What is a Baker's cyst?
John E. Zvijac, MD
Orthopedic Surgery
A Baker’s cyst, also called a popliteal cyst, is caused by a buildup of synovial fluid, the fluid that lubricates the knee joint. It can cause a bulge and a feeling of tightness behind your knee. The pain can get worse when you fully flex or extend your knee or when you’re active. If the cyst ruptures, it can cause pain, swelling and bruising on the back of the knee and calf. 

A Baker’s cyst is a bulge of swelling that forms in the back (posterior) of the knee. The swelling is synovial (joint) fluid. The condition is caused by an opening in the joint capsule which allows fluid from the joint capsule to enter and enlarge the bursa, a small fluid sack normally present in this area of the knee.

This answer provided for NATA by Loras College.

Anthony L. Komaroff, MD
Internal Medicine
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.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.