Magnetic Resonance Imaging of the Proximal
Metacarpal Region in Endurance Horses: Investigation of the Effect of Training

Background: Injury to the proximal palmar cortex of the third metacarpal bone (PcMcIII) and the suspensory ligament (SL) is common in endurance horses. Exercise-induced changes in the proximal metacarpal region have not been documented. 

Objectives: To describe magnetic resonance imaging (MRI) changes in the proximal PcMcIII and SL after 6 months of endurance training and competing in novice and experienced horses. 

Study design: Prospective descriptive study. 

Methods: Low-field MR images of both proximal metacarpal regions were acquired from six novice (never competed at >80km) and six experienced horses (completed ≥two 120 km rides), beforeand after 6 months of training and competing. Measurements were acquired from T1-weighted transverse MR images at four levels, including thickness of the proximal PcMcIII (at 25%, 50% and 75% of its mediolateral width) and mediolateral width and dorsopalmar depth of the entire SL and its lobes. Measurements obtained at the two examinations and in novice and experienced horses were compared with independent or paired t-tests. 

Results: The medial aspect of PcMcIII was thicker in experienced than in novice horses at 2 (p = 0.002; MD [mean difference] = 1.02 mm; 95% confidence interval [CI] 0.42-1.61) and 3 cm (p < 0.001; MD = 1.92 mm; 95%CI 0.71-1.87) distal to the carpometacarpal joint. The PcMcIII was thicker medially than laterally (p = 0.006; MD = 0.58 mm; 95%CI 0.17-0.99) and its thickness increased from proximal to distal (p < 0.001; MD = 2.41 mm; 95%CI 1.89-2.92). No significant difference in the SL measurements was observed. 

Main limitations: Small sample size and short study duration. Low resolution of MR images may limit detection of subtle changes. 

Conclusions: The medial aspect of PcMcIII was thicker in experienced than in novice endurance horses, reflecting the cumulative effect of long-term exercise and possibly age. Six months of endurance training were insufficient to induce low- field MRI changes in the PcMcIII or SL.

Ethical animal research: Approved by the Ethical and Animal Welfare Committee of the University of Veterinary Medicine Budapest. 

Informed consent: Informed owner consent was obtained. 

Competing interests: None. 

Sources of funding: This study was co-financed by the European Social Fund (grant agreement no. EFOP-3.6.3- VEKOP-16-2017-00005), and also supported by Hallmarq Veterinary Imaging. The National Research, Development and Innovation Office, Hungary, financed the equipment.

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