Joint

Joints, in anatomy, regions of union between bones or cartilages in the skeleton. Synarthroses are rigid, immovable joints, such as the connections between the bones of the skull; symphyses are slightly movable joints, such as the junction of the bones making up the front of the pelvis; and diarthroses are movable joints, such as the meeting of the bones of the limbs with those of the trunk.

Immovable joints are held together by actual intergrowth of bone or by strong fibrous cartilage. Slightly movable joints are held together by elastic cartilage. Typical movable joints consist of an external layer of fibrous cartilage giving rise to strong ligaments that support the separate bones. The bones of movable joints are covered with smooth cartilage and are lubricated by a thick fluid, called synovial fluid, produced between the bones in membranous sacs, known as bursae. Bursitis, or inflammation of the bursae, is a common painful condition of movable joints. See also Arthritis.

The human body has several types of movable joints. Ball-and-socket joints, which allow free movement in all directions, are found in the hip and shoulder. Hinge joints, allowing movement in one plane only, are found in the elbows, knees, and fingers. Pivot joints, permitting rotation only, are found between the first two vertebrae; the head rotates from side to side on a joint of this type called the axis. Gliding joints, in which the surfaces of the bones move a short distance over each other, are found between the various bones of the wrist and ankle.