October 15, 2018
Knee injury
Ask the Experts

What Is a Baker's Cyst?

by Berkeley Wellness  

Q: I have swelling behind my knee, which sometimes makes my knee feel stiff. Any idea what it may be?

A: It sounds like a Baker’s cyst, though you should have your doctor check it out to make sure it’s not a more serious condition (such as a blood clot or infection) and then discuss treatment options. Imaging studies, including ultrasounds and MRIs, can be used if the diagnosis is uncertain or to confirm a Baker’s cyst.

Also called a popliteal cyst, a Baker’s cyst occurs when the synovial fluid that normally lubricates the knee joint is produced in excess, most often due to arthritis flare-ups, a cartilage (meniscus) or internal ligament tear in the knee, or anything else that causes swelling in the joint. The fluid gets trapped between the bones of the joint and is squeezed out into the sac (bursa) behind the knee, causing a visible bulge, sort of like a small water-filled balloon.

First described by the surgeon William Morrant Baker in the 1870s, Baker’s cysts are actually quite common: One study that used ultrasounds reported that 26 percent of 399 people with knee pain had them.

Though often symptomless, Baker’s cysts can cause tightness, achiness, and pain in the knee, especially when bending or fully extending it. On occasion, large cysts burst, leading to fluid accumulation and swelling in the calf.

A Baker’s cyst that causes no problems does not need to be treated; it may go away on its own. If you have discomfort, however, try icing the area, elevating the leg when possible, taking anti-inflammatory drugs, and wearing a compression knee sleeve. Physical therapy may help increase pain-free range of motion and strengthen surrounding muscles.

Treating the underlying knee problem (if any) may clear the cyst by stopping the inflammation that’s causing it. If needed, treatment directed at the cyst itself may include draining the fluid (needle aspiration) and a corticosteroid shot into the knee joint. Or the cyst itself—especially if it’s large and painful—can be surgically removed, though it may come back if an underlying joint abnormality has not been adequately treated.

Also see Are Sebaceous Cysts Dangerous?