I don't think you could get a lawyer to take the case. Plaintiff lawyers work for 25-30% of a judgment. Your damages are $500. 30% of that is only $150. Could a lawyer resolve your claim in an hour or two? Doubtful.
I'd try writing a demand letter to the doctor. You could complain to insurance, leave bad reviews, maybe file a complaint with your state's dental board. Failing that, most states have small claims courts where lawyers are unnecessary. The jurisdiction of those courts is normally limited by the amount of damages, but I could see professional malpractice cases also being excepted.
In your shoes, I'd probably write a couple of strongly worded letters asking for relief threatening to leave reviews and leave complaints with insurance/etc. I'd expect to get no response or at least not a satisfactory one, but it's worth a shot. I wouldn't take them to court.