Multiscale alterations in bone matrix quality increased fragility in steroid induced osteoporosis

Utility and Limitations of TALLYHO/JngJ as a Model for Type 2 Diabetes-Induced Bone Disease

Type 2 diabetes mellitus (T2DM) increases risk of fractures due to bone microstructural and material deficits, though the mechanisms remain unclear. Preclinical models mimicking diabetic bone disease are required to further understand its pathogenesis. The TALLYHO/JngJ (TH) mouse is a polygenic model recapitulating adolescent-onset T2DM in humans. Due to incomplete penetrance of the phenotype ~25% of male TH mice never develop hyperglycemia, providing a strain-matched nondiabetic control. We performed a comprehensive characterization of the metabolic and skeletal phenotype of diabetic TH mice and compared them to either their nondiabetic TH controls or the recommended SWR/J controls to evaluate their suitability to study diabetic bone disease in humans. Compared to both controls, male TH mice with T2DM exhibited higher blood glucose levels, weight along with impaired glucose tolerance and insulin sensitivity. TH mice with/without T2DM displayed higher cortical bone parameters and lower trabecular bone parameters in the femurs and vertebrae compared to SWR/J. The mechanical properties remained unchanged for all three groups except for a low-energy failure in TH mice with T2DM only compared to SWR/J. Histomorphometry analyses only revealed higher number of osteoclasts and osteocytes for SWR/J compared to both groups of TH. Bone turnover markers procollagen type 1 N-terminal propeptide (P1NP) and tartrate-resistant acid phosphatase (TRAP) were low for both groups of TH mice compared to SWR/J. Silver nitrate staining of the femurs revealed low number of osteocyte lacunar and dendrites in TH mice with T2DM. Three-dimensional assessment showed reduced lacunar parameters in trabecular and cortical bone. Notably, osteocyte morphology changed in TH mice with T2DM compared to SWR/J. In summary, our study highlights the utility of the TH mouse to study T2DM, but not necessarily T2DM-induced bone disease, as there were no differences in bone strength and bone cell parameters between diabetic and non-diabetic TH mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Characterization of three-dimensional bone-like tissue growth and organization under influence of directional fluid flow

The transition in the field of bone tissue engineering from bone regeneration to in vitro models has come with the challenge of recreating a dense and anisotropic bone-like extracellular matrix (ECM). Although the mechanism by which bone ECM gains its structure is not fully understood, mechanical loading and curvature have been identified as potential contributors. Here, guided by computational simulations, we evaluated cell and bone-like tissue growth and organization in a concave channel with and without directional fluid flow stimulation. Human mesenchymal stromal cells were seeded on donut-shaped silk fibroin scaffolds and osteogenically stimulated for 42 days statically or in a flow perfusion bioreactor. After 14, 28, and 42 days, constructs were investigated for cell and tissue growth and organization. As a result, directional fluid flow was able to improve organic tissue growth but not organization. Cells tended to orient in the tangential direction of the channel, possibly attributed to its curvature. Based on our results, we suggest that organic ECM production but not anisotropy can be stimulated through the application of fluid flow. With this study, an initial attempt in three-dimensions was made to improve the resemblance of in vitro produced bone-like ECM to the physiological bone ECM.

Mineralized Collagen Fibrils: An Essential Component in Determining the Mechanical Behavior of Cortical Bone

Bone comprises mechanically different materials in a specific hierarchical structure. Mineralized collagen fibrils (MCFs), represented by tropocollagen molecules and hydroxyapatite nanocrystals, are the fundamental unit of bone. The mechanical characterization of MCFs provides the unique adaptive mechanical competence to bone to withstand mechanical load. The structural and mechanical role of MCFs is critical in the deformation mechanisms of bone and the marvelous strength and toughness possessed by bone. However, the role of MCFs in the mechanical behavior of bone across multiple length scales is not fully understood. In the present study, we shed light upon the latest progress regarding bone deformation at multiple hierarchical levels and emphasize the role of MCFs during bone deformation. We propose the concept of hierarchical deformation of bone to describe the interconnected deformation process across multiple length scales of bone under mechanical loading. Furthermore, how the deterioration of bone caused by aging and diseases impairs the hierarchical deformation process of the cortical bone is discussed. The present work expects to provide insights on the characterization of MCFs in the mechanical properties of bone and lays the framework for the understanding of the multiscale deformation mechanics of bone.

