Date of Award
Doctor of Philosophy (PhD)
Temporomandibular joint (TMJ) disorder affects over 10 million people in the US each year. The signs and symptoms of temporomandibular joint disorders (TMDs) include limited mouth opening, clicking and locking of the jaw, and significant pain in the craniofacial region. In patients who seek treatment for TMDs, over 70% have TMJ disc displacement related to disc degeneration. The TMJ disc is interposed between the mandible condyle and the glenoid fossa of the temporal bone. The disc reduces contact stresses within the joint and provides lubrication to the joint. It is generally believed that pathological mechanical loadings, such as sustained jaw clenching or malocclusion, trigger a cascade of molecular events that lead to TMJ disc degeneration. A deeper understanding of the biomechanics, i.e. mechanical environment and effect on the nutrient environment, could lead to developments in TMD diagnosis and management. Therefore, the objective of this research study was to determine the mechanical and transport properties of the human TMJ disc and begin to model joint biomechanics using patient specific finite element models. Our central hypothesis is that sustained mechanical loading can alter solute transport and nutrient levels in the TMJ disc as well as mechanical function resulting in disc derangement and degeneration. The TMJ disc is a large, avascular structure. The nutrients required by disc cells for maintaining disc health are supplied by synovial fluid at the margins of the disc as well as nearby blood vessels through diffusion. The first study investigated the effect of mechanical strain on small solute diffusion in human TMJ discs using the electrical conductivity method. From ion exchange theory, electrical conductivity is proportional to diffusivity assuming the tissue is uncharged. The results indicated that mechanical strain significantly impeded solute diffusion through the disc, which was consistent with our similar porcine study. Female conductivity was higher than male conductivity and was affected more by mechanical strain. This study suggested that female TMJ disc tissue could have slight differences in composition and porosity responsible for differences in electrical conductivity. In addition to investigating diffusion under mechanical strain, the charged matrix of the TMJ disc was investigated by determining the fixed charge density (FCD) of human TMJ discs. The FCD of cartilaginous tissues has been shown to contribute to high osmotic swelling pressure responsible for mechanical properties in addition to electrokinetic effects such as streaming potential. The fixed charge density was determined using a two point electrical conductivity approach and correlated to the glycosaminoglycan (GAG) content. The FCD in the TMJ disc was most similar to annulus fibrosis tissues found in the intervertebral disc of the spine. This study suggested that the TMJ disc is similar to other fibrocartilage tissues, with the dense collagen matrix possibly contributing a more significant role in mechanical loading than GAG content. Finally, the human TMJ disc was characterized for viscoelastic tensile properties under incremental stress relaxation tests. The disc exhibited a linear stress response to incremental strain. The instantaneous and relaxed modulus values were lower than values found for human patellar tendon, human supraspinatus tendon and porcine TMJ discs studied previously. In addition, the disc did not exhibit an anisotropic response to loading as found in similar studies; nor were there differences in male and female results. This study suggested that the human TMJ disc (cadaver age ~69) likely exhibits a lower tensile modulus than young porcine TMJ discs (<1 year), although in this study no age effects were determined from the cadaver age range between 58 to 82 years. The results of these studies are being used to construct a patient specific finite element model. The human material properties are being combined with patient specific anatomy from MRI/CT scans. In the future, these models may be used by the TMJ research community to track and monitor the progression of TMDs.
Wright, Gregory, "Biomechanical Characterization and Modeling of Human TMJ Disc" (2015). All Dissertations. 1533.