Symptoms:

A large cyst may cause some discomfort or stiffness, but there are often no symptoms. There may be a painless or painful swelling behind the knee.

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

It is important to know whether pain or swelling is caused by a Baker's cyst or a blood clot. A blood clot (deep venous thrombosis) 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 show that the growth is fluid filled.

If the mass grows quickly, or you have night pain, severe pain, or fever, you will need more tests to make sure you do not have other types of tumors.

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

MRIs can help the health care provider see the cyst and look for any meniscal injury.