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

Collagen pre-strain discontinuity at the bone—Cartilage interface

The bone-cartilage unit (BCU) is a universal feature in diarthrodial joints, which is mechanically-graded and subjected to shear and compressive strains. Changes in the BCU have been linked to osteoarthritis (OA) progression. Here we report existence of a physiological internal strain gradient (pre-strain) across the BCU at the ultrastructural scale of the extracellular matrix (ECM) constituents, specifically the collagen fibril. We use X-ray scattering that probes changes in the axial periodicity of fibril-level D-stagger of tropocollagen molecules in the matrix fibrils, as a measure of microscopic pre-strain. We find that mineralized collagen nanofibrils in the calcified plate are in tensile pre-strain relative to the underlying trabecular bone. This behaviour contrasts with the previously accepted notion that fibrillar pre-strain (or D-stagger) in collagenous tissues always reduces with mineralization, via reduced hydration and associated swelling pressure. Within the calcified part of the BCU, a finer-scale gradient in pre-strain (0.6% increase over ~50μm) is observed. The increased fibrillar pre-strain is linked to prior research reporting large tissue-level residual strains under compression. The findings may have biomechanical adaptative significance: higher in-built molecular level resilience/damage resistance to physiological compression, and disruption of the molecular-level pre-strains during remodelling of the bone-cartilage interface may be potential factors in osteoarthritis-based degeneration.

Nanoscale Imaging and Analysis of Bone Pathologies

Understanding the properties of bone is of both fundamental and clinical relevance. The basis of bone’s quality and mechanical resilience lies in its nanoscale building blocks (i.e., mineral, collagen, non-collagenous proteins, and water) and their complex interactions across length scales. Although the structure–mechanical property relationship in healthy bone tissue is relatively well characterized, not much is known about the molecular-level origin of impaired mechanics and higher fracture risks in skeletal disorders such as osteoporosis or Paget’s disease. Alterations in the ultrastructure, chemistry, and nano-/micromechanics of bone tissue in such a diverse group of diseased states have only been briefly explored. Recent research is uncovering the effects of several non-collagenous bone matrix proteins, whose deficiencies or mutations are, to some extent, implicated in bone diseases, on bone matrix quality and mechanics. Herein, we review existing studies on ultrastructural imaging—with a focus on electron microscopy—and chemical, mechanical analysis of pathological bone tissues. The nanometric details offered by these reports, from studying knockout mice models to characterizing exact disease phenotypes, can provide key insights into various bone pathologies and facilitate the development of new treatments.

Decellularized pericardium tissues at increasing glucose, galactose and ribose concentrations and at different time points studied using scanning X-ray microscopy

Diseases like widespread diabetes or rare galactosemia may lead to high sugar concentrations in the human body, thereby promoting the formation of glycoconjugates. Glycation of collagen, i.e. the formation of glucose bridges, is nonenzymatic and therefore cannot be prevented in any other way than keeping the sugar level low. It relates to secondary diseases, abundantly occurring in aging populations and diabetics. However, little is known about the effects of glycation of collagen on the molecular level. We studied in vitro the effect of glycation, with d-glucose and d-galactose as well as d-ribose, on the structure of type 1 collagen by preparing decellularized matrices of bovine pericardia soaked in different sugar solutions, at increasing concentrations (0, 2.5, 5, 10, 20 and 40 mg ml-1), and incubated at 37°C for 3, 14, 30 and 90 days. The tissue samples were analyzed with small- and wide-angle X-ray scattering in scanning mode. We found that glucose and galactose produce similar changes in collagen, i.e. they mainly affect the lateral packing between macromolecules. However, ribose is much faster in glycation, provoking a larger effect on the lateral packing, but also seems to cause qualitatively different effects on the collagen structure.

A multiscale study of structural and compositional changes in a natural nanocomposite: Osteoporotic bone with chronic endogenous steroid excess

