5, 55 On average, female individuals have 25% less muscle mass than male individuals, which can focus forces on to a smaller area of bone with less muscle protection. 35, 46, 71, 72, 90 Anatomically, a wider pelvis and more common genu valgum results in a compensatory increased Q-angle and often foot pronation. 102įemale individuals have a higher incidence of stress fractures. 96, 102 Continued training after muscle fatigue changes loading patterns of the foot based on force plate studies and is postulated to be a factor in stress-related injuries. 35, 46, 55 Several studies have described the biomechanical effects of muscle fatigue as a possible factor in the development of stress fractures. 35, 46, 55 Extrinsic factors such as type of activity, excessive or new training regimens, equipment and footwear issues, training surfaces and techniques, and nutrition can also play a role. Intrinsic factors consist of the patient’s anatomy and biology including cavus feet, leg length discrepancies, excessive forefoot varus tarsal coalitions, a prominent posterior calcaneal process, tight heel cords, poor bone density or vascular supply, and abnormal hormonal levels. Intrinsic and extrinsic factors have been described when determining the etiology of a stress fracture ( Table 1). 12 In a study of 320 athletes, the tibia (49.1%), tarsals (25.3%), and metatarsals (8.8%) were the most frequently involved bones affected by a stress fracture. 8, 12, 13, 17, 35 Most bones have reported cases of stress fractures, but the lower extremities have the highest prevalence. 12, 13 This mechanism of injury explains the higher incidence of stress fractures among military recruits, runners, and those involved in jumping sports, though any activity with repetitive loading can lead to stress fractures. 12, 13 If the repetitive loading continues, the stress reaction can go on to a true stress fracture. This diagnosis is made in a symptomatic patient who has bone scan or magnetic resonance imaging (MRI) evidence of bone reactive changes without a true fracture line. 12, 13 The initial stage of bone failure is generally called a stress reaction.
12 A bone responds to stress on a continuum from a stress reaction to a fracture. In contrast to acute fractures, which typically occur with a single maximal load, stress fractures occur due to repetitive, submaximal loading of a bone, leading to microfractures that are unable to heal due to bone resorption and bone formation imbalances. 12, 35 The incidence of these injuries is rising due to earlier and longer participation in sports, the emergence of more extreme sporting activities, and the heightened awareness of the diagnosis. Stress fractures are relatively uncommon injuries, accounting for approximately 1% to 7% of all athletic injuries. Introduction, Epidemiology, and Basic Science
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