Synovium

The synovial tissue of the TMJ is the internal lining of the external capsule. Thus, one would normally expect to find synovial tissue at the periphery of the joint. Generally, it is accepted that synovial tissue may be of several types, depending on what intra-articular tissue it covers. For example, most joints have loose connective tissue, dense fibrous or an adipose type of synovial tissue. Therefore, that part of the synovium that rests on the non-articulating retrodiscal attachment would be of the loose connective type. More fibrous synovial tissue would be found around the medial and lateral condylodiscal ligaments and the tendinous insertion of the upper belly of the lateral pterygoid muscle. Where villi of synovial tissue project into the joint space from the perimeter, there usually is fatty tissue in the stalks of the villi, and this is classic adipose synovial tissue. Larger villous folds will be found to contain blood vessels.

The synovial fluid is generally a dialysate of blood that also contains mucin, lymphocytes, monocytes, and macrophages. The non-vascularized tissues of the joint are dependent on synovial fluid for nutrition. Hence, the thinner mid-portion of the disc and the articular cartilage covering the condyle, fossa and eminence are dependent on the pumping of synovial fluid. As in most other synovial diarthroses, the synovial fluid must be compressed by the articulating surfaces and thereby be driven into deeper layers of these tissues. Thus, the mechanism of synovial nutrition in the TMJ is dependent on three factors. First, the disc must be compressed against the adjacent articulating surfaces in normal juxtaposition between the condyle and the opposing superior osseous structures. Second, there must be compressive loading of structures against each other, and the articulating structures must be firm yet pliable enough to drive the synovial fluid into the tissues effectively. Third, the tissue being penetrated by synovial fluid must be relatively thin. Thus, the thicker anterior and posterior bands of the disc will have their own internal blood supply. Additionally, in most synovial joints, the intra-articular cortical bone is nourished at least in part by synovial fluid.