Baker's Cyst

A Baker’s cyst is a membrane-lined sac of fluid that forms behind the knee.

A Baker’s cyst is formed when synovial fluid (fluid that is produced by the joint lining to lubricate and protect the joints) escapes from the knee joint capsule (also called the synovial sac or synovium) and forms a new sac outside the joint at the back of the knee.

There may be a slight bulge or noticeable lump, most visible when standing.

Baker’s Cyst is named after the British physician, Dr. William Baker (1839-1896), who published the first reports on synovial cysts of the knee joint. A Baker’s cyst is also called popliteal cyst as it occurs in the popliteal space - the area behind the knee joint.

Causes:

Conditions that cause an excessive amount of synovial fluid to be secreted, such as arthritis or torn cartilage in the knee, may lead to a Baker’s cyst. Advanced osteoarthritis is the most common cause of a Baker’s cyst in older adults.

Synovial fluid (a thick, sticky fluid that lubricates and protect the joints) is secreted by membranes lining the knee joint and is contained in the knee joint capsule. Excess synovial fluid may accumulate and the pressure may cause the knee joint capsule to herniate into the area in the back of the knee.

(Excessive fluid within the knee joint is called knee effusion and is often referred to as water on the knee)

Even if the knee is healthy, a Baker’s cyst may form if a normal bursa at the back of the knee (bursae are sacs containing synovial fluid that reduce friction in moving parts of joints) forms a tunnel that connects to the knee joint capsule and synovial fluid drains into the bursa at the back of the knee and causes it to swell. This cause is more common in children.

Symptoms:

In some cases there are no symptoms. The amount of swelling varies. Large cysts are obviously more problematic. Symptoms include discomfort or pain behind the knee and/or a feeling of fullness or tightness that worsens when standing. There may be difficulty bending the knee.

Occasionally a Baker’s cyst ruptures, causing acute pain behind the knee and pain, swelling, and redness of the calf. Since the symptoms of a ruptured Baker’s cyst are similar to those of a deep vein thrombosis (a blood clot in a deep vein) in the leg, immediate medical attention is needed to rule the latter out.

Treatment:

Treating the cause of the Baker’s cyst, e.g. arthritis or torn cartilage, usually reduces the swelling. If the Baker’s cyst is caused by torn cartilage, surgery to repair the cartilage may be necessary.

Reducing inflammation in the knee, whether from arthritis or an injury, may be achieved by temporarily reducing activity, elevating the knee above heart level (placing the knee on a pillow while lying down), applying ice (wrap ice in cloth and apply 20 minutes at a time, every three or four hours), wrapping the knee in an ACE bandage to provide compression, and anti-inflammatory medication. For more information on knee osteoarthritis, click here.

If the Baker’s cyst is not causing problems, no treatment is necessary. Baker’s cysts often eventually disappear without treatment. If the cyst is extremely large or painful it may be drained or surgically removed.

Aspiration: A Baker’s cyst can be drained using a needle/syringe but usually recurs if the primary problem causing the Baker’s cyst is still present.

Injection of corticosteroids: Cortisone (a powerful anti-inflammatory) may be injected to reduce the amount of fluid being produced. Again, this may only help temporarily.

Surgery: A Baker’s cyst can be surgically removed if problems persist despite treatment of the condition that caused the Baker’s cyst. Surgery is usually not necessary.

In rare cases, a lump behind the knee may be a tumor. Always seek a proper diagnosis from a qualified physician.

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