Joints are the junctions where two or more bones meet, providing the skeletal system with the mobility required for movement. The pivot joint is a type of synovial joint, meaning it possesses a fluid-filled cavity that allows for free movement. This joint design enables a specific and efficient type of motion, distinguishing it from other joints like the hinge or ball-and-socket types.
Defining the Structural Components
The pivot joint, also known as a trochoid joint, is defined by its unique anatomical arrangement. Structurally, one bone has a rounded or cylindrical surface that fits precisely into a ring. This ring is typically formed by the second bone and a strong surrounding ligament. This construction classifies the pivot joint as a uniaxial joint, restricting movement to a single axis of motion. The movement occurs around the long axis of the central, cylindrical bone, which acts like a rotating peg within the surrounding ring.
Like all synovial joints, the bone ends are covered with articular cartilage and the cavity is filled with synovial fluid for lubrication and friction minimization. The stability of the pivot joint relies heavily on the surrounding fibrous capsule and ligaments. The ligamentous ring holds the rotating bone firmly in place while still permitting its specialized motion.
Primary Movement Capabilities
The specific structure of the pivot joint dictates its function: allowing rotation around a central, longitudinal axis. This rotational movement is a spinning or swiveling motion, where the bone turns on its own axis. Unlike a hinge joint, which permits angular movements like flexion and extension, the pivot joint is restricted to this one degree of freedom. In the forearm, this rotation is termed pronation and supination. Supination involves turning the palm forward or upward, while pronation turns the palm backward or downward.
Major Locations in the Skeletal System
Two major locations in the human body illustrate the structure and function of the pivot joint. The first is the atlantoaxial joint, found between the first two cervical vertebrae in the neck. The dens, a projection from the second vertebra (C2 or the axis), fits into a ring formed by the first vertebra (C1 or the atlas) and its transverse ligament. This articulation allows for approximately 30 degrees of axial rotation, enabling the head to turn from side to side.
The second example is the proximal radioulnar joint, located near the elbow. Here, the head of the radius rotates within the radial notch of the ulna, held in place by the annular ligament. This rotation of the radial head is responsible for the movements of pronation and supination in the forearm.