The articulation point between different parts of the skeleton, whether bone or cartilage. Joints are divided into those which are fixed or relatively fixed (fibrous and cartilaginous joints), and those which allow free movement (synovial joints). In the former, such as the sutures between the bones of the skull, a layer of cartilage or fibrous tissue lies between the bones, binding them firmly together. Joints like those between the vertebral bodies (see SPINAL COLUMN), have a thick disc of fibro-cartilage between the bones. Although the individual joint is capable of very little movement, a series of these makes the spinal column flexible.
All movable joints involve four structures: the bones whose junction forms the joint; a layer of cartilage covering the ends of these, making them smooth; a fibrous sheath, the capsule, thickened at various points into bands or ligaments, which hold the bones together; and, finally, the synovial membrane, which lines the capsule and produces synovial fluid, lubricating the movements of the joint. In addition, the bones are kept in position at the joints by the various muscles passing over them and by atmospheric pressure. Where the ends of the bones do not quite correspond, a subsidiary disc of fibro-cartilage may help to adapt the ends of the bones more perfectly to each other. Larger cavities may be filled by movable pads of fat under the synovial membrane, giving additional protection to the joint.
Diagram of synovial joints. (Left) A right shoulder-joint (simple synovial) from the front. (Right) Right knee-joint (synovial with articular disc) from the front.
In gliding joints, such as the wrist and ankle, the bones have flat surfaces capable of only a limited amount of movement. In hinge joints, such as the elbow and knee, movement takes place around one axis. Ball-and-socket joints, such as the shoulder and hip, allow free movement in any direction. Subsidiary varieties are named according to the shape of the bones which enter the joint.