Stress fractures account for 0.5%-21% of all injuries in recreational or competitive athletes presenting to sports medicine clinics (53). The highest incidence of stress fractures occurs among track and field athletes (6%-20% of total injuries), compared with athletes in other sports such as football, basketball, soccer, or rowing (1,29,53).

The site of injury varies from sport to sport. For example, among track athletes, stress fractures of the navicular, tibia, and metatarsals are the most common, whereas in distance runners, the most common locations are the tibia and fibula, and in dancers, the metatarsals (19,53).

A stress fracture may best be described as an accelerated bony remodeling in response to repetitive submaximal stress. Histologic studies (37,54) have shown that repetitive response to stress leads to osteoclastic activity that surpasses the rate of osteoblastic new bone formation, resulting in temporary weakening of bone.

It has been debated whether stress fractures occur due to the increased load after fatigue of supporting structures or to contractile muscular forces acting across and on the bone (12,54); in principle, both factors contribute.

If the activity continues, trabecular microfractures result, which are believed to explain the early bone marrow edema seen on magnetic resonance imaging (MRI) scanning (16). The bone responds by forming new periosteal bone for reinforcement. Eventually, if the osteoclastic activity continues to exceed the rate of osteoblastic new bone formation, a full cortical break occurs. It is important to recognize this process as a continuum accumulation of damage that results clinically in a spectrum of injuries, from the early stages called stress reactions, to stress fractures, and ultimately complete bone fractures (19,58).

Alteration in the training program is considered one of the most important factors related to the occurrence of stress fractures. Any rapid change in mileage, pace, intensity, or some other factor inserted into the program without adequate time for physiologic adaptation may predispose to stress fractures (27,39).

Failure to follow intensive training days with easy ones also can contribute to injury. Hard or cambered training surfaces and training in older shoes are also important precursors to lower extremity overuse injuries (16,25).

Anatomic variables such as narrow width of the tibia, smaller calf girth, and less muscle mass in the lower limb have been associated with stress fractures. In a study (11) comparing 23 running athletes with a history of tibial stress fractures and 23 healthy runners, the stress fracture group had a significantly smaller tibial cross-sectional area. Bony geometry plays a role in stress fracture development. In runners, repetitive loading in a single plane leads to asymmetric cross-sectional geometry of the tibia in contrast to soccer players who load in multiple directions and are found to have a more robust and symmetric bone geometry (8).

Statistics suggest that women are at greater risk for sustaining stress fractures than men (53). The female athlete triad (menstrual irregularity, disordered eating, and osteopenia) emphasizes this susceptibility in this group of athletes. Other studies have failed to show a significant gender difference (28).

The physical examination typically reveals local tenderness over the involved bone (12). Other tests for the clinical detection of stress fracture such as the fulcrum test (femur), hop test (tibia), and spinal extension test (pars interarticularis) are helpful but not as reliable as direct palpation (29).

The fulcrum test is performed by gradually applying a force across the distal femur in the seated position using the edge of the examination table as a fulcrum while fixing the proximal femur (15). In the hop test, patients are asked to repeat single-legged hops over the affected leg. The spinal extension test is performed with the patient standing, balancing on one leg to increase the load over the ipsilateral pars interarticularis. Pain provocation with these maneuvers is a positive finding.

Assessment of biomechanical factors such as varus alignment of the lower extremity, true leg length discrepancies, femoral neck anteversion, muscle weakness, excessive Q angles, excessive subtalar pronation, or a pes cavus style is recommended, because these may contribute to the mechanical load imparted to the affected site (41).

Radiographic findings are insensitive in the detection of early-stage stress injuries, and their usefulness is limited to the late phase. The time from onset of pain to positive radiographic evidence of a stress fracture can vary from 2-12 weeks (45). Early radiographic findings of a stress fracture in the long bones may include the visualization of a faint fracture radiolucency in the cortical bone (46). As the bone remodels, the endosteum can become ill defined, thickened, and sclerotic. As the fracture heals and remodels, periosteal reaction follows both on the cortical and endosteal surfaces. Stress fractures present differently in trabecular and cortical bone. The former is characterized by a predictable pattern manifested by a line of sclerosis perpendicular to the trabeculae, whereas the latter demonstrates periosteal reaction or a cortical fracture line.