Glucocorticoid (or steroid) induced osteoporosis (GIOP) is the leading form of secondary osteoporosis, affecting up to 50% of patients receiving chronic glucocorticoid therapy. Bone quantity (bone mass) changes in GIOP patients alone are inadequate to explain the increased fracture risk, and bone material changes (bone quality) at multiple levels have been implicated in the reduced mechanics. Quantitative analysis of specific material-level changes is limited. Here, we combined multiscale experimental techniques (scanning small/wide-angle X-ray scattering/diffraction, backscattered electron imaging, and X-ray radiography) to investigate these changes in a mouse model (Crh^-120/+) with chronic endogenous steroid production. Nanoscale degree of orientation, the size distribution of mineral nanocrystals in the bone matrix, the spatial map of mineralization on the femoral cortex, and the microporosity showed significant changes between GIOP and the control, especially in the endosteal cortex. Our work can provide insight into the altered structure-property relationship leading to lowered mechanical properties in GIOP. SIGNIFICANCE STATEMENT: As a natural nanocomposite with a hierarchical structure, bone undergoes a staggered load transfer mechanism at the nanoscale. Disease and age-related deterioration of bone mechanics are caused by changes in bone structure at multiple length scales. Although clinical tools such as dual-energy X-ray absorptiometry (DXA) can be used to assess the reduction of bone quantity in these cases, little is known about how altered bone quality in diseased bone can increase fracture risk. It is clear that high-resolution diagnostic techniques need to be developed to narrow the gap between the onset and diagnosis of fracture-related changes. Here, by combining several scanning probe methods on a mouse model (Crh^-120/+) of glucocorticoid-induced osteoporosis (GIOP), we developed quantitative and spatially resolved maps of ultrastructural changes in collagen fibrils and mineral nanocrystals, mineralization distribution (microscale), and morphology (macroscale) across femoral osteoporotic bone. Our results indicate that the altered bone remodelling in GIOP leads to 1) heterogeneous bone structure and mineralization, 2) reduced degree of orientation of collagen fibrils and mineral nanocrystals, and 3) reduced length and increased thickness of mineral nanocrystals, which contribute to mechanical abnormalities. The combined multiscale experimental approach presented here will be used to understand musculoskeletal degeneration in aging and osteoporosis.

High resolution imaging in bone tissue research-review

This review article focuses on imaging of bone tissue to understand skeletal health with regards to bone quality. Skeletal fragility fractures are due to bone diseases such as osteoporosis which result in low bone mass and bone mineral density (BMD) leading to high risk of fragility fractures. Recent advances in imaging and analysis technologies have highly benefitted the field of biological sciences. In particular, their application in skeletal health has been of significant importance in understanding bone mechanical behavior (structure and properties) at the tissue level. While synchrotron based microCT technique has remained the gold standard for non-destructive evaluation of structure in material and biological sciences, several lab based microCT systems have been developed to provide high resolution imaging of specimens with greater access, and ease of use in laboratory settings. Lab based microCT scanners are widely used in the bone field as a standard tool to evaluate three-dimensional (3D) morphologies of bone structure at image resolutions appropriate for bone samples from small animals to bone biopsy specimens from humans. Both synchrotron and standard lab based microCT systems provide high resolution imaging ex vivo for a small sized specimen. A few X-ray based systems are also commercially available for in vivo scanning at relatively low image resolutions. Synchrotron-based CT microscopy is being used for various ultra-high-resolution image analyses using complex 3D software. However, the synchrotron-based CT technology is in high demand, allows only limited numbers of specimens, expensive, requires complex additional instrumentation, and is not easily available to researchers as it requires access to a synchrotron source which is always limited. Therefore, desktop laboratory scanners (microXCT, Zeiss/Xradia, Scanco, SkyScan. etc.), mimicking the synchrotron based CT technology or image resolution, have been developed to solve the accessibility issues. These lab based scanners have helped both material science, and the bone field to investigate bone tissue morphologies at submicron mage resolutions. Considerable progress has been made in both in vivo and ex vivo imaging towards providing high resolution images of bone tissue. Both clinical and research imaging technologies will continue to improve and help understand osteoporosis and other related skeletal issues in order to develop targeted treatments for bone fragility. This review summarizes the high resolution imaging work in bone research.

Reduction of fibrillar strain-rate sensitivity in steroid-induced osteoporosis linked to changes in mineralized fibrillar nanostructure

As bone is used in a dynamic mechanical environment, understanding the structural origins of its time-dependent mechanical behaviour - and the alterations in metabolic bone disease - is of interest. However, at the scale of the mineralized fibrillar matrix (nanometre-level), the nature of the strain-rate dependent mechanics is incompletely understood. Here, we investigate the fibrillar- and mineral-deformation behaviour in a murine model of Cushing's syndrome, used to understand steroid induced osteoporosis, using synchrotron small- and wide-angle scattering/diffraction combined with in situ tensile testing at three strain rates ranging from 10-4 to 10-1 s-1. We find that the effective fibril- and mineral-modulus and fibrillar-reorientation show no significant increase with strain-rate in osteoporotic bone, but increase significantly in normal (wild-type) bone. By applying a fibril-lamellar two-level structural model of bone matrix deformation to fit the results, we obtain indications that altered collagen-mineral interactions at the nanoscale - along with altered fibrillar orientation distributions - may be the underlying reason for this altered strain-rate sensitivity. Our results suggest that an altered strain-rate sensitivity of the bone matrix in osteoporosis may be one of the contributing factors to reduced mechanical competence in such metabolic bone disorders, and that increasing this sensitivity may improve biomechanical performance.

