Fragility of bone material controlled by internal interfaces

Bone material is built in a complex multiscale arrangement of mineralized collagen fibrils containing water, proteoglycans and some noncollagenous proteins. This organization is not static as bone is constantly remodeled and thus able to repair damaged tissue and adapt to the loading situation. In preventing fractures, the most important mechanical property is toughness, which is the ability to absorb impact energy without reaching complete failure. There is no simple explanation for the origin of the toughness of bone material, and this property depends in a complex way on the internal architecture of the material on all scales from nanometers to millimeters. Hence, fragility may have different mechanical origins, depending on which toughening mechanism is not working properly. This article reviews the toughening mechanisms described for bone material and attempts to put them in a clinical context, with the hope that future analysis of bone fragility may be guided by this collection of possible mechanistic origins

Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering

The springback effect during ambient pressure drying of aerogels is an interesting structural phenomenon, consisting of a severe shrinkage followed by almost complete re-expansion. The drying of gels causes shrinkage, whereas re-expansion is believed to be linked to repelling forces on the nanoscale. A multi-scale structural characterization of this significant volume change is key in controlling aerogel processing and properties. In this work, hydrophobic, monolithic silica aerogels with high specific surface areas were synthesized by modification with trimethylchlorosilane and ambient pressure drying. Here, we report a multi-method approach focusing on in-situ X-ray scattering to observe alterations of the nanostructured material during the drying of surface-modified and unmodified silica gels. Both show a porous fractal nanostructure, which partially collapses during drying and only recovers in surface-modified samples during the springback effect. Distinct changes of the X-ray scattering data were reproducibly associated with the shrinkage, re-expansion and drying of the gel network. Our findings may contribute to tailor aerogels with specific functionality, as the springback effect has a direct influence on properties (e.g., porosity, pore size distribution), which is directly affected by the degree of re-expansion

A 3D network of nanochannels for possible ion and molecule transit in mineralizing bone and cartilage

During crucial growth stages of vertebrate long bones, calcified cartilage beneath the growth plate is anchored to bone by a third mineralized component, the cement line. Proper skeletal development is contingent on the interplay of these three constituents, yet their mineralization processes and structural interactions are incompletely understood, in part from limited knowledge of their meso- and nanoscale features. Herein, focused ion beam-scanning electron microscopy (FIB-SEM) with serial surface imaging is applied to examine the cartilage–bone interface of mouse femoral heads at an unprecedented scale: FIB-SEM provides 3D, nanometer resolution of structural details for volumes encompassing metaphyseal calcified cartilage, bone, and the intervening cement line. A novel and complex structural network is revealed, comprising densely packed nanochannels smaller than bone canaliculi (≈10–50 nm diameter) within the calcified cartilage and bone extracellular matrices, but absent in the cement line. A structural correlation is demonstrated between the nanochannels and ellipsoidal mineral domains, which appear to coalesce during mineralization in a process analogous to powder sintering in metallurgy. A mineralization process is proposed, supported by energy-dispersive X-Ray spectroscopy of nanochannel contents, in which these unreported structures offer ion and molecule conduits to access the extracellular matrices of calcified cartilage and bone

Breast cancer–secreted factors perturb murine bone growth in regions prone to metastasis

Breast cancer frequently metastasizes to bone, causing osteolytic lesions. However, how factors secreted by primary tumors affect the bone microenvironment before the osteolytic phase of metastatic tumor growth remains unclear. Understanding these changes is critical as they may regulate metastatic dissemination and progression. To mimic premetastatic bone adaptation, immunocompromised mice were injected with MDA-MB-231–conditioned medium [tumor-conditioned media (TCM)]. Subsequently, the bones of these mice were subjected to multiscale, correlative analysis including RNA sequencing, histology, micro–computed tomography, x-ray scattering analysis, and Raman imaging. In contrast to overt metastasis causing osteolysis, TCM treatment induced new bone formation that was characterized by increased mineral apposition rate relative to control bones, altered bone quality with less matrix and more carbonate substitution, and the deposition of disoriented mineral near the growth plate. Our study suggests that breast cancer–secreted factors may promote perturbed bone growth before metastasis, which could affect initial seeding of tumor cells

Dendritic polyglycerol anions for the selective targeting of native and inflamed articular cartilage

The destruction of articular cartilage is a critical feature in joint diseases. An approach to selectively target the damaged tissue is promising for the development of diagnostic and therapeutic agents. We herein present the interaction of dendritic polyglycerol (dPG) anions with native and inflamed cartilage. Confocal laser scanning microscopy revealed the inert character of dPG and low functionalized dPG bisphosphonate (dPGBP7%) toward cartilage in vitro. An enhanced binding was observed for highly functionalized dPG bisphosphonate, sulfate, and phosphate, which additionally showed a higher affinity to IL-1β treated tissue. The mixed anion containing sulfate and bisphosphonate groups exhibited an exceptionally high affinity to cartilage and strongly bound to collagen type II, as shown by a normalized fluorescence-based binding assay. All polyglycerol anions, except dPGBP7%, were taken up by chondrocytes within 24 h and no cytotoxicity was found up to 10−5 M. In a rheumatoid arthritis model, dPGBP7% accumulated in mineralized compartments of inflamed joints and showed an increasing affinity to cartilage with higher clinical scores, as evident from histological examinations. For dPGS no interaction with bone but a strong binding to cartilage, independent of the score, was demonstrated. These results make dPG anions promising candidates for the selective targeting of cartilage tissue

