© 2006 Elsevier Inc. All rights reserved. normal bone to excessive repetitive stress. Fatigue stress fractures tend to be seen in a young, active, healthy population such as recreational and professional athletes and members of the military. Insufficiency fractures tend to occur in bones that are predisposed to fracture based upon osteopenia and osteoporosis, and these conditions are more commonly seen in the elderly population or in patients who have secondary causes of demineralization [3].

Most stress fractures occur 4 to 5 weeks after the onset of a new exercise. Muscles normally provide biomechanical dissipation of stress from the bones, but fatigued muscle may decrease this protective contribution, and this can result in the transmission of increased stress to the bones [4]. The incidence of stress fractures increase with advancing age [5]. The location of stress fractures in runners tends to also vary with age, with femoral and tarsal stress fractures occurring in older patients, and fibular and tibial stress fractures occurring in the younger athlete [6].

Runners are particularly at risk because of the multitude of factors that can result in the increased or altered stressors to the skeletal system. Stress fractures occur in 13% to 37% of runners [7]. There is a decreasing incidence of these fractures in the tibia (33%), navicular (20%), metatarsals (20%), femur (11%), fibula (7%), and pelvis (7%); in 75%, the medial tibial crest is involved [8]. Factors found to be associated with stress fractures include training errors, distance, age, running surfaces, shoes, diet factors such as malnutrition and anorexia nervosa, smoking, alcohol use, a history of overuse injuries, and hormonal alterations such as amenorrhea, inhaled corticosteroids, and hypotha-lamic dysfunction [7-11]. Certain biomechanical factors have been found to be associated with patients experiencing multiple stress fractures. These include a high longitudinal arch of the foot, leg-length inequality, and excessive forefoot varus. Females who have menstrual irregularities seem to have an increased risk of recurrent stress fractures. Runners who have high weekly training mileage have also been found to have an increased risk of recurrent stress fractures of the lower extremities [12].

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