THERMAL-STRESS TO THE COCHLEA DURING LASE R STAPEDOTOMY .1. CONTINUOUS-WAVE LASERS

As a consequence of perforating the footplate during laser stapedotomy , direct radiation to the inner ear will warm perilymph and adjacent s tructures. To determine the possible thermal dangers to cochlear struc tures from different laser parameters, heat transport mechanisms, temp erature increases and temperature fields were investigated in a model system approximating caloric and physiologic changes in the inner ear. The temperature-time course of local cochlear warming showed a rapid convection-dependent increase that reached a peak at about the end of the laser impulse. An increase in power density caused an elevation of the temperature in all laser systems used. Maximum temperatures varie d widely for CO2 lasers at the same wavelength, but a different beam-t ime behavior was found at a distance of 2 mm behind the perforation by using low-power densities. Heat values were lowest at a pulse duratio n of 50 ms in the superpulse (< 5 degrees> C) and continuous wave (cw) modes (< 9 degrees C), while the highest value was found in the pulse r mode (to 21 degrees C). After argon laser irradiation at high-power densities, temperatures were nearly independent of location (5.5-13 de grees C). When considering risks of possible inner ear damage from the rmal stress during laser stapedotomy, application of the CO2 superpuls e and cw laser appears to be safe over a large power-density range. Lo w energies using a small-beam diameter and short pulse durations (50-1 00 ms) are recommended. In contrast, use of the CO2 laser in a pulser mode may result in inner-ear damage because of the high temperatures p roduced. Structures located at a greater distance can be endangered by direct irradiation with the argon laser.