LAUSR

BACKGROUND Abnormalities in vascular channel appearance within the proximal sesamoid bone (PSB) are the most common findings in Thoroughbred yearling presale radiographs, and are often evaluated on radiographs of adult racehorses. Despite this, their pathogenesis and clinical significance are poorly understood, and associations with racing performance are inconsistent. OBJECTIVES To determine microstructural characteristics of the PSBs associated with the radiographic appearance of vascular channels using microcomputed tomography (µCT), and to determine associations with past racing performance in mature horses. STUDY DESIGN Cross-sectional. METHODS One pair of PSBs were isolated from a forelimb of 59 Thoroughbred racehorses undergoing post-mortem examination. Each PSB (n = 118) was radiographed, assigned a vascular channel grade using previously published and novel grading systems, then imaged using µCT. Associations between radiographic, µCT and performance variables were investigated with uni- and multivariable generalised linear models. RESULTS All PSBs had at least one vascular channel (mean 3.6 ± 0.89) observed on µCT originating from the abaxial border, yet in only 63.6% (75/118) were channels observed radiographically. Proximal sesamoid bones with a higher bone volume fraction (odds ratio [OR] 1.08; 95% confidence interval [CI] 1.01-1.15; P = 0.03) and wider channel diameter (mm) on µCT (OR 20.67; 95% CI 3.29-130.00; P = 0.001) were more likely to have vascular channels identified on radiographs. Greater radiographic channel number (OR 0.96; 95% CI 0.92-1.00; P = 0.04) and channel diameter (mm; OR 0.96; 95% CI 0.92-1.00; P = 0.04) were associated with fewer career placings. MAIN LIMITATIONS Radiographs of isolated bones avoided the normal superimposition of tissue encountered in the live horse. CONCLUSIONS The ability to identify vascular channels radiographically indicates widening of channels and densification of the PSB. More radiographic channels and greater channel diameter were associated with similar or poorer measures of past performance, suggesting that these changes are not desirable.