UC Irvine

Cartilage undergoes characteristic mechanical stress relaxation following laser irradiation below the ablation threshold. Porcine auricular cartilage (1-2 mm thickness) was irradiated with a Nd:YAG laser (λ = 1.32 μm) at two power levels (W/cm2). Surface temperature (S(c)(t) (°C)) (monitored using a single element HgCdTe infrared detector, 10-14 μm spectral range), and integrated back scattered light intensity I(t) were measured during laser irradiation. A HeNe laser beam (λ = 632.8 nm) was incident on the back surface of the cartilage specimen and fractional integrated backscattered light intensity was measured using an integrating sphere and a silicon photodiode. Laser irradiation (5.83 W/cm2, 50 Hz pulse repetition rate (PRR)) continued until surface temperature reached approximately 70°C, during which cartilage mechanical stress relaxation was observed. Integrated back scattered light intensity reached a plateau at about 70°C). At higher laser power (39.45 W/cm2, 50 Hz PRR), a feedback-controlled cryogen spray was used to maintain surface temperature below 5O°C. A similar plateau response was noted in integrated backscattered light intensity. This signal may be used to optimise the process of stress relaxation in laser cartilage reshaping. Several clinical applications involving reconstructive surgery are proposed.