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

Contents 1 Overview 2 Composition 3 Health and disease 3.1 Classification 3.2 Pathology 4 Additional images 5 External links and references

The inner membrane of synovial joints is called the synovial membrane, which secretes synovial fluid into the joint cavity. This fluid forms a thin layer (approximately 50 micrometres) at the surface of cartilage, but also seeps into microcavities and irregularities in the articular cartilage surface, filling any empty space ^[1]. The fluid within articular cartilage effectively serves as a synovial fluid reserve. During normal movements, the synovial fluid held within the cartilage is squeezed out mechanically (so-called weeping lubrication) to maintain a layer of fluid on the cartilage surface.

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 ^[2]. 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.^[3]

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.^[4]

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	High
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 ^{[5] [6]}:

• Noninflammatory (Group I)
  • Osteoarthritis, degenerative joint disease
  • Trauma
  • Rheumatic fever
  • Chronic gout or pseudogout
  • Scleroderma
  • Polymyositis
  • Systemic lupus erythematosus
  • Erythema nodosum
  • Neuropathic arthropathy (with possible hemorrhage)
  • Sickle-cell disease
  • Hemochromatosis
  • Acromegaly
  • Amyloidosis
• Inflammatory (Group II)
  • Rheumatoid arthritis
  • Reiter's syndrome
  • Psoriatic arthritis
  • Acute rheumatic fever
  • Acute gout or pseuodgout
  • Scleroderma
  • Polymyositis
  • Systemic lupus erythematosus
  • Ankylosing spondylitis
  • Inflammatory bowel disease arthritis
  • Infection (viral, fungal, bacterial) including Lyme disease
  • Acute crystal synovitis
• Septic (Group III)
  • Pyogenic bacterial infection
  • Septic arthritis
• Hemorrhagic
  • Trauma
  • Tumors
  • Hemophilia/coagulopathy
  • Scurvy
  • Ehlers-Danlos syndrome
  • Neuropathic arthropathy

Synovial joint

1. ^ http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm
2. ^ http://www.glycoforum.gr.jp/science/hyaluronan/HA01/HA01E.html
3. ^ http://www.orthop.washington.edu/arthritis/general/joints/04
4. ^ http://arthritis-research.com/content/5/S3/5
5. ^ http://www.fpnotebook.com/RHE64.htm
6. ^ http://www.rheumatology.org/publications/primarycare/number6/hrh0033698.asp?aud=mem

• 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]
• Hyaluronan: structure and properties
• Synovial fluid analysis, from the American College of Rheumatology
• FPnotebook RHE62
• FPnotebook RHE64 - white blood cell count
• Normal joint structure, from University College London