The Third Metacarpal Bone of Horse Bones May Be More Resistant to Racing Injuries

horse bones

Horses are a uniquely adapted species, genetically evolved to move over long distances. Their long, thin limbs are designed to store and transfer energy through the flexion phase of the stride. Their short head and neck are positioned to balance this biomechanical extreme with the rest of their body. The result is that horses can carry up to 1,000 pounds at 40 miles per hour while moving their legs in ways that would be dangerous or even fatal to a human. But, despite the fact that their long bones are genetically adapted to the biomechanical challenges of running, horses face serious problems with their articular bone structure (the tarsal and metacarpal bones), particularly with their third metacarpal bone or cannon bone. Racehorses in particular operate at a biomechanical extreme and are susceptible to severe, often career-ending injuries on the track. A recent study suggests that fostering adaptation in this bone with training may increase the ability of the cannon bone to resist the stresses of racing and reduce serious, sometimes life-threatening injuries.

The third metacarpal or cannon bone of the equine limb is a long, slender, elliptical bone that connects the elbow joint to the carpus or wrist bone. The cannon bone is susceptible to fractures, splints and other conditions that can cause lameness. In a recently published study comparing the third metacarpal bones of a group of Thoroughbred racehorses with those of American Quarter Horses and feral Assateague Island ponies, Johns Hopkins Medicine scientists found that the cannon bone of racing horses had more cracks than the other two groups. The cannon bone of the racing group was also more brittle and had lower mineral density. The researchers speculate that these differences are due to the specialized biomechanical environment that is unique to the racehorses and their training regimens.

When horses are not in motion, the cannon bone is surrounded by a network of tendons that help to maintain the shape and function of the limb. During exercise, however, these tendons are loaded to an even greater extent and the structure of the cannon bone changes as a result. Backscattered electron microscopic studies of the Mc3 bone have shown that increased stiffness in horses is a result of an accumulation of new bone in the form of secondary osteons that are deposited on resorption cavities and incompletely filled osteons in the trabecular bone (Boyde and Firth, 2005).

This increases the mechanical load on the bone, which promotes further resorption of the existing bone and creates a pattern of woven bone. It is this woven bone that is most resistant to fatigue failure in the cannon bone, as opposed to cracking within the cortical layer of the bone. The woven bone is supported by a matrix of type 1 collagen fibrils that are oriented in longitudinal bundles with wavy portions along the fibres called crimps. Tendons that have been injured will have disturbed crimp morphology and will not respond as well to loading, as those with normal crimp morphology (Boyde and Firth, 2007).