An example of a pivot joint in the human skeletal system is the rotation of the atlas around the axis. The uppermost cervical vertebra of the spine, the atlas sits on top of the axis and forms the joint that enables the various movements of the head, such as nodding and rotation. A pivot joint is one in which a bone, such as the atlas, fits into a ring formed by another.
Pivot joints belong to the classification of joints called synovial joints. These joints are characterized by a synovial cavity filled with fluid between the articulating bones. Freely movable, the surfaces of the bones' connecting areas are covered with articular cartilage, which reduces friction. The bones comprising a synovial joint also rely upon ligaments to help hold them together. The synovial membrane, which lines the interior of the synovial cavity connecting the articulating bones, secretes a lubricant called synovial fluid.
In addition to pivot joints, the synovial joints include ball and socket, gliding, condyloid, hinge and saddle joints. Synovial joints are also referred to as "freely movable" joints and represent the largest number of articulations in the human skeletal system. The field of science that studies joints, which is called arthroscopy, divides joints by their functional classifications: immovable, slightly movable and freely movable. The respective formal medical terms for the three bone-joint functions are synarthrosis, amphiarthrosis and diarthrosis.