The temporomandibular joint (TMJ) is a load-bearing joint consisting of the condyle of the mandibular bone, the fossa eminence of the temporal bone, and a fibrocartilaginous disc held in between the bone surfaces by ligaments. The TMJ disc serves to distribute stress, lubricate movement, and protect the articular surfaces of the joint. Over ten million Americans suffer from TMJ disorders (TMD) that affect the movement and function of the joint, making everyday tasks like talking and eating difficult and painful. A wide variety of treatments and surgeries have been proposed and undertaken with limited success based on the varying degree of joint dysfunction. The fibrocartilage disc has become a major focus of study because disc displacement and degeneration are the primary causes of TMD, so a better understanding of the disc is required before more effective diagnostic techniques and treatment approaches can be developed. Some of these properties include the tissue biomechanical behavior under various loading conditions, the cellular composition of the disc, and basic cellular metabolic (energy) rates.
The TMJ disc has been found to be distinct from other cartilage types found in the body in regards to primary cell types, extracellular matrix components (ECM), and mechanical properties. These significant differences are attributed to the unique environment and loading conditions of the joint. It is generally believed that pathological mechanical loadings (e.g. sustained jaw clenching or traumatic impact) trigger a cascade of molecular events leading to TMJ disc degeneration and derangement, which are central to many TMJ disorders and pathophysiology. Therefore, the objective of this research is to investigate the effect of sustained mechanical loading on nutrient transport and cell nutrition of the TMJ disc in order to better understand the biomechanical etiology of TMD. Our general hypothesis is that sustained mechanical loading can alter solute transport and nutrient concentrations in the TMJ disc, resulting in changes to the cellular metabolism, tissue composition, and mechanical function, ultimately leading to disc pathologies.
First, the biphasic mechanical properties of porcine and human TMJ discs were measured to characterize the complex mechanical environment of the joint. Compression and shear experiments were developed to validate the use of the porcine model and to correlate mechanical function with biochemical structure. Significant correlation between aggregate modulus and permeability with water content was found in human confined compression studies. Fluid pressurization was found to play a major role in the load support during dynamic compression and significant frequency dependence during dynamic testing was indicative of the viscoelastic nature of the tissue. These studies highlighted the unique biochemical and mechanical properties of the TMJ disc compared with other cartilage types.
Due to the avascular nature of TMJ disc tissue, transport of nutrients and removal of waste is a major difficulty. The rate of small nutrient (i.e., oxygen and glucose) transport in the TMJ disc is mainly governed by their diffusivities, which depends on solute size, matrix composition, and local mechanical strain. The transport of nutrients was investigated to develop new constitutive relationships between solute diffusivity and tissue hydration to establish strain-dependent transport properties. Our studies showed that solute diffusivities in the TMJ disc were significantly lower than in other cartilaginous tissues and that compressive strain further impeded diffusion. These findings suggest that a steeper nutrient gradient exists in the TMJ disc and is likely vulnerable to pathological events such as sustained loading due to jaw clenching.
The nutrient gradients are dependent on the balance between the diffusion rates into the TMJ disc and the uptake and utilization by disc cells. TMJ disc cellular consumption rates of oxygen and glucose were measured in a variety of environmental conditions to develop functional relationships between nutrient consumption rates, oxygen tension, glucose concentration, and pH value. Consumption rates were found to be highly substrate dependent with increased concentrations resulting in increased consumption rates. Cell proliferation and matrix protein production were severely inhibited at low oxygen and glucose concentrations suggesting that nutrient environment heavily dictated cell responses and metabolism.
The objective of this project was to characterize the mechanical, biochemical, transport, and consumption properties of the TMJ disc in an effort to better understand TMJ disorders related to pathological loading and disc derangement. Future work for this research involves incorporating the TMJ disc properties into a predictive 3D finite element model of the in vivo TMJ environment. This model can be further developed into a TMD diagnostic tool based on patient specific magnetic resonance images (MRI) and jaw tracking data. This work will help to build new strategies for TMD treatment and can be applied to tissue engineering approaches in other cartilaginous tissues. Therefore, it is necessary to characterize the TMJ disc and its surrounding tissues via experimental and theoretical research to accurately model the complex properties of native tissue before useful applications can be developed.
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