An investigation into the effects of the hierarchical structure of tendon fascicles on micromechanical properties

Abstract During physiological loading, a tendon is subjected to tensile strains in the region of up to 6 per cent. These strains are reportedly transmitted to cells, potentially initiating specific mechano-transduction pathways. The present study examines the local strain fields within tendon fascicles subjected to tensile strain in order to determine the mechanisms responsible for fascicle extension. A hierarchical approach to the analysis was adopted, involving micro and macro examination. Micro examination was carried out using a custom-designed rig, to enable the analysis of local tissue strains in isolated fascicles, using the cell nuclei as strain markers. In macro examination, a video camera was used to record images of the fascicles during mechanical testing, highlighting the point of crimp straightening and macro failure. Results revealed that local tensile strains within a collagen fibre were consistently smaller than the applied strain and showed no further increase once fibres were aligned. By contrast, between-group displacements, a measure of fibre sliding, continued to increase beyond crimp straightening, reaching a mean value of 3.9 per cent of the applied displacement at 8 per cent strain. Macro analysis displayed crimp straightening at a mean load of 1 N and sample failure occurred through the slow unravelling of the collagen fibres. Fibre sliding appears to provide the major mechanism enabling tendon fascicle extension within the rat-tail tendon. This process will necessarily affect local and cellular strains and consequently mechanotransduction pathways.