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Synovial fluid is a biological substance that appears in the knuckles and other joints of the body. Its function is to provide lubrication and cushioning to the connected bones in the joint during activity. Synovial fluid is generated and maintained by the surrounding synovial membrane in each joint. It is present in the fingers and toes, knees, elbows, hips and other major joints involved in bodily movement. The fluid also creates the commonly heard “cracking” or “popping” of knuckles and joints.
The joints that contain this fluid are called synovial joints. Like many biological structures, these joints are very complex; they allow a wide range of movement with great precision and even grace. Each joint is capable of innumerable movements over the course of a normal lifetime. Synovial fluid is what allows these movements to occur without wearing down the ends of the various bones through friction. The fluid is dense enough to provide cushion to the bones when necessary, becoming a biological shock absorber.
Bones do not meet evenly in a joint; each bone has a small cavity on its end. Under normal circumstances, this cavity is filled with a synovial membrane that is attached to the nearby bones and cartilage. When the joint bends, the synovial fluid in the membrane ensures that contact between the bones is smooth and minimal. This fluid is periodically cleansed and replenished by the synovial membrane. Its composition is unlike that of other bodily fluids, with a unique viscosity, or thickness, that is well suited to joint maintenance.
Rheumatism and other forms of arthritis are inflammations of the joints. These are sometimes caused by the breakdown of joint tissue over time. Rheumatoid arthritis is an inflammation of the synovial membrane itself. This causes a buildup of synovial fluid that can result in chronic pain and an inability to use the affected joints. No cure has been found for this disease, but early diagnosis and treatment can reduce long-lasting damage to the joints.
Synovial fluid contains carbon dioxide, the same substance that makes bubbles in soda. An unusual side effect occurs when a joint is placed under steady pressure: the carbon dioxide forms a bubble within the fluid that pops noisily. This is called cavitation, and it is responsible for the common phenomenon known as “cracking knuckles.” This habit does not cause arthritis, as is commonly believed, but there is some evidence that it can contribute to other joint problems in later life. Some people find knuckle cracking distasteful or rude, while others consider it pleasing or even therapeutic.