COMBINATION OF FIBER-GUIDED PULSED ERBIUM AND HOLMIUM LASER-RADIATIONFOR TISSUE ABLATION UNDER WATER

Because of the high absorption of near-infrared laser radiation in bio logical tissue, erbium lasers and holmium lasers emitting at 3 and 2 p m, respectively, have been proven to have optimal qualities for cuttin g or welding and coagulating tissue. To combine the advantages of both wavelengths, we realized a multiwavelength laser system by simultaneo usly guiding erbium and holmium laser radiation by means of a single z irconium fluoride (ZrF4) fiber. Laser-induced channel formation in wat er and poly(acrylamide) gel was investigated by the use of a time-reso lved flash-photography setup, while pressure transients were recorded simultaneously with a needle hydrophone. The shapes and depths of vapo r channels produced in water and in a submerged gel after single erbiu m and after combination erbium-holmium radiation delivered by means of a 400-mu m ZrF4 fiber were measured. Transmission measurements were p erformed to determine the amount of pulse energy available for tissue ablation. The effects of laser wavelength and the delay time between p ulses of different wavelengths on the photomechanical and photothermal responses of meniscal tissue were evaluated in vitro by the use of hi stology. It was observed that the use of a short (200-mu s, 100-mJ) ho lmium laser pulse as a prepulse to generate a vapor bubble through whi ch the ablating erbium laser pulse can be transmitted (delay time, 100 mu s) increases the cutting depth in meniscus from 450 to 1120 mu m a s compared with the depth following a single erbium pulse. The results indicate that combination of erbium and halmium laser radiation preci sely and efficiently cuts tissue under water with 20-50-mu m collatera l tissue damage. (C) 1996 Optical Society of America