DYNAMICS OF LASER-INDUCED CHANNEL FORMATION IN WATER AND INFLUENCE OFPULSE DURATION ON THE ABLATION OF BIOTISSUE UNDER WATER WITH PULSED ERBIUM-LASER RADIATION

The ability to use fiber-delivered erbium-laser radiation for non-cont act arthroscopic meniscectomy in a liquid environment was studied. The laser radiation is transmitted through a water-vapor channel created by the leading part of the laser pulse. The dynamics of the channel fo rmation around a submerged fiber tip was investigated with time-resolv ed flash photography. Strong pressure transients with amplitudes up to a few hundreds of bars measured with a needle hydrophone were found t o accompany the channel formation process. Additional pressure transie nts in the range of kbars were observed after the laser pulse associat ed with the collapse of the vapor channel. Transmission measurements r evealed that the duration the laser-induced channel stays open, and th erefore the energy transmittable through it, is substantially determin ed by the laser pulse duration. The optimum pulse duration was found t o be in the range between 250 and 350 mus. This was confirmed by histo logical evaluations of the laser incisions in meniscus: Increasing the pulse duration from 300 to 800 mus leads to a decrease in the crater depth from 1600 to 300 mum. A comparison of the histological examinati on after laser treatment through air and through water gave informatio n on the influence of the vapor channel on the ablation efficiency, th e cutting quality and the induced thermal damage in the adjacent tissu e. The study shows that the erbium laser combined with an adequate fib er delivery system represents an effective surgical instrument liable to become increasingly accepted in orthopedic surgery.