http://www.ccohs.ca/oshanswers/diseases/ganglion.html

What is a ganglion cyst?
A ganglion cyst is a bump or mass that forms under the skin. Most commonly, ganglions are seen on the wrist (usually the back side) and fingers, but they can also develop around joints on the shoulder, elbow, knee, hip, ankle and foot. Ganglion cysts form when tissues surrounding certain joints become inflamed and swell up with lubricating fluid. They can increase in size when the tissue is irritated and often can "disappear" spontaneously. These masses or cysts appear to grow sometimes but they are not tumours or cancerous.

What are the symptoms?
Ganglions can be painless; however, they often are associated with tenderness and pain which may restrict the range of movements.

How are ganglion cysts recognized?
The diagnosis of ganglion cyst is made by physical examination. Medical tests such as x-rays may be used to confirm the diagnosis.

How are ganglion cysts treated?
The treatment can consist of rest, splinting the affected joint and, in some cases, aspiration of fluid is recommended. If a ganglion cyst tends to reoccur, surgical removal may be recommended.

What are the causes of ganglion cysts?

The cause of ganglions is not always clear. Non-occupational factors or conditions such as rheumatoid arthritis have been associated with ganglion cysts. Occupational factors play also an important role in the development of ganglions. Those occupations that require workers to excessively overuse certain joints such as the wrist and fingers pose the risk for ganglion cysts.