A fistula from a piercing connects two surface exits to each other, like a wormhole through your body. Once complete, the piercing touches nothing internal to you, just skin.

But in this case, the prosthetic must penetrate through the skin to connect to bone.

I'd have the same question about them. I'm not disputing that it works, I just don't understand how the body isn't constantly fighting it. Or leaking.

In the case of the Metal Mohawk, it actually was difficult for the owner to maintain. He kept bumping the spikes on things and eventually had them removed. Granted, his was a cosmetic change and not a functional one. I would be willing to put up with extra care to effectively replace a hand. Additionally, consider that people who receive donor transplants will use immune suppressants for the remainder of their life. That said, material selection is also important.