Structural gradients and strain partitioning across the mouse Achilles tendon enthesis revealed by in situ X-ray scattering
The enthesis is the insertion site of tendon into bone and exhibits a high mechanical durability despite the large mismatch in material properties between the two tissues. This durability stems from gradients in composition, structure and organization on multiple hierarchical length scales. Despite extensive research on enthesis structure and mechanics, the local deformation mechanisms are poorly understood. Synchrotron scanning small- and wide-angle X-ray scattering was combined with in situ tensile testing of the mouse Achilles tendon enthesis to extensively map the mechanical response of the collagen fibrils and molecules as well as the hydroxyapatite mineral particles and crystals. Gradients in nano- and molecular scale structure and a stronger and more immediate deformation response towards the interface compared to further away were observed in both the soft and mineralized tissue. The strain decreased progressively across hierarchical levels; with an applied tissue strain of 20% the nanoscale fibrils were strained by ~1-2%, the collagen molecules by ~0.5% and the hydroxyapatite crystals by ~0.05%, thus following an approximate ratio of 1 : 0.1 : 0.01 : 0.001. These results show that load transfer across the enthesis is both spatially heterogeneous and hierarchy-dependent. This indicates that the graded attachment accommodates deformation through region-specific load sharing and hierarchical strain partitioning, consistent with a contribution from dissipation within the non-collagenous matrix. In doing so, the enthesis can mitigate stress concentrations and maintain mechanical integrity across the tendon-to-bone transition.