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

The research conducted a comparative study using equine deep digital flexor tendon (DDFT) as a model to understand the internal sliding mechanism of the human Achilles tendon. It explored the significant differences in strain among its two joined structures, which could be a factor in the development of tendon diseases.

Research Objective and Model

• The study aimed to understand the internal sliding behavior or intra-tendinous shear mechanism of the human Achilles tendon, primarily composed of three smaller sub-tendons.
• Given the difficulty of directly studying the human Achilles tendon, the researchers made use of the equine deep digital flexor tendon (DDFT) and its accessory ligament (AL) as the model. The horse’s DDFT and AL were chosen because they have a comparable sub-bundle structure, are subjected to intense and frequent asymmetric loads, and are natural sites of injury similar to human Achilles tendons.

Methods and Procedures

• Whole tendon and fascicle level quasi-static mechanical tests were conducted on collected equine AL-DDFT.
• The researchers performed whole tendon level testing by sequentially loading through the proximal AL and then the proximal DDFT, recording the regional strain in the free structures and the connected DDFT and AL.
• The fascicle level test was performed focusing on the inter-sub-bundle matrix between the two structures at the junction point.

Results and Conclusions

• The results showed a significant difference in the regional strain between the connected DDFT and AL.
• A higher transmission of force was observed from the AL to the DDFT than vice versa, partially explained by the mechanical properties and geometry of the two structures and the differences in the properties of the interfascicular matrices.
• Therefore, this tendon model effectively showed that a high displacement discrepancy occurs between the two structures.
• The study concluded that the equine tendon model can serve as an accessible and practical model to study intra-tendinous shear mechanics at the sub-tendon level.

Significance of the Study

• The researchers emphasized the potential role of their model in further understanding the complex behavior of sub-tendons within the human Achilles tendon.
• This understanding can play a critical role in the pathogenesis of tendon disease, given the proven differences in the material stiffness between the equine AL and DDFT that derive largely from differences in the inter-fascicular matrix.
• Furthermore, this study suggested that the load distribution between sub-structures is largely dependent on their morphological relationship, giving valuable insights into Achilles tendon mechanical behaviour, injury mechanisms, and rehabilitation.