Adaptation to achilles tendon insertion in human calcaneus bone sample

Abstract

The attachment of soft tissues to bone occurs through a complex multi-tissue region called enthesis. Entheses have the challenging task to allow a smooth transmission of forces from very dissimilar tissues. Indeed, soft tissues such as tendons differ from bone not only in material properties, but also in composition and structure. Many remarkable adaptation strategies are present to manage this intricate mechanical environment, including the presence of a thin layer of fibrocartilage. However, despite the numerous joint pathologies involving the enthesis, there are still a lot of unknown about this region, especially regarding the behaviour of bone beneath the insertion of the tendon. Building on previous research on enthesis in rodents, this thesis proposes a framework for analysing the structural adaptations of bone at the enthesis, focusing on the Achilles tendon-calcaneus system in human samples. Our project is based on a combination of clinical and research X-ray tomographs allowing for different resolutions and fields of view. Our aims are to provide workflows to investigate human calcaneus morphology using high resolution peripheral quantitative computed tomography combined with micro-computed tomography, as well as to compare adaptation strategies at the same location in humans and rats. First, high resolution peripheral quantitative computed tomography provided an initial overview of calcaneal microarchitecture. Following a specific image segmentation protocol based on standard filters and morphological operators, a spatially resolved study of bone architecture was performed. The results revealed notable adaptations in both bone density and orientation within the trabecular bone beneath the enthesis. Indeed, despite an heterogenous distribution of local bone density within the calcaneus, even with regions showing very low values, in all samples the bone density at the insertion region seemed to be preserved, and trabeculae were shown to be thicker than away from the insertion, and to align preferentially towards the direction of the tendon. Secondly, a localised examination of the enthesis and periosteal regions was conducted exploiting microcomputed tomography. The observed global anisotropy in trabecular orientation was confirmed through a local analysis involving the skeletonisation of the trabecular network, enabling to assess the morphology of each trabecula individually. Moreover, this characterization highlighted that the longer were the trabeculae, the higher was their alignment. These findings suggest that, similarly to rodents, human calcaneus bone is showing specific and adaptive three-dimensional microstructure at the insertion of the Achilles tendon. Interestingly, where rodents exhibit anisotropy in vascular and fibre structure, in humans, this adaptation occurs at a higher length scale, through the alignment of the trabeculae. Further investigations should extend these observations to populations with varying ages and genders, as well as joint pathologies, in order to validate and generalise the observed strategies.