**12.2 Stress fractures**

Stress fracture occurs when a bone breaks after being subjected to repeated tensile or compressive stresses, none of which would be large enough to cause individually the bone to fail, in a person who is not known to have an underlying disease that would be expected to cause abnormal bone fragility (De Weber, 2011). The incidence of stress fractures is less than 1 % in the general population. The reported incidence in athletic populations varies with the type of athlete. Among military recruits the incidence ranges from 1 to 31 %, among runners 13 to 52 %, and among participants in collegiate team sports 1 to 8 % (De Weber, 2011; as cited in Bennel, 1997). In most instances, the individuals who suffer stress fractures have been engaging in vigorous activity to which they have not yet become conditioned. The failure to recognize the characteristic clinical and imaging findings of a stress fracture and the continued excessive exercise by the athlete will occasionally lead to a complete fracture (Collier et al, 1993). Imaging is needed when high risk stress fractures are suspected or a definitive diagnosis is necessary. The sites at high risk complications are the pars interarticularis of the lumbar spine, femoral head, superior side of the femoral neck, patella, anterior cortex of the tibia, medial malleolus, talus, tarsal navicular, proximal fifth metatarsal, great toe sesamoids, and the base of the second metatarsal bone (De Weber, 2011).

Fig. 8. Scintigraphy: localized uptake in tibial metaphysis and both internal femoral condyles.

Three-phase bone scan has traditionally been used for diagnosis of stress fractures because it can show evidence of stress fracture within 2 to 3 days of injury and has high sensitivity. Acute stress fractures appear as discrete, localized, sometimes linear areas of increased uptake on all three phases (angiographic, soft tissue, and delayed phases) of a Tc-99m-MDP bone scan (**Fig. 8**). However, the specificity of bone scan is low. Approximately 40 % of positive findings occur at asymptomatic sites (De Weber, 2011; as cited in Bennell et al., 1999). Bone scan can also be falsely positive with shin splints, despite shin splints are typically positive only during the delayed phase of the scan (De Weber, 2011; as cited in Deutsch, 1997). Areas of increased uptake may represent subclinical sites of bone remodeling or stress reactions. Increased uptake can also appear in the setting of bone tumors, osteomyelitis, or avascular necrosis. Although rare, there are reports of false-negative bone scans (De Weber, 2011; as cited in Gaeta et al., 2005; Spitz & Newberg, 2002). Because of these limitations, MRI is supplanting bone scan as the diagnostic tool of choice when plain radiographs are negative and confirmation of suspected stress fracture is needed.