Hyperglycemia compromises Rat Cortical Bone by Increasing Osteocyte Lacunar Density and Decreasing Vascular Canal Volume

Uncontrolled diabetes is associated with increased risk of bony fractures. However, the mechanisms have yet to be understood. Using high-resolution synchrotron micro-CT, we calculated the changes in the microstructure of femoral cortices of streptozotocin-induced hyperglycemic (STZ) Wistar Albino rats and tested the mechanical properties of the mineralized matrix by nanoindentation. Total lacunar volume of femoral cortices increased in STZ group due to a 9% increase in lacunar density. However, total vascular canal volume decreased in STZ group due to a remarkable decrease in vascular canal diameter (7 ± 0.3 vs. 8.5 ± 0.4 µm). Osteocytic territorial matrix volume was less in the STZ group (14,908 ± 689 µm³) compared with healthy controls (16,367 ± 391 µm³). In conclusion, hyperglycemia increased cellularity and lacunar density, decreased osteocyte territorial matrix, and reduced vascular girth, in addition to decreasing matrix mechanical properties in the STZ group when compared with euglycemic controls.

Birol Ay et al. use high-resolution synchrotron radiation micro-CT to calculate the changes in the microstructure of femoral cortices in STZ-induced hyperglycemic rats. They show that hyperglycemia increases lacunar density due to a reduction in osteocytic territorial matrix volume but decreases total vascular canal volume due to a decrease in canal diameter.

Matrix-induced pre-strain and mineralization-dependent interfibrillar shear transfer enable 3D fibrillar deformation in a biogenic armour

The cuticle of stomatopod is an example of a natural mineralized biomaterial, consisting of chitin, amorphous calcium carbonate and protein components with a multiscale hierarchical structure, and forms a protective shell with high impact resistance. At the ultrastructural level, cuticle mechanical functionality is enabled by the nanoscale architecture, wherein chitin fibrils are in intimate association with enveloping mineral and proteins. However, the interactions between these ultrastructural building blocks, and their coupled response to applied load, remain unclear. Here, we elucidate these interactions via synchrotron microbeam wide-angle X-ray diffraction combined with in situ tensile loading, to quantify the chitin crystallite structure of native cuticle - and after demineralization and deproteinization - as well as time-resolved changes in chitin fibril strain on macroscopic loading. We demonstrate chitin crystallite stabilization by mineral, seen via a compressive pre-strain of approximately 0.10% (chitin/protein fibre pre-stress of ∼20 MPa), which is lost on demineralization. Clear reductions of stiffness at the fibrillar-level following matrix digestion are linked to the change in the protein/matrix mechanical properties. Furthermore, both demineralization and deproteinization alter the 3D-pattern of deformation of the fibrillar network, with a non-symmetrical angular fibril strain induced by the chemical modifications, associated with loss of the load-transferring interfibrillar matrix. Our results demonstrate and quantify the critical role of interactions at the nanoscale (between chitin-protein and chitin-mineral) in enabling the molecular conformation and outstanding mechanical properties of cuticle, which will inform future design of hierarchical bioinspired composites. STATEMENT OF SIGNIFICANCE: Chitinous biomaterials (e.g. arthropod cuticle) are widespread in nature and attracting attention for bioinspired design due to high impact resistance coupled with light weight. However, how the nanoscale interactions of the molecular building blocks - alpha-chitin, protein and calcium carbonate mineral - lead to these material properties is not clear. Here we used X-ray scattering to determine the cooperative interactions between chitin fibrils, protein matrix and biominerals, during tissue loading. We find that the chitin crystallite structure is stabilized by mineral nanoparticles, the protein phase prestresses chitin fibrils, and that chemical modification of the interfibrillar matrix significantly disrupts 2D mechanics of the microfibrillar chitin plywood network. These results will aid rational design of advanced chitin-based biomaterials with high impact resistance.

A soluble bone morphogenetic protein type 1A receptor fusion protein treatment prevents glucocorticoid-Induced bone loss in mice

