Heating and structural alterations in cartilage under laser radiation

We have developed a theoretical model to calculate the temperature field an d the spatial distribution of modified cartilage following laser irradiatio n. The model incorporates both thermal and mass transfer in a tissue with b ulk absorption of laser radiation, water evaporation from the surfaces of a slab, and temperature dependence of the diffusion coefficient. We propose that water undergoes a bound-to-free phase transition in cartilage heated t o about 70 degrees C and the mobility of proteoglycan units in the cartilag e matrix increases. Movement of the proteoglycan units takes place only whe n the temperature exceeds 70 degrees C and results in tissue denaturation, Using our model, we show: 1) the maximal temperature is reached not on the surface irradiated but rather at some distance below; 2) surface temperatur e reaches its asymptotic value quicker than the maximal temperature; and 3) the depth of the denatured tissue volume strongly depends on laser fluency , wa wavelength, exposure time, and cartilage thickness. The model allows f or the prediction and control of temperature and depth of structural altera tions during the course of laser reshaping and treatment of cartilage.