Predicting and Preventing Fractures in Racehorses

A study of the skeletons of various horses shows that racehorses are not adequately prepared for the extreme mechanical stresses they endure during a race. The resulting injuries, which are often fatal, could be prevented with proper training that strengthens the bones.   Racehorses are pushed to extreme biomechanical limits. These animals, weighing about 500 kg, move at speeds of nearly 65 km/h. Selected over generations, the long, slender legs that give them this speed are, unfortunately, fragile. Accidents happen quickly, and fractures can cause irreparable tissue damage, often leading to the animal’s euthanasia. Seventy percent of these injuries occur in the third metacarpal bone, located between the horse’s knee and pastern. In an effort to find a way to predict and prevent these fractures, researchers at Johns Hopkins Medicine sought to understand how the third metacarpal bone responds to mechanical stresses, such as those encountered during races. They compared the sizes, densities, and ability to bend without breaking of the third metacarpal bone in three different horse breeds to better understand how it is affected by the animal’s lifestyle and training. Specifically, Thoroughbreds—which are trained to run long distances around turns—were compared to American Quarter Horses, which sprint short distances in a straight line, and to Assateague Island ponies, which are short and stocky and live in the wild without training. And although the size of the third metacarpal bone varies among the three horse breeds, the researchers were surprised to find that the bone’s strength and structure relative to body size were remarkably similar across all three types of horses. They had expected that racehorses would have certain areas of their bones much stronger as a result of their training and physical exertion—a certain degree of bone remodeling to help the skeleton respond to the mechanical stresses it undergoes. Just as weightlifting strengthens human bones, exposure to the stress of racing around turns should have created anatomical differences between Thoroughbreds and other breeds that would prepare the bones to resist fracture. But the third metacarpals of Thoroughbreds are neither denser nor stronger than those of the other two breeds. Thoroughbred bones are simply not adapted to the forces experienced during races. While this study highlights the relationship between mechanical stresses on bone and its structural response in an animal pushed to its physiological limits, it points to a lack of preparation in the horse for the conditions it will face. The researchers therefore suggest focusing racehorse training on exercises that would increase bone density, such as taking tighter turns at higher speeds.