# Objective, Longitudinal Computed Tomographic Evaluation of the Metacarpal Condyles in Non-Lame Thoroughbred Racehorses

## TL;DR

This study tracks bone density changes in racehorses' cannon bones over two years of training using CT scans to understand normal adaptations and improve lameness diagnosis.

## Contribution

Provides longitudinal, objective CT data on metacarpal condyle density changes in non-lame Thoroughbred racehorses during training.

## Key findings

- Bone density in metacarpal condyles increased with training, especially in the first six months.
- Dorsal regions and medial condyles showed higher density compared to palmar and lateral regions.
- Higher body weight:height ratio and race starts correlated with increased bone density.

## Abstract

Lesions in the lower end of the cannon bone are a major cause of lameness and catastrophic injuries in Thoroughbred racehorses. Computed tomography (CT) is increasingly used in diagnosis, but longitudinal and objective data in non-lame horses are scarce. Knowledge of the findings in non-lame horses is essential for interpretation of images in lame horses. Forty Thoroughbred yearlings were examined using CT at five time points over two years, starting before race training. Attenuation, reflecting bone density, in specific regions of the front cannon bones was measured objectively. Attenuation increased with time spent in training, particularly in the first six months, reflecting adaptation to exercise. Differences were observed among regions: condyles had higher density than parasagittal grooves; the front half was denser than the back; and the front half of the inside condyle was denser than the outside. Increased body weight:height ratio and the number of race starts were associated with higher bone density. These changes in bone density reflect modelling of the bone associated with training exercise. This study demonstrated that CT can be used to monitor bone adaptation. Understanding normal adaptive changes is important for distinguishing them from pathological changes and it aids accurate interpretation of images of lame horses.

There are limited data on sequential computed tomographic (CT) evaluation and objective CT assessment of the metacarpal condyles in Thoroughbred racehorses. This longitudinal study aimed to document changes in attenuation of the metacarpal condyles during the first two years of training and racing. Fan-beam CT examination of the metacarpophalangeal regions was performed on 40 non-lame Thoroughbred yearlings, and repeated four more times, approximately six months apart. Mean Hounsfield Unit (HU) measurements were obtained on sagittal reconstructions of the dorsal and palmar halves of the medial and lateral condyles and parasagittal grooves. One-way ANOVA with a post hoc Tukey’s Test was used to investigate differences between mean HU values over time at the different regions of interest. Multivariable mixed-effects linear regression models assessed the association between dorsal and palmar HU and potential explanatory variables. Mean HU increased significantly with training, especially during the first six months, with a maximal sequential mean increase found in the medial parasagittal groove (119.8 [95% confidence interval 85.3, 154.30], p < 0.001). Dorsal regions had higher HU than palmar regions, with the highest HU recorded in the dorsal aspect of the medial condyle at time 3 (mean HU 1120.1 ± 63.4). Condyles had higher HU than parasagittal grooves (p < 0.001), the palmar half of the right condyles had higher HU than the left (p = 0.045) and the dorsal aspect of the medial condyle had higher HU than the lateral (p < 0.001). An increasing number of race starts and higher body weight:height ratio were associated with higher HU (p < 0.001). The main limitation was the loss of horses to follow-up as the study progressed. In conclusion, density of most regions of the metacarpal condyles increased with time spent in training, reflecting adaption to racehorse training.