The effect of water content on specific heat capacity was examined using temperature modulated Differential Scanning Calorimetry (TMDSC). This research was motivated in part by the development laser cartilage reshaping operations, which use photothermal heating to accelerate stress relaxation and shape change. Deposition of thermal energy leads to mechanical stress relaxation and redistribution of cartilage internal stresses, which may lead to a permanent shape change. The specific heat of cartilage specimens (dia: 3 mm and thickness 1-2 mm) was measured using a heating rate of 2 degree(s)C/min for conventional DSC and 2 degree(s)C/min with an amplitude 0.38-0.45 degree(s)C and a period 60-100 sec for TMDSC. The amount of water in cartilaginous tissue was determined using thermogravimetry analysis (TGA) under ambient conditions. In order to correlate changes in heat flow with alterations in cartilage mechanical behavior, dynamic mechanical temperature analysis (DMTA) was used to estimate the specific transition temperatures where stress relaxation occurs. With decreasing water content, we identified a phase transition that shifted to a higher temperature after 35-45% water content was measured. The phase transition energy increased from 0.12 J/g to 1.68 J/g after a 45% weight loss. This study is a preliminary investigation focused on understanding the mechanism of the stress relaxation of cartilage during heating. The energy requirement of such a transition estimated using TMDSC and temperature range, where cartilage shape changes likely occur, was estimated.
In this study, the rheological and phase behavior of porcine nasal cartilage were determined using dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry, and thermogravimetric analysis and the principles of thermal analysis (TA). The results were then incorporated in a finite element analysis used to estimate thermal residual stress and temperature distributions during laser irradiation. The finite element analysis was conducted by using a commercially available code ABAQUS (Hibbitt, Karlsson & Sorensen, Inc, USA) to solve the fully coupled thermo-mechanical equations. Arrhenius kinetics were used to obtain the activation energy associated with the phase transition as determined using DMTA and the results were compared with the energy of the phase transformation calculated using DSC. Laser-induced stress relaxation produced an increase in the von Mises stress within the simulation. The values generated during photo thermal heating were calculated assuming cartilage as an isotropic linear visoelastic material. The advantages and limitations of this approach technique are discussed, in particular with relevance to optimizing this procedure for use in clinical settings.