THERMAL-STRESS TO THE COCHLEA DURING LASE R STAPEDOTOMY .2. PULSED LASERS

Apart from ablation properties at the stapes footplate, the degree of thermal stress of the inner ear is important when considering the suit ability of pulsed lasers for stapedotomy. The aim of the present study was to compare the heating of cochlear structures with presently avai lable pulsed laser systems during stapedotomy under reproducible condi tions. Temperature increases and heat-exchange processes were examined in a physiologic model of the cochlea using various laser parameters effective for footplate perforations. With all systems, increases in e nergy density, number of pulses and resultant total energy led to high er temperatures. In the effective energy density range, the highest te mperature increases achieved with the requisite number of pulses at a distance of 2 mm behind the footplate perforation were 30 degrees and 26 degrees C with the pulsed CO2 and Ho:YAG lasers, respectively. The lowest temperature recorded was 5.5 degrees C with the Er:YAG and < 5 degrees C with the Er:YSGG laser. The excimer laser investigated at on ly one energy density showed maximum temperatures of 10 degrees C. Wit h regard to possible inner ear damage from thermal stress during laser stapedotomy. the Er:YSGG laser can be used safely over a relatively b road energy density range. The Er:YAG laser investigated also appears suitable for stapedotomy when considering thermal effects. In contrast , application of the pulsed CO2 laser at parameters effective for stap edotomy leads to high temperatures and wide scattering to compromise i ts use. The Ho:YAG laser also appears to be unsuitable because of the higher energy density and pulse rate required for sufficient perforati on and the resultant higher temperature increases in the perilymph. Al though footplate perforations can be achieved with only slight tempera ture increases in the fluid of the cochlea model, the excimer laser do es not seem to be appropriate for stapedotomy because of the long peri od of heat exposure required and the lower ablation rates at the stape s.