Actually it is a medical fact. Bones are made up of numerous calcium bonds that break apart when the bone suffers a trauma or a break. When the bonds reform they multiply thus increasing bone density and strengthening the bone. This is how many athletes strengthen their bones, not necessarily by breaking them but by causing trauma to their bones and forcing the calcium bonds to break and re-build. This is how Muay-thai practicioners strengthen their legs. The reason many athletes are prone to re occuring bone injuries is because of the constant use of those bones. Just because the bone is stronger does not mean that it is impervious to breaks. If Franklin is going to be slamming is fist into someones face for 5 rounds then I would say there is a good chance of him breaking it again even if it is fully healed. Also I do not think that the re-breaks athletes suffer occur in the same spot on the bone.