Development and clinical significance of side bones in cold-blooded trotters

Sammanfattning: Ossification of ungular cartilages (OUC) is apparent on fossil P3 bones and seems to be as old as the history of Equus. OUC has been scientifically studied at least since the 18th century. Traditional diagnostic techniques, palpation and pathology, have more recently been replaced by radiographic techniques, scintigraphy and magnetic resonance imaging. However, these new techniques have not yet been fully validated for OUC and have added little to the current understanding of how the ossification process arises, develops and influences performance in a population perspective. Based on results from studies using palpation as a diagnostic tool, stallions with a high degree of OUC are not allowed to breed and material from horses with OUC cannot be stored in gene banks. In this thesis, different grades of OUC in 649 cold-blooded trotters, with performance parameters from 23,556 races, were studied and compared. The results revealed significant relationships between gender and performance, but not between different grades of OUC and performance. The proximal ossification process of OUC decreased significantly after three years of age, when only a few of 2,591 cartilages examined changed their grade of OUC. An improved grading system to achieve more consistent and reliable assessment of radiological findings was devised. This system, encompassing four grades instead of six, does not use navicular bone and palmar level of distal interphalangeal joint as reference points. It is thus more forgiving of individual anatomical variations in P3 and of slightly uneven positioning/loading of hooves at time of exposure. Overall, the results suggest that OUC is either a physiological variation or an adaptation to unknown stimuli early in life, as many years of intense training and racing appeared not to affect the presence or development of OUC, confirming previous reports of moderate to high heritability. Based on these findings, excluding stallions with high OUC grades will not improve equine welfare. However, the influence of environmental factors before three years of age on the extent of OUC warrants future study.

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