May also be called: Joint Swelling; Swollen Joints

Joint effusion (ih-FYOO-zuhn) is swelling in a joint due to fluid moving into the soft tissues surrounding the joint.

More to Know

The body makes many different fluids, including blood, lymph (part of the body's immune system), and synovial fluid, which helps lubricate joints. Effusion is swelling that happens when fluid leaks out of a vein, artery, lymph vessel, or synovial membrane into the surrounding tissue. This causes the tissue to expand, or swell.

When effusion happens in a joint — commonly the knee — excess fluid can pool in a part of the joint called the synovial cavity. It then leaks out into the soft tissue around the joint. This can happen as a result of injury, infection, or arthritis:

• Injuries to joints can rupture blood vessels or lymph vessels, causing blood or lymph to build up under the skin. Effusion following an injury can sometimes be a sign of a broken bone or a tear in a ligament or tendon. Joint effusion caused by an injury often is accompanied by joint pain.
• In effusions due to infection, germs (bacteria, viruses, or fungi) move from an injury into the bloodstream. When they reach a joint, they can escape to the bloodstream and cause pain, fever, redness, and warmth.
• Effusion caused by arthritis can happen in autoimmune diseases such as juvenile idiopathic arthritis where general inflammation in the body also affects the joints and can cause both redness and warmth.

Joint effusions also can happen in diseases such as gout, where crystals deposit in the joint, or in cancerous and noncancerous tumors.

Treatment for joint effusion depends on the cause and often includes resting the joint and applying ice packs to reduce swelling. In some cases, fluid may be removed from the joint to reduce swelling and increase the joint's range of motion. This fluid can be checked for signs of infection, crystals, and other substances to help find out the cause of the effusion.

Keep in Mind

Effusion is a symptom of an injury or other condition affecting a joint. In almost all cases, if the underlying condition is identified and treated, the effusion will go away. Joint effusion that happens for no apparent reason or with a fever should be checked by a doctor as soon as possible.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.