Local adaptation of bone micro-structure and canal network to tendon insertion investigated by image-based micro-FE simulations

Tendons anchor to bone through a multi-material region called enthesis, showing several strategies to cope with the challenging task of joining dissimilar tissues. Less understood is whether bone microstructure also displays specific features to facilitate force transmission from tendon to bone. Such knowledge is of clinical relevance as local mechanical stresses play a crucial role in avulsion
fractures and interface pathologies. Here we characterized the microstructure of bone close to tendon insertion in calcanei of adult rats (n=5) with micro-computed tomography (SkyScan, Bruker; 5 to 1.2 micrometer voxel size). After aligning the virtual bones along the main axes of inertia, we investigated a bony tuberosity connecting the Achilles tendon with the plantar fascia ligament. We performed a spatially resolved analysis of trabecular microstructure, indicating that trabecular network (BV/TV and anisotropy) is not significantly influenced by the tendon insertion (P = 0.69). Conversely, high resolution images revealed that bone beneath the attachment region exhibits a highly oriented canal network, aligned with the pulling direction of the tendon (P b 0.005). Image-based micro-finite element analysis was used to calculate stresses within the bone induced by tendon loading. Simulations highlighted a non-trivial stress pattern within the bone with high stresses mainly flowing within the tuberosity (27% higher than in the rest of the bone, P b 0.005). Our work suggests that not only the enthesis but also the underlying bone is well-adapted to accommodate tendon loading.