What Is a Baker's Cyst?

Medically Reviewed

Q. For some weeks I’ve had a painful swelling behind my left knee that I’ve been told is a Baker’s cyst. Does this condition require surgery?

A. A Baker’s cyst is technically not a real cyst—a term used for a sac or pocket lined with cells that secrete fluid. In a Baker’s cyst, synovial fluid from the knee joint flows into tissues behind the joint, including the bursa, a small pouch (one of many around the body’s joints) that protects tendons and muscles from irritation.

In older people, the flow of fluid into the bursa is often caused by arthritis inflammation in front of the knee. Torn cartilage in the knee is another cause. The cyst may feel like a water-filled balloon, and as fluid accumulates, puffiness and pain behind the knee and in the upper calf typically increase, especially when bending or straightening the knee.

Occasionally a Baker’s cyst ruptures, suddenly causing more pain, swelling, and bruising on the back of the knee and calf. Often a Baker’s cyst will resolve on its own. If it doesn’t, and if your doctor determines the cyst is related to arthritis, a corticosteroid injection into the knee joint or cyst may get rid of the cyst.

Draining a persistent cyst with a needle can also produce relief. However, the cyst can recur. Surgery is rarely necessary, but an extremely large, painful cyst may require arthroscopic surgery to decompress the cyst and treat any torn cartilage.