When it comes to legal questions you should always ask a lawyer.
That said, I think you're fine to do what you describe as long as you do not use the name of any professional, professional association or regulatory board in the domain. Additionally, you should let visitors know that you are not endorsed by the same and refrain from using any of their copyrighted / trade marked logos or other media.
Its not just copyright or trademark violations that you need to worry about. Also consider a user that had a bad experience with a dentist that they found using your web site. You would be amazed how easy it is to find a lawyer to sue someone for just about anything.
We get back to talking to a lawyer, who could help you write a suitable disclaimer and make sure that you stay out of trouble.