Glucocorticoid-induced osteoporosis (GIOP) is a frequent complication of systemic glucocorticoid (GC) therapy, is the most common form of secondary osteoporosis, and is associated with skeletal fragility and increased fracture risk. A soluble form of BMP receptor type 1A fusion protein (mBMPR1A-mFc) acts as an antagonist to endogenous BMPR1A and could increase bone mass in both ovariectomized and ovary-intact mice, but its effects in GIOP mice remained unclear. The aim of this study was to evaluate the effects of mBMPR1A-mFc on the skeleton in experimental models of GIOP. mBMPR1A-mFc treatment could increase the bone mineral density (BMD), trabecular bone volume, thickness, and number, and cortical thickness, and reduce the structure model index and trabecular separation in GIOP mice. mBMPR1A-mFc treatment could also prevent bone loss and enhance biomechanical strength in GIOP mice by promoting osteoblastic bone formation and inhibiting osteoclastic bone resorption. Mechanistic studies revealed that mBMPR1A-mFc treatment increased murine osteoblastogenesis by activating the Wnt/β-catenin signaling pathway while decreasing osteoclastogenesis by inhibiting the RANK/RANKL/osteoprotegerin (OPG) signaling pathway. These findings demonstrate that mBMPR1A-mFc treatment in GIOP mice improves bone mass, microarchitecture, and strength by enhancing osteoblastic bone formation and inhibiting osteoclastic bone resorption in GIOP mice and offers a promising novel alternative for the treatment of GIOP.

Estimation of the elastic modulus of child cortical bone specimens via microindentation

Purpose: Non-pathological child cortical bone (NPCCB) studies can provide clinicians with vital information and insights. However, assessing the anisotropic elastic properties of NPCCB remains a challenge for the biomechanical engineering community. For the first time, this paper provides elastic moduli values for NPCCB specimens in two perpendicular directions (longitudinal and transverse) and for two different structural components of bone tissue (osteon and interstitial lamellae). Materials and Methods: Microindentation is one of the reference methods used to measure bone stiffness. Here, 8 adult femurs (mean age 82 ± 8.9 years), 3 child femurs (mean age 13.3 ± 2.1 years), and 16 child fibulae (mean age 10.2 ± 3.9 years) were used to assess the elastic moduli of adult and child bones by microindentation. Results: For adult specimens, the mean moduli measured in this study are 18.1 (2.6) GPa for osteons, 21.3 (2.3) GPa for interstitial lamellae, and 13.8 (1.7) GPa in the transverse direction. For child femur specimens, the mean modulus is 14.1 (0.8) GPa for osteons, lower than that for interstitial lamellae: 15.5 (1.5) GPa. The mean modulus is 11.8 (0.7) GPa in the transverse direction. Child fibula specimens show a higher elastic modulus for interstitial lamellae 15.8 (1.5) than for osteons 13.5 (1.6), with 10.2 (1) GPa in the transverse direction. Conclusion: For the first time, NPCCB elastic modulus values are provided in longitudinal and transverse directions at the microscale level.

Immunogenicity analysis of decellularized cardiac scaffolds after transplantation into rats

Aim: Cardiac extracellular matrix (cECM) scaffolds are promising biomaterials for clinical applications. Our aim is to determine the immunogenicity of decellularized scaffolds from different sources for use as artificial organs during organ transplantation. Materials & methods: We transplanted Lewis rats with syngeneic (Lewis rat cECM), allogeneic (BN rat cECM) or xenogeneic (hamster cECM) decellularized cardiac scaffolds. Acute vascular and cellular rejection was quantified by immunohistochemistry and immune cell infiltration. Results: BN rat and hamster hearts were rejected following transplantation. BN and hamster cECMs had similarly low immunogenicity compared with Lewis rat cECMs and did not lead to increased rejection. Conclusion: We found that scaffolds from all sources did not induce vascular or cellular rejection and exhibited low immunogenicity.

Mouse Models and Online Resources for Functional Analysis of Osteoporosis Genome-Wide Association Studies

Osteoporosis is a complex genetic disease in which the number of loci associated with the bone mineral density, a clinical risk factor for fracture, has increased at an exponential rate in the last decade. The identification of the causative variants and candidate genes underlying these loci has not been able to keep pace with the rate of locus discovery. A large number of tools and data resources have been built around the use of the mouse as model of human genetic disease. Herein, we describe resources available for functional validation of human Genome Wide Association Study (GWAS) loci using mouse models. We specifically focus on large-scale phenotyping efforts focused on bone relevant phenotypes and repositories of genotype-phenotype data that exist for transgenic and mutant mice, which can be readily mined as a first step toward more targeted efforts designed to deeply characterize the role of a gene in bone biology.

