Hey guys, I just thought of this amazing idea, instead of Madara using some technique himself, what if Kakuzu found him and helped heal him?
I say this because, he lived at a time where he had a mission to assassinate the first hokage, but he failed, so he went back to his village and killed the elders and learned his secret techniques. He must carry a great hatred towards Hashirama, and may have wanted to help Madara because they shared a common enemy.
Also I remember that Kakuzu stitched back Hidan's head, so perhaps he has good healing techniques to use on mortals? Wat do you think?