The pivot joint is a specialized type of synovial joint in the human body. This means the joint is freely movable and encased in a fluid-filled capsule. The primary function of the pivot joint is to enable rotational movement, allowing one bone to spin relative to another.
The Unique Structure of the Pivot Joint
The physical configuration of the pivot joint is designed to facilitate rotation while limiting other movements. One articulating bone possesses a rounded or cylindrical projection. This end fits precisely into a ring-shaped structure formed by the second bone and surrounding ligaments.
The ligamentous ring securely holds the cylindrical bone in place, providing stability for the joint. The entire structure is enclosed within a fibrous capsule. Within this capsule, synovial fluid is secreted, which minimizes friction between the smooth, cartilage-coated bone surfaces as they move. This setup ensures that the bone can pivot smoothly around its longitudinal axis without slipping or sliding.
Defining Rotational Movement
The movement allowed by the pivot joint is strictly rotational, defining it as a uniaxial joint. This means motion occurs around only a single, central axis. This axis runs down the long length of the cylindrical bone, acting as a fixed point around which the bone spins.
The action is a pure rotation, where the bone turns like a spindle within its fixed ring enclosure. This movement does not involve significant translation. Specific terminology describes the rotational movements enabled by this joint class, such as pronation and supination in the forearm.
Primary Locations and Enabled Actions
The pivot joint’s specialized rotation is utilized in highly important locations in the body.
Atlantoaxial Joint
One primary example is the atlantoaxial joint, located between the first two cervical vertebrae in the neck, the atlas (C1) and the axis (C2). The dens, a bony projection on the axis (C2), acts as the pivoting cylinder, rotating within a ring formed by the atlas (C1) and the transverse ligament. This articulation is responsible for approximately 40 to 70% of the head’s total axial rotation. The movement allows us to turn our head side-to-side, such as shaking the head “no.”
Proximal Radioulnar Joint
The second major location is the proximal radioulnar joint, found near the elbow between the radius and ulna bones of the forearm. At this joint, the head of the radius rotates within a collar formed by the ulna’s radial notch and the annular ligament. This rotation enables the movements known as pronation and supination. Pronation turns the palm to face downward, while supination turns the palm to face upward. These actions are fundamental for tasks like using a screwdriver or turning a doorknob.