Combining in silico and in vitro experiments to characterize the role of fascicle twist in the Achilles tendon

Vickie B Shim,David G Lloyd,Geoff G Handsfield,Thor F Besier,Justin W Fernandez

doi:10.1038/s41598-018-31587-z

Vickie B Shim, David G Lloyd + Show 3 more

Open Access

https://doi.org/10.1038/s41598-018-31587-z

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Abstract

The Achilles tendon (AT), the largest tendon in the human body has a unique structural feature, that is the fascicles in the AT display spiral twist. However, their functional and structural roles are still unclear. We used subject-specific computational models and tissue mechanical experiment to quantitatively characterize the role of fascicle twist in the Achilles tendon. Ten subject-specific finite element (FE) models of the Achilles tendon were developed from ultrasound images. Fascicle twist was implemented in these models using the material coordinate system available in our FE framework. Five different angles (0~60°) were implemented and material property optimization was performed for each of them (total 50 sets) using results from uniaxial stretch experiment. We showed that fascicle twist allows for even distribution of stress across the whole tendon, thus improving tissue strength. The predicted rupture load increased up to 40%. A number of connective tissues display similar fascicle twists in their structure. The resulting non-uniform strain distribution has been hypothesized as a primary factor in tissue degeneration and injuries. Therefore, our technique will be used to design biomechanically informed training and rehabilitation protocols for management of connective tissue injuries and degeneration.

Highlights

An interesting feature of the Achilles tendon is that it is actuated by three muscles, that is medial gastrocnemius, lateral gastrocnemius and soleus
We hypothesised that the presence of fascicle twist allows for a more even distribution of stress across the whole tendon under the differential forces from the triceps surae muscles, improving tissue strength
The fascicle twist angles implemented using the material coordinate system in our model are shown in Fig. 1, which shows the rotation of the fascicles in the tendon

Summary

Introduction

An interesting feature of the Achilles tendon is that it is actuated by three muscles, that is medial gastrocnemius, lateral gastrocnemius and soleus. Quantitative and systematic analysis of the role of fascicle twist in the Achilles tendon has not been conducted, and there is currently no definitive answer to the question of why the human Achilles is twisted and how this twist influences tendon behavior This knowledge is important as recent studies have indicated that the nature of tendon deformation, heterogeneous strain distributions, may contribute to the development of overuse injuries and tendinopathy[14,15]. We hypothesised that the presence of fascicle twist allows for a more even distribution of stress across the whole tendon under the differential forces from the triceps surae muscles, improving tissue strength Another important structural feature in the Achilles tendon is the sliding between sub-tendons[16]. We hypothesize that CSA would have greater effects on tendon rupture load than either fascicle twist or tissue stiffness

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