prosthesis (n.)

1550s, in grammar, "addition of a letter or syllable to a word," from Late Latin, from Greek prosthesis "a putting to, an addition," from prostithenai "add to," from pros "to" (see pros-) + tithenai "to put, to place" (from reduplicated form of PIE root *dhe- "to set, put").

From 1706 in medical arts as "the addition of an artificial part to supply a defect of the body" on the notion of "that which is added to" the body. The sense was extended to "artificial body part" by 1900. Plural prostheses.