Temperature distributions in Nd:YAG (λ=1.32 μm) laser-irradiated cartilage grafts accompanied by cryogen spray cooling

Laser irradiation of mechanically deformed cartilage results in permanent shape change. Under optimal conditions, the cartilage graft must be rapidly heated to the critical transition temperature, required for reshaping, while maintaining a spatially uniform temperature distribution. These two conditions are not easily satisfied, as increasing irradiance results in larger temperature gradients between irradiated and non-irradiated surfaces. Moreover, such a gradient increases with cartilage thickness. inasmuch as uncontrolled heating of the irradiated surface may result in overt chondrocyte death, the temperature in cartilage must be monitored and controlled. We propose feedback-controlled cryogen spray cooling as a technique to allow rapid laser heating within the cartilage while protecting the irradiated surface. In this investigation, we measured radiometric surface temperatures on irradiated and non-irradiated surfaces of porcine septal cartilage specimens (thickness 1.5, 2.0, 2.5 and 3 mm) during Nd:YAG laser exposure (lambda=1.32 mu M, 50 pulse repetition rate, 5-50 W/cm(2))with and without cryogen spray cooling. Along with surface temperatures, back-scattered light from an amplitude-modulated HeNe probe laser (lambda=632.8, 10 mW) was collected by a fiber and detected with a silicon photoreceiver using a lock-in amplifier. Previously, we have demonstrated that an increase followed by a sharp decline in the back-scattered light intensity signal correlates closely with characteristic alterations in cartilage thermal and mechanical properties accompanying the reshaping process, which suggest the occurrence of a phase transformation. The difference between irradiated and non-irradiated radiometric surface temperatures was determined as a function of time and related to incident laser irradiance and specimen thickness. To minimize excessive heating and produce a more uniform temperature distribution, a cryogen (10 ms) spurt was delivered at the surface coincident with the area of laser irradiation. The cryogen spurt was triggered whenever the radiometric surface temperature on the irradiated surface reached 50 degrees C. Back-scattered HeNe light was noted to increase, peak, and subsequently decrease during sustained laser irradiation in the presence and absence of cryogen spray cooling (CSC). Laser irradiation was terminated after the peak in the back-scattered HeNe signal was observed on the lock-in amplifier. The critical transition radiometric surface temperature increased with laser irradiance. In the absence of CSC, temperatures on irradiated and nonirradiated surfaces of a 1.5 mm section of cartilage differed by 1.5, 7, and 10 degrees C for 5, 15, and 30 W/cm(2), respectively, at the peak in light scattering. During laser irradiation with CSC, the radiometric surface temperature on the irradiated surface varied between 25-50 degrees C. Radiometric surface temperatures on the non-irradiated surface did not exceed 70 degrees C. A peak in the light scattering curve was still consistently observed for CSC irradiated tissue and was noted to occur when the non-irradiated surface temperature reached 45 degrees C. The use of CSC during laser irradiation permits rapid and homogenous heating within the cartilage while potentially protecting the irradiated surface.