User blog comment:AYET/Shanks missing arm/@comment-26195997-20120221105628/@comment-4754162-20120221114723

I agree, the blow by Akainu was aimed at the guts (stomach, lungs, heart) and it burned long enough (Akainu left his arm in him for quite a long time there) to prevent any possibility of regeneration by Ace. Haki... I would not want to go on that but if I am to take a wild guess, one's haki cancels the other, thus the direct blow, thus pretty much what Evan-kun said above