Synovial fluid is a clear, viscous liquid found in the cavities of synovial joints (such as the knee, hip, shoulder, and elbow). It lubricates the joint surfaces, reduces friction during movement, and provides nutrients to the articular cartilage. Under normal conditions, only a small amount of synovial fluid is present and it is not typically visible on ultrasound. When the amount increases due to injury, infection, or inflammatory conditions like rheumatoid arthritis or gout, it becomes detectable as a joint effusion — an anechoic or hypoechoic fluid collection within the joint space. Ultrasound is highly sensitive for detecting even small effusions and can guide aspiration (arthrocentesis) of the fluid for diagnostic testing to determine the cause of the effusion.
Sonography Term