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In this study, we attempted to determine the critical temperature [To] at which accelerated stress relaxation occurred during laser mediated cartilage reshaping. During laser irradiation, mechanically deformed cartilage tissue undergoes a temperature dependent phase transformation which results in accelerated stress relaxation. When a critical temperature is attained, cartilage becomes malleable and may be molded into complex new shapes that harden as the tissue cools. Clinically, reshaped cartilage tissue can be used to recreate the underlying cartilaginous framework of structures such as the ear, larynx, trachea, and nose. The principal advantages of using laser radiation for the generation of thermal energy in tissue are precise control of both the space-time temperature distribution and time-dependent thermal denaturation kinetics. Optimization of the reshaping process requires identification of the temperature dependence of this phase transformation and its relationship to observed changes in cartilage optical, mechanical, and thermodynamic properties. Light scattering, infrared radiometry, and modulated differential scanning calorimetry (MDSC) were used to measure temperature dependent changes in the biophysical properties of cartilage tissue during fast (laser mediated) and slow (conventional calorimetric) heating. Our studies using MDSC and laser probe techniques have identified changes in cartilage thermodynamic and optical properties suggestive of a phase transformation occurring near 60°C.

To develop methods to compare carbon dioxide (CO 2) resurfacing lasers, fluence, and debridement effects on tissue shrinkage and histological thermal denaturation. Design: In vitro human or in vivo porcine skin samples received up to 5 passes with scanner or shortpulsed CO 2 resurfacing lasers. Fluences ranging from 2.19 to 17.58 J/cm 2 (scanner) and 1.11 to 5.56 J/cm 2 (short pulsed) were used to determine each laser's threshold energy for clinical effect. Variable amounts of débridement were also studied. Main Outcome Measures: Tissue shrinkage was evaluated by using digital photography to measure linear distance change of the treated tissue. Tissue histological studies were evaluated using quantitative computer image analysis. Results: Fluence-independent in vitro tissue shrinkage was seen with the scanned and short-pulsed lasers above threshold fluence levels of 5.9 and 2.5 J/cm 2 , respectively. Histologically, fluence-independent thermal depths of damage of 77 µm (scanner) and 25 µm (pulsed) were observed. Aggressive debridement of the tissue increased the shrinkage per pass of the laser, and decreased the fluence required for the threshold effect. In vivo experiments confirmed the in vitro results, although the in vivo threshold fluence level was slightly higher and the shrinkage obtained was slightly lower per pass.

In this study, we examined 1) the effect of partial denaturation and 2) repetitive irradiation on porcine septal cartilage during Nd:YAG laser (?= 1 .32 jim,25 W/cm2, 5 mm spot size) exposure. Diffuse reflectance from a HeNe probe laser and internal stress were measured in mechanically deformed cartilage specimens (2 x 10 x 25 mm) during Nd:YAG laser irradiation. Specimens were first partially denaturation in heated saline water baths at selected temperatures (50° C, 70° C, and 1 00° C all for 30 minutes). Native and water bath heated specimens were sequentially irradiated three times (pulse duration varying from 5-1 2 seconds, determined by noting the onset of accelerated stress relaxation) with a 5 minute cooling interval between pulses. After the first laser pulse, diffuse reflectance and internal stress changes occurred synchronously (coupled); the peak in internal stress occurred within less than 1 second following observation of the peak in diffuse reflectance. With repeated laser irradiation, this time interval lengthens with eventual decoupling of the diffuse reflectance and internal stress. With decoupling, internal stress increases during laser heating, and abruptly decreases when irradiation is terminated. Decoupling occurs with greater frequency in specimens pre-heated in the water bath. With the first laser exposure, only 1 5% of control, 8% of 50° C heated, 0% of 70° C heated, and 8% of 1 00° C specimens exhibited decoupling. However, after two laser exposures, decoupling was observed in 83% and 60% of specimens heated in water baths at 70° C (N=12) and 1 00° C (N=12), respectively; in contrast, decoupling was observed in less than 20% of the native (N=24) and 50° C (N=l 2) water bath pre-heated specimens with two laser irradiations. The effect of partial denaturation using water bath immersion mimics findings observed with sequential laser irradiation. Hence cartilage likely undergoes partial denaturation during laser reshaping, and that sustained laser irradiation may result in irreversible changes in the tissue matrix.

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