Nearly 20% of fatal musculoskeletal injuries in Thoroughbred atheletes are due to complete humeral (forearm bone) fractures, which often occur early in training or following an prolonged layup. Rachel Entwistle, BS, of the University of California, Davis, discussed the wastage that occurs with humeral fractures in these atheletes at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Cailf. Complete fractures are typically characterized by a pre-existing, underlying stress fracture (periosteal bone callus). In Thoroughbred athletes this is often a bilateral disease (occurring in both front legs).

Entwistle explained that the maturity of the callus that bridges the fracture line is the best predictor of bone strength at the injury site. She noted that bone strength decreases in early stages of callus formation and attempts to return to normal with maturation as bone modifies its structure relative to its loading environment. Bone-tissue changes can be detected early by nuclear scintigraphy or MRI, and they can be seen later by radiography.

The study objective was to determine the functional consequences of stress fracture at both a common site (actual point of stress fracture) and distal (lower) site in the humerus, as well as to examine the relationship of bone strength and exercise history. Entwistle and colleagues hypothesized that bone tissue strength at the stress fracture site would be altered, yet the distal site would not be affected by stress fracture. They also hypothesized that bone tissue strength is altered by microstructural tissue changes likely affected by exercise regimes