Combination of nanoindentation and quantitative backscattered electron imaging revealed altered bone material properties associated with femoral neck fragility

Cortical and trabecular mechanical properties in the femoral neck vary differently with changes in bone mineral density

Graphical Abstract.

Composition and micromechanical properties of the femoral neck compact bone in relation to patient age, sex and hip fracture occurrence

Current clinical methods of bone health assessment depend to a great extent on bone mineral density (BMD) measurements. However, these methods only act as a proxy for bone strength and are often only carried out after the fracture occurs. Besides BMD, composition and tissue-level mechanical properties are expected to affect the whole bone's strength and toughness. While the elastic properties of the bone extracellular matrix (ECM) have been extensively investigated over the past two decades, there is still limited knowledge of the yield properties and their relationship to composition and architecture. In the present study, morphological, compositional and micropillar compression bone data was collected from patients who underwent hip arthroplasty. Femoral neck samples from 42 patients were collected together with anonymous clinical information about age, sex and primary diagnosis (coxarthrosis or hip fracture). The femoral neck cortex from the inferomedial region was analyzed in a site-matched manner using a combination of micromechanical testing (nanoindentation, micropillar compression) together with micro-CT and quantitative polarized Raman spectroscopy for both morphological and compositional characterization. Mechanical properties, as well as the sample-level mineral density, were constant over age. Only compositional properties demonstrate weak dependence on patient age: decreasing mineral to matrix ratio (p = 0.02, R2 = 0.13, 2.6 % per decade) and increasing amide I sub-peak ratio I∼1660/I∼1683 (p = 0.04, R2 = 0.11, 1.5 % per decade). The patient's sex and diagnosis did not seem to influence investigated bone properties. A clear zonal dependence between interstitial and osteonal cortical zones was observed for compositional and elastic bone properties (p < 0.0001). Site-matched microscale analysis confirmed that all investigated mechanical properties except yield strain demonstrate a positive correlation with the mineral fraction of bone. The output database is the first to integrate the experimentally assessed microscale yield properties, local tissue composition and morphology with the available patient clinical information. The final dataset was used for bone fracture risk prediction in-silico through the principal component analysis and the Naïve Bayes classification algorithm. The analysis showed that the mineral to matrix ratio, indentation hardness and micropillar yield stress are the most relevant parameters for bone fracture risk prediction at 70 % model accuracy (0.71 AUC). Due to the low number of samples, further studies to build a universal fracture prediction algorithm are anticipated with the higher number of patients (N > 200). The proposed classification algorithm together with the output dataset of bone tissue properties can be used for the future comparison of existing methods to evaluate bone quality as well as to form a better understanding of the mechanisms through which bone tissue is affected by aging or disease.

Association between nanoscale strains and tissue level nanoindentation properties in age-related hip-fractures

Measurement of the properties of bone as a material can happen in various length scales in its hierarchical and composite structure. The aim of this study was to test the tissue level properties of clinically-relevant human bone samples which were collected from donors belonging to three groups: ageing donors who suffered no fractures (Control); untreated fracture patients (Fx-Untreated) and patient who experienced hip fracture despite being treated with bisphosphonates (Fx-BisTreated). Tissue level properties were assessed by (a) nanoindentation and (b) synchrotron tensile tests (STT) where strains were measured at the 'tissue', 'fibril' and 'mineral' levels by using simultaneous Wide-angle - (WAXD) and Small angle- X-ray diffraction (SAXD). The composition was analysed by thermogravimetric analysis and material level endo- and exo-thermic reactions by differential scanning calorimetry (TGA/DSC3+). Irrespective of treatment fracture donors exhibited significantly lower tissue, fibril and mineral strain at the micro and nanoscale respectively and had a higher mineral content than controls. In nanoindentation only nanohardness was significantly greater for Controls and Fx-BisTreated versus Fx-Untreated. The other nanoindentation parameters did not vary significantly across the three groups. There was a highly significant positive correlation (p < 0.001) between organic content and tissue level strain behaviour. Overall hip-fractures were associated with lower STT nanostrains and it was behaviour measured by STT which proved to be a more effective approach for predicting fracture risk because evidently it was able to demonstrate the mechanical deficit for the bone tissue of the donors who had experienced fractures.

