"orthodontia, the branch of dentistry concerned with the treatment of irregularities of the teeth and jaws," 1909, from orthodontic (adj.); also see -ics.
Entries linking to orthodontics
orthodontic adj.
"serving to correct the positions of the teeth," 1905, from orthodontia + -ic.
-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.