The only thing I know that is really beneficial for trismus is exercising for the mouth, mouth stretches, and then Dynasplint, Therabite, wooden tongue depressors, bite plug, chewing gum especially the first several months. There is surgery for this at the jaw joint, coroidectomy, which controls the jaw opening, but radiation is done is such a wide area, that it usually effects more than this area, and not that effective in the long run. There is electrical type stimulation that may be beneficial, which name I don't recall off hand. Did they rule out infection, recurrence, osteoradionecrosis?