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Synovial Fluid

Definition: What is Synovial fluid?

The synovial fluid is the one that is found inside the synovial joints; with a viscous consistence, it has the purpose of lubricating the interior of the joint and also of reducing the friction between the joint cartilage the actual bone joint during different types of movement.


The synovial fluid is actually secreted by the synovial membrane, which coats the interior of the synovial joints. What happens is that the synovial fluid coats the surface of the joint cartilage, by forming a thin layer over it. Given the fact that the joint cartilage might present certain irregularities and small cavities, the synovial fluid will cover all of them, filling each empty space inside the joint. In a way, the synovial fluid found inside a joint is actually a reserve for the rest of the joints. When a joint is moved, the synovial fluid is going to be used in order to ensure the lubrication of the respective joint (mechanical squeeze over the surface of the cartilage)


The composition of the synovial fluid includes: hyaluronic acid, which is secreted by cells that resemble fibroblasts and which are part of the synovial membrane and interstitial fluid, the latter being filtered from the plasma.

It should be noted that the composition of the synovial fluid is 100% sterile, being primarily organized of connective tissue that is vascularized. It is composed from two main types of cells. The first type – type A – refers to the cells that derive from monocytes (blood cells); these cells work to remove the debris that results from the wear and tear process, which occurs normally with aging (these are also known as phagocytic cells). The second type – type B – are the cells that actually produce the synovial fluid. Apart from hyaluronic acid, the synovial fluid also contains other substances, such as lubricin (hence the lubrication properties), proteinases and collagenases.

In healthy people, this is the composition of the synovial fluid: hyaluronic acid, disaccharides and glucosamine. Keep in mind that the hyaluronic acid is produced by the cells of the synovial membrane and that it has different purposes: increased viscosity of the synovial fluid, better elasticity for the joint cartilage and lubrication of the articular surface, commonly found between the synovial membrane and the actual cartilage.

Lubricin is an essential component of the synovial fluid, being secreted by the fibroblasts of the synovial membrane and guaranteeing its lubricating properties. The main purpose of lubricin is to reduce the friction that would otherwise occur between the different cartilaginous surfaces. Recent studies have shown that lubricin might also help with the regulation of the growth process for the synovial cells.

Function of Synovial Fluid

The main function of the synovial fluid is to reduce the friction that can occur between the articular cartilage and the actual bone joint, during movement. In order to reduce the friction, the synovial fluid covers the joint cartilage and lubricates the moving joints.

There is one more function that is essential for the body, meaning the one of shock absorption. The interesting thing is that, the more pressure is applied to a joint, the more viscous the synovial fluid is going to become, thus absorbing all the shocks on the respective joints. This is often noticed at the levels of the diarthrotic joints, with synovial fluid becoming thicker in order to absorb the shocks and protect the joints. When there is no shock to be absorbed, the synovial fluid returns to its normal consistency, resuming its lubrication function. Apart from that, you might be interested to know that the synovial fluid also provides the joints with the necessary oxygen and nutrients. It also removes the carbon dioxide and metabolic waste that results from the activity of the chondrocytes, contributing thus to the health of the respective joints.

Synovial fluid analysis

In analysis the synovial fluid, one will find that, in healthy patients, the levels of glucose are equal to the ones commonly found in the serum. Apart from that, these are the tests that entail the synovial fluid analysis:

  • Mucin clot test
    • Useful to determine the inflammatory type of synovial fluid
    • Acetic acid is added to the synovial fluid collected from the joint
    • If the synovial fluid is healthy, the hyaluronic acid will congeal, forming the mucin clot
    • In case of inflammatory cells, the mucin clot does not form, suggesting the degeneration of the hyaluronic acid
  • Lactate levels
    • Elevated lactate levels – septic arthritis diagnosis (over 250 mg/Dl)
  • Complement factors
    • Decreased levels – rheumatoid or lupus arthritis diagnosis
  • Microscopic analysis
    • Assessment of cell count and crystals
    • Crystals assessed include: corticosteroid crystals, hydroxyapatite, calcium pyrophosphate and monosodium urate
      • Monosodium urate crystals – gout, arthritis caused by gout
      • Calcium pyrophosphate crystals – pseudogout
      • Hydroxyapatite crystals – calcific tendinitis
      • Corticosteroid crystals – after therapeutic injections with corticosteroids

How to increase synovial fluid?

