive vaporization. In vitro tissue effects, ablation depth, and extent of tissue damage on meniscus treated under water and on cornea treated in air were investigated and examined histologically. Er:YSGG radiation, due to its 100 times higher absorption than Ho:YAG radiation, exhibited a high tissue ablation efficiency with a relatively small zone of coagulated tissue (Q-switched 4-10 mum, free-running less than 100 mum), whereas the coagulated tissue zone was 300-1000 mum after free-running and 100-120 mum after Q-switched Ho:YAG laser impact