orthodontic (adj.)

"the branch of dentistry concerned with the treatment of irregularities of the teeth and jaws," 1849, from ortho- "straight, regular" + Greek odon (genitive odontos) "tooth" (from PIE root *dent- "tooth") + abstract noun ending -ia.

Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).

In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.

"orthodontia, the branch of dentistry concerned with the treatment of irregularities of the teeth and jaws," 1909, from orthodontic (adj.); also see -ics.