Soft tissues such as tendons or ligaments are attaching to bone thanks to a multi-material interface exhibiting gradients in composition, structure, and mechanical components. This complex region is called enthesis and is able to efficiently transmit stresses between two materials displaying a mismatch in material properties of at least two orders of magnitude.

Musculoskeletal injuries are frequent and whenever they require the reattachment of soft tissue to bone, the failure rate is high, due the lack of integrative solutions for the repair of entheses. Therefore, an improvement in our understanding of mechanisms allowing those interfacial regions to reach their efficiency is still required for clinical purposes, but also regarding the development of biomimetic strategies for engineering practise. This thesis proposes a new approach for the investigation of entheses functioning. Research has been mainly focused on understanding composition and structure of the interface, which is a very tiny region of about 500 [µm]. Our project aims at studying structural and mechanical adaptation of the bone beneath the insertion, with the main assumption that this region plays a pivotal role for facilitating the force transmission from tendon to bone. Our work is based on high resolution micro-computed tomography images of calcaneus bone of rats.

Bone morphology is first assessed by characterizing three features:​ bone porosity, anisotropy and the roughness at the surface. This analysis teaches us that porosity is gradually increasing from the insertion site to the interior of the bone. This gradual increase seems to be more controlled than when considering another location on the bone surface far from the insertion point. We also quantify that the insertion region shows significantly more anisotropy than the rest of the bone:​ bone micro-architecture seems to be well align with the loading coming from the tendon. Finally, we prove that the bone surface at the insertion is characterized by a significantly higher roughness than other locations. A higher surface roughness is known to increase the toughness of bimaterial interfaces.

Second, we develop a microstructural finite element model of the Achilles tendon and bone in order to evaluate biomechanical behavior arising from the interfacing of such dissimilar materials. It turns out that the transmission of the stresses from the tendon results in a non-trivial pattern within the bone, generating compression regions aligned with the trabeculae of the growth plate, and tension regions perpendicular to those ones. Tension regions are also found to be aligned with the micro-architecture located directly beneath the insertion, corroborating assumptions of a "line-of-force" between Achilles tendon insertion and the insertion of the plantar fascia located at the bottom of the foot. We also show that the material mismatch generates high, but localized, stresses at the interface. Finally, we demonstrate that the biological insertion is robust towards changes in the orientation of the force that may occur during the physiological range of motion. This concept is demonstrated by conducting a "thought" experiment consisting in comparing the behavior at the biological insertion with a "wrong" insertion located at the top of the bone.