What is periosteal reaction in a stress fracture?

A runner with a stress fracture called me for a telemedicine visit because she was confused. The doctor told her that she has a stress fracture. But there was no crack on the x-ray. The doctor said she only had a “periosteal reaction” which suggested she had a metatarsal stress fracture. She wanted to know if a periosteal reaction meant the foot was really broken, or not. The periosteal reaction is one of the earliest findings that you can see on your x-ray when you have a stress fracture in one of the metatarsal bones in your foot, before you even see a crack. Today on the Doc On The Run Podcast we're talking about periosteal reaction in a runner with a stress fracture.