A stress fracture is a small crack in a bone that occurs when bone tissue has to absorb more weight and impact than it can handle. This type of injury can occur over time in a well-conditioned bone that is overused, or suddenly in an underconditioned bone that is placed under undue stress in a short period.1 Stress fractures most frequently occur in the feet, ankles, and lower legs, though they can also occur in other parts of the body, such as the pelvis, hip, ribs, sacrum, clavicle, and upper extremities (arms).2,3
Stress fractures can affect many different types of athletes, but they are most common in those who participate in activities involving heavy, repetitive impact on the feet and lower legs, such as long-distance running, track and field, tennis, dancing, gymnastics, basketball, and soccer.4 The injury is common, comprising up to 15 percent of all sports-related injuries.5
How a stress fracture happens
Like most tissues in the body, bone is constantly renewing and replacing itself—a process called bone remodeling. Weight-bearing exercise that places moderate levels of stress on the bones is one way to further stimulate bone-tissue renewal and to increase bone density.2 An interesting example that illustrates the importance of weight load on bone density is the significant decrease in bone density seen in astronauts after spending time away from Earth.
However, if a bone absorbs so much weight and/or impact stress in a short period of time that the normal remodeling process cannot keep up, an imbalance develops between bone damage and bone remodeling.2
In an imbalance situation, instead of the bone becoming stronger and denser due to moderate exercise, the bone becomes thinner and weaker as it becomes overloaded, a phenomenon called bone fatigue. Bone fatigue is often expedited by muscle fatigue, in which muscles that normally serve a stabilizing role become fatigued, causing instability that transfers stress to the bones.6
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The body typically adds new bone cells during rest periods, so the mix of intense exertion without adequate rest causes this imbalance to develop.7
Common situations that may trigger a bone-stress imbalance include:
- Runners who extend their daily mileage totals too quickly (for example, faster than the “two-mile-a-week rule,” which governs how much runners should add to their single-session running distances each week)
- Ballet dancers who train ten to twelve hours a day for an upcoming performance without adequate rest periods
- Military recruits doing marching drills during basic training
During a bone-stress imbalance, the bone first undergoes a stress reaction that an athlete may feel as a mild ache in the affected bone, or not at all. This indicates that the bone tissue is damaged on a microscopic level.3 If the stress imbalance continues, that microscopic damage may become more significant, to the point it can be detected on clinical examination.
A stress fracture is also known as a hairline fracture because the bone remains essentially intact around the thin crack formed by the stress reaction. If a stress fracture remains untreated, however, the affected bone(s) may break completely, a far more serious type of injury.7-9
This article provides a detailed overview of stress fracture causes, symptoms, risk factors, and treatments.