How can a bone stress reaction turn into malunion from running?

I was just talking to a runner over webcam who had been researching metatarsal stress fractures, but heard something about a "malunion" and knew that a fracture malunion was a really bad thing. 

He had been diagnosed with a stress reaction in one of his metatarsals. 

If you continue to stress a metatarsal stress reaction enough, it can progress to a full-on metatarsal fracture. 

So, he wanted to know, "If I run on my stress reaction, is it going to turn into a malunion?"

How can a bone stress reaction turn into a malunion, from running? 

Well, that's what we're talking about today on the Doc On The Run Podcast.