synovia

Synovia is the term used for the lubricating substances found within diarthrodial joints and, more broadly, for the synovial membranes that produce them. In common medical usage, synovia most often refers to synovial fluid, the viscous, non-Newtonian liquid that fills joint cavities, as well as the synovial membrane lining the joint capsule that secretes it.

The synovial membrane consists of an intimal lining of specialized synoviocytes and a sublining connective tissue layer. Synoviocytes include type A cells with macrophage-like functions and type B cells that resemble fibroblasts. The fluid itself is produced by ultrafiltration of plasma across the capillary-rich membrane and active secretion by type B cells, particularly of hyaluronic acid and lubricin, which determine viscosity and lubrication.

Normal synovial fluid is a clear, pale yellow, highly viscous liquid rich in hyaluronic acid. It provides boundary lubrication, reduces friction between articular surfaces, and supplies nutrients to avascular cartilage. The composition and volume can vary with age, activity, and joint health; in inflammatory states, neutrophils and protein levels rise, and viscosity decreases.

Clinical significance: Analysis of synovial fluid (arthrocentesis) aids diagnosis. Normal fluid is viscous and low in cell count; septic joints show lower viscosity, turbidity, high neutrophils, and positive cultures; inflammatory arthritis shows elevated white cells and protein; crystals may indicate gout or pseudogout. Chronic synovitis may accompany various arthritides and lead to pannus formation.