Synovial fluid, also known as joint fluid, is a viscous, clear fluid produced by the synovial membrane that fills the joint cavity of synovial joints, such as the knee, hip, and shoulder. Its primary function is to lubricate the articular surfaces, reducing friction during movement, while also providing nourishment to the articular cartilage, which lacks its own blood supply and depends on the diffusion of nutrients through the synovial fluid.
In addition, synovial fluid acts as a shock absorber, protecting the joint surfaces from mechanical stress during weight-bearing and movement. The quantity and quality of synovial fluid may change in pathological conditions such as osteoarthritis, rheumatoid arthritis, or following joint injuries, often resulting in increased fluid accumulation within the joint (joint effusion) and visible swelling.
Analysis of synovial fluid through arthrocentesis (joint aspiration) is an important diagnostic tool for determining the cause of a joint effusion, as it helps distinguish between inflammatory, infectious, and degenerative joint disorders. In certain cases, intra-articular injections of substances such as hyaluronic acid are also used as a therapeutic approach, aiming to improve joint lubrication, reduce symptoms, and enhance joint function.