Synovial fluid is a viscous, non-Newtonian fluid found in the cavities of synovial joints. With its yolk-like consistency ("synovial" partially derives from ovum, Latin for egg), 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.[page needed] This fluid forms a thin layer (roughly 50 μm) 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:
- reduction of friction — synovial fluid lubricates the articulating joints[page needed]
- 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 instantaneously 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 (type A and type B) are present: Type A is derived from blood monocytes, and it removes the wear-and-tear debris from the synovial fluid. Type B produces 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 and the fluid is not linearly viscous. Synovial fluid has 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.[unreliable medical source?] 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.[unreliable medical source?]
Synovial fluid contains lubricin (also known as PRG4) as a second lubricating component, secreted by synovial fibroblasts. 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.[unreliable medical source?]
It also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint.
Health and disease
Synovial fluid may be collected by syringe in a procedure termed arthrocentesis, also known as joint aspiration.
Synovial fluid may be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic:
|WBC/mm3||<200||<2,000||5,000-75,000||>50,000||Similar to blood level|
|Polys (%)||<25||<25||50-70||>70||Similar to blood level|
|Gram stain||Negative||Negative||Negative||Often positive||Negative|
- Noninflammatory (Group I)
- Inflammatory (Group II)
- Rheumatoid arthritis
- Reactive arthritis
- Psoriatic arthritis
- Acute rheumatic fever
- Acute gout or pseudogout
- Systemic lupus erythematosus
- Ankylosing spondylitis
- Inflammatory bowel disease arthritis
- Infection (viral, fungal, bacterial) including Lyme disease
- Acute crystal synovitis
- Septic (Group III)
The cytological and biochemical analysis of human synovial fluid began around 1940 using cadaver-derived fluid and comparing characteristics to those of, for instance, bovine synovial fluid.
The mucin clot test is a very old approach to determining if an inflammatory infiltrate is present. In this test, acetic acid is added to the synovial fluid specimen. In a normal specimen, this should lead to a congealing of the hyaluronic acid, forming a 'mucin clot.' If inflammation is present, a mucin clot is not formed (the hyaluronic acid is degraded).
Lactate is elevated in septic arthritis, usually above 250 mg/dL.
Complement factors are decreased in rheumatoid arthritis and lupus arthritis.
Monosodium urate crystals are seen in gout/gouty arthritis and appear as needle-shaped negatively birefringent crystals varying in length from 2 to 20 µm. With negative birefringence, the crystals appear yellow in parallel light and blue with perpendicular light.
Calcium pyrophosphate crystals are seen in pseudogout (also known as calcium pyrophosphate deposition disease or, CPPD). These crystals are rod-shaped or rhomboids varying in length from 2 to 20 µm and with positie birefringence (blue with parallel light, yellow with perpendicular light).
Hydroxyapatite crystals are small and negatively birefringent. They are usually only detectable with an Alizarin Red S stain.
When the two articulating surfaces of a synovial joint are separated from one other, the volume within the joint capsule increases and a negative pressure results. The volume of synovial fluid within the joint is insufficient to fill the expanding volume of the joint and gases dissolved in the synovial fluid (mostly carbon dioxide) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble. This process is known as cavitation. Cavitation in synovial joints results in a high frequency 'cracking' sound.
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