Physiological and pathological osteocytic osteolysis

Osteocytes, the most abundant bone cell in the adult skeleton, can function as mechanosensors directing osteoblast and osteoclast function in order to maintain optimal load bearing bone in addition to functioning as endocrine cells regulating phosphate metabolism. A controversial function, previously overlooked or denied, has been osteocytes as regulators of calcium metabolism. Early histologists upon observing enlarged osteocyte lacunae in bone sections proposed that mature osteocytes could remove their perilacunar matrix, a term called "osteocytic osteolysis". New insights into this process have occurred during the last decade using novel technology thereby providing a means to identify molecular mechanisms responsible for osteocytic osteolysis. As release of calcium from a mineralized matrix requires a more acidic pH and specialized enzymes, it was proposed that osteocytes may utilize similar molecular mechanisms as osteoclasts to remove mineral. The idea that a cell descended from mesenchymal progenitors (the osteocyte) could function similarly to a cell descended from hematopoietic progenitors (the osteoclast) was challenged as being improbable. Here we review the molecular mechanisms behind this osteocyte function, the role of osteocytic osteolysis in health and disease, and the capacity of the osteocyte to reverse the osteolytic process by replacing the removed matrix, a revived osteoblast function.

Effect of SR-microCT radiation on the mechanical integrity of trabecular bone using in situ mechanical testing and digital volume correlation

The use of synchrotron radiation micro-computed tomography (SR-microCT) is becoming increasingly popular for studying the relationship between microstructure and bone mechanics subjected to in situ mechanical testing. However, it is well known that the effect of SR X-ray radiation can considerably alter the mechanical properties of bone tissue. Digital volume correlation (DVC) has been extensively used to compute full-field strain distributions in bone specimens subjected to step-wise mechanical loading, but tissue damage from sequential SR-microCT scans has not been previously addressed. Therefore, the aim of this study is to examine the influence of SR irradiation-induced microdamage on the apparent elastic properties of trabecular bone using DVC applied to in situ SR-microCT tomograms obtained with different exposure times. Results showed how DVC was able to identify high local strain levels (> 10,000 µε) corresponding to visible microcracks at high irradiation doses (~ 230 kGy), despite the apparent elastic properties remained unaltered. Microcracks were not detected and bone plasticity was preserved for low irradiation doses (~ 33 kGy), although image quality and consequently, DVC performance were reduced. DVC results suggested some local deterioration of tissue that might have resulted from mechanical strain concentration further enhanced by some level of local irradiation even for low accumulated dose.

Bone matrix development in steroid-induced osteoporosis is associated with a consistently reduced fibrillar stiffness linked to altered bone mineral quality

Glucocorticoid-induced osteoporosis (GIOP) is a major secondary form of osteoporosis, with the fracture risk significantly elevated - at similar levels of bone mineral density - in patients taking glucocorticoids compared with non-users. The adverse bone structural changes at multiple hierarchical levels in GIOP, and their mechanistic consequences leading to reduced load-bearing capacity, are not clearly understood. Here we combine experimental X-ray nanoscale mechanical imaging with analytical modelling of the bone matrix mechanics to determine mechanisms causing bone material quality deterioration during development of GIOP. In situ synchrotron small-angle X-ray diffraction combined with tensile testing was used to measure nanoscale deformation mechanisms in a murine model of GIOP, due to a corticotrophin-releasing hormone promoter mutation, at multiple ages (8-, 12-, 24- and 36 weeks), complemented by quantitative micro-computed tomography and backscattered electron imaging to determine mineral concentrations. We develop a two-level hierarchical model of the bone matrix (mineralized fibril and lamella) to predict fibrillar mechanical response as a function of architectural parameters of the mineralized matrix. The fibrillar elastic modulus of GIOP-bone is lower than healthy bone throughout development, and nearly constant in time, in contrast to the progressively increasing stiffness in healthy bone. The lower mineral platelet aspect ratio value for GIOP compared to healthy bone in the multiscale model can explain the fibrillar deformation. Consistent with this result, independent measurement of mineral platelet lengths from wide-angle X-ray diffraction finds a shorter mineral platelet length in GIOP. Our results show how lowered mineralization combined with altered mineral nanostructure in GIOP leads to lowered mechanical competence.

SIGNIFICANCE STATEMENT

Increased fragility in musculoskeletal disorders like osteoporosis are believed to arise due to alterations in bone structure at multiple length-scales from the organ down to the supramolecular-level, where collagen molecules and elongated mineral nanoparticles form stiff fibrils. However, the nature of these molecular-level alterations are not known. Here we used X-ray scattering to determine both how bone fibrils deform in secondary osteoporosis, as well as how the fibril orientation and mineral nanoparticle structure changes. We found that osteoporotic fibrils become less stiff both because the mineral nanoparticles became shorter and less efficient at transferring load from collagen, and because the fibrils are more randomly oriented. These results will help in the design of new composite musculoskeletal implants for bone repair.

