Thermo-optical response of cartilage during feedback controlled laser-assisted reshaping

Cartilage undergoes characteristic deformation following laser irradiation below the ablation threshold. Measurements of surface temperature and integrated scattered light intensity were performed during laser irradiation. Porcine auricular cartilage (1-2 mm thickness) was irradiated with an Nd:YAG laser (lambda = 1.32 mu m) with varying dose (J/cm(2)). Surface temperature was monitored using a single element HgCdTe infrared detector, responsive between 10-14 mu m. A HeNe laser beam (lambda = 632.8 nm) was incident on the back surface of the cartilage specimen and fractional integrated back scattered light intensity was measured using an integrating sphere and a silicon photodiode. Laser irradiation (2W, 5.83W/cm(2), 50 Hz PRR) was allowed to proceed until surface temperature reached 70 degrees C. Cartilage deformation was observed in each instance. Integrated scattered light intensity reached a plateau before the peak temperature (70 degrees) was reached. At increased laser power (10 W, 39.45 W/cm(2), 50 Hz PRR), a feedback controlled cryogen spray was used to maintain surface temperature below 50 degrees C. A similar plateau response was also noted in integrated scattered light intensity. This signal may be used to optimize the process of stress relaxation in laser cartilage reshaping. Several clinical applications are discussed.