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Catastrophic injuries in the fetlock joint of Thoroughbred racehorses are usually preceded by stress related pathology. Early detection of subchondral bone injury is therefore clinically important. Magnetic resonance imaging (MRI) and computed tomographic (CT) studies of the fetlock are available in both lame and non-lame Thoroughbred racehorses. However, subchondral bone plate thickness of the proximal phalanx and its changes over time have not previously been assessed objectively in non-lame Thoroughbreds.

The main objective of this study was to document changes in subchondral bone thickness of the proximal phalanx in Thoroughbred racehorses during their first two years of training and racing. Sixty-nine forelimbs from 38 Thoroughbred racehorses were included in the study. Computed tomographic examination of the front fetlocks were performed five times at approximately six months’ intervals. Subchondral bone plate thickness of the proximal phalanx was measured at nine locations: at the sagittal groove, medially, and laterally within each of the dorsal, middle, and palmar regions. Differences in subchondral bone plate thickness between examination time points were assessed using the Kruskal–Wallis test, followed by post hoc Dunn’s tests with Bonferroni correction. Associations between the measurements and age, bodyweight, height at the withers, and bodyweight-to-height ratio were assessed using Spearman’s rank correlation. Differences between sexes and between the left and right forelimbs were evaluated using Kruskal–Wallis test, followed by post hoc Dunn’s tests.

The subchondral bone plate was the thickest at the medial and lateral palmar measurement sites and the thinnest at the sagittal palmar site. Lateral palmar subchondral bone plate thickness was significantly greater at the third (p=0.005) and fifth (p=0.010) examinations compared with the first examination, while medial palmar subchondral bone plate thickness was significantly greater at the third examination (p=0.011). Bodyweight and bodyweight-to-height ratio were positively associated with the sagittal middle (p<0.001) and sagittal palmar measurements (p=0.005 and p=0.002, respectively). Stallions had significantly greater measurements than mares at the middle lateral (p=0.035) and medial (p=0.023), sagittal dorsal (p=0.030), and palmar lateral (p=0.024) and medial (p<0.001) measurement sites.

The results of our study will help accurate interpretation of CT studies of lame horses. Based on our findings, subchondral bone plate thickness of the proximal phalanx increased during training and racing, particularly in the palmar region. Subchondral bone plate thickness was also influenced by the horses’ body size and sex.