Should I use "prosthetics" or "prostheses" for a plural noun?

I haven't found a distinction between the noun prosthesis and the noun prosthetic, or their plural forms prostheses and prosthetics, when used to mean "artificial body parts." They are synonyms, like dyslexic and dyslectic. Prosthetic(s) is listed in the Oxford English Dictionary with this meaning, with several citations from medical or surgical sources, so I don't see any grounds for considering this usage non-technical or incorrect. Perhaps it could be viewed as an abbreviation of "prosthetic device(s)".

As you've mentioned, these words can also be used in some other ways, and they are not always synonyms in those other senses. Only prosthetic can serve as an adjective. When used as a noun referring to the general field, it seems that prosthetics is used with the suffix -s (analogous to nouns like physics, obstetrics), while prosthesis is used in its singular form (analogous to nouns like synthesis).

Even though they evidently can be used as synonyms, some people do seem to distinguish the meanings of the words; I ran across an example in the following question from the Hinduism Stack Exchange: Artificial Limb Transplant ( Prothesis) in Rig-Veda?

In medicine, a Prosthesis , is an artificial device that replaces a missing body part lost through trauma, disease, or congenital conditions. prosthetics is simply replacement of missing limbs.

I don't know of course what the original poster's background is, but I think this is evidence showing that some people share your impression about the difference between the two words.

As someone currently working with prostheses, the professional accepted use is prosthesis is the noun and prosthetic is only used as an adjective. However, colloquially both are understood.