Characterization of temperature dependent mechanical behavior of cartilage.

BACKGROUND AND OBJECTIVES Few quantitative studies have investigated the temperature dependent viscoelastic properties of cartilage tissue. Cartilage softens and can be reshaped when heated using laser, RF, or contact heating sources. The objectives of this study were to: (1) measure temperature dependent flexural storage moduli and mechanical relaxation in cartilage, (2) determine the impact of tissue water content and orientation on these mechanical properties, and (3) use these measurements to estimate the activation energy associated with the mechanical relaxation process. STUDY DESIGN/MATERIALS AND METHODS Porcine nasal septal cartilage specimens (30 x 10 x 2 mm) were deformed using a single cantilever arrangement in a dynamic thermomechanical analyzer. Stress relaxation measurements were made at discrete temperatures ranging from 25 to 70 degrees C in response to cyclic deformation (within the linear viscoelastic region). The time and temperature dependent behavior of cartilage was measured using frequency multiplexing techniques (10-64 Hz), and these results were used to estimate the activation energy for the phase change using the Williams-Landel-Ferry (WLF) equation and the Arrhenius kinetic equation. In addition, the effect of tissue orientation was examined with specimens oriented in both transverse and longitudinal directions at room temperature. RESULTS The storage moduli of porcine cartilage decreased with increasing temperature, and a critical change in mechanical properties was observed between 58 and 60 degrees C with a reduction in the storage modulus by 85-90%. The shift of the stress relaxation behavior from viscoelastic solid to viscoelastic liquid was observed between 50 and 57 degrees C and likely corresponds to the transition temperature region in which structural changes in the tissue occur. The storage moduli for transverse and longitudinally oriented specimens were 19-22 and 14-16 MPa, respectively at ambient temperature. Reducing the water content (<10% mass loss) by allowing it to dry under ambient conditions resulted in reduction in the storage modulus by 31-36%. The activation energy associated with the mechanical relaxation of cartilage was 147 kJ/mole at 60 degrees C. This value was calculated by measuring stress-strain relationship under conditions where linear viscoelastic behavior was observed (0.09-0.15% of strain) within the transition temperature region (58-60 degrees C). CONCLUSIONS The anisotropic mechanical behavior of cartilage was quantitatively analyzed in the transversely and longitudinally oriented specimens. Viscoelastic behavior appeared to be strongly dependent on the water content. Using empirically determined estimates of the transition zone temperature range accompanying stress relaxation, the activation energy for stress relaxation was calculated using time and temperature superposition theory and WLF equation. Further investigation of the molecular changes, which occur during laser irradiation, may assist in understanding the thermal and mechanical behavior of cartilage and how the reshaping process might to be optimized.