Interfascicular matrix-mediated transverse deformation and sliding of discontinuous tendon subcomponents control the viscoelasticity and failure of tendons.

In the present article, we investigated the sliding of discontinuous tendon subcomponents and the variation of nonhomogeneous deformation in the human Achilles tendon (AT) over time using uniaxial tensile and relaxation tests. The deformation and the resulting strain distribution under uniaxial tension are examined using a vision-based 3-D digital image correlation (DIC) system, which allows estimation of the strain field in the axial and lateral directions. Relaxation test under B-mode ultrasound imaging with the use of DIC method provides information about the local strain variation over time in the axial and anteroposterior directions. The observed nonhomogeneous deformation, a result from the twisted structure of the tendon, shows both compressive and tensile transverse strains that can generate interfascicular matrix (IFM) failure and initiate water accumulation in the course of tendinopathy. Moreover, using B-mode elastography with the DIC method, we have observed areas of low stiffness when the strain values exceed the strength limits, and this could correspond to IFM carrying the load between discontinuous tendon subcomponents. Thus, IFM carrying complex multiscale stresses may be responsible for the strength and viscoelastic properties of the AT. The results presented here reveal a new pathomechanism of AT failure. This could be useful in further studies on tendinopathy as well as effective planning of the AT therapy.