The Collaborative International Dictionary of English v.0.48:

orthodontist \orthodontist\ n.
   a dentist specializing in the prevention or correction of
   irregularities of the teeth; one specializing in
   orthodontics.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

orthodontist
    n 1: a dentist specializing in the prevention or correction of
         irregularities of the teeth