The human body contains specialized structures called articulations, or joints, which allow for a wide range of motion. Among the various types of freely movable synovial joints is the pivot joint, also known anatomically as a trochoid joint. This unique joint type is characterized by its ability to facilitate rotation around a single axis.
Mechanical Definition of a Pivot Joint
A pivot joint is classified as a uniaxial joint because its structural design permits movement in only one plane. This structure involves a rounded, cylindrical bony process that fits precisely within a ring formed by another bone and a strong surrounding ligament. The bony process acts like an axle, while the ligamentous and bony ring acts as a sleeve, permitting the axle to spin or rotate. The primary mechanical function is to allow rotational movement, where one bone spins relative to the other. The secure fit of the internal bone within the ring maintains stability and restricts movement to only the intended rotation.
Rotation in the Neck
One notable example is the atlantoaxial joint, located high in the neck between the first two cervical vertebrae. This articulation allows the head to turn from side to side. The first cervical vertebra, the atlas (C1), supports the skull, and the second, the axis (C2), provides the rotational anchor.
The axis features the dens, or odontoid process, a tooth-like projection that extends upward into the ring of the atlas. This dens acts as the central pivot around which the atlas rotates, carrying the skull with it. A powerful transverse ligament holds the dens securely in place, preventing displacement and protecting the delicate spinal cord.
The unique structure of the atlantoaxial articulation allows for approximately 50% of the total rotation that occurs in the neck. The superior articular facets of the axis allow the atlas to glide smoothly during rotation, maximizing the range of motion.
Movement in the Forearm
The second main location for pivot joints is found in the forearm, where two such joints work together to allow the hand to rotate.
Proximal Radioulnar Joint
The proximal radioulnar joint is situated near the elbow, where the head of the radius articulates with the radial notch of the ulna. This interaction is the primary location for the rotational movement of the forearm. The annular ligament forms a strong, fibrous ring that encircles the head of the radius, holding it firmly against the ulna. This arrangement creates the classic pivot structure, allowing the rounded radial head to spin within the ring-like ligament. The head of the radius can turn approximately 180 degrees, allowing the palm to face upward or downward without moving the upper arm.
Distal Radioulnar Joint and Function
The distal radioulnar joint, located near the wrist, complements the proximal joint, allowing the radius to cross over the ulna during rotation. When the palm is turned upward, the radius and ulna lie parallel to one another; this movement is termed supination. Conversely, when the radius rotates across the ulna, the palm turns downward, a movement known as pronation.
This action effectively changes the orientation of the hand, allowing tasks like pouring a drink or turning a doorknob. The intricate interplay between the radius and ulna is managed by an interosseous membrane, a sheet of connective tissue that runs between the two bones. This membrane helps distribute forces transmitted through the forearm while still allowing the necessary rotation. The combined function of the proximal and distal pivot joints facilitates the complex, coordinated movements of the human hand and wrist.