Chicken or egg......
I had what was loosely called TMJ problems for many years prior to diagnosis, my brother also had the same, he has no other health issues. Sinemet prescribed from dx time onwards has mostly cleared up TMJ problems, or minimized them. The only exception to this is if I go to bed highly medicated, then there is a return to bruxism, and clenched jaw and facial pain in the next a.m. This will resolve easily over the next few hours of wakefulness, and I put it down to a wearing off issues.
I do not see it as causative of PD, I see it as an early symptom of PD type condition, along with other issues that also resolved with use of ldopa, such as crampy feet that had been diagnosed as caused by heel spurs, and neck problems attributed to cervical spondylosis, and right hand/arm pain attributed to RSI. I also had a long standing issue with right sided chest pain and stiffness that was driving me crazy with discomfort especially after sleep which was diagnosed as intercostal chondritis (the doctor who decided on that had an 'aha' moment with that, loudly exclaiming "Ah, I KNOW what that is!') It to resolved with ldopa.
So which really came first? I have an unsure diagnosis as I am dopa responsive, but atypical, and my neuro treats me for PD. I think he is right to do that, as it is more likely that I have one whole body condition than a host of bits of me that have spontaneously 'gone wrong'.