Biomechanics of the tendon-bone interface
Project description
Joining dissimilar materials is a challenging and frequent requirement in nature as well as in engineering. Thanks to evolution and natural selection through millions of years, biological materials have probed different strategies to fulfill such task, hence becoming a source of inspiration and curiosity for a broad range of researchers. A fascinating example actually lies in the human body and is called the enthesis. This region is essential for the musculoskeletal system, ensuring efficient transitions at the interfaces between soft tissues and bones, often through a layer of fibrocartilage. Entheses therefore have a crucial role for the healthy functioning of our joints, yet they are much less investigated than single individual tissues such as tendon or bone. Limited regeneration abilities leading to poor clinical outcomes in case of reattachment surgeries, combined with injuries, inflammation and degenerative changes make entheses highly relevant in the clinical context.
In this project, we propose a multimodal correlative investigation of the tendon-bone interface, with the global aim to enrich the understanding of enthesis structure and properties. As biological system, the Achilles tendon insertion into calcaneus bone of adult rats is analyzed. This anatomical location does not only feature fibrocartilage at the enthesis, anchoring tendon to bone, but also another fibrocartilage, called periosteal, facilitating tendon sliding. The two tissues are adjacent but sustain distinct mechanical loading, and are therefore a valid model system to explore the impact of the mechanical environment on tissue properties at different hierarchical level.
Local anisotropy in mineralized fibrocartilage and subchondral bone beneath the tendon-bone interface
Micro-computed tomography (micro-CT) at two different length scales, combined with histology and electron microscopy imaging, highlight specificities of tissue organization, microstructure and microporosity at enthesis and periosteal regions. We show that the insertion of the tendon occurs at a dedicated bony protrusion that also allows a direct loading transfer from the tendon to the plantar fascia without the involvement of other regions of the calcaneus bone. A strong structural anisotropy of bone vascular channels can be observed within the tuberosity, which is also a characteristic feature of fibrochondrocyte lacunae, forming highly aligned columns. At the enthesis, a high roughness of the interface connecting unmineralized to mineralized fibrocartilage is additionally observed. Periosteal fibrocartilage exhibits very different features: besides reduced waviness at the mineralizing interface, the tissue is more heterogeneous and displays an isotropic channel network together with randomly arranged fibrochondroycte lacunae.
Structural and functional heterogeneity of mineralized fibrocartilage at the Achilles tendon-bone insertion
To further explore structure-property relationships within enthesis and periosteal fibrocartilage at the material level, we combined nanoindentation with second harmonic generation (SHG) and quantitative backscattered electron imaging (qBEI), allowing to spatially correlate mechanical properties with mineralization and fibrous matrix organization. SHG images show that microstructural anisotropy is mirrored into highly aligned fibers at the insertion, further impacting the local mechanical behavior. Indeed, anisotropic regions are associated with a close spatial correspondence between mineral content and tissue modulus and a remarkably constant spatial profile within mineralized fibrocartilage towards bone. Those findings suggest that the enthesis fibrocartilage should provide high strength to sustain the tensile loading of the tendon. The more isotropic porosity of periosteal area is also reflected in the arrangement of the collagenous matrix. Indeed, the tissue displays an intricate fiber organization which results in a gradual increase in tissue modulus, probably to resist compressive loading. Despite their distinct spatial modulation of mechanical properties, both fibrocartilages are less mineralized than bone and yet quite efficient as they attain their stiffness with less calcium that bone would require. Finally, qBEI images show a considerable entanglement between fibrocartilage and bone at the interface to increase anchoring ability.
Bone-fibrocartilage crosstalk and osteocyte lacuno-canalicular behavior at the tendon-bone insertion
Confocal laser scanning microscope on stained samples allows to visualize and investigate the osteocyte lacuno-canalicular network behavior close to the interface with fibrocartilage. As staining infiltrates throughout all the accessible pores, it can also be exploited (in combination with previously used techniques such as micro-CT, qBEI and SHG) to track communication pathways between bone and mineralized fibrocartilage.
Contacts
Related publications
📃 A. Tits, D. Ruffoni*, Joining soft tissues to bone: insights from modeling and simulations. Bone Reports 2020; 100742. [PDF]
📃 A. Tits, E. Plougonven, S. Blouin, M. A. Hartmann, J-F Kaux, P. Drion, J. Fernandez, G.H. van Lenthe, D. Ruffoni*. Local anisotropy in mineralized fibrocartilage and subchondral bone beneath the tendon-bone interface. Scientific Reports 2021; 11:16534. [PDF]
Collaborations
👥 Jean-François Kaux (Department of Physical Medicine and Sports Traumatology, University of Liège)
👥 Erwan Plougonven (Chemical Engineering Department, University of Liege, Belgium)
👥 Pierre Drion (Department of Biomedical and Preclinical Sciences, University of Liege, GIGA, Belgium)
👥 Peter Varga (AO Research Institute Davos, Switzerland)
👥 G. Harry Van Lenthe (Department of Mechanical Engineering, KU Leuven, Belgium)
👥 Stéphane Blouin (Ludwig Boltzmann Institute of Osteology, Hanusch Hospital of OEGK and AUVA Trauma Centre Meidling, 1st Medical Department Hanusch Hospital, Vienna, Austria)
👥 Markus A. Hartmann (Ludwig Boltzmann Institute of Osteology, Hanusch Hospital of OEGK and AUVA Trauma Centre Meidling, 1st Medical Department Hanusch Hospital, Vienna, Austria)
👥 Maximilian Rummler (Max Planck Institute of Colloids and Interfaces, Potsdam, Germany)
👥 Richard Weinkamer (Max Planck Institute of Colloids and Interfaces, Potsdam, Germany)
