Synovial

• Synovial fluid


  Viscous fluid within the space of movable joints or synovial joints
• Synovial joints


  • Assist greatly in movement and locomotion
  • Ends of bones are lined by smooth hyaline cartilage
  • Separated by a cavity that contains the synovial fluid
  • Entire joint is enclosed in a fibrous joint capsule lined by a synovial membrane
• Synoviocytes


  Specialized cells that secrete a mucopolysaccharide containing hyaluronic acid and a small amount of protein into the synovial fluid
• Arthritis


  Damage to the joints that can produce pain and stiffness
• Categories of arthritis


  • Noninflammatory
  • Inflammatory
  • Infectious
  • Hemorrhagic
• Analyzing synovial fluid is useful in determining the cause of arthritis to guide treatment and management
• Septic arthritis (due to Staphylococcus aureus) is a medical emergency that requires immediate treatment with antibiotics and fluid drainage
• Risk factors for septic arthritis

  • Contiguous spread (skin infection, cutaneous ulcers)
  • Direct inoculation (previous intraarticular injection, prosthetic joint)
  • Hematogenous spread (diabetes, HIV, immunosuppression, IV drug abuse, osteoarthritis, other sepsis, prosthetic joint, rheumatoid arthritis, sexual activity)
  • Age older than 80 years
• Crystal-induced arthritis must also be detected promptly as they are treatable with the right medications
• Arthrocentesis


  1. Performed under sterile conditions

  2. Collect synovial fluid by needle aspiration

  3. Large joints normally contain no more than 4 mL of synovial fluid
• Plastic syringes should be used to avoid contamination by birefringent particulates that can interfere with crystal identification
• Synovial fluid does not normally clot, but diseased joints may contain fibrinogen and clot
• Recommended tubes for synovial fluid collection

  • Sterile, heparinized tube (Gram stain, culture)
  • Heparin or EDTA tube (cell counts, differential)
  • Non-anticoagulated tube (chemical analysis)
• Powdered anticoagulants must not be used as they may produce artifacts that interfere with crystal analysis
• Normal synovial fluid


  • Colorless to pale yellow
  • Viscosity resembles egg whites
• Abnormal synovial fluid colors

  • Deeper yellow (noninflammatory, inflammatory)
  • Greenish, yellow, brownish (septic arthritis)
  • Red-brown (hemorrhagic arthritis)
• Traumatic aspirations can cause uneven distribution of blood in the specimen
• Ochronosis


  Metabolic disorder where pigments are deposited into the synovium, giving a brown-black "ground pepper" appearance
• "Rice bodies"


  Fragments of degenerating synovial cells or microinfarcted synovium that can contribute to turbidity
• Viscosity of synovial fluid

  Comes from polymerization of hyaluronic acid, essential for joint lubrication
• Rope's test (mucin clot test)

  • Measures degree of hyaluronate polymerization
  • Normal fluid forms a solid clot with clear surrounding fluid
  • Abnormal fluid has less firm clot and more turbid surrounding fluid
• Cell counting of synovial fluid

  1. Total WBC count is most frequently performed

  2. Prevent cellular lysis by performing counts immediately or refrigerating

  3. Dilute with normal saline, not acetic acid
• The key lies in the identification of a questionable fluid as being synovial fluid
• Microscopic examination


  1. Cell counting

  2. Pretreatment of very viscous synovial fluid

  3. Differential counting

  4. Crystal identification

  
• Total WBC count


  • Most frequently performed cell count on synovial fluid
  • One of the parameters assessed to properly categorize the groups of joint disorders
  • To prevent cellular lysis or disintegration, counts should be performed as soon as possible or if this cannot be done, the fluid must be refrigerated
• Pretreatment of very viscous synovial fluid

  1. Add a pinch of hyaluronidase to 0.5 mL of fluid or one drop of 0.05% hyaluronidase in phosphate buffer per mL of fluid

  2. Subject the mixture to 37°C incubation for 5 minutes
• Manual counts


  Done using the Neubauer counting chamber
• Acetic acid cannot be used on synovial fluid because it causes the formation of mucin clots
• Diluents for synovial fluid

  • Normal saline
  • Hypotonic saline (0.3%) or saline containing saponin (if the fluid is bloody and RBCs must be lysed to visualize and count the WBCs)
• Methylene blue may be added to the normal saline to stain WBC nuclei when counts are done on mixed specimens
• Some highly specialized instruments are able to perform automated counts on synovial fluid but things to watch out for include the possibility of blocking the equipment apertures with highly viscous fluid and the presence of debris and tissue cells may also falsely elevate WBC counts
• Normal WBC count


  Less than 200 cells/L
• WBC count in severe infections and some inflammatory forms of arthritis

  May reach 100,000 cells/L or higher
• Counting using Neubauer counting chamber

  1. Include in the count those cells that touch the left and top borders of the square

  2. Use the four (4) corner squares to count, and if the rough estimate was more than 200 cells per large square, include counting the central smaller squares as well

  3. Make necessary adjustments to the formula based on the number of squares counted
• Differential counting


  1. To know the proportion of a particular cell type or what the composite cellular elements are

  2. Performed on cytocentrifuged preparations or on very thinly smeared slides that are stained with Romanowsky type stains (Wright, Giemsa)

  3. Prior to slide preparation, the fluid should ideally be incubated with hyaluronidase, as viscosity will hamper the process
• Cytocentrifugation


  1. Involves centrifuging a small aliquot of the fluid after treatment with hyaluronidase

  2. The cells are concentrated into a small area of the slide and the cell morphology is preserved

  3. Time consuming, technical process
• Direct smear


  1. Fluid is thinly smeared onto a glass slide, ideally after treatment with hyaluronidase

  2. Very operator-dependent; cells may exhibit crushing and smearing artifacts
• Primary cells seen in a normal synovial fluid

  • Mononuclear cells like monocytes
  • Macrophages
  • Synovial tissue cells
• Neutrophils


  Should comprise less than 25% (20% in Henry's) of the differential count
• Lymphocytes


  Should comprise less than 15% of the differential count