Human Anatomy - Kotsan I. Ya. 2009

Bones and Their Joints
Discontinuous bone joints - diarthroses or joints

Interrupted joints (synovial joints) are characterized by A number of distinct features. Each joint consists of primary (essential) elements and secondary, or auxiliary, structures.

The primary elements of a joint include: the articular surfaces of the bones, the articular capsule, the joint cavity, and synovial fluid (Fig. 18).

The articular surfaces of bones (facies articulares) generally match each other in shape, being congruent. Thus, if one surface is convex, the opposing one is concave. They are covered with Cartilage, the smooth surface of which minimizes friction and acts as a Shock absorber.

The articular capsule (capsula articularis) hermetically seals the articular surfaces of the bones. It consists of two layers: an outer fibrous layer and an inner synovial layer. The outer layer is formed of Cytology/practical/45.html">Dense Connective Tissue. It represents a continuation of the periosteum from one articulating bone to the other and provides the Joint Capsule with strength, containing Blood Vessels and nerves. The inner layer is composed of loose connective tissue. It produces synovial fluid and also performs its resorption (absorption), ensuring a continuous metabolic exchange.

The joint cavity (cavitas articularis) is a narrow, slit-like space between the articular surfaces of the bones, hermetically enclosed by the articular capsule and containing synovial fluid.

Synovial fluid, or synovia (synovia), is a viscous substance (a derivative of Blood Plasma and Lymph) that acts as a lubricant to reduce friction between bones and helps cushion shocks and impacts. Furthermore, due to the cohesion of the articular surfaces, synovia assists in stabilizing the joints. The pressure within the joint cavity is negative (below atmospheric pressure). This also plays a vital role in joint stability, as atmospheric pressure prevents the articular surfaces from pulling apart. If the articular capsule is damaged, air enters the joint cavity, causing the articular surfaces of the bones to separate immediately.

Auxiliary joint elements include: discs, menisci, articular Lips, ligaments, synovial folds, synovial membrane projections (bursae), and villi.

Discs are cartilaginous plates, typically oval or triangular in shape, located within the joint cavity and dividing it into two compartments (chambers). They enhance the range of movements (e.g., the temporomandibular joint).

Menisci are also cartilaginous structures, but unlike discs, they are incomplete rings with an opening in the center. Menisci increase bone congruence, absorb shocks, and contribute to a variety of movements. They are crescent-shaped (e.g., the knee joint).

Articular lips are fibrocartilaginous rims located along the margins of articular sockets. They Complement and expand the joint surface and improve the conformity of the articulating surfaces (e.g., the shoulder joint).

Ligaments are structures made of dense connective tissue that connect bones. They are classified into capsular, extracapsular, and intracapsular ligaments.

Folds and projections of the synovial membrane (bursae) fill the empty spaces within joints when articular surfaces are incongruent. They contain adipose tissue and can function as shock absorbers (e.g., the knee joint).

Villi are found in large numbers on the inner surface of the synovial layer of the articular capsule. They serve as the site for the production and resorption of synovial fluid.

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Fig. 18. Diagram of a joint

1 — fibrous layer of the articular capsule, 2 — synovial layer of the articular capsule, 3 — periosteum, 4, 5 — articular surfaces of the bones, 6 — joint cavity