Pathology of, and risk factors for, forelimb proximal sesamoid bone fractures in thoroughbred racehorses

Forelimb proximal sesamoid bone (PSB) fractures are the most common fatal racing and training musculoskeletal injuries in California Thoroughbred racehorses. PSB fractures are believed to be preceded by fatigue-induced structural bone damage and repetitive musculoskeletal overuse during training. The goal of this project was to determine if underlying changes in the PSBs predispose them to fracture, and if risk factors could be determined for PSB fracture.;Repetitive injury, stress fractures and incomplete healing precede complete, fatal fractures of the humerus, third metacarpal bone, pelvis and vertebrae in Thoroughbred racehorses. Exercise intensity, hoof conformation, and horseshoe characteristics are known risk factors for suspensory apparatus failure, and are likely similar for PSB fractures.;In our first study, we proposed that there were significant differences for types and distribution of PSB fractures between left and right limbs, medial and lateral bones, and for horses of different age and sex. We also predicted that radiographically detectable changes of osteophytes and vascular channels were positively associated with PSB fractures. Of the 328 equine cadavers, 136 (41.5%) had at least 1 fractured PSB. Few differences in fracture characteristics were apparent between left and right forelimbs. Medial bones appeared preferentially affected with complete transverse or split transverse simple fractures. Fractured medial bones tended to have indistinct fracture margins; > 1 vascular channel > 1 mm in width; and osteophytes in abaxial wing, basilar middle, or basilar abaxial locations. Oblique, partial transverse and axial fractures were detected predominantly in lateral bones. Fracture prevalence relative to horses actively training and racing increased with age up to 5 years. Osteophytes and vascular channels were common in fractured and intact proximal sesamoid bones. Odds for incurring a fracture in a PSB were lower in bones with osteophytes and vascular channels than in bones without these features.;The second study evaluated whether exercise, hoof conformation, and horseshoe traction devices increased the likelihood of bone damage and thus acted as risk factors for PSB fractures. Compared with horses that died without PSB fractures, horses that died with PSB fractures were more likely to be sexually intact males, spend more time in active training and racing, complete more events, train and race longer since their last lay-up, have higher exercise intensities during the 12 months prior to death, and have greater cumulative distances for their career. Horses with PSB midbody fractures were more likely to be sexually intact males, train and race longer since their last lay-up, and have higher exercise intensities during the 12 months prior to death.;Transverse fractures of the PSBs were suspected to be acute manifestations of a more chronic process. We examined PSBs from Thoroughbred racehorses with a PSB midbody fracture, the contralateral intact PSB from the same horse and intact PSBs from horses matched by age and gender to the fracture cases to find evidence of microstructural features that might predispose horses to acute PSB fractures. Compaction of trabecular bone and lower bone porosity were observed in fractured PSBs and their contralateral intact PSBs compared to intact PSBs from the age- and gender-matched horses, particularly in the medullary region of the bone. Fracture margin incongruity was evident on the palmar aspect of the fracture margins in 6 of 8 horses with PSB fracture. An additional focal defect, consistent with bone remodeling, was found in medial, fractured and contralateral intact PSBs in 2 of the 6 horses with fracture margin incongruity. Fracture appeared to initiate from sites of previous remodeling in the palmar flexor region and propagate towards the articular surface.