Effect of pre-stress on the dynamic tensile behavior of the TMJ disc.

Previous dynamic analyses of the temporomandibular joint (TMJ) disc have not included a true preload, i.e., a step stress or strain beyond the initial tare load. However, due to the highly nonlinear stress-strain response of the TMJ disc, we hypothesized that the dynamic mechanical properties would greatly depend on the preload, which could then, in part, account for the large variation in the tensile stiffnesses reported for the TMJ disc in the literature. This study is the first to report the dynamic mechanical properties as a function of prestress. As hypothesized, the storage modulus (E') of the disc varied by a factor of 25 in the mediolateral direction and a factor of 200 in the anteroposterior direction, depending on the prestress. Multiple constant strain rate sweeps were extracted and superimposed via strain-rate frequency superposition (SRFS), which demonstrated that the strain rate amplitude and strain rate were both important factors in determining the TMJ disc material properties, which is an effect not typically seen with synthetic materials. The presented analysis demonstrated, for the first time, the applicability of viscoelastic models, previously applied to synthetic polymer materials, to a complex hierarchical biomaterial such as the TMJ disc, providing a uniquely comprehensive way to capture the viscoelastic response of biological materials. Finally, we emphasize that the use of a preload, preferably which falls within the linear region of the stress-strain curve, is critical to provide reproducible results for tensile analysis of musculoskeletal tissues. Therefore, we recommend that future dynamic mechanical analyses of the TMJ disc be performed at a controlled prestress corresponding to a strain range of 5­10%.

Stress relaxation behavior of mandibular condylar cartilage under high-strain compression.

During temporomandibular joint (TMJ) function, the mandibular condylar cartilage plays a prime role in the distribution and absorption of stresses generated over the condyle. Biomechanical characterization of the tissue under compression, however, is still incomplete. The present study investigates the regional variations in the elastic and equilibrium moduli of the condylar cartilage under high strains using unconfined compression and stress relaxation, with aims to facilitate future tissue engineering studies. Porcine condylar cartilages from five regions (anterior, central, lateral, medial, and posterior) were tested under unconfined compression. Elastic moduli were obtained from the linear regions of the stress-strain curves corresponding to the continuous deformation. Equilibrium moduli were obtained from the stress relaxation curves using the Kelvin model. The posterior region was the stiffest, followed by the middle (medial, central, and lateral) regions and the anterior region, respectively. Specifically, in terms of the equilibrium modulus, the posterior region was 1.4 times stiffer than the middle regions, which were in turn 1.7 times stiffer than the anterior region, although only the difference between anterior and posterior regions was statistically significant. No significant differences in stiffness were observed among the medial, central, lateral, and posterior regions. A positive correlation between the thickness and stiffness of the cartilage was observed, reflecting that their regional variations may be related phenomena caused in response to cartilage loading patterns. Condylar cartilage was less stiff under compression than in tension. In addition, condylar cartilage under compression appears to behave in a manner similar to the TMJ disc in terms of the magnitude of moduli and drastic initial drop in stress after a ramp strain.

Biomechanical properties of the mandibular condylar cartilage and their relevance to the TMJ disc.

Mandibular condylar cartilage plays a crucial role in temporomandibular joint (TMJ) function, which includes facilitating articulation with the TMJ disc, reducing loads on the underlying bone, and contributing to bone remodeling. To improve our understanding of the TMJ function in normal and pathological situations, accurate and validated three-dimensional (3-D) finite element models (FEMs) of the human TMJ may serve as valuable diagnostic tools as well as predictors of thresholds for tissue damage resulting from parafunctional activities and trauma. In this context, development of reliable biomechanical standards for condylar cartilage is crucial. Moreover, biomechanical characteristics of the native tissue are important design parameters for creating functional tissue-engineered replacements. Towards these goals, biomechanical characteristics of the condylar cartilage have been reviewed here, highlighting the structure-function correlations. Structurally, condylar cartilage, like the TMJ disc, exhibits zonal and topographical heterogeneity. Early structural investigations of the condylar cartilage have suggested that the tissue possesses a somewhat transversely isotropic orientation of collagen fibers in the fibrous zone. However, recent tensile and shear evaluations have reported a higher stiffness of the tissue in the anteroposterior direction than in the mediolateral direction, corresponding to an anisotropic fiber orientation comparable to the TMJ disc. In a few investigations, condylar cartilage under compression was found to be stiffer anteriorly than posteriorly. As with the TMJ disc, further compressive characterization is warranted. To draw inferences for human tissue using animal models, establishing stiffness-thickness correlations and regional evaluation of proteoglycan/glycosaminoglycan content may be essential. Efforts directed from the biomechanics community for the characterization of TMJ tissues will facilitate the development of reliable and accurate 3-D FEMs of the human TMJ.

Hyaline cartilage cells outperform mandibular condylar cartilage cells in a TMJ fibrocartilage tissue engineering application.