Instrumented nanoindentation in musculoskeletal research

Musculoskeletal tissues, such as bone, cartilage, and muscle, are natural composite materials that are constructed with a hierarchical structure ranging from the cell to tissue level. The component differences and structural complexity, together, require comprehensive multiscale mechanical characterization. In this review, we focus on nanoindentation testing, which is used for nanometer to sub-micrometer length scale mechanical characterization. In the following context, we will summarize studies of nanoindentation in musculoskeletal research, examine the critical factors that affect nanoindentation testing results, and briefly summarize other commonly used techniques that can be conjoined with nanoindentation for synchronized imaging and colocalized characterization.

Multiscale Femoral Neck Imaging and Multimodal Trabeculae Quality Characterization in an Osteoporotic Bone Sample

Although multiple structural, mechanical, and molecular factors are definitely involved in osteoporosis, the assessment of subregional bone mineral density remains the most commonly used diagnostic index. In this study, we characterized bone quality in the femoral neck of one osteoporotic patients as compared to an age-matched control subject, and so used a multiscale and multimodal approach including X-ray computed microtomography at different spatial resolutions (pixel size: 51.0, 4.95 and 0.9 µm), microindentation and Fourier transform infrared spectroscopy. Our results showed abnormalities in the osteocytes lacunae volume (358.08 ± 165.00 for the osteoporotic sample vs. 287.10 ± 160.00 for the control), whereas a statistical difference was found neither for shape nor for density. The osteoporotic femoral head and great trochanter reported reduced elastic modulus (Es) and hardness (H) compared to the control reference (−48% (p < 0.0001) and −34% (p < 0.0001), respectively for Es and H in the femoral head and −29% (p < 0.01) and −22% (p < 0.05), respectively for Es and H in the great trochanter), whereas the corresponding values in the femoral neck were in the same range. The spectral analysis could distinguish neither subregional differences in the osteoporotic sample nor between the osteoporotic and healthy samples. Although, infrared spectroscopic measurements were comparable among subregions, and so regardless of the bone osteoporotic status, the trabecular mechanical properties were comparable only in the femoral neck. These results illustrate that bone remodeling in osteoporosis is a non-uniform process with different rates in different bone anatomical regions, hence showing the interest of a clear analysis of the bone microarchitecture in the case of patients’ osteoporotic evaluation.

Three-dimensional mapping of cortical porosity and thickness along the superolateral femoral neck in older women

Although several studies have analyzed inter-individual differences in the femoral neck cortical microstructure, intra-individual variations have not been comprehensively evaluated. By using microCT, we mapped cortical pore volume fraction (Ct.Po) and thickness (Ct.Th) along the superolateral femoral neck in 14 older women (age: 77.1 ± 9.8 years) to identify subregions and segments with high porosity and/or low thickness—potential “critical” spots where a fracture could start. We showed that Ct.Po and Ct.Th significantly differed between basicervical, midcervical, and subcapital subregions of the femoral neck (p < 0.001), where the subcapital subregion showed the lowest mean Ct.Th and the highest mean Ct.Po. These cortical parameters also varied substantially with age and with the location of the analyzed microsegments along the individual’s neck (p < 0.001), showing multiple microsegments with high porosity and/or low thickness. Although the highest ratio of these microsegments was found in the subcapital subregion, they were also present at other examined subregions, which may provide an anatomical basis for explaining the fracture initiation at various sites of the superolateral neck. Given that fractures likely start at structurally and mechanically weaker spots, intra-individual variability in Ct.Po and Ct.Th should be considered and the average values for the entire femoral neck should be interpreted with caution.

Impaired bone quality in the superolateral femoral neck occurs independent of hip geometry and bone mineral density

