Mechanobehavior and Ontogenesis of the Temporomandibular Joint

Abstract

Craniofacial secondary cartilages of the mandibular condyle and temporomandibular joint (TMJ) eminence grow in response to the local mechanical environment. The intervening TMJ disc distributes normal loads over the cartilage surfaces and provides lubrication. A better understanding of the mechanical environment and its effects on growth, development, and degeneration of the TMJ may improve treatments aimed at modifying jaw growth and preventing or reversing degenerative joint disease (DJD). This review highlights data recorded in human subjects and from computer modeling that elucidate the role of mechanics in TMJ ontogeny. Presented data provide an approximation of the age-related changes in jaw-loading behaviors and TMJ contact mechanics. The cells of the mandibular condyle, eminence, and disc respond to the mechanical environment associated with behaviors and ultimately determine the TMJ components' mature morphologies and susceptibility to precocious development of DJD compared to postcranial joints. The TMJ disc may be especially prone to degenerative change due to its avascularity and steep oxygen and glucose gradients consequent to high cell density and rate of nutrient consumption, as well as low solute diffusivities. The combined effects of strain-related hypoxia and limited glucose concentrations dramatically affect synthesis of the extracellular matrix (ECM), which limit repair capabilities. Magnitude and frequency of jaw loading influence this localized in situ environment, including stem and fibrocartilage cell chemistry, as well as the rate of ECM mechanical fatigue. Key in vivo measurements to characterize the mechanical environment include the concentration of work input to articulating tissues, known as energy density, and the percentage of time that muscles are used to load the jaws out of a total recording time, known as duty factor. Combining these measurements into a mechanobehavioral score and linking these to results of computer models of strain-regulated biochemical events may elucidate the mechanisms responsible for growth, maintenance, and deterioration of TMJ tissues.

Shear Wave Elastography in the Evaluation of Temporomandibular Joint Disorders

“…The receiver operative characteristic curve for the articular disc stiffness in region of interest 3 (displaced portion) less than 8.667 kPa as a factor distinguishing between patients with temporomandibular disorders and healthy controls. undergoes degenerative changes and deformations (Nickel et al 2018), and its elasticity in various regions can change considerably (Roberts and Stocum 2018). The results of our study suggest that a change in the disc's biomechanics in TMD patients is primarily a consequence of a lesser stiffness in its anteriorly displaced portion, rather than a result of a hardening in the intermediate zone as previously reported in rats (Wang et al 2014).…”

The TMJ disc is a common ancestral feature in all mammals, as evidenced by the presence of a rudimentary disc during monotreme development

“…However, unlike in therian mammals, the monotreme fibrocartilage failed to separate from the condylar to form an articular disc in the TMJ. Interactions with musculature, both mechanical [13,30–32] and molecular [12,13,15,33–37], have been suggested to be responsible for the proper formation of the TMJ disc. Lack of mechanical force in monotremes might therefore result in the disc remaining attached to the condylar.…”

Mechanobehavior and mandibular ramus length in different facial phenotypes

“…The three-dimensional shapes and relationships of growing, loaded temporomandibular joint (TMJ) surfaces, are affected by in situ mechanics. 9,10 Mandibular forward positioning, as in dentofacial orthopedic treatment, likely decreases TMJ hard tissue surfacematching. 11 This increases TMJ stress-concentrations (MPa), leading to increased energy densities (ED, mJ/ mm 3 ), which are measures of the mechanical work input per volume between condyle and temporal eminence loading areas.…”

“…The variables of TMJ ED and DF have been combined as a mechanobehavior score (MBS ¼ ED 2 x DF, mJ mm 3 À Á 2 %) previously, and showed that MBS were significantly higher in women with, compared to without, TMJ disc displacement. 15 As a first step to characterize variables that influence mandibular condyle ontogeny, 10 the current study investigated MBS and mandibular ramus length (Condylion-Gonion [Co-Go], mm) in adults with varying facial phenotypes. That is, the hypotheses tested were that MBS: (1) was correlated with Co-Go and (2) differed between facial phenotypes.…”