"Prosthesis" vs. "prosthetic"
The two words "prosthesis" and "prosthetic" are closely related, the noun and adjective versions of the same idea. So it's an odd phenomenon that the adjective is widely known and used, while the noun is largely the domain of medical professionals.
Of course it isn't wrong to use an adjective as a noun -- people are always saying things like "The blue car was in the shop so we drove the gray." But it's funny that most people without formal medical training never use the noun, even when it would flow more naturally, as in the Onion's fake headline "'Digger' Prosthetic Artist Recalls Grueling Process Of Having Tom Cruise Sit Still" (which makes it sound like it was the artist, rather than the foam-rubber face makeup, that was fake).
"Prosthesis" is a tricky word, so you wouldn't expect everybody to know it. But if you've mastered "prosthetic", you've handled the hard parts anyway -- like that "sth". So it's not at all like the pattern of doctors saying "myocardial infarction" while laypeople say "heart attack". I find it a bit of a mystery.