Synovial fluid is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency, the principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.
The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity. The fluid contains hyaluronic acid secreted by fibroblast-like cells in the synovial membrane and interstitial fluid filtered from the blood plasma. This fluid forms a thin layer (roughly 50 micrometre) at the surface of cartilage and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space. The fluid in articular cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out mechanically to maintain a layer of fluid on the cartilage surface (so-called weeping lubrication). The functions of the synovial fluid include:
- Reducing friction - Synovial fluid lubricates the articulating joints.
- Shock absorption - As a dilatant fluid, synovial fluid is characterized by the rare quality of becoming more viscous under applied pressure. The synovial fluid in diarthrotic joints becomes thick the moment shear is applied in order to protect the joint and subsequently thins to normal viscosity to resume its lubricating function between shocks.
- Nutrient and waste transportation - The fluid supplies oxygen and nutrients and removes carbon dioxide and metabolic wastes from the chondrocytes within the surrounding cartilage.
Synovial tissue is sterile and composed of vascularized connective tissue that lacks a basement membrane. Two cell types are present:
- Type A is derived from blood monocytes; it removes the wear-and-tear debris from the synovial fluid.
- Type B produces synovial fluid, which is made of hyaluronic acid and lubricin, proteinases, and collagenases.
Synovial fluid exhibits non-Newtonian flow characteristics: The viscosity coefficient is not a constant, and the fluid is not linearly viscous. Instead, it exhibits thixotropic characteristics: Viscosity decreases and the fluid thins over a period of continued stress.
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 to lubricate the surfaces between synovium and cartilage.
Synovial fluid contains lubricin secreted by synovial cells. Chiefly, it is responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There also is some evidence that it helps regulate synovial cell growth. Synovial fluid also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint.