Synovial fluid is a viscous fluid in the cavities of the synovial joints, which include the knees and elbows. The main purpose of synovial fluid is to reduce friction between cartilage layers and bones during joint movement. In a healthy synovial joint, bones slide smoothly around each other within the joint cavity. An articular cartilage layer on the bone ends, and synovial fluid in the joint cavity, provide a buffer to allow this smooth movement and prevent bone damage.
Synovial joints are the only joints in the body where the bones are not connected. Instead, a cavity exists between them. Fibrous connective tissue forms the walls of the cavity, creating an articular capsule. The connective tissue attaches to bones on either side of the joint, just below the articulating surface -- the end of the bone that moves inside the joint cavity.
Articular cartilage exists in a thin layer over the ends of bones that interact within synovial joint cavities. This cartilage contains chondrocytes, cells that produce a thick liquid component of synovial fluid -- lubricin. There are no blood vessels within articular cartilage, so nutrients are obtained from synovial fluid.
The thin synovial membrane lines the articular capsule. Cells within this membrane produce albumin and hyaluronan. These two substances give synovial fluid an egg white consistency and slickness. Synovium is a porous connective tissue layer under the synovial membrane. Synovium exchanges nutrients between blood and synovial fluid and aid in injury healing processes.
Synovial joints include ball-and-socket joints in the hip and shoulder, hinge and pivot joints in the elbows, plane joints in the wrist, condyloid joints in fingers and saddle joints in thumbs, and the temporomandibular joint in the jaw. Each joint contains a very small amount of synovial fluid. Cartilage and small fat pads in the joints disperse and absorb the fluid.
Synovial fluid lubricates joints and performs other related functions. It keeps bones apart to avoid deterioration of cartilage layers and absorbs impact to shield cartilage and bones from damage. Furthermore, the substance filters nutrients for articular cartilage while preventing toxins and other harmful substances from entering the joint cavity. When joints move and bear weight, synovial fluid is squeezed out of the articular cartilage and drawn in the direction of movement to keep a layer of fluid between the cartilage surfaces.
Synovial fluid is somewhat sticky, straw-colored and transparent, with no visible particulate matter. Cloudy synovial fluid is a sign of white blood cells (indicating infection), bacteria, or crystals. Jointed affected by gout commonly contain crystals, which cause the associated pain. Synovial fluid is thinner and lacks viscosity when inflammation is present.
Rheumatoid arthritis is characterized by inflammation of the joint membrane. The autoimmune disease causes constant irritation and breakdown of the synovial membrane. The resulting inflammation disrupts synovial fluid production and allows unwelcome substances to enter the joint cavity. Osteoarthritis is a common disorder of synovial joints and is often related to age. The condition occurs when the cartilage between bones breaks down, and the bone itself starts to wear away. Excessive synovial fluid within joints is a painful symptom of osteoarthritis.
The best way to maintain healthy joints and synovial fluid production is to stay hydrated and eat a well-balanced diet. Doctor-recommended exercises and stretches can protect and improve joint function. Fatty acids in fish and nuts are beneficial to joint health, and supplements containing glucosamine and chondroitin are very popular. However, it is best to consult a doctor before adding new supplements to your diet.
Doctors can extract synovial fluid from joints using needle aspiration. This procedure is arthrocentesis or synovial fluid analysis, and the doctor can perform it right in the examination room. The fluid is sent to a lab or hospital for testing as part of the diagnostic procedure for numerous conditions and injuries of the joints and can also help diagnose more general bleeding and inflammatory disorders.
Hemarthrosis is the technical term for blood inside a joint. This usually happens as a result of injury, which prompts a rapid increase in the production of synovial fluid. This causes pain at the time of injury, but cells called phagocytes absorb the blood within a few days. Fat can also enter the joint if the synovial membrane is torn -- this is more difficult for the body to remove. Joints can recover from minor injuries on their own, but severe injuries may require medical intervention.
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