Tomographic X‐ray scattering based on invariant reconstruction: analysis of the 3D nanostructure of bovine bone

Small-angle X-ray scattering (SAXS) is an effective characterization technique for multi-phase nanocomposites. The structural complexity and heterogeneity of biological materials require the development of new techniques for the 3D characterization of their hierarchical structures. Emerging SAXS tomographic methods allow reconstruction of the 3D scattering pattern in each voxel but are costly in terms of synchrotron measurement time and computer time. To address this problem, an approach has been developed based on the reconstruction of SAXS invariants to allow for fast 3D characterization of nanostructured inhomogeneous materials. SAXS invariants are scalars replacing the 3D scattering patterns in each voxel, thus simplifying the 6D reconstruction problem to several 3D ones. Standard procedures for tomographic reconstruction can be directly adapted for this problem. The procedure is demonstrated by determining the distribution of the nanometric bone mineral particle thickness (T parameter) throughout a macroscopic 3D volume of bovine cortical bone. The T parameter maps display spatial patterns of particle thickness in fibrolamellar bone units. Spatial correlation between the mineral nano­structure and microscopic features reveals that the mineral particles are particularly thin in the vicinity of vascular channels

The contribution of the pericanalicular matrix to mineral content in human osteonal bone

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.The osteocyte lacunar-canalicular network (LCN) penetrates bone and houses the osteocytes and their processes. Despite its rather low volume fraction, the LCN represents an outstanding large surface that is possibly used by the osteocytes to interact with the surrounding mineralized bone matrix thereby contributing to mineral homeostasis. The aim of this study was to quantitatively describe such contributions by spatially correlating the local density of the LCN with the mineral content at the same location in micrometer-sized volume elements in human osteons. For this purpose, 65 osteons from the femur midshaft from healthy adults (n = 4) and children (n = 2) were structurally characterized with two different techniques. The 3D structure of the LCN in the osteons was imaged with confocal laser scanning microscopy after staining the bone samples with rhodamine. Subsequent image analysis provided the canalicular length density, i.e. the total length of the canaliculi per unit volume (μm/μm3). Quantitative information on the mineral content (wt%Ca) from the identical regions was obtained using quantitative backscattered electron imaging. As the LCN-porosity lowers the mineral content, a negative correlation between Ca content and network density was expected. Calculations predict a reduction of around −0.97 fmol Ca per μm of network. However, the experiment revealed for 62 out of 65 osteons a positive correlation resulting in an average additional Ca loading of +1.15 fmol per μm of canalicular network, i.e. an accumulation of mineral has occurred at dense network regions. We hypothesize that this accumulation happens in the close vicinity of canaliculi forming mineral reservoirs that can be utilized by osteocytes. Significant differences found between individuals indicate that the extent of mineral loading of the reservoir zone reflects an important parameter for mineral homeostasis.German Federal Ministry of Education and ResearchAUVA (Research Funds of the Austrian Workers Compensation Board, Austria)WGKK (Viennese sickness insurance funds, Austria)

Heterogeneity of the osteocyte lacuno-canalicular network architecture and material characteristics across different tissue types in healing bone

Various tissue types, including fibrous connective tissue, bone marrow, cartilage, woven and lamellar bone coexist in healing bone. Similar to all bone tissue type, healing bone contains a lacuno-canalicular network (LCN) housing osteocytes that are known to orchestrate bone remodeling in healthy bone by sensing mechanical strains and translating them into biochemical signals. The structure of the LCN is also hypothesized to influence mineralization processes. Hence, the aim of the present study was to visualize and correlate spatial variations in the LCN topology with mineral characteristics, within and at the interfaces of the different tissue types that comprise healing bone. We applied a correlative multi-method approach to visualize the LCN architecture and quantify mineral particle size and orientation within healing femoral bone in a mouse osteotomy model (26 weeks old C57BL/6 mice). This approach revealed structural differences across several length scales during endochondral ossification within the following regions: calcified cartilage, bony callus, cortical bone and the transition zone between the cortical region and callus that developed during 21 days after the osteotomy. In this transition zone, we observed a continuous convergence of mineral characteristics and osteocyte lacunae shape as well as discontinuities in the lacunae volume and LCN connectivity. The bony callus exhibits a 34% higher lacunae number density with 40% larger lacunar volume compared to cortical bone. The presented correlations between LCN architecture and mineral characteristics improves our understanding of how bone develops during healing and may indicate a contribution of osteocytes to bone (re)modeling