Proximal sesamoid bone (PSB) injuries account for approximately 50% of all fatal catastrophic musculoskeletal injuries in Thoroughbred racehorses. In addition, PSB fractures are not uncommon in Thoroughbred foals and yearlings as a result of energetic or intense free exercise. The ability to interpret some PSB pathology assessments, however, is more difficult given the limited information published on the normal development and maturation of these paired sesamoid bones. In addition, the level of normal variation in PSB morphological and structural parameters within the Thoroughbred population as a function of age, body size, gender, and inter-animal variation are not well documented, complicating accurate assessments of PSB pathology.

These knowledge gaps were addressed through postmortem analyses of PSB anatomy from 36 horses representing an age range from birth to skeletal maturity. All study subjects were believed to represent normal appendicular skeletal structure for the Thoroughbred breed and were assessed using clinical computed tomography (cCT) evaluation (n=18 horses, 21 months to six years of age) and micro–computed tomography (μCT) bone morphometry evaluation (n=22 horses, birth to six years of age). Medial PSBs were found to be shorter and deeper with an increased bone mineral density compared to lateral PSBs. Trabecular separation and degree of anisotropy were also greater in medial than lateral PSBs. All of the measured μCT parameters changed with increasing age, clearly documenting a postnatal maturation process. Connectivity density and degree of anisotropy changed rapidly after birth, while the parameters of total volume, bone volume, bone volume fraction, trabecular number, and trabecular thickness changed at a slower rate. Importantly, however, all of these parameters approached a horizontal asymptote before 24 months of age. The only μCT parameter that continued to change beyond two years of age was trabecular separation. These findings greatly advance the understanding of postnatal development and maturation timelines of PSBs in Thoroughbred horses, as well as documenting structural differences between medial and lateral PSBs. The data also indicate that PSBs in Thoroughbred horses have reached skeletal maturity by two years of age, a time that corresponds with the onset of high speed exercise and racing competition. The results define normal structural and morphological characteristics of Thoroughbred PSBs, as well as a preliminary appreciation for the extent of inter-animal variation. This knowledge will provide an essential baseline for the assessment of equine PSB pathologies.