Shoulder impalement and arm paralysis
So for this story I'm writing, MC impales the antagonist's shoulder with a spear.
However, this antagonist comes back later with a prosthetic arm and a lust for vengeance. So far my explanation is just that because of how long it took for anyone to find him, it would've been too late to try to save the arm, and it's permanently paralyzed. So he ends up just getting it amputated and replaced with this prosthetic.
Important to note that he doesn't really have access to a hospital until much later (hence the amputation and prosthetic.) and he lives in something of a forest commune. So any medical attention he could get would be very primitive and rudimentary.
Edit: also important to note that this is a superhero story taking place in modern times but with some extra technological advancements. So this antagonist has powers and can resist the infection from his injury, but not the nerve damage.
How realistic of a possibility is this? And if I want to be really specific, what part of the shoulder exactly would it have to be to result in complete arm paralysis?