An equine tendon model for studying intra-tendinous shear in tendons that have more than one muscle contribution

Human Achilles tendon is composed of three smaller sub-tendons and exhibits non-uniform internal displacements, which decline with age and after injury, suggesting a potential role in the development of tendinopathies. Studying internal sliding behaviour is therefore important but difficult in human Achilles tendon. Here we propose the equine deep digital flexor tendon (DDFT) and its accessory ligament (AL) as a model to understand the sliding mechanism. The AL-DDFT has a comparable sub-bundle structure, is subjected to high and frequent asymmetric loads and is a natural site of injury similar to human Achilles tendons. Equine AL-DDFT were collected and underwent whole tendon level (n=7) and fascicle level (n=7) quasi-static mechanical testing. Whole tendon level testing was performed by sequentially loading through the proximal AL and subsequently through the proximal DDFT and recording regional strain in the free structures and joined DDFT and AL. Fascicle level testing was performed with focus on the inter-sub-bundle matrix between the two structures at the junction. Our results demonstrate a significant difference in the regional strain between the joined DDFT and AL and a greater transmission of force from the AL to the DDFT than vice versa. These results can be partially explained by the mechanical properties and geometry of the two structures and by differences in the properties of the interfascicular matrices. In conclusion, this tendon model successfully demonstrates that high displacement discrepancy occurs between the two structures and can be used as an easy-access model for studying intra-tendinous shear mechanics at the sub-tendon level. Our study provides a naturally occurring and easily accessible equine model to study the complex behaviour of sub-tendons within the human Achilles tendon, which is likely to play a critical role in the pathogenesis of tendon disease. Our results demonstrate that the difference in material stiffness between the equine AL and DDFT stems largely from differences in the inter-fascicular matrix and furthermore that differences in strain are maintained in distal parts of the tightly joined structure. Furthermore, our results suggest that distribution of load between sub-structures is highly dependent on the morphological relationship between them; a finding that has important implications for understanding Achilles tendon mechanical behaviour, injury mechanisms and rehabilitation.