I am fortunate to have a dentist who fully understands how rheumatoid arthritis impacts the ability to keep one's mouth open for longer dental procedures.
Rheumatoid arthritis can affect any joint in the body, including the jaw.
He knows that when I give the signal, he is to stop work. Even with the rubber dam in place, I can slightly close my mouth, thus relaxing my jaw. A few moments makes all the difference. This definitely changes an "Oww!" to an "Ahh!"
Speak to your dentist about how difficult is to keep your mouth open for extended periods of time. Decide upon a signal to use that indicates that you need to take a break in order to rest your jaw.