You might want to post this in Orthopedics...
But it my opinion, the bone will fuse wherever it is still attached, severely broken or no. If the fracture is breaking the skin, left untreated, I'm sure gangrene would eventually take over and it would have to be amputated. Some hairline fractures can repair themselves if the person takes care not to put any sort of strain upon it until it is healed. It is more likely to successfully repair itself in babies and young children than in adults. Some broken bones, such as bones in the foot, sometimes are left untreated.
http://homeemergency.wordpress.com/2007/06
/28/fractures-broken-bones/