A Multi-modality Approach Towards Elucidation of the Mechanism for Human Achilles Tendon Bending During Passive Ankle Rotation

Ryuta Kinugasa,Mineko Fujimiya,Masaki Katayose,Keigo Taniguchi,Naoto Yamamura,Shantanu Sinha,Shu Takagi,V Reggie Edgerton

doi:10.1038/s41598-018-22661-7

Abstract

The in vitro unconstrained Achilles tendon is nearly straight, while in vivo experiments reveal that the proximal region of the Achilles tendon, adjacent to Kager’s fat pad, bends ventrally during plantarflexion but remains nearly straight during dorsiflexion. Tendon bending is an important factor in determining the displacement of the foot compared to the shortening of the muscle fibers. The objective of this study was to elucidate the various mechanisms that could cause tendon bending, which currently remain unknown. Examination of Thiel-embalmed cadavers, with preservation of native articular joint mobility, revealed that the Achilles tendon still bent ventrally even when its surrounding tissues, including the skin surface, Kager’s fat pad, and distal portions of the soleus muscle were removed. Shear modulus and collagen fiber orientation were distributed homogeneously with respect to the longitudinal line of the tendon, minimizing their causative contributions to the bending. Given that tendon bending is not caused by either the nature of the deformations of the tissues surrounding the Achilles tendon or its physical properties, we conclude that it results from the geometric architecture of the Achilles tendon and its configuration with respect to the surrounding tissues.

Highlights

In turn acts as an amplifier for joint excursion
Our most recent study[8] explained the creation of the curvature by reproducing it using sophisticated three-dimensional (3D) finite element modeling (FEM) and demonstrated that the fiber architecture of the triceps surae muscles, which play an important role in the final resultant motion of the muscle, impacted the magnitude and direction of Achilles tendon curvature during active plantarflexion
Since active contraction would cause complex tendon behavior resulting from fiber orientation-induced muscle deformation[8], paired with independent loading of each triceps surae muscle[11,12,13]; it was necessary to first simplify the problem and understand how the Achilles tendon bends when subjected to relatively simple passive movement, since even passive ankle rotation causes bending of the Achilles tendon[5]

Summary

Introduction

Using a simple geometrical model, this study showed that the mechanical consequence of such an obstruction would be to constrain the movement of the Achilles tendon and thereby amplify the displacement of the tendon’s end point compared to its start point This speculation was subsequently successfully verified and observed in vivo, as a curvature of the Achilles tendon was seen when using an MRI spin-tag technique[5]. The objective of this study, was to elucidate the role that the following factors play in causing the bending of the human Achilles tendon during passive ankle rotation: (1) the different tissues surrounding the Achilles tendon, including the superficial skin surface, the Kager’s fat pad, and the soleus muscle. Failure of the above mechanisms to explain the bending of the Achilles tendon would require us to consider the third and final possible factor, namely, the structural dynamics of the tendon upon ankle rotation as determined by its geometric architecture and its configuration with respect to the surrounding tissues

Objectives

Methods

Results

Discussion

Conclusion