Veterinary session

Background: Proximal palmar metacarpal pain, associated with proximal suspensory desmitis and/or stress pathology of the proximal palmar cortex of the third metacarpal bone (McIII), is one of the most common causes of lameness in endurance horses. However, the effect of exercise on these structures remains poorly elucidated.

Objectives: The main aim of this study was to objectively describe training-related adaptive changes in the proximal palmar cortex of McIII and in the proximal aspect of the suspensory ligament (SL) of novice and experienced endurance horses, before and after six months of training.

Materials and Methods: Six novice and six experienced non-lame endurance horses with no history of proximal metacarpal pain were selected for the study. Clinical and magnetic resonance imaging (MRI) examinations were performed twice, approximately six months apart. Between examinations horses received endurance training appropriate to their age and competition level. All measurements were made at 2, 3, 5 and 7 cm distal to the level of the carpometacarpal joint on transverse T1-weighted gradient echo images. The thickness of the palmar cortex of McIII was measured at 25, 50 and 75% of the lateromedial width of the palmar cortex. Measurements of the lateromedial width and dorsopalmar depth of the SL and its lobes were also obtained.

Paired samples t-test and independent samples t-test or their non-parametric equivalents were used to test for difference in the thickness of the palmar cortex of McIII, the lateromedial width and dorsopalmar depth of the SL and its lobes between novice and experienced horses, pre- and post season measurements and horses of different age categories.

Results: The medial aspect of the palmar cortex of McIII was significantly thicker in experienced than in novice horses at 2 and 3 cm distal to the carpometacarpal joint (p<0.05). Horses of ≥8 years of age also had a thicker medial palmar cortex of McIII than younger horses at these levels (p<0.05). There was no statistically significant difference between pre- and post-season measurements of the thickness of the palmar cortex of McIII in any location. No significant changes were observed in lateromedial width and dorsopalmar depth of the SL or its lobes, between pre and post season measurements, novice and experienced horses or between different age categories.

Conclusions: Low-field MRI was unable to detect objective changes in the proximal palmar cortex of McIII or the proximal SL following six months of endurance training and competition. However, the significant difference in the proximal palmar cortex measurements between novice and experienced horses gives evidence of long-term effect of exercise. Further studies on a larger number of horses and using a more sensitive diagnostic imaging method are indicated to detect more subtle changes induced by exercise over short period of time.