Synovial fluid
Synovial fluid is a viscous, non-Newtonian fluid
Non-Newtonian fluid
A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity of non-Newtonian fluids is not independent of shear rate or shear rate history...

 found in the cavities of synovial joint
Synovial joint
A Synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones....

s. With its yolk-like consistency ("synovial" partially derives from ovum, Latin for egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

), 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 μm
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is 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
The articular bone is part of the lower jaw of most tetrapods, including amphibians, sauropsids and early synapsids. In these animals it is connected to two other lower jaw bones, the suprangular and the angular...

 cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out mechanically
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....

 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
  • shock absorption - as a dilatant
    A dilatant material is one in which viscosity increases with the rate of shear strain. Such a shear thickening fluid, also known by the acronym STF, is an example of a non-Newtonian fluid....

     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
    Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...


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 B produces synovial fluid. Synovial fluid is made of hyaluronic acid and lubricin, proteinases, and collagenases. Synovial fluid exhibits non-Newtonian flow
Non-Newtonian fluid
A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity of non-Newtonian fluids is not independent of shear rate or shear rate history...

 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
Hyaluronan is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues...

 (hyaluronic acid), a polymer of disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...

s composed of D-glucuronic acid and D-N-acetylglucosamine
Glucosamine is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of the polysaccharides chitosan and chitin, which compose the exoskeletons of crustaceans and other arthropods, cell walls in fungi and...

 joined by alternating beta-1,4 and beta-1,3 glycosidic bond
Glycosidic bond
In chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....

s. 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
Synovial membrane is the soft tissue found between the articular capsule and the joint cavity of synovial joints....

 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.

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.

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


Synovial fluid may be collected by syringe in a procedure termed arthrocentesis
Arthrocentesis is the clinical procedure of using a syringe to collect synovial fluid from a joint capsule. It is also known as joint aspiration. Arthrocentesis is used in the diagnosis of gout, arthritis, and synovial infections....

, 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
The knee joint joins the thigh with the leg and consists of two articulations: one between the fibula and tibia, and one between the femur and patella. It is the largest joint in the human body and is very complicated. The knee is a mobile trocho-ginglymus , which permits flexion and extension as...

Normal Noninflammatory Inflammatory Septic Hemorrhagic
Volume (ml) <3.5 >3.5 >3.5 >3.5 >3.5
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

High High Low Mixed Low
Clarity Clear Clear Cloudy Opaque Mixed
Color Colorless/straw Straw/yellow Yellow Mixed Red
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...

<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


Many synovial fluid types are associated with specific diagnoses:
  • Noninflammatory (Group I)
    • Osteoarthritis
      Osteoarthritis also known as degenerative arthritis or degenerative joint disease, is a group of mechanical abnormalities involving degradation of joints, including articular cartilage and subchondral bone. Symptoms may include joint pain, tenderness, stiffness, locking, and sometimes an effusion...

      , degenerative joint disease
    • Trauma
    • Rheumatic fever
      Rheumatic fever
      Rheumatic fever is an inflammatory disease that occurs following a Streptococcus pyogenes infection, such as strep throat or scarlet fever. Believed to be caused by antibody cross-reactivity that can involve the heart, joints, skin, and brain, the illness typically develops two to three weeks after...

    • Chronic gout
      Gout is a medical condition usually characterized by recurrent attacks of acute inflammatory arthritis—a red, tender, hot, swollen joint. The metatarsal-phalangeal joint at the base of the big toe is the most commonly affected . However, it may also present as tophi, kidney stones, or urate...

       or pseudogout
    • Scleroderma
      Systemic sclerosis or systemic scleroderma is a systemic autoimmune disease or systemic connective tissue disease that is a subtype of scleroderma.-Skin symptoms:...

    • Polymyositis
      Polymyositis is a type of chronic inflammation of the muscles related to dermatomyositis and inclusion body myositis.-Signs and symptoms:...

