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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.

Contents

Overview

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 and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space.[1] 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).

Composition

Synovial tissue is sterile and 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.[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 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]

Its functions are:

reducing friction by lubricating the joint, absorbing shocks, and supplying oxygen and nutrients to and removing carbon dioxide and metabolic wastes from the chondrocytes within articular cartilage.[citation needed]

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

Health and disease

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Collection

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

Classification

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

Pathology

Many synovial fluid types are associated with specific diagnoses:[5][6]

Joints cracking

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.[7] This process is known as cavitation. Cavitation in synovial joints results in a high frequency 'cracking' sound.[8][9]

Additional images

References

  1. ^ http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm
  2. ^ GlycoForum / Science of Hyaluronan-1
  3. ^ Arthritis - UW Medicine - Department of Orthopaedics and Sports Medicine
  4. ^ Arthritis Research & Therapy | Full text | Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints
  5. ^ Lupus Anticoagulant
  6. ^ American College of Rheumatology
  7. ^ 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. doi:10.1136/ard.30.4.348. PMID 5557778. 
  8. ^ 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.
  9. ^ Howstuffworks "What makes your knuckles pop?"

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