The synovial fluids can be increased by:

  • Eating fish or taking fish oil supplements (Omega 3 and 6 fatty acids)
  • Include soy or soy-based products in your diet, so as to stimulate the production of hyaluronic acid
  • Eat foods that are rich in magnesium, such as: fruits (apple, pear, melon, banana), veggies (tomatoes)
  • Reduce your intake of red meat and also the amount of dairy
  • Avoid high intakes of carbs, including pasta
  • Take supplements that have the following ingredients in their composition: collagen, glucosamine sulfate, chondroitin sulfate, hyaluronan and methyl-sulphonyl-methane.

Clinical significance

The procedure through which the synovial fluid is collected from a joint is known as arthrocentesis or joint aspiration. The doctor will use a special syringe in order to collect the synovial fluid from the respective joint capsule. The procedure can be used with diagnosis purposes, identifying medical problems such as gout, inflammatory changes at the level of a joint (arthritis) or infectious conditions (septic arthritis).

The synovial fluid can be classified into different types, including:

  • Normal – volume <3.5 ml; high viscosity; clear; colorless/straw; <200 WBC/mm3; <25% polys; negative gram stain
  • Non-inflammatory – volume >3.5 ml; high viscosity; clear; straw/yellow; <2000 WBC/mm3; <25% polys; negative gram stain
  • Inflammatory – volume >3.5 ml; low viscosity; cloudy; yellow; 5000-75.000 WBC/mm3; 50-70% polys; negative gram stain
  • Septic – volume >3.5 ml; mixed viscosity; opaque; mixed color; >50.000 WBC/mm3; >70% polys; negative gram stain
  • Hemorrhagic – volume >3.5 ml; low viscosity; mixed clarity; red color; WBC/mm3 and polys similar to blood levels; negative gram stain.

The viscosity of the synovial fluid can be a good indicator of different pathologies. However, there are medical conditions in which the synovial fluid remains normal, such as: arthritis as a result of trauma, arthritis due to age-related degeneration and inflammation of the synovial membrane (condition known as pigmented villonodular synovitis). The viscosity of the synovial fluid can remain within normal consistency or be reduced in patients who have been diagnosed with systemic lupus erythematosus. As for the conditions in which the viscosity of the synovial fluid is always reduced, these are: rheumatic fever, rheumatoid arthritis, gout and other types of arthritis (septic, tubercular).

The non-inflammatory type synovial fluid is encountered in the following medical conditions: amyloidosis, acromegaly, hemochromatosis, sickle cell anemia, arthropathy of neuropathic causes, erythema nodosum, systemic lupus erythematosus, polymyositis, scleroderma, gout or pseudogout (chronic condition), rheumatic fever, trauma and degenerative joint diseases, such as osteoarthritis.

The inflammatory type synovial fluid is associated with the following medical conditions: acute crystal synovitis, Lyme disease, infections caused by bacteria, viruses or fungi; arthritis associated with IBS (spastic colon or inflammatory bowel disease), ankylosing spondylitis, systemic lupus erythematosus, polymyositis, scleroderma, gout or pseudogout (acute condition), acute rheumatic fever, arthritis of different types (psoriatic, rheumatic and rheumatoid).

The septic type synovial fluid is found in patients who suffer from pyogenic bacterial infections and septic arthritis. The hemorrhagic type synovial fluid is found in patients who suffer from: neuropathic arthropathy, Ehlers-Danlos syndrome, scurvy, hemophilia or other coagulopathies, neoplastic growths and trauma.

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