Skeletal adaptation is substantially influenced by mechanical loads. Osteocytes and their lacuno-canalicular network have been identified as a key player in load sensation and bone quality regulation. In the femoral neck, one of the most common fracture sites, a complex loading pattern with lower habitual loading in the superolateral neck and higher compressive stresses in the inferomedial neck is present. Variations in the femoral neck-shaft angle (NSA), i.e., coxa vara or coxa valga, provide the opportunity to examine the influence of loading patterns on bone quality. We obtained femoral neck specimens of 28 osteoarthritic human subjects with coxa vara, coxa norma and coxa valga during total hip arthroplasty. Bone mineral density (BMD) was assessed preoperatively by dual energy X-ray absorptiometry (DXA). Cortical and trabecular microstructure and three-dimensional osteocyte lacunar characteristics were assessed in the superolateral and inferomedial neck using ex vivo high resolution micro-computed tomography. Additionally, BMD distribution and osteocyte lacunar characteristics were analyzed by quantitative backscattered electron imaging (qBEI). All groups presented thicker inferomedial than superolateral cortices. Furthermore, the superolateral site exhibited a lower osteocyte lacunar density along with lower lacunar sphericity than the inferomedial site, independent of NSA. Importantly, BMD and corresponding T-scores correlated with microstructural parameters at the inferomedial but not superolateral neck. In conclusion, we provide micromorphological evidence for fracture vulnerability of the superolateral neck, which is independent of NSA and BMD. The presented bone qualitative data provide an explanation why DXA may be insufficient to predict a substantial proportion of femoral neck fractures. STATEMENT OF SIGNIFICANCE: The femoral neck, one of the most common fracture sites, is subject to a complex loading pattern. Site-specific differences (i.e., superolateral vs. inferomedial) in bone quality influence fracture risk, but it is unclear how this relates to hip geometry and bone mineral density (BMD) measurements in vivo. Here, we examine femoral neck specimens using a variety of high-resolution imaging techniques and demonstrate impaired bone quality in the superolateral compared to the inferomedial neck. Specifically, we found impaired cortical and trabecular microarchitecture, mineralization, and osteocyte properties, regardless of neck-shaft angle. Since BMD correlated with bone quality of the inferomedial but not the superolateral neck, our results illustrate why bone densitometry may not predict a substantial proportion of femoral neck fractures.

Globular structure of the hypermineralized tissue in human femoral neck

Bone becomes more fragile with ageing. Among many structural changes, a thin layer of highly mineralized and brittle tissue covers part of the external surface of the thin femoral neck cortex in older people and has been proposed to increase hip fragility. However, there have been very limited reports on this hypermineralized tissue in the femoral neck, especially on its ultrastructure. Such information is critical to understanding both the mineralization process and its contributions to hip fracture. Here, we use multiple advanced techniques to characterize the ultrastructure of the hypermineralized tissue in the neck across various length scales. Synchrotron radiation micro-CT found larger but less densely distributed cellular lacunae in hypermineralized tissue than in lamellar bone. When examined under FIB-SEM, the hypermineralized tissue was mainly composed of mineral globules with sizes varying from submicron to a few microns. Nano-sized channels were present within the mineral globules and oriented with the surrounding organic matrix. Transmission electron microscopy showed the apatite inside globules were poorly crystalline, while those at the boundaries between the globules had well-defined lattice structure with crystallinity similar to the apatite mineral in lamellar bone. No preferred mineral orientation was observed both inside each globule and at the boundaries. Collectively, we conclude based on these new observations that the hypermineralized tissue is non-lamellar and has less organized mineral, which may contribute to the high brittleness of the tissue.

Mineral density differences between femoral cortical bone and trabecular bone are not explained by turnover rate alone

Bone mineral density distributions (BMDDs) are a measurable property of bone tissues that depends strongly on bone remodelling and mineralisation processes. These processes can vary significantly in health and disease and across skeletal sites, so there is high interest in analysing these processes from experimental BMDDs. Here, we propose a rigorous hypothesis-testing approach based on a mathematical model of mineral heterogeneity in bone due to remodelling and mineralisation, to help explain differences observed between the BMDD of human femoral cortical bone and the BMDD of human trabecular bone. Recent BMDD measurements show that femoral cortical bone possesses a higher bone mineral density, but a similar mineral heterogeneity around the mean compared to trabecular bone. By combining this data with the mathematical model, we are able to test whether this difference in BMDD can be explained by (i) differences in turnover rate; (ii) differences in osteoclast resorption behaviour; and (iii) differences in mineralisation kinetics between the two bone types. We find that accounting only for differences in turnover rate is inconsistent with the fact that both BMDDs have a similar spread around the mean, and that accounting for differences in osteoclast resorption behaviour leads to biologically inconsistent bone remodelling patterns. We conclude that the kinetics of mineral accumulation in bone matrix must therefore be different in femoral cortical bone and trabecular bone. Although both cortical and trabecular bone are made up of lamellar bone, the different mineralisation kinetics in the two types of bone point towards more profound structural differences than usually assumed.

