The Collaborative International Dictionary of English v.0.48:

orthodontia \orthodontia\ n. (Med.)
   The treatment of the teeth so as to make the teeth align
   properly within the mouth, usually involving the wearing of
   braces and sometimes oral surgery.
   [PJC]

WordNet (r) 3.0 (2006):

orthodontia
    n 1: the branch of dentistry dealing with the prevention or
         correction of irregularities of the teeth [syn:
         orthodontics, orthodontia, orthodonture, dental
         orthopedics, dental orthopaedics]