Ptychographic X-ray CT characterization of the osteocyte lacuno-canalicular network in a male rat's glucocorticoid induced osteoporosis model

Ptychographic X-ray computed tomography (PXCT) is a quantitative imaging modality that non-destructively maps the 3D electron density inside an object with tens of nanometers spatial resolution. This method provides unique access to the morphology and structure of the osteocyte lacuno-canalicular network (LCN) and nanoscale density of the tissue in the vicinity of an osteocyte lacuna. Herein, we applied PXCT to characterize the lacunae and LCN in a male Wistar rat model of glucocorticoid-induced osteoporosis (GIO). The ptychographic images revealed significant (p < 0.05) differences in the number of canaliculi originating from the lacuna per ellipsoidal surface unit, Ca.Nb (p = 0.0106), and the 3D morphology of the lacuna (p = 0.0064), between GIO and SHAM groups. Moreover, the mean canalicular diameter, Ca.Dm, was slightly statistically un-significantly smaller in GIO (152 ± 6.5) nm than in SHAM group (165 ± 8) nm (p = 0.053). Our findings indicate that PXCT can non-destructively provide detailed, nanoscale information on the 3D organization of the LCN in correlative studies of pathologies, such as osteoporosis, leading to improved diagnosis and therapy.

The Role of Matrix Composition in the Mechanical Behavior of Bone

PURPOSE OF REVIEW

While thinning of the cortices or trabeculae weakens bone, age-related changes in matrix composition also lower fracture resistance. This review summarizes how the organic matrix, mineral phase, and water compartments influence the mechanical behavior of bone, thereby identifying characteristics important to fracture risk.

RECENT FINDINGS

In the synthesis of the organic matrix, tropocollagen experiences various post-translational modifications that facilitate a highly organized fibril of collagen I with a preferred orientation giving bone extensibility and several toughening mechanisms. Being a ceramic, mineral is brittle but increases the strength of bone as its content within the organic matrix increases. With time, hydroxyapatite-like crystals experience carbonate substitutions, the consequence of which remains to be understood. Water participates in hydrogen bonding with organic matrix and in electrostatic attractions with mineral phase, thereby providing stability to collagen-mineral interface and ductility to bone. Clinical tools sensitive to age- and disease-related changes in matrix composition that the affect mechanical behavior of bone could potentially improve fracture risk assessment.

Nano-scale mechanisms explain the stiffening and strengthening of ligament tissue with increasing strain rate

Ligament failure is a major societal burden causing disability and pain. Failure is caused by trauma at high loading rates. At the macroscopic level increasing strain rates cause an increase in failure stress and modulus, but the mechanism for this strain rate dependency is not known. Here we investigate the nano scale mechanical property changes of human ligament using mechanical testing combined with synchrotron X-ray diffraction. With increasing strain rate, we observe a significant increase in fibril modulus and a reduction of fibril to tissue strain ratio, revealing that tissue-level stiffening is mainly due to the stiffening of collagen fibrils. Further, we show that the reduction in fibril deformation at higher strain rates is due to reduced molecular strain and fibrillar gaps, and is associated with rapid disruption of matrix-fibril bonding. This reduction in number of interfibrillar cross-links explains the changes in fibril strain; this is verified through computational modelling.

Bone quality changes associated with aging and disease: a review

Bone quality encompasses all the characteristics of bone that, in addition to density, contribute to its resistance to fracture. In this review, we consider changes in architecture, porosity, and composition, including collagen structure, mineral composition, and crystal size. These factors all are known to vary with tissue and animal ages, and health status. Bone morphology and presence of microcracks, which also contribute to bone quality, will not be discussed in this review. Correlations with mechanical performance for collagen cross-linking, crystallinity, and carbonate content are contrasted with mineral content. Age-dependent changes in humans and rodents are discussed in relation to rodent models of disease. Examples are osteoporosis, osteomalacia, osteogenesis imperfecta (OI), and osteopetrosis in both humans and animal models. Each of these conditions, along with aging, is associated with increased fracture risk for distinct reasons.

Towards in situ determination of 3D strain and reorientation in the interpenetrating nanofibre networks of cuticle

Determining the in situ 3D nano- and microscale strain and reorientation fields in hierarchical nanocomposite materials is technically very challenging. Such a determination is important to understand the mechanisms enabling their functional optimization. An example of functional specialization to high dynamic mechanical resistance is the crustacean stomatopod cuticle. Here we develop a new 3D X-ray nanostrain reconstruction method combining analytical modelling of the diffraction signal, fibre-composite theory and in situ deformation, to determine the hitherto unknown nano- and microscale deformation mechanisms in stomatopod tergite cuticle. Stomatopod cuticle at the nanoscale consists of mineralized chitin fibres and calcified protein matrix, which form (at the microscale) plywood (Bouligand) layers with interpenetrating pore-canal fibres. We uncover anisotropic deformation patterns inside Bouligand lamellae, accompanied by load-induced fibre reorientation and pore-canal fibre compression. Lamination theory was used to decouple in-plane fibre reorientation from diffraction intensity changes induced by 3D lamellae tilting. Our method enables separation of deformation dynamics at multiple hierarchical levels, a critical consideration in the cooperative mechanics characteristic of biological and bioinspired materials. The nanostrain reconstruction technique is general, depending only on molecular-level fibre symmetry and can be applied to the in situ dynamics of advanced nanostructured materials with 3D hierarchical design.

