The Micromechanical Environment of the Impinged Achilles Tendon Insertion
Mechanical deformation applied to tendon at the tissue-scale is transferred to the microscale -- including the extracellular matrix (ECM), the pericellular matrix (PCM), the cell and the nucleus -- through a process known as strain transfer. Microscale strains, in turn, trigger biological activity that plays an important role in the maintenance of tendon phenotype and homeostasis. Although tendon predominantly experiences longitudinal tensile forces, transverse forces due to bony impingement have been implicated in both physiological (e.g., maintenance of the tendon insertion) and pathophysiological (e.g. insertional Achilles tendinopathy) processes. However, to our knowledge, prior studies have not characterized the micromechanical strain environment in the context of tendon impingement. Therefore, the objective of this study was to characterize the micromechanical strain environment in the impinged Achilles tendon insertion using a novel mouse hindlimb explant model in combination with finite element (FE) modeling. We hypothesized that impingement would generate large magnitudes of transverse compressive strain at the local matrix, PCM, and cell scales. Mouse hindlimb explants were imaged on a multiphoton microscope, and image stacks of the same population of tendon cells were obtained at the Achilles tendon insertion before and after dorsiflexion-induced impingement. Using an innovative multiphoton elastography approach, three-dimensional Green-Lagrange and principal strains were measured at the matrix scale, while longitudinal strain and aspect ratio were measured at the PCM and cell scales. Our results demonstrate that impingement generated substantial transverse compression at the matrix-scale, which led to longitudinal stretching of cells, an increase in cell aspect ratio, and -- surprisingly -- longitudinal compression of the tendon PCM. These experimental results were corroborated by an FE model developed to simulate the micromechanical environment in impinged regions of the Achilles tendon. Moreover, in both experiments and simulations, impingement-generated microscale stresses and strains were highly dependent on initial cell-cell gap spacing. Understanding the factors that influence the microscale strain environment generated by impingement could contribute to a more mechanistic understanding of impingement-induced tendinopathies and inform the development of approaches that disrupt the progression of pathology.