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What Is Joint Effusion?

Abnormal Accumulation of Fluid


Updated May 16, 2014

What Is Joint Effusion?
melodija (iStock)

Question: What Is Joint Effusion?

Joint effusion (not to be confused with joint fusion) is the medical term used when there is an abnormal accumulation of fluid in or around a joint. When the knee is affected, the fluid build-up is also referred to as "water on the knee" and "fluid on the knee". What is the significance of joint effusion in terms of arthritis and related conditions?


Is Fluid Normal in Joints?

A small amount of fluid exists in normal joints. When a joint is affected by inflammatory arthritis, such as rheumatoid arthritis, abnormal fluid build-up is not uncommon. When this occurs, the joint appears swollen. The fluid is produced by the tissues that line the joint (i.e. synovium).

Why Drain Fluid From a Joint?

Your doctor may drain fluid from the affected joint to help relieve pressure in the joint while reducing pain and other symptoms. In addition, if infection is not suspected, your doctor may decide to inject a small amount of cortisone in the affected joint to reduce inflammation and prevent the fluid from building up again.

Why Is the Fluid Tested?

Studying the aspirated fluid can help with diagnosis. For example, if the patient has a warm and red knee, there is a chance that the joint is infected. Sending the fluid to the laboratory for culture can help confirm or rule out an infection. In addition, a blood count on the fluid can be obtained. If there are a lot of white blood cells in the specimen, the fluid may be cloudy. Cloudy fluid may develop in patients with infection, rheumatoid arthritis, or gout.

Gout fluid may have a golden color. In patients with osteoarthritis, the fluid is often clear. In patients who have injured their knee, the fluid may be clear or bloody.

Dr. Zashin is clinical assistant professor at University of Texas Southwestern Medical School and an attending physician at Presbyterian Hospitals of Dallas and Plano. Dr. Zashin is author of Arthritis Without Pain - The Miracle Of TNF Blockers. The book is useful for anyone on one of the biologic drugs or considering the biologic drugs.

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