Synovial Fluid (Joint Fluid) is a typical body fluid modified by components secreted by the joint tissue.
The major difference between synovial fluid and other body fluids is the high concentration of hyaluronic acid (“HA”) in the synovial fluid.
The composition of synovial fluid is:
HA is produced by synovial lining cells and is secreted into the synovial fluid.
Lubricin is secreted from synovial fluid cells.
Synovial Fluid has two main functions:
At room temperature, synovial fluid assumes a gelatine like appearance.
Synovial fluid may be thixotropic, a property of certain gels to become fluid when shaken.
Lubrication of synovial surfaces by synovial fluids (all moving joints have synovial membranes) requires HA and occurs due to a boundary phenomenon.
Boundary lubrication occurs when each load bearing surface is coated or impregnated with a thin layer of lubricant that keeps the sliding surfaces apart allowing ease of motion and a low co-efficient of friction.
Boundary lubrication of articular cartilage is very effective in preventing wear and tear due to motion but loses its protective abilities under high loads (weight and activity).
The lubricating properties of the synovial fluid are directly related to the HA properties:
Viscous solutions without HA do not lubricate nearly as well as solutions containing HA of equal or lower viscosity.
Degradation of HA decreases its lubricating ability.
Vascular and synovial membrane permeability are altered by inflammation which changes the composition of diseased synovial fluid.
In healthy joints, HA in the synovial fluid is constantly broken down and replaced (the average 70kg (154lb) person has approximately 15 grams of HA in the body, 33% of which is turned over every day).
In osteoarthritis, breakdown occurs faster than replacement which results in less viscous synovial fluid that doesn’t function properly allowing for: