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Bone

The osseous elements of the TMJ are the condyle, fossa and articular eminence. The mandibular condyle forms the inferior articulating structure and is the convex member of the joint. The condyle receives its nourishment from three sources. The most superficial part of the intra-articular cortical bone is nourished by synovial fluid. The part of the condylar cortex and marrow that is contained within the joint capsule is dependent on the perforating blood vessels from the insertion of the lateral pterygoid muscle for its blood supply. Those parts of the condylar head and neck that are extra-articular will receive a more direct blood supply from the periosteum and adjacent muscle tissues. Hence, the parts of the condyle that are most demanding of the blood supply are posterior and superior. Furthermore, the outflow of marrow blood is generally to more dependent parts of the cancellous bone, and if there is to be adequate blood flow into the condyle, there must also be equal flow of blood out through the condylar neck. The narrower the neck or the tighter the trabecular bone of the neck, the more vulnerable the condyle may be to venous outflow sludging. This blood will back into the arterial side, and a diminished condylar blood supply will result.

The superior articulating structures are the fossa and the eminence. The fossa is the concave portion of the joint, and functionally, in all synovial joints, the majority of movements take place with the convex and concave surfaces in articulation. The fossa generally is well-vascularized extracapsularly by periosteal and muscular attachments, and blood supply is usually not a problem. The medial half of the fossa is beneath the middle cranial fossa, and in this area, the bone is quite thin.

The articular eminence is part of the temporal bone and is convex from anterior to posterior. As such, this part of the articulation is not structured to function for long periods with the convex condyle. Even though the disc is normally interposed, the two convex surfaces will have one point of contact that will overload the joint locally as these areas. This, in turn, could result in osteochondritis dissecans and localized collapse of these areas. It is not infrequently found that mastoid air cells extend into the eminence and as such pose a potential communication between the joint and the mastoid sinuses. Again, the blood supply to the eminence is generally favorable because of the abundant adjacent muscle attachments. Of one final note is the potential occlusion of the middle meningeal artery with antecomedial displacement of TMJ structures.

 

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