all three of my letters were from dental school faculty. (one was my research mentor, one student research group advisor I worked with, one was a pedo professor I got along well with)
I think that unless you worked for the orthodontist and they know you really well and can write a great letter and add something significant to your application I would avoid a private practice guy.
That being said, if your orthodontist is an alumus of a particular school that you are interested in and is friends or adjunct with the program director, then I'd have them write a letter just to that particular program.
If I had to do it over again, I would have done research in the ortho dept with a well known faculty member and busted my butt so that he could write me a great letter.