A gliding joint is a synovial joint in which the bony surfaces that the joint holds together are flat, or only slightly rounded. (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 interpretation of the international Latin anatomical term for the gliding joint would be "joint that joins flat bony surfaces." The wrists have good examples of gliding joints (as well as joints of other types).
If the bony surfaces of two bones which meet at a joint actually touched each other, then motion would cause friction that 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 joins bones together (indeed, 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 synovial fluid is in a sense the equivalent of the motor oil which 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 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 in 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 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.
A gliding joint allows three different kinds of motion: linear motion, such as smooth sliding of bone past bone (the bones seem to glide past each other, hence the name "gliding" joint), angular motion such as bending and stretching, and circular motion. The ends of the bones that a gliding joint joins are nearly flat or only slightly curved and thus facilitate the characteristic sliding, bending, and twisting movements.
Role in human health
The role of gliding 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.
Common diseases and disorders
The gliding 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 gliding joints are rare. If such growths occur, they usually involve non-cancerous (benign) growths of cartilage or of tendons and their sheaths at the joints of the wrists. 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 joint.
Articulation— A synonym for "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.
Metacarpals— The five bones of the hand that are joined to the carpal bones of the wrist and to the digits.
Neoplasm— New and abnormal growth of tissue, which may be non-cancerous (benign) or cancerous (malignant)
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.)
Tarsals— The seven bones located between the bones of the lower leg and the metatarsals.
Vertebrae— Bones of the spine.
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).
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〉 (29 January 2001).