Cysts that occur on wrists and ankles (and sometimes the joints of the fingers) are called ganglions. The cause is unknown, as many people get them for no apparent reason. But many people also get them after a trauma injury to the joint. For some people, they come and go or get bigger and smaller as fluid goes in or out, for others they stay the same. Also sometimes, they get so full/hard that they can almost feel like a piece of bone. Most of the time, nothing is done about them unless they hurt or are cosmetically ugly. If they do hurt, the first course of treatment is to drain the fluid out of the cyst. Usually this will take care of it. Rarely are they removed.
In any case, look up ganglion on the net. Good luck
Looking for name of cyst so I can do research on it. 
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