Arthrocentensis: practical and valuable (Proceedings)

Synovial membrane lines all diarthrodial joints. Synoviocytes are macrophage like cells which phagocytize foreign materials and produce synovial fluid which contains the two lubricants hyaluronic acid and polysulfated glycosaminoglycans. The normal synovial membrane is a poor filter, allowing all components of blood into the joint fluid except cells, platelets, and large molecules such as fibrinogen.

Normal synovial fluid is ≤ 1/2 milliliter (even in large joints of large dogs), clear, transparent, and has viscosity enough to cause the fluid to stretch 9 inches or more when dropped from a needle before breaking.

Arthritis includes degenerative joint disease (DJD) aka osteoarthritis (OA); immune mediated arthritis (e.g. rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE)); and infectious arthritis.

For all of these types of arthritis, the first response to the insulting etiology is inflammation of the synovial membrane. The result in increased volume of synovial fluid, coloration (yellow color with DJD, yellow or other colors with infectious or immune mediated arthritis), turbidity, and loss of viscosity. In addition, larger than normal molecules, including fibrinogen, enter the joint fluid resulting in the joint fluid clotting after collection unless placed in an EDTA or other anti-coagulant coated blood collection tube.

Analysis of joint fluid is very sensitive in identifying that a joint is normal or is abnormal. Joint fluid is less helpful in identifying the specific pathology present. However, identifying a joint as unquestionably normal or unquestionably abnormal is very helpful to the diagnostic process via ruling in or ruling out a joint as the (or a) anatomic location of pathology. Recognize that joint fluid is specific to the intra-articular joint, not surrounding joint anatomy.

     • Normal joint fluid: volume ≤ 1/2 milliliter, clear or slightly yellow, transparent, stretch of > 9 inches.

     • DJD: volume > 1 milliliter, distinct yellow, transparent, stretch of 6 - 9 inches.

     • Immune or infectious: > 1 ml, various colors, loosing or not transparent, stretch of ≤ 2 inches.

     • Note: streaks of blood are iatrogenic, well mixed blood was present before the needle entered.

Gross properties of joint fluid are typically all that is needed; although clinical pathology and cytologic analysis of joint fluid can be helpful in a few circumstances.

Cytology of Joint Fluid may be helpful in identifying immune mediated vs infectious arthritis.

Difficulty comes in separating immune mediated arthritis from septic arthritis. If WBC count is higher than 20,000, then is not DJD. If see toxic neutrophils and/or phagocytized bacteria, then is infectious (bacteria outside of PMN's may be contamination of cytologic stain(s). Bacterial culture of joint fluid or joint membrane if positive is reliable, but if negative is meaningless since there is a 50% false negative culture rate with joint cultures. If in doubt, treat with antibiotics and if no response to treatment in 2-3 days, then can reasonably treat as immune disease with corticosteriods (corticosteroid and or delay of antibiotics is devastating to an infected joint). Collection of synovial fluid is demonstrated on video.