Synovial fluid is a viscous fluid found in the cavities of synovial joint"synovial" partially derives from ovum,(egg)• The principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.• It also provides nutrition to the articular cartilage and disc
GENERATION AND CLEARANCE OF SYNOVIAL FLUID• The Starling equation reads as follows:• J= K ([Pc − Pi] − σ[πc − πi])• ([Pc − Pi] − σ[πc − πi]) is the net driving force,• Kf is the proportionality constant, and• Jv is the net fluid movement between compartments.
• According to Starlings equation, the movement of fluid depends on six variables:• Capillary hydrostatic pressure ( Pc )• Interstitial hydrostatic pressure ( Pi )• Capillary oncotic pressure ( πc )• Interstitial oncotic pressure ( πi )• Filtration coefficient ( Kf )• Reflection coefficient ( σ )
• Synovial fluid is a mixture of a protein-rich ultrafiltrate of plasma and hyaluronan synthesized by synoviocytes.• Generation of this ultrafiltrate depends on the difference between intracapillary and intra- articular hydrostatic pressures and between colloid osmotic pressures of capillary plasma and synovial tissue fluid
• Proteins are present in synovial fluid at concentrations inversely proportional to molecular size, with synovial fluid albumin concentrations being about 45% of those in plasma• Synovial fluid is cleared through lymphatics in the synovium, assisted by joint movement
• Normal synovial fluid — Typical charecteristics• * Highly viscous• * Clear• * Essentially acellular• * Protein concentration approximately 45% that of plasma• * Glucose concentration similar to that in plasma• Concentrations of electrolytes and small molecules are equivalent to those in plasma
• Hyaluronic acid is synthesized by fibroblast-like synovial lining cells, and it appears in high concentrations in synovial fluid at around 3 g/L, compared with a plasma concentration of 30 μg/L.• Lubricin, a glycoprotein that assists articular lubrication, is another constituent of synovial fluid that is generated by the lining cells• Their function is to provide lubrication to the articular surfaces
• In normal joints, intra-articular pressures are slightly subatmospheric at rest (0 to -5 )• During exercise, hydrostatic pressure in the normal joint may decrease further• Resting intra-articular pressures in rheumatoid joints are around 20 mm Hg, whereas during isometric exercise, they may increase to greater than 100 mm Hg, well above capillary perfusion pressure and, at times, above arterial pressure
• Synovial fluid may be collected by syringe in a procedure termed arthrocentesis, also known as joint aspiration• Categories of joint effusions — Results of synovial fluid analysis can be used to categorize the fluid as noninflammatory, inflammatory, septic, or hemorrhagic based upon the clinical and laboratory analysis
• COMPONENTS OF SYNOVIAL FLUID ANALYSIS• Gross appearance — The volume, clarity, color, and viscosity of joint fluid are noted.• Clarity — Increased opacity of the fluid is usually due to abnormally large numbers of nucleated or red blood cells. However, translucent or even opaque fluid may be the result of acellular material. Examples include lipids in fat necrosis, cholesterol crystals in chylous effusions, or innumerable monosodium urate crystals aspirated from gouty tophi
• Color — Colorless, clear fluid is normal, while increasing amounts of plasma and nucleated cells contribute to the yellow or yellow-green appearance of inflammatory or septic fluids. Bright red, rusty, or chocolate brown fluids are indicative of fresh or old blood• Viscosity — As joint fluid is expelled from the syringe and allowed to drop into a suitable receptacle normal fluid will produce a long string- like extension as it falls. Release of proteolytic enzymes into inflamed synovial fluid typically generally causes a decrease in viscosity. However, frankly purulent (septic) effusions may also be viscous
• Microscopic examination — The microscopic examination of synovial fluid may be performed on as little as one drop of uncentrifuged fluid (wet mount), however, examination of the sediment of a centrifuged specimen may improve the sensitivity of the microscopic examination for crystals
• Cell count — Normal synovial fluid is nearly acellular. Inflammatory and septic synovial fluids are characterized by increasing numbers of leukocytes. Bacterial joint infections typically are purulent with leukocyte counts (most of which are neutrophils) of 50,000 to 150,000 cells/mm3
• Crystal search — Examination of synovial fluid for monosodium urate (MSU) crystals and calcium pyrophosphate dihydrate (CPPD) crystals is facilitated by having a microscope with polarizing filters and a quarter wave plate• Other crystals — Crystals other than MSU and CPPD may have a role in the pathogenesis of some diseases. Examples include crystals of cholesterol, hydroxyapatite, and basic calcium phosphate.
• Gram stain — The synovial fluid Gram stain is an easily performed test that can provide immediate, useful information concerning the diagnosis and therapy (Gram positive versus Gram negative coverage) of septic arthritis• Despite its utility, the sensitivity and specificity of synovial fluid Gram stain is not known precisely. In nongonococcal bacterial arthritis, the sensitivity of Gram stain has been estimated to range from 50 to 70 percent . In gonococcal arthritis the sensitivity is much lower, probably <10 percent .
• Routine bacterial culture — The synovial fluid samples should be routinely sent for culture of the common nongonococcal causes of bacterial arthritis: staphylococci followed by streptococci and Gram negative bacteria• Antibiotics should generally not be given prior to joint aspiration. If they have the likelihood of recovering a pathogenic microorganism from synovial fluid may be increased if the fluid is first inoculated into a commercial culture systems that contain antibiotic-binding beads
• When should cultures be sent for unusual organisms? — The history may reveal clues suggesting the possibility of an unusual cause of septic arthritis:•• * A history of tuberculosis exposure• * A history of trauma• * Travel to or living in an area endemic with fungal infections or Lyme disease• * The presence of immune suppression• * A monoarthritis that is refractory to conventional therapy
bloody fluid with a thicker layer of lipid material separated aftercentrifugation was aspirated from a patient with a tibial fracture into the joint space
This is the colorless, clear synovial fluid from a patient with osteoarthritis accompanied by a low synovial-fluid white cell count.
These fluid collections which serve as good samples of cloudy but translucent inflammatory synovial fluid were taken from a patient with rheumatoid arthritis (left) and gout (right) respectively
this fluid is a good example of a cloudy, pus-like fluid aspirated from a patient with acute bacterial infectious arthritis
Monosodium urate crystal
• SUMMARY AND RECOMMENDATIONS * Synovial fluid analysis may be diagnostic in patients with bacterial joint infection and crystal-induced arthritis. This analysis is indicated in febrile patients with an acute flare of already established arthritis and in other situations in which the cause of a joint effusion is uncertain or septic arthritis is suspected
• * The volume of synovial fluid removed is noted along with the clarity, color, and viscosity.• * The most valuable components of laboratory analysis of synovial fluid are: the white cell count, differential count, cultures, Gram stain, and crystal search using polarized light microscopy• * Normal synovial fluid is viscous, clear, colorless and nearly acellular. Abnormal synovial fluids are categorized into those that are noninflammatory, inflammatory, septic, and hemorrhagic as a means to reduce the number of possible causes of effusions to consider in the differential diagnosis. However, for each category there is significant diagnostic overlap.• * Gonococcal, Borrelial (Lyme disease), mycobacterial, or fungal joint infections should be suspected when routine bacterial cultures of synovial fluid do not yield a pathogenic organism. Additional diagnostic tests are suggested when these diseases are suspected.
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References• Kelley’s Textbook of Rheumatology,8th Edition• UptoDate ,2011• CURRENT Rheumatology Diagnosis & Treatment,second edition