Heating and structural alterations in cartilage under laser radiation

We have developed a theoretical model to calculate the temperature field and the spatial distribution of modified cartilage following laser irradiation. The model incorporates both thermal and mass transfer in a tissue with bulk 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 to about 70 degrees C and the mobility of proteoglycan units in the cartilage matrix increases. Movement of the proteoglycan units takes place only when 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 temperature 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 for the prediction and control of temperature and depth of structural alterations during the course of laser reshaping and treatment of cartilage.