I don't know what "back revision" surgery is but that seems...bad!
It depends upon the surgery. If a discetomy it means they took out a little of that material they left behind. If a fusion it makes they added some more bone replacement at the original site to build it up some more. In either event it’s impressive he can return in three to four months. In thr peroeonal injury world he’d never work again and would need extensive rehab and perpetual pain management (which of course would never happen once the case is resolved but I digress)….