synovial fluid

synovial fluid

synovial fluid: see joint.
Synovial fluid is a thick, stringy fluid found in the cavities of synovial joints. With its egg-like consistency ("synovial" partially derives from ovum, Latin for egg), synovial fluid reduces friction between the articular cartilage and other tissues in joints to lubricate and cushion them during movement.


The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity. This fluid forms a thin layer (roughly 50 μm) at the surface of cartilage, but also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space . The fluid within articular cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held within the cartilage is squeezed out mechanically to maintain a layer of fluid on the cartilage surface (so-called weeping lubrication).


Synovial tissue is composed of vascularized connective tissue that lacks a basement membrane. Two cells type (type A and type B) are present: type B produce synovial fluid. Synovial fluid is made of hyaluronic acid and lubricin, proteinases and collagenases. Synovial fluid exhibits non-Newtonian flow characteristics. The viscosity coefficient is not a constant, the fluid is not linearly viscous, and its viscosity increases as the shear rate decreases.

Normal synovial fluid contains 3-4 mg/ml hyaluronan (hyaluronic acid), a polymer of disaccharides composed of D-glucuronic acid and D-N-acetylglucosamine joined by alternating beta-1,4 and beta-1,3 glycosidic bonds . Hyaluronan is synthesized by the synovial membrane and secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages and lubricate the surfaces between synovium and cartilage.

Synovial fluid also contains lubricin secreted by synovial cells. It is chiefly responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There is also some evidence that it helps regulate synovial cell growth.

Health and disease


Synovial fluid can be collected by syringe in a procedure termed arthrocentesis, also known as joint aspiration.


Synovial fluid can be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic:
Classification of synovial fluid in an adult knee joint
Normal Noninflammatory Inflammatory Septic Hemorrhagic
Volume (ml) <3.5 >3.5 >3.5 >3.5 >3.5
Viscosity High High Low Mixed Low
Clarity Clear Clear Cloudy Opaque Mixed
Color Colorless/straw Straw/yellow Yellow Mixed Red
WBC/mm3 <200 200-2,000 2,000-75,000 >100,000 Same as blood
Polys (%) <25 <25 >50 >75 Same as blood
Gram stain Negative Negative Negative Often positive Negative


Many synovial fluid types are associated with specific diagnoses :

Joints cracking

When two parts forming a joint are pulled away from each other, the joint capsule increases in volume but the synovial fluid in the capsule no longer fills it all. Gases dissolved in the fluid quickly fill the empty space causing a sharp cracking sound. The general term for this is cavitation.

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