Ultrastructural Pathology of Atherosclerosis, Calcific Aortic Valve Disease, and Bioprosthetic Heart Valve Degeneration: Commonalities and Differences

Atherosclerosis, calcific aortic valve disease (CAVD), and bioprosthetic heart valve degeneration (alternatively termed structural valve deterioration, SVD) represent three diseases affecting distinct components of the circulatory system and their substitutes, yet sharing multiple risk factors and commonly leading to the extraskeletal calcification. Whereas the histopathology of the mentioned disorders is well-described, their ultrastructural pathology is largely obscure due to the lack of appropriate investigation techniques. Employing an original method for sample preparation and the electron microscopy visualisation of calcified cardiovascular tissues, here we revisited the ultrastructural features of lipid retention, macrophage infiltration, intraplaque/intraleaflet haemorrhage, and calcification which are common or unique for the indicated types of cardiovascular disease. Atherosclerotic plaques were notable for the massive accumulation of lipids in the extracellular matrix (ECM), abundant macrophage content, and pronounced neovascularisation associated with blood leakage and calcium deposition. In contrast, CAVD and SVD generally did not require vasculo- or angiogenesis to occur, instead relying on fatigue-induced ECM degradation and the concurrent migration of immune cells. Unlike native tissues, bioprosthetic heart valves contained numerous specialised macrophages and were not capable of the regeneration that underscores ECM integrity as a pivotal factor for SVD prevention. While atherosclerosis, CAVD, and SVD show similar pathogenesis patterns, these disorders demonstrate considerable ultrastructural differences.

Investigation of bone matrix composition, architecture and mechanical properties reflect structure-function relationship of cortical bone in glucocorticoid induced osteoporosis

Glucocorticoid induced osteoporosis (GIOP) is the most common negative consequence of long-term glucocorticoid treatment, leading to increased fracture risk followed by loss of mobility and high mortality risk. These biologically induced changes in bone quality at molecular level lead to changes both in bone matrix architecture and bone matrix composition. However, the quantitative details of changes in bone quality - and especially their link to reduced macroscale mechanical properties are still largely missing. In this study, a mouse model for glucocorticoid-induced osteoporosis (GIOP) was used to investigate mechanical and material alterations in bone cortex (natural nanocomposite) at different scale. By combining quantitative backscattered electron (qBSE) imaging, nanoindentation and high brilliance synchrotron X-ray nanomechanical imaging on a genetically modified mouse model of GIOP, we were able to quantify the local indentation modulus, mineralization distribution and the alterations of nanoscale structures and deformation mechanisms in the mid-diaphysis of femur, and relate them to the macroscopic mechanical changes. Our results showed clear and significant changes in terms of material quality of bone at nanoscale and microscale, which manifests itself in development of spatial heterogeneities in mineralization and indentation moduli across the bone organ, with potential implications for increased fracture risk.

Mechanical properties of stingray tesserae: High-resolution correlative analysis of mineral density and indentation moduli in tessellated cartilage

