Stress Fractures

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Stress fractures are the result of repetitive, cyclic loading of bone, which has overwhelmed the reparative ability of the bone. They occur when there is a failure of the bone to adapt to mechanical loads.

Risk factors for stress fractures and possible mechanisms :
Low bone density  decreased bone strength; Small bone size  decreased bone strength; Skeletal alignment  elevated bone strain, unaccustomed bone strain, muscle fatigue; Body size and composition  elevated bone strain, menstrual disturbances, muscle fatigue, low bone density; Bone turnover  low bone density, elevated bone strain, inadequate repair of microdamage; Muscle flexibility and joint range  elevated bone strain, unaccustomed bone strain, muscle fatigue; Muscle strength and endurance  elevated bone strain, unaccustomed bone strain; Low calcium intake  greater rate of bone turnover, low bone density, inadequate repair of microdamage; Nutritional factors  altered body composition, low bone density, greater rate of bone turnover, reduced calcaium absorption, menstrual disturbances, inadequate repair if microdamage; Menstrual disturbances  low bone density, greater rate of bone remodelling, increased calcium excretion; Training  elevated bone strain, unaccstomed boen strain, greater number of loading cycles, muslce fatigue, inadequate time for repair of microdamage, menstrual disturbances, altered body composition; Inappropriate surface  elevated bone strain, unaccustomed bone strain, muscle fatigue; Inappropriate footwear  elevated bone strain, unaccustomed bone strain, muscle fatigue; Higher external loading  elevated bone strain, muscle fatigue; Genetic factors  low bone density, greater rate of bone remodellin, psychological traits; Psychological traits  excessive training, nutritional intake and eating disorders.

Imaging of stress fractures
Plain x-ray:
• poor sensitivity and high specificity
• seen as new periosteal bone formation; area of sclerosis; callus formation
• do not appear unless 2-3 weeks after appearance of symptoms

Bone scan:
• indicated if x-ray is negative and stress fracture is highly suspected
• high sensitivity and low specificity
• bone scan changes appear 48-72 hours after appearance of symptoms

CT:
• useful for differentiating a positive bone scan

MRI
• can not image cortical bone as well as CT

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