A ganglion cyst is a fluid-filled sac. It is usually attached to the membrane that surrounds a tendon or a joint lining. Ganglion cysts usually appear on the back of the wrist. They may also be on the underside of the wrist, hand, fingers, or feet. Ganglion cysts are not cancerous.

Ganglion Cyst
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The cause of ganglion cysts is not known.