Hi!

This looks to me like some sort of response to infection/inflammation. I can't see any evidence of a cloaca, but with the bone being broken I guess we can't rule out the possibility that there was one on the piece that's broken off. However, since we don't have a sinus on the bit we can see and the bone at first glance does not appear to be overly swollen, it's probably not osteomyelitis. The reaction appears to be more on the surface of the shaft than in the bone marrow cavity. So that would suggest periostitis. It seems like quite a significant case though. It probably came from more than a mild bump or bruise.