I still remember a LIFE magazine article, I think it was early 90s, maybe even last 80s. Drunk guy hit by train, one arm severed and sent hundreds of feet away, and the other arm had the hand crushed. They found the severed arm, brought it and the person to the hospital. The doctor made a quick decision to take the hand off of the severed arm, and use it to replace the crushed hand.
The photos looked so weird! I can't find any images of it.
The doctor rewired the nerves in reverse, so that when he wanted to move the thumb, that's what moved, etc. Of course, the thumb is on the bottom.