Regional and disease-related differences in properties of the equine temporomandibular joint disc

Temporomandibular joint (TMJ) disorders are increasingly recognized in animals. The articular disc plays a major role in TMJ disorders in people, but its role in the pathogenesis of TMJ disease in animals is not well understood. This study characterizes properties of the equine TMJ disc, including regional variation within discs as well as differences in properties associated with naturally-occurring TMJ osteoarthritis.

Methods
Data represent the results of characterization of mechanical and biochemical properties of TMJ discs obtained from sixteen horses euthanized for reasons unrelated to this research and with owner consent for research use. Each joint was dissected and evaluated for TMJ osteoarthritis and the articular disc removed. Samples from the rostral, caudal, lateral, central, and medial regions of each disc were tested for compressive properties and biochemical composition. Orthogonal samples from the lateral, central, and medial regions were tested for tensile properties. Compressive properties were evaluated via stress-relaxation in unconfined compression at 10% adn 20% strain. Data were fit to a viscoelastic solution for a standard linear solid and the instantaneous modulus, relaxation modulus, and coefficient of viscosity calculated. Tensile properties were assessed by strain to failure with a strain rate of 1% per second. Tensile Young's modulus was calculated from the linear portion of the stress-strain curve and the ultimate tensile strength defined as the peak stress. Collagen was measured using a colorometric hydroxyproline assay, and sulfated glycosaminglycans were measured using a dimethylmethylene blue binding assay.