CALCULATION OF THE KINETICS OF HEATING AND STRUCTURAL-CHANGES IN THE CARTILAGINOUS TISSUE UNDER THE ACTION OF LASER-RADIATION

A theoretical model is developed for the calculation of the temperatur e fields and determination of the size of a zone with structural chang es in the cartilaginous tissue. The model is based on a simultaneous a nalysis of the heat and mass transfer processes and it takes into acco unt the bulk absorption of laser radiation by the tissue, surface evap oration of water, and temperature dependences of the diffusion coeffic ients. It is assumed that under the influence of a phase transition be tween free and bound water, caused by heating of the cartilage to 70 d egrees C, the proteoglycans of the cartilage matrix become mobile and, as a result of such mass transfer, structural changes are induced in the cartilaginous tissue causing relaxation of stresses or denaturatio n. It is shown that the maximum temperature is then reached not on the irradiated surface but at some distance from it, and that the size of the zones of structural changes (denaturation depth) depends strongly on the energy density of the laser radiation and its wavelength, on t he duration of the irradiation, and on the cartilage thickness. This m odel makes it possible to calculate the temperature field and the dept h of structural changes in laser-induced relaxation of stresses and ch anges in the shape of the cartilaginous tissue.