Microstructural properties of the proximal sesamoid bones of Thoroughbred racehorses in training.
Authors: Ayodele Babatunde A, Hitchens Peta L, Wong Adelene S M, Mackie Eleanor J, Whitton R Christopher
Journal: Equine veterinary journal
Summary
# Microstructural properties of the proximal sesamoid bones of Thoroughbred racehorses in training Proximal sesamoid bone fractures represent one of the most devastating catastrophic injuries in racing Thoroughbreds, yet the mechanistic link between training-induced bone adaptation and fracture risk remains poorly understood. Babatunde and colleagues used micro-computed tomography to characterise the internal bone architecture of 63 racehorses' sesamoid bones, measuring bone volume fraction and mineral density across four anatomical regions and correlating these findings with individual racing histories and surface pathology. The research revealed substantial regional variation within the sesamoids, with the dorsal midbody region showing greater density than apical, palmar or basilar zones—a distribution pattern reflecting load-bearing during racing. Perhaps most intriguingly, horses with greater bone densification (higher bone volume fraction) showed markers of bone fatigue including surface microcracks and resorption, earlier career commencement, and lower competitive performance ratings, whereas animals with lower densification demonstrated improved longevity and success. These associations suggest that excessive mineralisation may represent a maladaptive response to early or intensive training rather than an indicator of bone strength. For practitioners, these findings challenge the assumption that denser sesamoid bones equate to greater fracture resistance and instead suggest that premature densification signals cumulative microdamage and potential fatigue. Monitoring young racehorses for evidence of accelerated sesamoid adaptation—through radiographic or ultrasound assessment—may help identify individuals at heightened fracture risk, informing decisions about training load, age at commencement, and preventative intervention strategies.
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Practical Takeaways
- •Young racehorses (especially those starting racing before age 4) show accelerated bone densification in sesamoid bones; monitoring early career stress may help prevent catastrophic fractures
- •The presence of microcracks and bone resorption alongside high bone density suggests fatigue damage rather than strength—horses with lower sesamoid density actually had longer careers and better performance
- •Farriers and veterinarians should be alert to regional variation in sesamoid bone properties; the midbody dorsal region bears different loading and shows different microstructure than other regions
Key Findings
- •Mean proximal sesamoid bone volume fraction (BVTV) was 0.79±0.08 with bone material density of 806.02±24.66 mg HA/ccm in Thoroughbred racehorses
- •Medial sesamoids had greater BVTV than lateral sesamoids (0.80 vs 0.79), with the midbody dorsal region showing highest density (0.86±0.06)
- •Sesamoid densification increased with presence of microcracks, bone resorption, lower handicap ratings, and racing commencement before age 4 years
- •Lower sesamoid bone volume fraction was associated with better racing longevity and performance, suggesting excessive densification indicates fatigue-related pathology