I read the info, sounds like what my GP called a synovial cyst behind my knee. Boy did that hurt!! Came up over night it seemed, couldn"t move my leg sideways and could hardly stand to walk. Had a MRI, then arthroscopic surgery. I had torn cartilage plus arthritis in that knee, but had not had much pain, just once in awhile it seemed to grab and hurt. Surgery removed all the cartalige plus scraping the bones to smooth them out. Lots of pain afterwards, and now almost constant pain in the knee. Nothing else was given as an option, surgery was done as out-patient.