Baker's Cyst (Popliteal Cyst)

When joint of synovial fluid builds up behind the knee if can form a cyst, called Baker's cyst, named for the surgeon who first described the condition. It is also sometimes called a popliteal cyst. Such cysts are not uncommon.

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Symptoms

Baker's cysts often have no symptoms, although there is sometimes slight swelling behind the knee, which may or may not cause pain. Cysts are usually soft and somewhat tender. The cyst behind the knee will feel like a water-filled balloon. In some cases, the cyst ruptures, causing pain, swelling, redness and bruising behind the knee and up into the calf. Pain, swelling and bruising behind the knee can also be a symptom of a blood clot, which requires immediate medical attention.


Cause and Risk Factors

Sometimes, especially in children, a normal lubricating fluid sac forms a connection with the knee, which can form a Baker's cyst. In adults, such cysts usually form by herniation of the knee joint capsule out into the back of the knee. When this occurs, the meniscal cartilage of the knee also commonly tears. In fact, almost any damage to cartilage in the knee can lead to the formation of a cyst. In older adults, Baker's cysts can form when there is degenerative arthritis in the knee.

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Diagnosis

A physical examination will examine the knee, especially the area behind the knee. Such cysts can often be seen if a patient is standing with the knee fully extended. Baker's cysts are most easily felt when the knee is bent. The physician sometimes will compare the affected knee with the other knee to attempt to find a smaller cyst. In some cases, a cyst can limit the range of motion of the knee or there may be signs of a meniscal tear.

An ultrasound can also determine the location and contents of a cyst.

If the mass grows rapidly, causes severe pain or is accompanied by a fever, it may be something other than a Baker's cyst. A more extensive workup is then required to rule out other possible causes of the symptoms. Although an X ray, will not show a cyst or a meniscal tear, it will show other abnormalities that may be causing the symptoms, including arthritis. An MRI will show a cyst and help show its size and location, as well as show any meniscal injury.

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Treatments

Treatment if often not required since such cysts are benign, although the cyst should be monitored for any changes over time. Cysts in children almost always disappear with time. Often just rest, ice (15 minutes at a time) and elevating the leg will reduce discomfort, although a corticosteroid may be prescribed to reduce inflammation. If the cyst causes pain, treatment usually seeks to deal with the underlying cause of the cyst, such as a meniscal tear or arthritis. Removing a cyst is usually not recommended, since surgical removal risks damaging blood vessels and nerves behind the knee. Surgery is usually only recommended in cases where the cyst causes severe pain. If the fluid in a cyst becomes hard and gel-like, then it might be drained (aspirated).

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