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Laser radiation can be used to reshape cartilage tissue into new morphologic configurations. When a critical temperature is attained, mechanically deformed cartilage becomes malleable and may be reshaped into new geometric configurations that harden as the tissue cools. This temperature-dependent process results in mechanical stress relaxation and is characteristic of a phase transformation. 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. In this study, we illustrate the utility of laser mediated cartilage reshaping in ex vivo porcine model of reconstructive nasal and laryngeal surgery, and attempt to determine the temperature range in which accelerated stress relaxation occurs during laser mediated cartilage reshaping. Optimization of the reshaping process requires identification of the temperature dependence of this phase transformation and its relationship to observed changes in cartilage optical (diffuse scattering), mechanical (internal stress), and thermodynamic properties (heat capacity). Light scattering, infrared radiometry, and modulated differential scanning calorimetry were used to measure temperature-dependent changes in the biophysical properties of cartilage tissue during fast (laser mediated) and show heating (conventional calorimetric heating). Our studies using MDSC and laser probe techniques have identified changes in tissue thermodynamic and optical properties suggestive of a phase transformation occurring near 60°C. Clinically, reshaped cartilage tissue can be used to recreate the underlying cartilagious framework of structures in the head and neck such as the ear, larynx, trachea, and nose.

Laser-assisted cartilage reshaping is mediated by thermally induced stress relaxation, and may be used to alter cartilage morphology for reconstructive surgical procedures in the upper airway and face without carving, morselizing, or suturing. Internal stress σ(t), integrated backscattered light intensity I(t) from a He-Ne probe laser (λ = 632.8 nm), and radiometric surface temperature Sc(t) were measured during the reshaping of porcine nasal septal cartilage using a pulsed Nd:YAG laser (λ = 1.32 μm). Internal stress and integrated backscattered light intensity were observed to increase, plateau, and then decrease in similar ways during laser irradiation. The plateau region occurred when the cartilage front surface temperature approached 65°C. I(t) was utilized in a feedback control procedure to reshape cartilage specimens from a flat to a curved geometry. Immediately following laser irradiation, the tissues were rehydrated in normal saline for 15 min while wrapped around a small dowel. A stable shape change was retained for 21 days while the specimens were stored in normal saline at 5°C. The backscattered light intensity signal mirrors underlying changes in internal stress, and further rate of change or slope of I(t) is nearly zero when the surface temperature reaches about 65°C. Measurements of I(t) (or, equivalently, the fractional change in integrated backscattered light intensity ΔI(t)/I0) may be used to control the process of laser-assisted cartilage reshaping and minimize nonspecific thermal injury due to uncontrolled heating. © 1998 Society of Photo-Optical Instrumentation Engineers.

In this study, we attempted to determine the critical temperature [Tc] 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.