Inflammatory and noninflammatory synovitis alters both the synovium and the composition of synovial fluid. In the synovial fluid of inflammatory joint diseases, the lysosomal enzymes released break down the hyaluronic acid “sieve” barrier.
The loss of the hyaluronic sieve allows large proteins to enter the synovial fluid. This protein-rich fluid can form clots resulting from the presence of fibrinogen. A decreased concentration of hyaluronic acid within the synovial fluid also reduces its viscosity.
In inflammatory conditions, leukocytes pass freely across the synovium and into the synovial fluid. As the white blood cell count within the synovial fluid increases, its clarity decreases. Fluid that contains large numbers of crystals also appears cloudy on gross examination and can be confused with the fluid of infection. In cases of infection or severe inflammation, impaired glucose delivery and increased metabolism by synovial components combine to decrease the glucose level in the synovial fluid relative to plasma concentration; Table 2 summarizes the characteristics of synovial fluid in different types of effusions. Note that parameters suggested in Table 2 are only rough guidelines, as the microscopic and macroscopic characteristics of synovial fluids vary widely depending on the condition. Host response, underlying disease states, and medications such as steroids can greatly alter the composition of synovial fluids.
Aspiration of the joint (arthrocentesis) hones the differential diagnosis. If there is a history of trauma, blood in the joint (hemarthrosis) suggests a ligament tear, fracture, retinacular tear (ie, patellar dislocation), or peripheral meniscal tear. If there is no history of trauma, a hemarthrosis suggests a clotting disease, hemangioma, or PVNS.