What do the initials pa and ms stand for behind a dentist name?

PA = It's a legal term to describe a separate legal entity - Professional Association. It's a form of incorporating a group of professionals, such as physicians, or dentists, or attorneys, or Accountants to allow the individuals protection from lawsuits. If you wanted to sue one of the members for malpractice, you'd have to sue the professional association rather than the individual.

MS = Medical Surgeon