Correlation of racing performance with radiographic changes in the proximal sesamoid bones of 487 Thoroughbred yearlings

Reasons for performing study: Proximal sesamoiditis has been a common and confusing radiographic diagnosis in Thoroughbred yearling surveys. This study categorised the radiographic appearance of yearling proximal sesamoid bones and determined their effect on the number of races started and earning potential for racing at ages 2 and 3 years. Hypothesis: Yearlings with moderate to severe proximal sesamoiditis have decreased performance at age 2 and 3 years. Methods: Survey radiographs of 487 Thoroughbred yearlings were reviewed. Proximal sesamoid bones were examined and findings placed into 7 categories, 5 of which measured size, shape, and number of vascular canals, and 2 examined the radiographic shape of the sesamoid abaxial surface. Records of horses age 2 and 3 years were reviewed to obtain the number of races completed and money won in those races. Results: Vascular canals with parallel sides <2 mm in width in any number were normal. Yearlings with 1 or 2 abnormally conformed vascular canals (nonparallel sides and >2 mm in width) had a decrease in number of race starts at age 2 years when compared to normal horses. This same change in the fore- or hindlimbs was associated with decreases in starts and earnings at age 2 years. If uniformly present in both fore- and hindlimbs there was no difference from normal horses. Yearlings with >2 irregular vascular canals had a decrease in number of race starts and earnings at age 2 and 3 years. Changes in proximal sesamoid bone contour did not affect performance. Conclusions: Horses with enlarged vascular canals, within their sesamoids as yearlings, start fewer races and earn less prize money than horses with normal vascular canals. Potential relevance: One or two enlarged canals affected horses as 2-year-olds. Three or more enlarged vascular canals affected horses at ages 2 and 3 years, which should be taken into account when examining radiographs for potential purchase of young horses.