Multimodal and multiscale characterization of bone and bone interfaces in health and disease

Abstract: Seeing is believing. Our understanding of phenomena often involves their direct observation. However, bone architecture is challenging to visualize given its multi-level hierarchical organization. In this thesis, bone and bone interfaces are characterized via multimodal and multiscale platforms, combining different techniques across several length scales. Imaging techniques across the micro-nano continuum are complemented by spectroscopy methods to explore, respectively, the structure and composition of bone and bone interfaces, using both light and electron probes. By applying a characterization methodology more typical of materials science, this thesis aims to unveil structural and compositional abnormalities of bone induced by disease [Papers I-II], and bone response to functionalized biomaterials in compromised conditions [Papers III-IV]. Additionally, it expands three-dimensional (3D) characterization opportunities at the nanoscale in both native and peri-implant bone [Papers V-VI]. This characterization approach uncovered changes in bone quality (structure and/or composition) in the compromised conditions under investigation in this thesis, i.e., leptin receptor (LepR) deficiency and medication-related osteonecrosis of the jaw (MRONJ) [Papers I-II]. In a preclinical model of LepR deficiency for type 2 diabetes/obesity, multimodal characterization of bone at the microscale showed structural abnormalities indicative of delayed skeletal development, despite unaffected bone matrix composition [Paper I]. A combination of multiscale imaging and spectroscopy techniques spanning the micro-to-nanoscale enabled a detailed study of the interface between necrotic bone and bacteria in a case of MRONJ, shedding light on possible mechanisms of bone degradation. When applied to bone-biomaterial interfaces, the application of a multimodal and multiscale characterization workflow informed perspectives on bone response to novel biomaterial solutions aimed to promote osseointegration in osteoporotic conditions via local drug delivery of phytoestrogens [Paper III] or anabolic agents [Paper IV]. This highlighted the importance of studying peri-implant bone at the mesoscale [Paper III] and of confirming biomaterial behaviour in vivo in the presence of surface functionalization [Paper IV]. Lastly, this thesis emphasized the importance of 3D imaging at the nanoscale with electron tomography to resolve bone ultrastructure at biomaterial interfaces [Paper V] and in native conditions [Paper VI]. Specifically, in Paper VI, artifact-free on-axis electron tomography resolved some long-debated aspects regarding the organization of mineralized collagen fibrils, the fundamental building block units of bone.