An Overview of
The interesting crystalline material structure of bone is the underpinning for the development of stress fractures. Fatigue fractures of a long bone are not uncommon under significant repeated load. It makes us realize that long bones are just like any brittle structure that can be overstressed. This goes back to the stress/strain curves in materials management in physics. This condition even occurs in the spine (see pars fractures and isthmic spondylolisthesis).
Bone, like any crystalline structure, has an absolute fatigue point for gross failure (a fall from a height breaking the leg). There is however the possibility of cumulative stress from constant impact that does not break the bone fully. Tiny fractures in the substance of the bone can occur that do not individually reach the gross failure point.
These stresses accumulate over time with repeated loads. This is a time and stress related function. Stress fractures overwhelm the ability of the bone to heal without rest (time).
An analogy would be taking a wire coat hanger and bending it continuously until it breaks. No one individual bend causes failure but the accumulation of bends breaks the wire. It is remarkable that bone has a self-healing process and can recover from these less than full structural failure injuries given time and rest.
Groups at Risk for
Two groups that have the greatest risk of
are the athletic and the military populations. In either population, prolonged impact activities (running, weight lifting and calisthenics) set the stage for these overuse fractures.
Insufficiency fractures due to inferior bone stock generally found in the elderly due to osteoporosis are also at high risk. These fractures also occur in the thoracic and lumbar vertebra.
Location of Lower Extremity Fractures
The typical location of these lower extremity fractures is generally found in long tubular bones. Tibial stress fractures, the most common are at 50% of all stress fractures. Metatarsal stress fractures (foot) occur under 20% of the time and femoral shaft and neck fractures exist at about a 10% occurrence rate.
There are potential risks and different healing outcomes for fractures in different locations. As would be expected, fractures with reduced vascularity due to location and anatomy (the anterior tibia and navicular bone in the foot) had poorer outcomes due to this relative lack of blood supply. If a fracture of the femoral neck is allowed to develop and then displace, the chance of avascular necrosis (bone death) to the femoral head is very high due to the retrograde vascularity of the femoral head.