Geometric Properties of the Third Metacarpal Bone: A Comparison Between Thoroughbred and Quarter Horse Racehorses
Authors: Goldstein Deanna M. , Engiles Julie B. , Rezabek Grant B. , Ruff Christopher B. 
Published: June 2021

A frequent reason that Thoroughbred racehorses are killed during racing or training is fracture of the third metacarpal (the cannon bone). Determining the factors involved in these catastrophic cannon bone fractures is imperative to improving both horse and jockey safety. Though seemingly spontaneous, catastrophic cannon bone fractures generally result from the combination of mechanical forces commonly present in racing Thoroughbreds, and generally materialize in the distal lateral condyle of the cannon bone. Thoroughbreds, however, are not the only breed of racehorse to experience catastrophic fractures—Quarter Horses also exhibit specific patterns of injury in more proximal limb bones. Different fracture patterns in these two breeds of galloping racehorse may indicate different loading patterns of their limbs, which could be a product of varied training programs, or variance in breed morphology. The present analysis compares geometric properties of the third metacarpal in Thoroughbred and Quarter Horse racehorses, using feral Assateague Island ponies as an outgroup, in order to clarify the reasons for distinctive fracture patterns in different breeds of racehorses.Six cannon bones from each breed were obtained from New Bolton Center (Thoroughbreds), Oklahoma State University (Quarter Horses), and the Assateague Island National Seashore (Assateague Island ponies). Peripheral Quantitative Computed Tomography (pQCT) generated cross sections at 40% and 20% (distal) of bone length, and at the midpoint of the distal articular surface. Cross‐sectional areas, second moments of area, and section moduli were calculated for the two diaphyseal sections, and trabecular bone mineral density and total bone mineral density were determined for the distal articular section. Approximate body masses were estimated from proximal articular breadths using a published formula.Analysis of covariance (ANCOVA) revealed that although the overall strength of cannon bones in Thoroughbreds and Quarter Horses did not differ relative to body size, Thoroughbreds had relatively higher anteroposterior bending strength in the diaphysis than Quarter Horses, and relatively lower mediolateral bending strength. Thoroughbreds also had lower values of trabecular and total bone densities in the distal articular section, which is the region where lateral condyle fractures are generally incurred by Thoroughbreds but not Quarter Horses. Both Thoroughbreds and Quarter Horses had lower overall safety factors compared to feral Assateague Island ponies. Assateague Island ponies exhibited relatively stronger bone shafts and a higher trabecular density in the distal articular section. Results suggest that breed‐specific bone structure differences contribute to the greater incidence of fractures in domestic racehorses and, relatedly, the common location of fractures in Thoroughbreds.Support or Funding InformationFunded by the American Society of Mammalogists.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.