OBJECTIVE: To compare temporomandibular joint (TMJ) condylar cartilage cells in vitro to hyaline cartilage cells cultured in a three-dimensional (3D) environment for tissue engineering of mandibular condylar cartilage. DESIGN: Mandibular condylar cartilage and hyaline cartilage cells were harvested from pigs and cultured for 6 weeks in polyglycolic acid (PGA) scaffolds. Both types of cells were treated with glucosamine sulfate (0.4 mM), insulin-like growth factor-I (IGF-I) (100 ng/ml) and their combination. At weeks 0 and 6, cell number, glycosaminoglycan (GAG) and collagen content were determined, types I and II collagen were visualized by immunohistochemistry and GAGs were visualized by histology. RESULTS: Hyaline cartilage cells produced from half an order to a full order of magnitude more GAGs and collagen than mandibular condylar cartilage cells in 3D culture. IGF-I was a highly effective signal for biosynthesis with hyaline cartilage cells, while glucosamine sulfate decreased cell proliferation and biosynthesis with both types of cells. In vitro culture of TMJ condylar cartilage cells produced a fibrous tissue with predominantly type I collagen, while hyaline cartilage cells formed a fibrocartilage-like tissue with types I and II collagen. The combination of IGF and glucosamine had a synergistic effect on maintaining the phenotype of TMJ condylar cells to generate both types I and II collagen. CONCLUSION: Given the superior biosynthetic activity by hyaline cartilage cells and the practical surgical limitations of harvesting cells from the TMJ of a patient requiring TMJ reconstruction, cartilage cells from elsewhere in the body may be a potentially better alternative to cells harvested from the TMJ for TMJ tissue engineering. This finding may also apply to other fibrocartilages such as the intervertebral disc and knee meniscus in applications where a mature cartilage cell source is desired.

Regional dynamic tensile properties of the TMJ disc.

Although the TMJ disc has been well-characterized under tension and compression, dynamic viscoelastic regional and directional variations have heretofore not been investigated. We hypothesized that the intermediate zone under mediolateral tension would exhibit lower dynamic moduli compared with the other regions of the disc under either mediolateral or anteroposterior tension. Specimens were prepared from porcine discs (3 regions/direction), and dynamic tensile sweeps were performed at 1% strain over a frequency range of 0.1 to 100 rad/sec. Generally, the intermediate zone possessed the lowest storage and loss moduli, and the highest loss tangent. This study further accentuates the known distinct character of the intermediate zone by showing for the first time that these differences also extend to dynamic behavior, perhaps implicating the TMJ disc as a structure primarily exposed to predominantly anteroposterior tension via anterior and posterior attachments, with a need for great distension mediolaterally across the intermediate zone.

Degenerative disorders of the temporomandibular joint: etiology, diagnosis, and treatment.

Temporomandibular joint (TMJ) disorders have complex and sometimes controversial etiologies. Also, under similar circumstances, one person's TMJ may appear to deteriorate, while another's does not. However, once degenerative changes start in the TMJ, this pathology can be crippling, leading to a variety of morphological and functional deformities. Primarily, TMJ disorders have a non-inflammatory origin. The pathological process is characterized by deterioration and abrasion of articular cartilage and local thickening. These changes are accompanied by the superimposition of secondary inflammatory changes. Therefore, appreciating the pathophysiology of the TMJ degenerative disorders is important to an understanding of the etiology, diagnosis, and treatment of internal derangement and osteoarthrosis of the TMJ. The degenerative changes in the TMJ are believed to result from dysfunctional remodeling, due to a decreased host-adaptive capacity of the articulating surfaces and/or functional overloading of the joint that exceeds the normal adaptive capacity. This paper reviews etiologies that involve biomechanical and biochemical factors associated with functional overloading of the joint and the clinical, radiographic, and biochemical findings important in the diagnosis of TMJ-osteoarthrosis. In addition, non-invasive and invasive modalities utilized in TMJ-osteoarthrosis management, and the possibility of tissue engineering, are discussed.

Tensile properties of the mandibular condylar cartilage.

Mandibular condylar cartilage plays a crucial role in temporomandibular joint (TMJ) function, which includes facilitating articulation with the temporomandibular joint disc and reducing loads on the underlying bone. The cartilage experiences considerable tensile forces due to direct compression and shear. However, only scarce information is available about its tensile properties. The present study aims to quantify the biomechanical characteristics of the mandibular condylar cartilage to aid future three-dimensional finite element modeling and tissue engineering studies. Porcine condylar cartilage was tested under uniaxial tension in two directions, anteroposterior and mediolateral, with three regions per direction. Stress relaxation behavior was modeled using the Kelvin model and a second-order generalized Kelvin model, and collagen fiber orientation was determined by polarized light microscopy. The stress relaxation behavior of the tissue was biexponential in nature. The tissue exhibited greater stiffness in the anteroposterior direction than in the mediolateral direction as reflected by higher Young's (2.4 times), instantaneous (1.9 times), and relaxed (1.9 times) moduli. No significant differences were observed among the regional properties in either direction. The predominantly anteroposterior macroscopic fiber orientation in the fibrous zone of condylar cartilage correlated well with the biomechanical findings. The condylar cartilage appears to be less stiff and less anisotropic under tension than the anatomically and functionally related TMJ disc. The anisotropy of the condylar cartilage, as evidenced by tensile behavior and collagen fiber orientation, suggests that the shear environment of the TMJ exposes the condylar cartilage to predominantly but not exclusively anteroposterior loading.