Baker's cyst is an accumulation of joint fluid (synovial fluid) that forms behind the knee.

A Baker's cyst may form by the connection of a normal bursa (a normal lubricating fluid sac) with the knee joint. This type is more common in children.

The condition can also be caused by the herniation of the knee joint capsule out into the back of the knee, which is more common in adults. This commonly occurs with a tear in the meniscal cartilage of the knee.

In older adults, Baker's cysts are frequently associated with knee arthritis.

A large cyst may cause some discomfort or stiffness but generally has no symptoms. There may be a painless or painful swelling behind the knee.

The cyst may feel like a water-filled balloon. Occasionally, the cyst may rupture, causing pain, swelling, and bruising on the back of the knee and calf.

It is important to tell the difference between a ruptured Baker's cyst and a blood clot (deep venous thrombosis), which can also cause pain, swelling, and bruising on the back of the knee and calf. A blood clot may be dangerous and requires immediate medical attention.

During a physical exam, the doctor will look for a soft mass in the back of the knee. If the cyst is small, comparing the affected knee to the normal knee can be helpful. There may be limitation in range of motion caused by pain or by the size of the cyst. In some cases there will be signs and symptoms of a meniscal tear.

Transillumination, or shining a light through the cyst, can demonstrate that the mass is fluid filled.

If the mass demonstrates any abnormal signs, like rapid growth, night pain, severe pain, or fever, a more involved work-up is indicated to rule out noncystic tumors that can grow in the back of the knee.

X-rays will not show the cyst or a meniscal tear but will show other abnormalities that may be present including arthritis.

MRIs can be helpful to visualize the cyst and to demonstrate any meniscal injury.

Often no treatment is necessary and the practitioner can observe the cyst over time. If the cyst is painful, treatment is usually aimed at correcting the underlying problem, such as arthritis or a meniscus tear. Removal of the cyst is generally not done because it may damage nearby blood vessels and nerves.

Sometimes, a cyst can be drained (aspirated) or, in rare cases, removed surgically, if the cyst becomes excessively large or causes symptoms.

A Baker's cyst will not cause any long-term harm, but can be annoying and painful. Baker's cysts usually go away on their own, but how fast this occurs varies from person to person.

Long-term disability is rare, as most cases improve with time or arthroscopic surgery.

Complications are unusual, but may include:

• Long-term pain and swelling
• Complications from associated injuries, like meniscal tears

Call for an appointment with your health care provider if there is a swelling behind the knee that becomes large or painful. Pain could be a sign of infection, which is not normally associated with Baker's cyst.