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Fractures--a preventable hazard of racing thoroughbreds?

Fractures are a common cause of loss among Thoroughbred racehorses. A large proportion of these injuries occurs in the absence of a specific traumatic event and show typical characteristics of stress fractures. The fractures show a high degree of consistency in their morphology; they frequently share the same locations as incomplete cracks and they are often associated with pre-existing pathology (periosteal and endosteal new bone formation and intracortical remodelling). Bone is able to adapt to changes in its mechanical environment. Studies of the Thoroughbred racehorse show modification of the geometric properties of the third metacarpal bone in response to training. These modifications are associated with reduced bone strains. Intense training before the adaptive response is completed and bone strain reduced increases the risk of fatigue damage. Fatigue of bone is associated with progressive microdamage, which is important in the pathogenesis of stress fractures. However, the biological repair mechanism of bone (remodelling) is also instrumental in the development of stress fractures. Horses exercised before bone repair is complete are likely to be at significantly greater risk of sustaining a catastrophic stress fracture. A number of key questions regarding the importance of microdamage, remodelling and training schedules in the prevention of stress fractures are addressed in this review.