    • Systemic lupus erythematosus
      Systemic lupus erythematosus
      Systemic lupus erythematosus , often abbreviated to SLE or lupus, is a systemic autoimmune disease that can affect any part of the body. As occurs in other autoimmune diseases, the immune system attacks the body's cells and tissue, resulting in inflammation and tissue damage...

    • Erythema nodosum
      Erythema nodosum
      Erythema nodosum is an inflammation of the fat cells under the skin characterized by tender red nodules or lumps that are usually seen on both shins...

    • Neuropathic arthropathy (with possible hemorrhage)
    • Sickle-cell disease
      Sickle-cell disease
      Sickle-cell disease , or sickle-cell anaemia or drepanocytosis, is an autosomal recessive genetic blood disorder with overdominance, characterized by red blood cells that assume an abnormal, rigid, sickle shape. Sickling decreases the cells' flexibility and results in a risk of various...

    • Hemochromatosis
    • Acromegaly
      Acromegaly is a syndrome that results when the anterior pituitary gland produces excess growth hormone after epiphyseal plate closure at puberty...

    • Amyloidosis
      In medicine, amyloidosis refers to a variety of conditions whereby the body produces "bad proteins", denoted as amyloid proteins, which are abnormally deposited in organs and/or tissues and cause harm. A protein is described as being amyloid if, due to an alteration in its secondary structure, it...

  • Inflammatory (Group II)
    • Rheumatoid arthritis
      Rheumatoid arthritis
      Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...

    • Reactive arthritis
      Reactive arthritis
      Reactive arthritis , is classified as an autoimmune condition that develops in response to an infection in another part of the body. Coming into contact with bacteria and developing an infection can trigger the disease. Reiter's syndrome has symptoms similar to various other conditions collectively...

    • Psoriatic arthritis
      Psoriatic arthritis
      Psoriatic arthritis is a type of inflammatory arthritis that, according to the National Psoriasis Foundation, will develop in up to 30 percent of people who have the chronic skin condition psoriasis...

    • Acute rheumatic fever
    • Acute gout or pseuodgout
    • Scleroderma
    • Polymyositis
    • Systemic lupus erythematosus
    • Ankylosing spondylitis
      Ankylosing spondylitis
      Ankylosing spondylitis , previously known as Bekhterev's disease, Bekhterev syndrome, and Marie-Strümpell disease is a chronic inflammatory disease of the axial skeleton with variable involvement of peripheral joints and nonarticular structures...

    • Inflammatory bowel disease arthritis
    • Infection
      An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

       (viral, fungal, bacterial) including Lyme disease
      Lyme disease
      Lyme disease, or Lyme borreliosis, is an emerging infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in the United States, whereas Borrelia afzelii and Borrelia garinii cause most...

    • Acute crystal synovitis
  • Septic (Group III)
    • Pyogenic bacterial infection
    • Septic arthritis
      Septic arthritis
      Septic arthritis is the purulent invasion of a joint by an infectious agent which produces arthritis. People with artificial joints are more at risk than the general population but have slightly different symptoms, are infected with different organisms and require different treatment. Septic...

  • Hemorrhagic
    • Trauma
    • Tumor
      A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

    • Hemophilia/coagulopathy
      Coagulopathy is a condition in which the blood’s ability to clot is impaired. This condition can cause prolonged or excessive bleeding, which may occur spontaneously or following an injury or medical and dental procedures.The normal clotting process depends on the interplay of various proteins in...

    • Scurvy
      Scurvy is a disease resulting from a deficiency of vitamin C, which is required for the synthesis of collagen in humans. The chemical name for vitamin C, ascorbic acid, is derived from the Latin name of scurvy, scorbutus, which also provides the adjective scorbutic...

    • Ehlers-Danlos syndrome
      Ehlers-Danlos syndrome
      Ehlers–Danlos syndrome is a group of inherited connective tissue disorders, caused by a defect in the synthesis of collagen . The collagen in connective tissue helps tissues to resist deformation...

    • Neuropathic arthropathy

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
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble. This process is known as cavitation
Cavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...

. Cavitation in synovial joints results in a high frequency 'cracking' sound.

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

External links

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