Biomechanics of TMJ

Sleeping sideways or on your stomach can unbalance the jaw, creating problems in the teeth and stomatognathic system, posture, and joints. Sleeping lying in the stomach up position, on the contrary, can prevent the appearance of more or less serious disorders such as halitosis, periodontitis, pyorrhea, resulting in tooth loss, and even serious changes in the temporomandibular joint, posture and the whole psychophysical balance.

Heterotopic cartilage develops in certain pathologic conditions, including those affecting the human temporomandibular joint (TMJ), but the underlying molecular mechanisms remain obscure. This is in part due to the fact that a reliable animal model of such TMJ diseases is not available. Here, we show that aberrant chondrocyte differentiation and ectopic cartilage formation occur spontaneously in proteoglycan 4 (Prg4) mutant TMJ discs without further invasive procedure. By 2 mo of age, mutant disc cells displayed chondrocyte transdifferentiation, accompanied by strong expression of cartilage master gene Sox9 and matrix genes aggrecan and type II collagen. By 6 mo, heterotopic cartilage had formed in the discs and expressed cartilage hypertrophic markers Runx2 and ColX. The ectopic tissue grew in size over time and exhibited regional mineralization by 12 mo. Bone morphogenetic protein (BMP) signaling was activated with
the ectopic chondrogenic cells and chondrocytes, as indicated by phosphorylated Smad 1/5/8 nuclear staining and by elevated expression of Bmp2, Bmpr1b, Bmpr2, and BMP signaling target genes. Likewise, we found that upon treatment with recombinant human BMP 2 in high-density micromass culture, mutant disc cells differentiated into chondrocytes and synthesized cartilage matrix more robustly than control cells. Importantly, a specific kinase inhibitor of BMP receptors drastically attenuated chondrogenesis in recombinant human BMP 2–treated mutant disc cultures. Unexpectedly, we found that Prg4 was expressed at joint-associated sites, including disc/muscle insertion and muscle/bone interface, and all these structures were abnormal in Prg4 mutants. Our data indicate that Prg4 is needed for TMJ disc integrity and function and that its absence leads to ectopic chondrogenesis and cartilage formation in conjunction with abnormal BMP signaling. Our findings imply that the BMP signaling pathway could be a potential therapeutic target for prevention or inhibition of ectopic cartilage formation in TMJ disease.

Like humans, domestic pigs are omnivorous and thus are a common model for human masticatory function. Prior attempts to characterize food–tooth interactions and jaw movements associated with mastication have been limited to aspects of the oral apparatus that are visible externally (with videography) and/or to 2D movements of oral structures (with monoplanar videofluoroscopy). We used XROMM, a 3D technique that combines CT-based morphology with biplanar videofluoroscopy, to quantify mandibular kinematics, tooth occlusion and mandibular condylar displacements within the temporomandibular joint (TMJ) during feeding. We observed that the pig TMJ moved detectably in only three of six possible degrees of freedom during mastication: two rotations, pitch and yaw; and one translation, protraction–retraction. Asymmetrical yaw around a dorsoventral axis produced the observed alternating left–right chewing cycles responsible for food reduction. Furthermore, the relative motions of the upper and lower premolars contained a substantial mesiodistal component in addition to the buccolingual component, resulting in an oblique (rather than a strictly transverse) power stroke. This research demonstrates the capacity of XROMM to explore the kinematic underpinnings of key masticatory movements, such as the occlusal power stroke, by integrating tooth, joint and rigid body jaw movements. XROMM also allowed us to test kinematic hypotheses based on skeletal anatomy with actual kinematics observed during naturalistic feeding behaviors. We observed that the soft tissue structures of the TMJ appear to play a significant role in limiting the range of motion of a joint, and thus analyses based solely on osseous morphology may over-estimate joint mobility.

Like humans, domestic pigs are omnivorous and thus are a common model for human masticatory function. Prior attempts to characterize food–tooth interactions and jaw movements associated with mastication have been limited to aspects of the oral apparatus that are visible externally (with videography) and/or to 2D movements of oral structures (with monoplanar videofluoroscopy). We used XROMM, a 3D technique that combines CT-based morphology with biplanar videofluoroscopy, to quantify mandibular kinematics, tooth occlusion and mandibular condylar displacements within the temporomandibular joint (TMJ) during feeding. We observed that the pig TMJ moved detectably in only three of six possible degrees of freedom during mastication: two rotations, pitch and yaw; and one translation, protraction–retraction. Asymmetrical yaw around a dorsoventral axis produced the observed alternating left–right chewing cycles responsible for food reduction. Furthermore, the relative motions of the upper and...