Radionuclide scanning is a more sensitive but less specific method for imaging bony stress injuries (36,45). Radionuclide technetium-99 diphosphonate triple-phase scanning can provide the diagnosis as early as 2-8 days after the onset of symptoms (16). In acute stress fractures, all three phases of the bone scan are positive. One must be aware of the possibility of increased uptake in nonpainful sites, indicating subclinical accelerated remodeling (5).

MRI is considered as the gold standard for the evaluation of stress injuries (5,35,45). Fat suppression technique allows for early detection of injuries improving sensitivity and accuracy. A four-stage grading system has been developed: A grade 1 injury simply shows periosteal edema on the fat suppressed or short tau inversion recovery (STIR) images. In grade 2 injuries, abnormal increased signal intensity is also seen within the marrow cavity or along the endosteal surface on fat-suppressed T2-weighted images. In grade 3 injuries, signal abnormalities are also present on T1-weighted images. Grade 4 injuries involve an actual fracture line often seen on both T1- and T2-weighted images (16). One study (2) showed that grade 3 and 4 injuries took longer to heal than grade 1 and 2 injuries and demonstrated that the grade of injury has prognostic implications regarding the time of healing. Reported false-negative MRI findings have been because of reader errors, suboptimal choice of imaging planes and sequences, inhomogeneities in fat suppression, and partial volume effects (19).

Before making therapeutic decisions, it is important to correlate MRI findings with clinical symptoms. Bergman and Fredericson (5) studied 21 asymptomatic runners with MRI of the tibia. Nine (43%) of them showed abnormalities indicating stress injuries. After 12 months, none of the asymptomatic runners developed a bone stress injury. Stress response to exercise may cause bone marrow edema, and interpretation should always be made in conjunction with the patient’s clinical history (45).

It is important to distinguish stress fractures at higher risk for delayed union, nonunion, displacement, or intra-articular component. These fractures require early diagnosis, aggressive treatment, and occasionally internal fixation. High-risk stress fractures include fractures of the femoral neck, patella, tibial diaphysis, fifth metatarsal diaphysis (Jones fracture), tarsal navicular, body of the talus, base of second metatarsal, sesamoids, and pars interarticularis.

Less critical or not-at-risk fractures can be treated with a two-phase protocol. Phase 1 includes pain control with analgesics and physical therapy modalities. Weight bearing is allowed for normal activities within the tolerance of pain.

A modified activity program such as elliptical, rowing, or cycling can be used to maintain strength and fitness but to reduce impact loading to the skeleton. A program of deep water training or pool running can be indicated. In our experience, we have found it useful to include an antigravity treadmill device as part of the cross-training recovery protocol. This device allows controlled, progressive weight bearing while permitting unrestricted mobility and natural mechanics and enables the athlete to preserve aerobic condition.

Phase 2, graduated return to sport, generally begins once the athlete has been pain free for 10-14 days. The athlete can return to running only every other day for the first 2 weeks. Then, over a 3- to 6-week period, a gradual increase in distance and frequency is permitted (17).

Functional foot orthoses may play a role in treatment and prevention of stress fractures (15). Orthoses are useful either for reducing abnormal pronation in patients with a markedly everted rear foot or providing better shock absorption in athletes with a rigid, inverted rear foot (10).

If there is a positive history of irregular menses or amenorrhea, then consideration should be given to obtaining a bone mineral density test and endocrine workup. Hormonal replacement therapy indication should be cautiously considered only if the athlete is unable to resume normal menses through weight gain and is not indicated as a long-term solution. Calcium and vitamin D supplementation and additional nutritional deficiencies should be assessed and corrected (20).

The role of calcium and vitamin D supplementation in the prevention of stress fractures is still to be determined. Recent evidence shows that a daily calcium dietary intake of 1,500 mg in female adolescent athletes reduced incidence of stress fractures and increased bone mineral density (55).