prosthesis [prostheses](artificial replacement for a body part) noun [UK: prɒs.ˈθiːs.ɪs] [US: prɑːs.ˈθiːs.ɪs]
prothesis [protheses](the addition of a sound or syllable at the beginning of a word without changing the word's meaning or the rest of its structure) noun [UK: prˈɒθəsˌɪs] [US: prˈɑːθəsˌɪs]