The joint capsules or articular capsules form complete envelopes for the freely movable bone joints.

Each capsule consists of two layers — a outer layer (stratum fibrosum) composed of white fibrous tissue, and an inner layer (stratum synoviale) which is a secreting layer, and is usually described separately as the synovial membrane.

Fibrous capsule

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The fibrous capsule is attached to the whole circumference of the articular end of each bone entering into the joint, and thus entirely surrounds the joint. It serves to keep certain proteins out.

Synovial membrane

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The synovial membrane invests the inner surface of the fibrous capsule, and is reflected over any tendons passing through the joint cavity, as the tendon of the popliteus muscle in the knee, and the tendon of the biceps brachii muscle in the shoulder.

It is composed of a thin, delicate, connective tissue, with branched connective-tissue corpuscles. It secretes synovial fluid which lubricates and provides nutrients to the joint. The fluid is thick, viscid and glairy, like the white of an egg, and is hence termed synovia, from the Latin for egg.

In the fetus this membrane is said, by Toynbee, to be continued over the surfaces of the cartilages; but in the adult such a continuation is wanting, excepting at the circumference of the cartilage, upon which it encroaches for a short distance and to which it is firmly attached.

In some of the joints the synovial membrane is thrown into folds which pass across the cavity; they are especially distinct in the knee. In other joints there are flattened folds, subdivided at their margins into fringe-like processes which contain convoluted vessels.

These folds generally project from the synovial membrane near the margin of the cartilage, and lie flat upon its surface.

They consist of connective tissue, covered with endothelium, and contain fat cells in variable quantities, and, more rarely, isolated cartilage cells; the larger folds often contain considerable quantities of fat.

The joint capsule is comprised of Type A and B synoviocytes. Impurities in the synovial fluid are removed by the Type A cells while the Type B cells secrete a lubricant called hyaluronan.

Diseases

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Frozen shoulder (adhesive capsulitis) is a disorder in which the shoulder capsule becomes inflamed and stiff.

Cápsula articular