From what I know , it's very competitive-- meaning that at first you have to have great grades at the dschool, and then go to the PhD which is very competitive; so , I think at first you have to get into the dschool at first and then you may go PhD . (actually, I've heard from dental students that by the end of D-4 80% of students changed their minds about specializing-probably, it was enough for them to spend 4 years in dental school😉