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Synovial fluid

Synovial fluid
A typical joint
Latin synovia
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Anatomical terminology

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.[1][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.[2] 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[3][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.[4]

Normal synovial fluid contains 3–4 mg/ml hyaluronan (hyaluronic acid),[5] 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.[6][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.[7][unreliable medical source?]

Synovial fluid contains lubricin (also known as PRG4) as a second lubricating component, secreted by synovial fibroblasts.[8] 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.[9][unreliable medical source?]

It also contains phagocytic cells that remove microbes and the debris that results from normal wear and tear in the joint.

Clinical significance


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:

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 <2,000[10] 5,000[10]-75,000 >50,000[10] Similar to blood level
Polys (%) <25 <25[10] 50[10]-70[10] >70[10] Similar to blood level
Gram stain Negative Negative Negative Often positive Negative

Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum.

Synovial fluid viscosity


  • Normal
  • Traumatic arthritis
  • Degenerative (Osteo) arthritis
  • Pigmented villonodular synovitis

Normal or decreased:

  • Systemic lupus erythematosus


  • Rheumatic fever
  • Rheumatoid arthritis
  • Gout
  • Pyogenic (Septic) arthritis
  • Tubercular arthritis


Many synovial fluid types are associated with specific diagnoses:[11][12]


Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum.

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.[13]


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).[14]

Lactate is elevated in septic arthritis, usually above 250 mg/dL.

Complement factors are decreased in rheumatoid arthritis and lupus arthritis.


Microscopic analysis of synovial fluid is performed to evaluate for cell count and crystals. Crystals include monosodium urate, calcium pyrophosphate, hydroxyapatite and corticosteroid crystals.[14]

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 positive 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.

Corticosteroid crystals may be seen following therapeutic corticosteroid injection into the joint space. They appear blunt, jagged, and show variable birefringence.[14]

Cracking joints

Main article: Cracking joints

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.[15] This process is known as cavitation. Cavitation in synovial joints results in a high frequency 'cracking' sound.[16][17]


  1. ^ Principles of Anatomy & Physiology, 12th Edition, Tortora & Derrickson, Pub: Wiley & Sons
  2. ^ Edwards, Jo, ed. (2000). "Normal Joint Structure". Notes on Rheumatology. University College London. Archived from the original on 19 Nov 2012. Retrieved 5 April 2013. 
  3. ^ McCracken, Thomas (2000). New Atlas of Human Anatomy. China: Metro Books. pp. 1-240. ISBN 1-58663-097-0.
  4. ^ Elert, Glenn. "Viscosity". The Physics Hypertextbook. Archived from the original on 23 Mar 2013. 
  5. ^ Hui, Alexander Y.; McCart, William J.; Masuda, Koichi; Firestein, Gary S.; Sah, Robert L. (Jan–Feb 2012). "A Systems Biology Approach to Synovial Joint Lubrication in Health, Injury, and Disease". Systems Biology and Medicine. Wiley Interdisciplinary Reviews 4 (1): 15–7. PMC 3593048. PMID 21826801. doi:10.1002/wsbm.157. 
  6. ^ "GlycoForum / Science of Hyaluronan". 15 December 1997. 
  7. ^ "Joints". University of Washington Medicine - Department of Orthopaedics and Sports Medicine. Retrieved 2013-02-04. 
  8. ^ Jay, GD; Britt, DE; Cha, CJ (March 2000). "Lubricin is a product of megakaryocyte stimulating factor gene expression by human synovial fibroblasts". J Rheumatol (abstract) 27 (3): 594–600. PMID 10743795. 
  9. ^ Warman M (2003). "Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints". Arthritis Research & Therapy 5 (Suppl 3): S2. doi:10.1186/ar804. 
  10. ^ a b c d e f g Table 6-6 in: Elizabeth D Agabegi; Agabegi, Steven S. (2008). Step-Up to Medicine (Step-Up Series). Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-7153-6. 
  11. ^ "Lupus Anticoagulant". Family Practice Notebook. Archived from the original on 18 January 2013. Retrieved 7 April 2013. 
  12. ^ American College of Rheumatology[dead link]
  13. ^ Ropes, Marian W.; Rossmeisl, Elsie C.; Bauer, Walter (November 1940), "The Origin and Nature of Normal HUman Synovial Fluid" (SCANNED PAGES), J Clin Invest 19 (6): 795–799, PMID 16694795, doi:10.1172/JCI101182, retrieved 6 April 2013  and references therein
  14. ^ a b c De Mais, Daniel (2009), Quick Compendium of Clinical Pathology (2nd ed.), Chicago: ASCP Press, ISBN 9780891895671, OCLC 692198047 [pages needed]
  15. ^ Unsworth A, Dowson D, Wright V. (1971). "'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint.". Ann Rheum Dis 30 (4): 348–58. PMC 1005793. PMID 5557778. doi:10.1136/ard.30.4.348. 
  16. ^ Watson P, Kernoham WG, Mollan RAB. A study of the cracking sounds from the metacarpophalangeal joint. Proceedings of the Institute of Mechanical Engineering [H] 1989;203:109-118.
  17. ^ Howstuffworks "What makes your knuckles pop?"

Further reading

  • Warman W. "Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints." Arthritis Res. Ther. 2003, 5(Suppl 3):5 [1]

External links

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