I have a friend with a metal plate right where her neck meets her spine. She has all kinds of back and shoulder problems and has received treatments with Tens units (milliamps) with no problems as long as she avoided the metal plates. She also has one on her hip.
However I put a micro-current device on her today and she started feeling nauseousness and having cold sweats, she said it could have been a panic attack because she started freaking out that it was going to cause problems with the metal plate on her spine.
I've also seen microcurrent spa treatment centers that say they wont treat anyone with a metal plate in them.