|An MRI reveals a tibial stress fracture|
Traditionally, overuse injuries to the bone in distance runners are divided into two distinct categories: stress reactions and stress fractures. Runners who develop pain along one of their bones hope desperately that they have the former and not the latter, since the usual prescription for stress fracture recovery is six to eight weeks of no running whatsoever. The usual restrictions for stress reactions depend on the doctor, but typically involve two to four weeks away from running. Some doctors, coaches, and runners eschew the term "stress reaction" entirely because, in their view, you either have a stress fracture, or you don't—that's all there is to it.
Normally, the story unfolds something like this: a high school runner develops a sharp, aching, localized pain somewhere along a bone in his lower body. It doesn't improve much with icing and lowered training volume, so his coach or trainer refers him to a doctor. The doctor orders an X-ray, examines it, but sees no evidence of calcification, so he orders the runner to ease back into training, but return if pain continues. The high schooler gives running a shot, but continues to have pain. The doctor then orders a bone scan or an MRI, which shows bone marrow edema or increased metabolic activity at the location of pain. This is deemed to be a stress fracture, and the runner is put in a boot, forbidden from running for six to eight weeks, and his season is effectively over. Sound familiar?
The reason for caution with stress fractures is well-known. If you have a stress fracture and continue to run recklessly on it, it can worsen and eventually lead to the bone splitting in two—a true fracture. This can lead to heaps of complications and could end your running career. There is also a category of "high risk" stress fractures that occur in particular areas like the femoral neck, the navicular, and the sesamoid bones, which are known to have a significant risk for poor healing or nonunion.1 These require even more time off and a much slower return to running.
Problems with the old model
However, doctors and physical therapists are starting to learn what coaches have already picked up on: the traditional approach to low risk stress fractures (as the vast majority are) is inadequate on a number of points.
The case for a new approach to bone injuries in runners was laid out in an exhaustive review article published in October of last year by Stuart Warden, Irene Davis, and Michael Fredericson, three extremely prolific running injury researchers.2 They propose using the term "bone stress injury" or BSI, which is intended to encompass all overuse injuries to bone that runners sustain.
Under Warden, Davis, and Fredericson's model, bone stress injuries exist on a continuum. On the most severe end of this spectrum are true stress fractures: a fracture line is observable on an MRI or CT scan, and there is edema (swelling) in the bone marrow and periosteum, the membrane that covers the surface of the bones. A stress fracture is accompanied by a sharp pain or ache during weight-bearing activity that sometimes persists even when you're resting.
The next step down the continuum of bone stress injury is the stress reaction: pain and aching during or after weight-bearing that is associated with bone marrow edema (on an MRI) or increased bone remodeling (as imaged by a bone scan), but lacks a visible fracture line.
Further down the bone stress injury spectrum lies asymptomatic areas of bone remodeling. As it turns out, if you were to schedule weekly MRIs for a group of high-level runners in heavy training—say, a college cross country team—you would quite often find runners developing transient areas of bone marrow or periosteal edema that would be indicative of a stress reaction, except that they have no pain associated with them, and never develop problems in the area.3
The biology of bone remodeling
One of the core paradoxes of stress fractures and stress reactions is why they occur in fairly experienced runners. All medical students can recite Wolff's law—bone responds to stress by becoming stronger. So, theoretically, running more should lead to stronger bones, not stress fractures.