I would advise seeing an orthopaedic surgeon to review your to determine the possibility of having an arthroscopic (keyhole) examination and repair (if required). This surgery can be very sucessfull for treating meniscal injuries and cysts and typical full recovery time is 4-6 weeks (depends on damage done and how sucessful surgery goes).
For more information on the procedure check "Knee arthroscopy" on wikipedia and also follow the link in the piece to "meniscal injury" for more specific information. As always with wikipedia, take what you read with a pinch of salt as I wouldn't agree 100% with whats written but it gives you an idea of whats involved.
I'm not sure where you are based geographically or your circumstances (private insurance?) but a good place to start is usually your GP.