Researchers Study Sesamoid Bone Shape Differences

The training and racing of 2-year-old Thoroughbreds has always been a source of debate in the racing world due to concerns that exerting young skeletons might make horses more likely to injure themselves. But recent study results from Italian researchers suggest that at least one set of bones in Thoroughbreds might not impacted by training as juveniles: the sesamoids.

The researchers, led by Francesca Beccati, DVM, PhD, of the University of Perugia, looked at 27 2-year-old Thoroughbreds' proximal sesamoid bones (PSB)—the two small bones that form the rear portion of the fetlock joints—studying the differences in bone height and width in the horses' forelegs.

Beccati and colleagues measured the bone dimensions before and after the horses partook in one year of exercise and racing. Before the horses hit the track, researchers found a significant difference between the shape of the medial and lateral (inside and outside, respectively) proximal sesamoids, as well as differences between the bones in the right and left legs.

Specifically, the team found that the inside proximal sesamoids were significantly shorter and wider than the outside sesamoids in both legs, while the inside proximal sesamoid in the right leg was, on average, wider than that of the inside proximal sesamoid in the left leg. The differences were not significant enough to see under close examination with the naked eye, but were easily measured when the joints were radiographed (X rayed).

Beccati said the proximal sesamoids withstand compressive impact as horses gallop, particularly as the horse tires, which puts them at a high risk of a variety of fractures.

“(According to previous studies) apical fractures (affecting the upper third of the sesamoid bone) are the most common type of PSB fracture in racing Standardbreds and Thoroughbreds, and medial apical (affecting the inner portion of the sesamoid bone) sesamoid fracture of the forelimb carries the worst prognosis, has the most difficult rehabilitation, and decreases the quality of the injured horse’s racing career in the Thoroughbred,” said Beccati.

After the horses completed a year of training and racing, the researchers reevaluated their sesamoid bones. The team ultimately excluded 10 horses that showed signs of sesamoiditis (inflammation of the sesamoid bones) after the training period. Of the remaining horses in the study, none showed significant changes in sesamoid bone shape after training and racing.

Because the bones in the fetlock have finished growing by the time a horse turns two, Beccati said horses will retain these differences in bone height and width throughout their lives.

As for the sesamoiditis cases identified in the study, Beccati said she does not believe the sesamoids' shape differences were to blame.

“I personally think that abnormalities in conformation (such as fetlock valgus or varus, or toed-out or toed-in) biomechanically are more important in asymmetrical stress on the distal limb, and the asymmetrical stress makes horses more prone to sesamoiditis,” she said.

The study, "Morphologic Radiographic Study of the Proximal Sesamoid Bones of the Forelimb in Thoroughbred Racehorses in Training," will appear in an upcoming issue of Anatomia, Histologia, Embryologia

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