Synovial fluid is a thick, stringy fluid found in the cavities of synovial joints. It reduces friction between the articular cartilage and other tissues in joints by lubricating and cushioning them during movement.
During movement, the synovial fluid held in the cartilage is squeezed out mechanically to maintain a layer of fluid on the cartilage surface. Normal synovium contains synovial lining cells that are 1-3 cells deep. Synovium lines all intracapsular structures except the contact areas of articular cartilage. 
The major difference between synovial fluid and other body fluids derived from plasma is the high content of hyaluronic acid in synovial fluid. The normal viscosity of the synovial fluid is due to the hyaluronic acid.
Functions of Synovial Fluid
Synovial fluid aids in the nutrition of articular cartilage by acting as a transport medium for nutritional substances, such as glucose, and provides lubrication of the articulating surfaces.
Secretion of Synovial Fluid
The synovial lining cells reside in a matrix rich in type I collagen and proteoglycans. There are two main types of synovial lining cells, but these can be differentiated only by electron microscopy.
- Type A cells are macrophage-like and have primarily a phagocytic function.
- Type B cells are fibroblast-like and produce hyaluronate, which accounts for the increased viscosity of synovial fluid.
Other cells found in the synovium include antigen-presenting cells called dendritic cells and mast cells.
Synovial tissue also contains fat and lymphatic vessels, fenestrated microvessels, and nerve fibers derived from the capsule and periarticular tissues.
Hyaluronic acid, synthesized by synovial lining cells (type B), is secreted into the synovial fluid, making the fluid viscous.
Composition of Synovial Fluid
Synovial fluid is made of hyaluronic acid and lubricin, proteinases, and collagenases.
The 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.
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. It may also have a role in synovial cell growth.
Vascular permeability and synovial membrane permeability are altered by inflammation, which accounts for protein content changes in the diseased synovial fluid. Immunoglobulins, immune complexes, and complement are produced by cells accumulating in the inflamed synovial membrane and periarticular lymph nodes and find their way to the synovial fluid.
Normal synovial fluid complement levels in humans are approximately 10% of the serum values. In the inflamed joint synovial fluid complement levels will vary.
Cell counts of canine synovial fluid vary from joint to joint but normally are low. Lymphocytes, monocytes, and neutrophils are also present normally, while macrophages are seen only occasionally. In pathologic fluids, chondrocytes, osteoblasts, and osteoclasts may be seen. Fragments of articular cartilage may contain chondrocytes within lacunae. The exposed subchondral bone may give osteoblasts and multinucleated osteoclasts access to the synovial fluid.
Erythrocytes are rarely seen in the normal joint fluid. Their presence usually indicates contamination of the sample by peripheral blood at the time of aspiration
Physical Characteristics of Normal Synovial Fluid
- Normal synovial fluid is clear, pale yellow, viscid, and does not clot.
- Amount – thin film covering surfaces of synovium and cartilage within joint space
- Cell count – <200/mm3 . Less than 25 percent are neutrophils
- Protein – 1.3-1.7 g/dl (20 % of normal plasma protein)
- Glucose – within 20 mg/dl of the serum glucose level
- Temperature – 32 degree Celsius (peripheral joints are cooler than core body temperature)
- String sign (a measure of viscosity) – 1-2 inches
- When a drop is taken between the thumb and finger as in pinching and then they are separated, the drop forms a string till 1-2 inches
- pH – 7.4
Examination of Synovial Fluid
Collection of fluid is done without adding an anticoagulant, except a few milliliters for a cytologic study to which is added 2 mg potassium oxalate per milliliter of fluid.
Gross Appearance
Note the color, turbidity, apparent viscosity, and tendency to clot. Blood streaks are caused by needle puncture. The viscosity may be estimated by allowing the fluid to drip from the end of a small syringe. Normally, the viscous fluid falls drop by drop. The thin, nonviscous fluid of inflammation flows freely and uninterruptedly.
As needed following studies may be conducted on the synovial fluid
Bacterial studies
Culture is done at the bedside and should include guinea pig inoculation.
Cytology
For a total cell count, saline is used as a diluent, since routine diluents contain an acid that will precipitate the mucin. Methylene blue is added to the saline to stain the nucleated cells. A standard hemacytometer is used.
The differential count is done on very thin dried smears stained with Wright’s stain. The mucinous amorphous deposit stains deeply basophilic.
In traumatic joint disease, the total cell count varies from a normal of 60 to 3000, seldom higher, and consists mainly of mononuclear cells. In inflammatory joint disease, the total count is above 3000, mainly of mononuclear cells. In inflammatory joint disease, the total count is above 3000, mainly polymorphs. One must be cautious in interpreting the fluid in mild rheumatoid arthritis, since this may resemble that of traumatic arthritis.
Sugar
The concentration of sugar in synovial fluid is the same as that in the blood. Values are normal in traumatic arthritis and reduced in rheumatoid and infectious arthritis,
Protein
In traumatic fluid, the total protein does not rise above 5.5 g/dl, and the albumin fraction determined electrophoretically is more than 60% of the total. In rheumatoid arthritis, the total protein may rise above 8 g, with a proportionately higher amount of globulin, so that the albumin concentration is lower, often below 50%.
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