Board Thread:Naruto Discussions/@comment-23915152-20180418082757/@comment-734582-20180418180127

Kyuu19 wrote: Naruto as a series never really followed the laws of physics and medicine either..I mean at the end there Obito just ripped out his old eye from Kakashi and popped it back in like it was an every day thing lol. Kakashi and Obito share a blood group. It's interesting that for all the weird stuff, he did pay attention to whether the blood groups of the various transplantees were feasible. Obito, Kakashi and Madara are all O type (hence Kakashi was compatible with Obito's sharingan, and so was Madara) while Itachi and Sasuke both had AB (so again, compatible). It's so random that among all the weird science, he did pay attention to that.

Madara, by the way, used healing ninjutsu when he put in the eye. So it's likely that he simply used a very fine chakra scalpel plus medical ninjutsu to cut out the eye precisely and heal it back with ease. It's hardly the most insane thing he's done.