Proximal sesamoid bone (PSB) fractures are one of the most commonly reported catastrophic musculoskeletal injuries (CMI) in Thoroughbred racing. Despite the seriousness of these injuries, an understanding of the structural changes that occur normally within PSBs in response to the biomechanical forces of high-speed exercise and how they relate to pathological events leading to fracture remain limited. To address this critical knowledge gap, PSBs from Thoroughbred horses were analyzed with both computed tomography (CT) and microcomputed tomography (µCT) to assess their morphological and structural parameters. Comparisons were made using samples from the left and right forelimbs of Thoroughbreds of both genders between the ages of 2-6 years across the following four experimental groups:​ 1) horses that have not entered racehorse training, 2) horses that were actively training and racing before being euthanized for non-skeletal health issues, 3) horses that were actively training and racing before being euthanized due to a CMI that did not include a PSB fracture, and 4) horses that were actively training and racing before being euthanized due to a CMI that included a PSB fracture. Following full limb clinical CT evaluation, individual PSBs underwent µCT and a bone morphometry evaluation to measure the following structural parameters:​ bone volume (BV), total volume (TV), bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), connectivity density (Conn.D), and degree of anisotropy (DA), as well as CT evaluation for volumetric bone mineral density (vBMD) and the presence of a focal subchondral lesion in the midbody articular region of the PSB. The hypothesis being tested was that PSBs respond to the altered biomechanical forces of high-speed exercise by increasing trabecular thickness, bone volume fraction, and connectivity density, and by decreasing trabecular separation and degree of anisotropy. Results of this study found that PSBs from horses racing and in training had increased bone mineral content compared to unraced and untrained horses, demonstrated by significantly increased BV and BV/TV values. Additionally, PSBs from those horses that experienced a PSB fracture demonstrated increased values for Tb.Th and decreased values for Conn.D, Tb.Sp, and Tb.N. Multiple significant differences by forelimb and between medial and lateral PSBs were observed, where BV and BV/TV were increased in right forelimbs compared to left forelimbs and BV, BV/TV, Tb.Th, Tb.Sp, DA, and vBMD were found to be increased in medial PSBs and Tb.N to be increased in lateral PSBs. The focal subchondral lesion was found to be present on PSBs from groups 2, 3 and 4, and also found to occur significantly more frequently in medial PSBs than lateral PSBs. The long-term objective of this research is to identify structural parameters that can be assessed with advance imaging techniques and have the potential to yield clinically informative assessments of PSB health in equine athletes.