Microstructural properties of the proximal sesamoid bones of Thoroughbred racehorses in training

BACKGROUND: Proximal sesamoid bone fractures are common catastrophic injuries in racehorses. Understanding the response of proximal sesamoid bones to race training can inform fracture prevention strategies. OBJECTIVES: To describe proximal sesamoid bone microstructure of racehorses and to investigate associations between microstructure and racing histories. STUDY DESIGN: Cross-sectional. METHODS: Proximal sesamoid bones from 63 Thoroughbred racehorses were imaged using micro-computed tomography. Bone volume fraction (BVTV) and bone material density (BMD) of the whole bone and four regions (apical, midbody dorsal, midbody palmar and basilar) were determined. Generalised linear regression models were used to identify associations between bone parameters and race histories of the horses. RESULTS: The mean sesamoid BVTV was 0.79±0.08 and BMD was 806.02±24.66 mg HA/ccm. BVTV was greater in medial sesamoids compared to lateral sesamoids (0.80±0.07 vs 0.79±0.08; P<0.001) predominantly due to differences in the apical region (medial - 0.76±0.08 vs. lateral - 0.72±0.07; P<0.001). BVTV in the midbody dorsal region (0.86±0.06) was greater than other regions (midbody palmar - 0.79±0.07, basilar - 0.78±0.06 and apical - 0.74±0.08; P<0.001). BVTV was greater in sesamoids with more microcracks on their articular surface (Coef. 0.005; 95% CI 0.001, 0.009; P=0.01), greater extent of bone resorption on their abaxial surface (Grade 2 - 0.82±0.05 vs. Grade 1 - 0.80±0.05 or Grade 0 - 0.79±0.06; P=0.006), in horses with a low (0.82±0.07) or mid handicap rating (0.78±0.08) compared to high rating (0.76±0.07; P<0.001), in 2 to 5 year old horses (0.81±0.07) compared with younger (0.68±0.08) or older horses (0.77±0.08; P<0.001), and in horses that commenced their racing career at less than four years of age (0.79±0.08 vs. 0.77±0.77; P<0.001). BMD was greater in the midbody dorsal (828.6±19.6 mg HA/ccm) compared to other regions (apical - 805.8±21.8, midbody palmar - 804.7±18.4, basilar - 785.0±17.1; P<0.001), in horses with a handicap rating (low - 812.1±20.0, mid - 821.8±21.3 and high - 814.6±19.4) compared with those with no rating (791.08±24.4, P<0.001), in females (806.7±22.0) and geldings (812.2±22.4) compared to entires (792.7±26.2; P=0.02), and in older horses (<2 year old - 763.7±24.8 vs. 2-5 year old - 802.7±23.4, and 6-12 year old - 817.8±20.0; P=0.002). MAIN LIMITATIONS: Data were cross-sectional. CONCLUSIONS: Densification of the proximal sesamoid bones is associated with the commencement of racing in younger horses and the presence of bone fatigue related pathology. Lower sesamoid bone volume fraction was associated with longevity and better performance.