Acoustic response of cartilage during laser-mediated stress relaxation

In this study, laser reshaping of porcine septal cartilage was performed using an Nd:YAG laser (λ=1.32 μm), while changes in acoustic waves were observed, in an attempt to develop technique to monitor the reshaping process. Concurrent measurements of strain (during tensile compression and tension, as well as flexure), temperature, and a 5-MHz ultrasonic signal were recorded during laser irradiation (λ=1.32 μm, 4 W, 13 s). The sample was set up in a water bath to enhance acoustic coupling. From the ultrasonic signal, both time of flight (TOF) and signal amplitude as reflected from the back wall of the cartilage were extracted and correlated with temperature and strain measurements. The onset of stress relaxation of the cartilage generally occurred between 50°C and 60°C. While TOF measurements indicated a generally constant increase in the speed of sound of the cartilage during the irradiation period, the amplitude of the reflected acoustic signal correlated directly with the stress relaxation of the cartilage. At the point of stress relaxation, the amplitude of the acoustic signal consistently attenuated to roughly 50% of its original magnitude.