A stress fracture is a fatigue-induced bone fracture caused by repeated stress over time. Instead of resulting from a single severe impact, stress fractures are the result of accumulated injury from repeated submaximal loading, such as running or jumping. Because of this mechanism, stress fractures are common overuse injuries in athletes. Stress fractures can be described as small cracks in the bone, or hairline fractures. Stress fractures of the foot are sometimes called "march fractures" because of the injury's prevalence among heavily marching soldiers. Stress fractures most frequently occur in weight-bearing bones of the lower extremities, such as the tibia and fibula (bones of the lower leg), calcaneus (heel bone), metatarsal and navicular bones (bones of the foot). Less common are stress fractures to the femur, pelvis, sacrum, lumbar spine (lower back), hips, hands, and wrists. Stress fractures make up about 20% of overall sports injuries. Treatment usually consists of rest followed by a gradual return to exercise over a period of months.
Signs and symptoms Stress fractures are typically discovered after a rapid increase in exercise. Symptoms usually have a gradual onset, with complaints that include isolated pain along the shaft of the bone and during activity, decreased muscular strength and cramping. In cases of fibular stress fractures, pain occurs proximal to the lateral malleolus, that increases with activity and subsides with rest. If pain is constantly present it may indicate a more serious bone injury. There is usually an area of localized tenderness on or near the bone and generalized swelling in the area. Pressure applied to the bone may reproduce symptoms and reveal crepitus in well-developed stress fractures. Anterior tibial stress fractures elicit focal tenderness on the anterior tibial crest, while posterior medial stress fractures can be tender at the posterior tibial border.
Causes Bones are constantly attempting to remodel and repair themselves, especially during a sport where extraordinary stress is applied to the bone. Over time, if enough stress is placed on the bone that it exhausts the capacity of the bone to remodel, a weakened site—a stress fracture—may appear on the bone. The fracture does not appear suddenly. It occurs from repeated traumas, none of which is sufficient to cause a sudden break, but which, when added together, overwhelm the osteoblasts that remodel the bone. Potential causes include overload caused by muscle contraction, amenorrhea, an altered stress distribution in the bone accompanying muscle fatigue, a change in ground reaction force (concrete to grass) or the performance of a rhythmically repetitive stress that leads up to a vibratory summation point. Stress fractures commonly occur in sedentary people who suddenly undertake a burst of exercise (whose bones are not used to the task). They may also occur in athletes completing high volume, high impact training, such as running or jumping sports. Stress fractures are also commonly reported in soldiers who march long distances. Muscle fatigue can play a role in the occurrence of stress fractures. The neuromuscular hypothesis is the leading theoretical framework on muscle fatigue's role in stress fractures. In a runner, each stride normally exerts large forces at various points in the legs. Each shock—a rapid acceleration and energy transfer—must be absorbed. Muscles and bones serve as shock absorbers. However, the muscles, usually those in the lower leg, become fatigued after running a long distance and lose their ability to absorb shock. As the bones now experience larger stresses, this increases the risk of fracture. Experimental evidence remains mixed, with some studies done on competitive distance runners concluding that shock absorption does not change after brief exhaustive running. Previous stress fractures have been identified as a risk factor. Along with history of stress fractures, a narrow tibial shaft, high degree of hip external rotation, osteopenia, osteoporosis, and pes cavus are common predisposing factors for stress fractures. Common causes in sport that result in stress fractures include:
Over training Going back to competition too soon after an injury or illness Going from one event to another without proper training for the second event Starting initial training too quickly Changing habits or the environment like training surface or shoes
Diagnosis X-rays usually do not show evidence of new stress fractures, but can be used approximately three weeks after onset of pain when the bone begins to remodel. A CT scan, MRI, or 3-phase bone scan may be more effective for early diagnosis. MRI appears to be the most accurate diagnostic test. Tuning forks have been advocated as an inexpensive alternative for identifying the presence of stress fractures. The clinician places a vibrating tuning fork along the shaft of the suspected bone. If a stress fracture is present, the vibration would cause pain. This test has a low positive likelihood ratio and a high negative likelihood ratio, meaning that it produces both false positive diagnoses and false negative diagnoses, so it should not be used as the only diagnostic method.
Prevention Altering the biomechanics of training and training schedules may reduce the prevalence of stress fractures. Orthotic insoles have been found to decrease the rate of stress fractures in military recruits, but it is unclear whether this can be extrapolated to the general population or athletes. On the other hand, some athletes have argued that cushioning in shoes actually causes more stress by reducing the body's natural shock-absorbing action, thus increasing the frequency of running injuries. During exercise that applies more stress to the bones, it may help to increase daily calcium (2,000 mg) and vitamin D (800 IU) intake, depending on the individual.
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