Prostheses are specifically not orthoses, although given certain circumstances a prosthesis might end up performing some or all of the same functionary benefits as an orthosis.
pɹˈɑːsθəsˌiːz ɑːɹ spəsˈɪfɪkli nˌɑːt ˈɔːɹθoʊzᵻz, ɔːlðˈoʊ ɡˈɪvən sˈɜːʔn̩ sˈɜːkəmstˌænsᵻz ɐ pɹˈɑːsθəsˌɪs mˌaɪt ˈɛnd ˌʌp pɚfˈɔːɹmɪŋ sˌʌm ɔːɹ ˈɔːl ʌvðə sˈeɪm fˈʌŋkʃənˌɛɹi bˈɛnɪfˌɪts æz ɐn ɔːɹθˈoʊsɪs.