In the middle ages most of the population didn't have a clue what went on inside the human body. So when someone had his arm wrenched off if the 'norm' was to ignore the flashing red LEDs leaking out and get to the witch doctor ASAP so he could fix you with his magic soldering iron - all that smoke and mystic ceremonies - then why would you question what 'cured' you. You'd just be thankful and go to the temple and thank the gods.
The doctors would hold the power and it would be a family business - the mysteries of the liquid metal solder passed down from generation to generation because maybe they were humans and it was in their interests to have the 'slaves' ignorant.