A longitudinal study of radiodensity and radiographic appearance of the proximal sesamoid bones in Thoroughbred racehorses.

Background: There are limited data on long-term follow-up of radiographic changes of the proximal sesamoid bones (PSBs) in Thoroughbred racehorses. The effect of training and racing on radiodensity of the PSBs in live Thoroughbreds has not been described.

Objectives: To follow radiographic changes of the PSBs in racehorses over 2 years and describe alterations in overall radiodensity.

Study design: Prospective cohort study.

Methods: Forty yearlings were enrolled at the first examination (T0). Re-examinations were performed four times, approximately 6 months apart, on 31 (T1), 23 (T2), 13 (T3) and 8 (T4) horses. Radiographic and fan-beam computed tomographic (CT) examinations of both metacarpophalangeal regions were performed. The PSBs were graded (0-3) based on the number of enlarged (width ≥2 mm) vascular channels. The presence of abaxial margin new bone formation, concavity, and irregularity were recorded. The mean Hounsfield Unit (HU) values of the PSBs were measured on multiplanar CT reconstructions. Multivariable mixed-effects regression models estimated the effect of horse signalment and radiographic appearance on mean HU (linear) and the association between radiographic appearance and horse signalment (logistic).

Results: Mean HU was lowest at T0 (1064.0 ± 65.1) and highest at T3 (1194.5 ± 78.6). Increasing mean HU was associated with an increasing number of total starts (p < 0.001), increasing bodyweight: height ratio (p < 0.001) and was higher in medial vs. lateral PSBs (p = 0.01). Abaxial new bone formation was associated with a higher grade (p = 0.004) and lateral PSBs (p = 0.04). Abaxial margin concavity was more likely in younger horses (p = 0.01), medial PSBs (p = 0.03) and the left forelimb (p = 0.03). Abaxial margin surface irregularity was more likely in younger horses (p = 0.02) and medial PSBs (p < 0.001).

Main limitations: Results may not be representative of all racehorse populations. A substantial number of horses were lost to follow-up. The PSBs were examined in isolation.

Conclusions: Increasing PSB radiodensity in racehorses was associated with the number of race starts and related training. Abaxial margin concavities and irregularities may be normal developmental features. Radiographic improvement of PSB grades is possible.