Pivot Joint

Definition

A pivot joint is a synovial joint in which the ends of two bones meet—one end being a central bony cylinder, the other end being a ring (or ring-like structure) made of bone and ligament. In some joints, the cylinder rotates inside the ring. In other joints, the ring rotates around the cylinder. The rotation of the skull is made possible by a pivot joint. (A synovial joint is the living material that holds two or more bones together, but also permits these bones to move relative to each other.)

A more precise rendering of the international Latin anatomical term for pivot joint would be "wheel joint." A wheel rotates around an axis or pivot (for example, the axle around which automobile tires rotate). The Latin term (itself borrowed from Greek) refers directly to the ring made of bone and ligament. The English term refers directly to the cylindrical end of the bone.

Description

Pivot joints hold the two bones of the forearm together. That is, a pivot joint, located near the elbow, joins the bones of the forearm (called the ulna and the radius) to each other. These two bones are also joined to each other near the wrist by another pivot joint. A different pivot joint, located at the base of the skull, joins the first vertebra of the spine to the second vertebra and thus permits the head to rotate (since the first vertebra is joined to the skull).

If the bony surfaces of two bones that meet at a joint actually touched each other, then motion would cause friction, which would soon produce wear and tear on the touching ends of the bones. An engineer designing a mechanical counterpart would arrange for lubricating oil to prevent such wear and tear and facilitate smooth movement between the two metal"bones." A joint thus holds bones together (it is called a "joint" because it "joins" them) but also keeps them slightly separated to prevent their damaging each other in motion.

A kind of cartilage special to joints covers the ends of the bones being joined. A membrane hermetically seals two (or more) bone-ends with their cartilage, enclosing them in a kind of living capsule. For the sake of simplicity, the following example discusses a joint with only two bones. Inside this membrane capsule, there is a short distance between the cartilage of one bone and the cartilage of the other, because even cartilage rubbing directly against cartilage would produce wear and tear. But the gap between the cartilage surfaces is not a vacuum and is not filled with air. It is filled with synovial fluid. This fluid is in a sense the equivalent of the motor oil that lubricates moving parts of an automobile engine.

The interior of a synovial joint has negative pressure in relation to air pressure. For this reason, air pressure pushes the bones together tightly into the membrane capsule while the fluid keeps them from actually touching. The hermetically sealed membrane capsule in this paradoxical fashion aids the tight joining while it ensures the slight separation.

This negative pressure in the joint continues to work even after death. Of course, the two bones are kept together in a living body not only by the membrane capsule and the synovial fluid, but also by the tissues around the bones. If, while dissecting a corpse, one removes the tissues leaving only the membrane capsule, the pair of bones will remain tightly joined. But if one pierces the capsule and allows air to rush inside, one then has normal atmospheric pressure inside the capsule instead of the negative pressure of the interior of the living joint when it is hermetically sealed by the capsule, and now the bones come easily apart.

Synovial fluid has another important quality. Most bodily tissues are nourished by blood vessels, but the cartilage on bone-ends in joints does not have blood vessels. Synovial fluid provides the nutrition for the cartilage that keeps it alive, strong, and healthy. The wall of the membrane capsule has two layers. The outer layer is fibrous. The inner layer produces the synovial fluid, and hence is called the synovial layer.

Function

A pivot joint allows movement in one plane, such as rotation about an axis. Pivot joints, for example, permit one, after bending the elbow, to turn the palm of one's hand upward or downward by rotating the forearm. The two bones of the forearm (the ulna and the radius) twist around each other using a pivot joint.

Role in human health

The role of pivot joints in human health (the same as that played by the other types of synovial joints) is to allow freedom of movement and thus provide flexibility to the skeleton.

KEY TERMS

Articulation— A synonym of "joint."

Carpals— The eight small bones that form the wrist and are joined to the metacarpals of the hand and to the bones of the forearm.

Humerus— The bone of the upper part of the arm.

Neoplasm— New and abnormal growth of tissue, which may be non-cancerous (benign) or cancerous (malignant).

Pronation— Motion of the forearm and hand by which, after one bends the elbow, the palm is turned downward.

Radius— One of the two bones of the forearm. A pivot joint joins it with the ulna near the elbow. A second pivot joint joins the other end of the radius to the other end of the ulna near the wrist. Supination—Motion by which, after one bends one's elbow, a palm is turned upward.

Synovial fluid— A transparent, sticky fluid that lubricates joints and nourishes the cartilage in a joint. (It is also found in tendons, sheaths, and bursae.)

Ulna— One of the two bones of the forearm. Two pivot joints join it to the radius, one near the elbow, one near the wrist.

Common diseases and disorders

The pivot joints (and the other joints) can be affected by such conditions as the following:

• Ankylosis: The fusion of bones across a joint. It is often a complication of arthritis.
• Ankylosing spondylitis: A type of inflammatory arthritis that progresses to ankylosis. It occurs chiefly in young men.
• Capsulitis: Inflammation of the membrane capsule that produces and encloses the synovial fluid.
• Dislocation: The displacing of a bone from its normal position, causing tendons to stretch and strain.
• Neoplasms: Abnormal growths (neoplasms) involving the pivot joints are rare. Such growths as do occur usually involve non-cancerous (benign) growths of cartilage or of tendons and their sheaths. Synovial sarcoma is a cancerous (malignant) growth of cells resembling those of the synovial layer of the membrane capsule. It is found at the contact surfaces of bones in a joint, usually in the larger joints of young adults.
• Rheumatoid arthritis: A common form of chronic inflammation of the joints. It causes swelling, pain, stiffness, elevated temperature, and redness of the joints. It is a disease of connective tissue and leads to the destruction of bone, cartilage, and ligaments in the joints.

Resources

BOOKS

Dimon, Theodore and Megan Day. Anatomy of the Moving Body: A Basic Course in Bones, Muscles, and Joints. Berkeley, CA: North Atlantic Books, 2001.

Hoffmann, David. Healthy Bones and Joints: A Natural Approach to Treating Arthritis, Osteoporosis, Tendinitis, Myalgia, Bursitis. Pownal, VT: Storey Books, 2000 (A Storey Medicinal Herb Guide).

OTHER

LaStayo, Paul C., Ph.D., P.T., C.H.T., Northern Arizona University, Physical Therapy Program. "Differentiating Joint and Muscle Disorders." 〈http://jan.ucc.nau.edu/∼pcl/diagnosispt680/jmd.htm〉 (2.5 KB, 29 January 2001).