I'm a big fan of treating trismus, and really like the Therabite device, which is dedicated to passively stretching the jaw muscle and is designed to do so safely and carefully. However, it's gotten very expensive -- over $500. I have had a lot of trouble finding a DME supplier willing to front the device to Medicare patients as they seem unsure whether they will be reimbursed (though they typically are if an articulate case is made).
The old-fashioned solution is to stack up tongue blades to place in the mouth for passive stretching exercise. The challenge with this approach is that the mouth has to open wide enough to accommodate the stack that is constructed to push the patient beyond what s/he can typically accomplish, so there is at least an initial stressor when inserting the stack into the mouth. Poor dentition can also interfere with this treatment, especially if there is a risk of breaking teeth.
A very important factor is that the stretching must be done SLOWLY, so as not to injure the muscle, which can be easy to do. The muscle can have a fairly rapid positive response to being passively stretched; however, it can also be injured easily if pushed too far too quickly.
Typically, with the Therabite I will have the patient (or caregiver) use the device to stretch the jaw muscle JUST to the point where a MILD stretch is felt. I will secure the device at this point to not open any further, and provide an exercise regimen that typically consists of stretches with a slow count to 7, 7 times in a row = 1 set, 3-4 sets per day. I instruct the family NOT to make ANY changes to the device settings until I see the patient again. Jaw muscle injuries are painful and take a long time to heal and I am very cautious about not causing them.