Joint aspiration (puncture) is a particularly important diagnostic procedure for unexplained joint swelling. Sometimes it is the only way to make a clear diagnosis. Diagnostic joint aspiration is most often performed on the knee.
Joints are enclosed by a protective layer, the joint capsule, which is lined by the synovial membrane (synovialis). The synovial membrane produces joint fluid (synovia), which keeps the joint surfaces smooth (“joint lubricant”). In joint inflammation, the amount of joint fluid often increases greatly and causes the skin over the joint to bulge (joint effusion). This collection of fluid can be punctured with a thin needle and joint fluid can be drawn off. Examining the joint fluid gives clues to the cause of the effusion, e.g. rheumatic inflammation, joint infection or wear and tear.
If a joint is inflamed, medicines can also be injected into the joint space. They then act directly in the inflamed joint (e.g. cortisone). The advantage of this treatment is that only very small amounts of the medicine enter the bloodstream, so side effects in the rest of the body are unlikely. To minimise risks, joint aspiration is prepared in the same way as a surgical procedure (thorough disinfection, surgical gloves and face mask).