If there's anything that need to be open source from day one, it's prosthesis.

It's more about the ability of prosthetic users being able to own the parts that has been implanted into their body than being able to modify it per se.

This way, prosthetic users cannot be barred from law or practicality to seek competent prosthetic specialists and professional that's not from the same company that makes the prosthesis. For example, if I have an retinal prosthesis from a company that went bankrupt ten years ago, I can still seek services from some other company, with a technical manual and blueprint in hand.