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.
