Humeral fractures account for more than 35 percent of fatal fractures sustained during training of Thoroughbred racehorses greater than two years of age1. Fractured humeri have periosteal callus adjacent to the fracture fragment surfaces indicating that bone damage precedes fracture 2. Humeral fractures typically occur with exercise after return from lay-up3. Previous research failed to elucidate whether or not exercise intensity correlates with severity of periosteal callus in the neck region of the equine humerus or if certain severities of humeral periosteal callus are associated with increased underlying osteoporosis or endosteal callus formation. Our study found that horses that fracture humeri are young and have few career events, yet undergo a high percentage of their total career distance just prior to complete fracture. An apparent progression of humeral periosteal callus stages has also been identified.;MicroCT confirms that as complexity of the periosteal callus increases, the porosity of cortical bone and thickness of endosteal callus also increase. These more advanced calluses seem to protect from complete fracture following layup because humeri without periosteal callus fracture sooner following lay-up than do humeri without callus. Horses with more advanced humeral callus stages also run greater distances in the year prior to death suggesting that calluses either serve a protective purpose or that callus is produced as a result of more athletic events. Identification of humeral stress remodeling prior to return to work following layup is crucial to prevent catastrophic breakdown. Humeral stress fracture and subsequent stress remodeling should be considered in racehorses returning from layup that are young and have undergone relatively few timed events in their careers.;Literature Cited: 1 Johnson, J., Stover, SM, Daft, BM, Kinde, H., Read, DH, Barr, BC, Anderson, Moore, J., Woods, L., Stoltz, J., Blanchard, P. (1994) Causes of death in racehorses over a 2 year period. Equine vet J. 26, 327--330. 2 Stover, SM, Johnson, BJ, Daft, BM, et al. (1992) An association between complete and incomplete stress fractures of the humerus in racehorses. Equine Vet J. 24, 260--263. 3 Carrier, TK, Estberg, L., Stover, SM, Gardner, IA, Johnson, BJ, Read, DH, Ardans, AA. (1998) Association between long periods without high-speed workouts and risk of complete humeral or pelvic fracture in Thoroughbred racehorses: 54 cases (1991--1994). J. Am. Vet. Med. Assoc. 10, 1582--1587.
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