When a joint causes pain, swells, is red or has a limited range of motion, a doctor may recommend using a needle and syringe to remove fluid from the joint. This procedure is called arthrocentesis, or joint aspiration, and it may be done to obtain fluid for diagnostic lab testing, to alleviate pressure and relieve joint pain, or both.
The most commonly aspirated joint is the knee. However, arthrocentesis can be performed on other joints, such as the hip, elbow, wrist, ankle, shoulder and the knuckle of the big toe (the first metatarsophalangeal joint).
Synovial Fluid (Joint Fluid)
The aspirated fluid is called synovial fluid, or joint fluid. It is normally slippery and viscous and can be found in joints and bursae.
Synovial Fluid in Joints
A small amount of synovial fluid is found in most joints. The fluid, along with cartilage, facilitates smooth joint movement and provides cushioning between the joint’s bones.
Synovial Fluid in Bursae
Thin, slippery sacs in the joints called bursae contain synovial fluid, and they provide cushioning and reduce friction between a joint’s bones and surrounding soft tissue (e.g. muscle and ligaments). When a bursa is inflamed, it is called bursitis, and the bursa may swell with excess fluid.
Depending on the circumstances, a doctor performing arthrocentesis will remove fluid from the joint or from a joint’s bursa.
Synovial fluid’s composition can change over time and reflects the relative health of joints and bursae.