It is a viscous liquid found in the cavities of the movable joints (diarthroses) or synovial joints.
It is formed as a nonselective ultrafiltrate (excluding high molecular weight proteins) of plasma across the synovial membrane.
It reduces friction and lubricates in the joints, provides nutrients to the vascular-deficient articular cartilage and lessens the shock of joint compression.
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Color | Clinical Significance |
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colorless to pale yellow | normal |
deeper yellow | noninflammatory and inflammatory effusions |
greenish tinge | bacterial infection |
w/ blood | hemorrhagic arthritis or traumatic aspiration |
turbid | WBCs, synovial cell debris or fibrin |
milky | crystals |
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Cell/Inclusion | Description | Clinical Significance | Picture |
---|---|---|---|
neutrophils | polymorphonuclear leukocyte | bacterial sepsis, crystal-induced inflammation | ![]() |
lymphocyte | mononuclear leukocyte | nonseptic inflammation | ![]() |
macrophage (monocyte) | large mononuclear leukocyte, may be vacuolated | normal, viral infection | ![]() |
synovial lining cell | similar to macrophage but may be multinucleated, resembles a mesothelial cell | normal | ![]() |
lupus erythematosus (LE) cell | contains ingested round body | lupus erythematosus | ![]() |
Reiter cell | vacuolated macrophage with ingested neutrophils | Reiter syndrome, nonspecific inflammation | ![]() |
rheumatoid arthritis cell (ragocyte) | neutrophil with dark cytoplasmic granules containing immune complexes | rheumatoid arthritis, immunologic inflammation | ![]() |
cartilage cells | large, multinucleated cells | osteoarthritis | ![]() |
rice bodies | macroscopically resemble polished rice, microscopically show collagen and fibrin | tuberculosis, septic and rheumatoid arthritis | ![]() |
fat droplets | refractile intracellular and extracellular globules, stained with Sudan dyes | traumatic injury, chronic inflammation | ![]() |
hemosiderin | inclusions within clusters of synovial cells | pigmented villonodular synovitis | ![]() |
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Crystal | Shape | Significance | Picture |
---|---|---|---|
monosodium urate | needles, found extracellularly or within the cytoplasm of neutrophils | gout (impaired metabolism or consumption of purines, chemotherapy, impaired excretion of uric acid) | ![]() |
calcium pyrophsphate | rhombic square or rodded, found within vacuoles of neutrophils | pseudogout (cartilage calcification from degenerative arthritis, endocrine disorders that increase serum calcium) | ![]() |
cholesterol | notched, rhombic plates | chronic inflammation | ![]() |
corticosteroid | flat plates of varied shapes | injections | ![]() |
calcium oxalate | envelopes | renal dialysis | ![]() |
hydroxyapatite (calcium phosphate) | macroscopic red splotches, does not polarize | calcified cartilage degeneration, osteoarthritis | ![]() |
microscopic thick jagged rods (in electron microscopy) | ![]() |
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