Just something came up to my mind, Since kabuto said he's suffering from an unknown disease. Could it be that this was a bone cancer complicated by his KKG?
Thats What I'm thinking, What if he naturally got it from he was youn, and the more he uses his kkg, the more complicated it gets, since cancer means rapid growth of cells, and using KKG would rapidly cause his bone cells to multiply to produce more bones, maybe the cancer cells gets affected in the process?
Or maybe cancer was unknown to NV, Just like itachi's
Maybe its to complicated as it is tied to his KKG, and no matter, how much they try, its just keep coming back because of the pressence of his chakra had his unusual anatomy.