Body wall structure in the starfish Asterias rubens

The body wall of starfish is composed of magnesium calcite ossicles connected by collagenous tissue and muscles and it exhibits remarkable variability in stiffness, which is attributed to the mechanical mutability of the collagenous component. Using the common European starfish Asterias rubens as an experimental animal, here we have employed a variety of techniques to gain new insights into the structure of the starfish body wall. The structure and organisation of muscular and collagenous components of the body wall were analysed using trichrome staining. The muscle system comprises interossicular muscles as well as muscle strands that connect ossicles with the circular muscle layer of the coelomic lining. The collagenous tissue surrounding the ossicle network contains collagen fibres that form loop-shaped straps that wrap around calcite struts near to the surface of ossicles. The 3D architecture of the calcareous endoskeleton was visualised for the first time using X-ray microtomography, revealing the shapes and interactions of different ossicle types. Furthermore, analysis of the anatomical organisation of the ossicles indicates how changes in body shape may be achieved by local contraction/relaxation of interossicular muscles. Scanning synchrotron small-angle X-ray diffraction (SAXD) scans of the starfish aboral body wall and ambulacrum were used to study the collagenous tissue component at the fibrillar level. Collagen fibrils in aboral body wall were found to exhibit variable degrees of alignment, with high levels of alignment probably corresponding to regions where collagenous tissue is under tension. Collagen fibrils in the ambulacrum had a uniformly low degree of orientation, attributed to macrocrimp of the fibrils and the presence of slanted as well as horizontal fibrils connecting antimeric ambulacral ossicles. Body wall collagen fibril D-period lengths were similar to previously reported mammalian D-periods, but were significantly different between the aboral and ambulacral samples. The overlap/D-period length ratio within fibrils was higher than reported for mammalian tissues. Collectively, the data reported here provide new insights into the anatomy of the body wall in A. rubens and a foundation for further studies investigating the structural basis of the mechanical properties of echinoderm body wall tissue composites.

Interfibrillar stiffening of echinoderm mutable collagenous tissue demonstrated at the nanoscale

The mutable collagenous tissue (MCT) of echinoderms (e.g., sea cucumbers and starfish) is a remarkable example of a biological material that has the unique attribute, among collagenous tissues, of being able to rapidly change its stiffness and extensibility under neural control. However, the mechanisms of MCT have not been characterized at the nanoscale. Using synchrotron small-angle X-ray diffraction to probe time-dependent changes in fibrillar structure during in situ tensile testing of sea cucumber dermis, we investigate the ultrastructural mechanics of MCT by measuring fibril strain at different chemically induced mechanical states. By measuring a variable interfibrillar stiffness (EIF), the mechanism of mutability at the nanoscale can be demonstrated directly. A model of stiffness modulation via enhanced fibrillar recruitment is developed to explain the biophysical mechanisms of MCT. Understanding the mechanisms of MCT quantitatively may have applications in development of new types of mechanically tunable biomaterials.

Synchrotron Imaging Assessment of Bone Quality

Bone is a complex hierarchical structure, and its principal function is to resist mechanical forces and fracture. Bone strength depends not only on the quantity of bone tissue but also on the shape and hierarchical structure. The hierarchical levels are interrelated, especially the micro-architecture, collagen and mineral components; hence, analysis of their specific roles in bone strength and stiffness is difficult. Synchrotron imaging technologies including micro-CT and small/wide angle X-ray scattering/diffraction are becoming increasingly popular for studying bone because the images can resolve deformations in the micro-architecture and collagen-mineral matrix under in situ mechanical loading. Synchrotron cannot be directly applied in vivo due to the high radiation dose but will allow researchers to carry out systematic multifaceted studies of bone ex vivo. Identifying characteristics of aging and disease will underpin future efforts to generate novel devices and interventional therapies for assessing and promoting healthy aging. With our own research work as examples, this paper introduces how synchrotron imaging technology can be used with in situ testing in bone research.