"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.
word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.
Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.
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<a href="https://www.etymonline.com/word/orthodontist">Etymology of orthodontist by etymonline</a>
Harper, D. (n.d.). Etymology of orthodontist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/orthodontist