A Mouth guard for TMJ can be flexible, they are purchased in stores, Dental offices, and online. They all can work very well. They are very good for quick relief of TMJ jaw pain, but may not be good for long term use.
They can be as effective but not as predictable as the custom made Hard splints that a Dentist can make, but they still can eliminate pain immediately.
They can eliminate pain just as well as the TMJ Splint, but not everytime nor as often as the Hard Splint. That can be sufficient for many people. Especially when they are less expensive to purchase, then the financial investment is not so high.
The draw backs with flexible guards are, that they can lose their effectiveness due to changes in muscle proprioception (How the Brain senses where interferences are and move the jaw to miss them). Also, they won't last as long either.
Also, many or most of the flexible guards for TMJ allow the teeth to "touch" the guard when they shouldn't touch. When ever teeth touch something, it causes the jaw muscles to contract more.
So, if the guard is interfering in the back teeth area when jaw movement is taking place...that will increase the likelihood of causing the jaw to clench or clamp down, and the clamping or clenching is the #1 (non-stress factor) cause of TMJ problems.
Let me give you an example of how mouth guards and splints work...
Here's one example...If you are having jaw pain (which is usually muscle pain), a specific portion of your jaw muscles are hurting from overuse... Let's say 35% of the muscle fibers in the back of the jaw are the muscle fibers that are hurting you. By putting a guard in your mouth it will change the way that your teeth distribute the clenching forces.
Now, 35% of the back muscle fibers that was hurting will now relax, stop working as much or contract less than before you put the guard in, so, now you have less or no pain. Now, maybe the front 35% portion of the muscles are contracting too much. It's possible that if you used the same mouthguard for 6 weeks or more, other or different muscle fibers might start to hurt.
That analogy is accurate for an analogy only, because when muscle fibers contract, the complete fiber contracts, not just 35% of them. (But, different muscle groups can contract when others don't) Also, front muscle fibers will rarely hurt because it is difficult to overuse the front portion muscle fibers.
So, most likely you'll have less pain using any mouth guard temporarily, unless you have extreme amounts of stress in your life, or your body's ability to withstand that stress is below average.
Some mouth guards, due to their design will keep the back portion of muscle fibers contracting and will not eliminate pain.
So, Flexible mouthguards for TMJ do not have the ability to be as predictable to keep pain away as a well as the custom made accurately adjusted hard acrylic splint can be that is made by a TMJ Dentist.
Flexible Mouthguards that cover all the teeth on one Arch (upper teeth only or lower teeth only) only, can relieve pain. Those same types of mouthguards can increase clenching, so, you may have immediate relief for a few days or weeks, but may end up having pain again when different muscle fibers are overworking also.
Bottom line is...that
flexible mouth guards can be as effective as Hard guards to relieve jaw
pain symptoms, but not as predictable as the hard acrylic splint or hard
Update: I have recently found, purchased and wrote up reviews on two TMJ Exercise programs. They are similar TMJ Exercises that my TMJ Physical Therapist uses to treat my TMJ patients. Click here to read reviews on TMJ Exercise products.