To tell you the truth, I'm not sure if it starts at the top or the bottom. I think it starts further away from the injury first, and then works towards the injury. But I'm not positive. That's only what makes SENSE -- but medicine doesn't always make sense! LOL

I've never seen it explained as to where it starts healing.
Wish I could help you more on this. But those are just my suspicions. Hugs, Lee