More than likely just an atrophic nonunion fracture of the fibula. The tibia and fibula were probably broken at the same time, if the fibular fracture looked like it didn't need to be set they probably didn't even bother putting hardware in.

You don't really need to have any serious health issues for this. Sometimes with bones that aren't weight bearing it's just bad luck, sometimes it's because you're smoking a little too much.