Q: What is a Baker's cyst? What's the best way to get rid of it?

A: A Baker's cyst is a collection of fluid behind the knee. It was named after a British surgeon, William Baker, who described these cysts in the late 1800s. It's sometimes called a popliteal cyst. They can be found based on symptoms and a physical examination, but ultrasound or MRI testing can confirm the diagnosis.

Baker's cysts usually develop due to a problem in the knee that causes joint fluid to accumulate in the joint. This collection of fluid causes pressure to build. When the pressure in the joint is high enough, fluid is forced backwards to form a cyst behind the knee. The anatomy of the knee "traps" the fluid behind the knee; the fluid cannot easily return to the joint.

Eventually, the cyst can become large enough to restrict movement of the knee or cause pain. It can also block the flow of blood through veins behind the knee, causing the lower leg to swell. This can mimic a blood clot in a leg vein. A Baker's cyst can also rupture, spilling fluid into the calf. This is also a cause of leg swelling that can mimic a blood clot.

The best way to treat a Baker's cyst depends on the reason it developed in the first place. Options for treatment include:

Rest. If the fluid developed due to trauma, injured cartilage or a torn ligament, the fluid may go away over time just by resting the joint.

Medical treatment. If the fluid accumulated due to osteoarthritis, rheumatoid arthritis (or a related condition) or an infection, treating the arthritis may lead to the cyst going away over time.

Removing the fluid from the knee and injecting a steroid. These treatments may lower the pressure in the joint, reduce the formation of additional fluid and speed recovery. However, steroid injections are avoided if infection is present. The steroids may delay recovery from the infection.

Surgery. Occasionally, surgery may be recommended for an infection that does not promptly improve, or for torn cartilage or ligaments. Surgery for a Baker's cyst is usually recommended only as a last resort, after other, more conservative options have failed.

If you've been diagnosed with a Baker's cyst, talk to your doctor about the most likely cause and the appropriate treatment options.

(Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center, Boston, Mass., and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center.)

(For additional consumer health information, please visit www.health.harvard.edu.)

(c) 2011. PRESIDENT AND FELLOWS OF HARVARD COLLEGE. ALL RIGHTS RESERVED. DISTRIBUTED BY TRIBUNE MEDIA SERVICES, INC.