Symptoms:

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.

Signs and tests:

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.