COMPARISON OF THE EFFECTS OF ABSORPTION-COEFFICIENT AND PULSE DURATION OF 2.12-MU-M AND 2.79-MU-M RADIATION ON LASER-ABLATION OF TISSUE

Erbium and holmium lasers are attractive for minimally invasive surgic al applications as they operate at wavelengths where tissues exhibit s trong absorption due to their water content and because these waveleng ths 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 mu m) and Ho:YAG (2.12 mu m) laser radiation are presented and compared. Q-switched (tau = 40 ns, E less than or equal to 50 mJ) and free-running (tau = 250 and 400 mu s) Er:YSGG (E = 100 mJ) and Ho:YAG (E less than or equal to 1 J) laser energy was delivered in water via a 400-mu m fiber. The dimension and lifetime of the expanding and collapsing bubbles and the laser-induce d pressure in water after each laser pulse were measured with fast-fla sh videography and time-resolved pressure measurements, Depending on t he absorption coefficient, pulse energy, and pulse duration, three dif ferent regimes were distinguished: evaporation, tensile-stress-induced cavitation, and explosive vaporization. In vitro tissue effects, abla tion depth, and extent of tissue damage on meniscus treated under wate r and on cornea treated in air were investigated and examined histolog ically, 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-snitched 4-10 mu m, free -running less than 100 mu m), whereas the coagulated tissue zone was 3 00-1000 mu m after free-running and 100-120 mu m after Q-switched Ho:Y AG laser impact.