Board Thread:Questions and Answers/@comment-7148753-20130313174114/@comment-1393830-20130313184332

First off, Aizen is very strong and very durable. It takes a lot to damage him. Second, he had already implanted the Hogyoku inside of himself, allowing for very rapid regeneration of wounds. Also, based on the positioning, it doesn't look like Aizen took the full force of the attack.