What is a ”bone bruise” at an old fracture site?

I was just giving 3 lectures on running injuries at a medical conference in Las Vegas.  

As is often the case, after one of my lectures one of the physicians in the audience approached me in the hallway to ask a question.

What do you do with activity level when somebody has an old fracture where the bone was broken long ago?

The runner recently had a re-injury at that spot. It has been painful, it's been swelling, and he's trying to figure out what to do.

What's a bone bruise at an old fracture site? Is it a big problem or a little problem?

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