The North American Thoroughbred racehorse was chosen as a model to study the pathogenesis of fatigue failure of bone. This species has a high incidence of spontaneous fatigue failure of bone (bucked shins) during its early training. In vivo strain gauge studies of the third metacarpal bone of four young racehorses running at racing speeds showed high principal compressive strains [-4,841 +/- 572 (SD) microstrain] while two older horses had lower principal compressive strains (-3,317 microstrain measured at racing speed, -3,250 microstrain extrapolated from a slower speed run). Previously reported inertial property measurements of the third metacarpal bone were related to the difference in bone strains seen in young and older horses. The high strains on the surface of the third metacarpal bone associated with young horses in training may lead to high strain, low cycle fatigue. The changing shape of the third metacarpal bone during maturation may be consistent with the lower strains recorded during high speed exercise in the older animals. This phenomenon may allow for the accumulation of additional strain cycles in older animals before failure occurs.

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To examine the effect of alendronate (4-amino-1-hydroxybutylidene bisphosphonate) on fracture repair, the drug was given to mature beagle dogs orogastrically at 2 mg/kg/day for 9 weeks preceding fracture. 16 weeks after fracture, or both before and after fracture (25 weeks). A transverse mid-diaphyseal fracture of the right radius was surgically induced and was stabilized by external coaptation splinting. Fracture healing and bone remodeling were evaluated by radiography, gross and histological examination, and bone histomorphometry. The mechanical properties of the fracture callus were determined by a four-point bending test. Radiographs and gross and microscopic examination demonstrated normal bone healing at the fracture site in all dogs. In dogs that received alendronate during the fracture healing period, at 16 weeks the calluses were approximately 2-3 times larger than those in dogs that received a placebo during the healing period. This is consistent with slower callus bone remodeling, an expected pharmacological effect of the compound. Bone histomorphometry demonstrated that treatment with alendronate did not inhibit bone formation or mineralization. Mechanical testing showed that the ultimate load at failure and the flexural rigidity of both the fractured and contralateral intact bone were unaffected by treatment with alendronate. Therefore, in this study, treatment with alendronate before or during fracture healing, or both, resulted in no adverse effects on the union, strength, or mineralization of bone in mature beagle dogs.

Relationships between different gaits and speeds in the training regimen influence the incidence of bucked shins. To reduce the incidence of bucked shins, trainers should consider allocating more training effort to regular short-distance breezing and less to long-distance galloping.

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