Hi Tom-
I think its both the scarring and the change in the muscles, that they become hard and leathery, and that limits the motion. I developed scar tissue in the muscles because of the surgery, and then the radiation made them stiffer on top of it. But even in the scar tissue/leathery parts there is still some room to make it more flexible. The therabite pushes your mouth open, and breaks a little bit of the scar tissue or stretches it. Then you keep on using it to maintain whatever you have gained. It will always stiffen up again but after a long time you get some permanent gain.
The first few months I used the therabite, I would press "until you feel a stretch"(that's what the instructions said). It opened it up some, but not all the way. When I started doing it harder it opened it up several more milimeters within a few weeks. It took a few months of pressing harder to get the bigger opening to stick. When I say pressing harder, I mean, you press down on the lever until you feel a stretch. Then press a little harder and see if you can open it wider, then more, then more. For me, it hurt the way when you have a charley horse in your leg, you take a deep breath and think "Ok, a few more seconds, a little more, I can do it". Just like gym glass.
I took tylenol afterwards if I was sore. But after about 2 weeks if doing this I found it wasn't really painful, and I could press the therabite lever all the way to the bottom, that's as far as you can go.
I used the therabite 8 times a day or so for 8 months. Now I use it twice a day and I also stretch my jaw all the time with my hand absentmindedly, in the car, etc. However, it's not how many times a day you do it, less would probably be just as good, it's that when you do it, you really sit down and give it a real effort.
So..long story short , I don't know about the surgery, etc. and the slack jaw thing sounds scary. But I love the therabite, and if you still get benefit from using it now, it's probably not too late to try to get more out of it. Also, if you measure your opening once a day and record it you can really see if you are making progress. I measured mine at night after the last stretch because that was the biggest number. Just be careful if you try pressing harder with dentures that they don't slip or anything. I don't know too much about dentures and it might be different than with you own teeth. My goodness, I have written a book here
Melissa