Skeletal tissues are built and shaped through complex, interacting active and passive processes. These spatial and temporal variabilities make interpreting growth mechanisms from morphology difficult, particularly in bone, where the remodeling process erases and rewrites local structural records of growth throughout life. In contrast to the majority of bony vertebrates, the elasmobranch fishes (sharks, rays, and their relatives) have skeletons made of cartilage, reinforced by an outer layer of mineralized tiles (tesserae), which are believed to grow only by deposition, without remodeling. We exploit this structural permanence, performing the first fine-scale correlation of structure and material properties in an elasmobranch skeleton. Our characterization across an age series of stingray tesserae allows unique insight into the growth processes and mechanical influences shaping the skeleton. Correlated quantitative backscattered electron imaging (qBEI) and nanoindentation measurements show a positive relationship between mineral density and tissue stiffness/hardness. Although tessellated cartilage as a whole (tesserae plus unmineralized cartilage) is considerably less dense than bone, we demonstrate that tesserae have exceptional local material properties, exceeding those of (mammal) bone and calcified cartilage. We show that the finescale ultrastructures recently described in tesserae have characteristic material properties suggesting distinct mechanical roles and that regions of high mineral density/stiffness in tesserae are confined predominantly to regions expected to bear high loads. In particular, tesseral spokes (laminated structures flanking joints) exhibit particularly high mineral densities and tissue material properties, more akin to teeth than bone or calcified cartilage. We conclude that these spokes toughen tesserae and reinforce points of contact between them. These toughening and reinforcing functions are supported by finite element simulations incorporating our material data. The high stresses predicted for spokes, and evidence we provide that new spoke laminae are deposited according to their local mechanical environment, suggest tessellated cartilage is both mutable and responsive, despite lacking remodeling capability. STATEMENT OF SIGNIFICANCE: The study of vertebrate skeletal materials is heavily biased toward mammal bone, despite evidence that bone and cartilage are extremely diverse. We broaden the perspective on vertebrate skeleton materials and evolution in an investigation of stingray tessellated cartilage, a curious type of unmineralized cartilage with a shell of mineralized tiles (tesserae). Combining high-resolution imaging and material testing, we demonstrate that tesserae have impressive local material properties for a vertebrate skeletal tissue, arguing for unique tissue organization relative to mammalian calcified cartilage and bone. Incorporating our materials data into mechanical models, we show that finescale material arrangements allow this cartilage to act as a functional and responsive alternative to bone, despite lacking bone's ability to remodel. These results are relevant to a diversity of researchers, from skeletal, developmental, and evolutionary biologists, to materials scientists interested in high-performance, low-density composites.

High-resolution large-area imaging of nanoscale structure and mineralization of a sclerosing osteosarcoma in human bone

Osteosarcoma is the most common primary bone cancer type in humans. It is predominantly found in young individuals, with a second peak later in life. The tumour is formed by malignant osteoblasts and consists of collagenous, sometimes also mineralized, bone matrix. While the morphology of osteosarcoma has been well studied, there is virtually no information about the nanostructure of the tumour and changes in mineralization on the nanoscale level. In the present paper, human bone tissue inside, next to and remote from a sclerosing osteosarcoma was studied with small angle x-ray scattering, x-ray diffraction and electron microscopy. Quantitative evaluation of nanostructure parameters was combined with high resolution, large area mapping to obtain microscopic images with nanostructure parameter contrast. It was found that the tumour regions were characterized by a notable reduction in mineral particle size, while the mineral content was even higher than that in normal bone. Furthermore, the normal preferential orientation of mineral particles along the longitudinal direction of corticalis or trabeculae was largely suppressed. Also the bone mineral crystal structure was affected: severe crystal lattice distortions were detected in mineralized tumour tissue pointing to a different ion substitution of hydroxyl apatite in tumorous tissue than in healthy tissue.

50 years of scanning electron microscopy of bone-a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

Bone is an architecturally complex system that constantly undergoes structural and functional optimisation through renewal and repair. The scanning electron microscope (SEM) is among the most frequently used instruments for examining bone. It offers the key advantage of very high spatial resolution coupled with a large depth of field and wide field of view. Interactions between incident electrons and atoms on the sample surface generate backscattered electrons, secondary electrons, and various other signals including X-rays that relay compositional and topographical information. Through selective removal or preservation of specific tissue components (organic, inorganic, cellular, vascular), their individual contribution(s) to the overall functional competence can be elucidated. With few restrictions on sample geometry and a variety of applicable sample-processing routes, a given sample may be conveniently adapted for multiple analytical methods. While a conventional SEM operates at high vacuum conditions that demand clean, dry, and electrically conductive samples, non-conductive materials (e.g., bone) can be imaged without significant modification from the natural state using an environmental scanning electron microscope. This review highlights important insights gained into bone microstructure and pathophysiology, bone response to implanted biomaterials, elemental analysis, SEM in paleoarchaeology, 3D imaging using focused ion beam techniques, correlative microscopy and in situ experiments. The capacity to image seamlessly across multiple length scales within the meso-micro-nano-continuum, the SEM lends itself to many unique and diverse applications, which attest to the versatility and user-friendly nature of this instrument for studying bone. Significant technological developments are anticipated for analysing bone using the SEM.

Melorheostotic Bone Lesions Caused by Somatic Mutations in MAP2K1 Have Deteriorated Microarchitecture and Periosteal Reaction

Melorheostosis is a rare non-hereditary condition characterized by dense hyperostotic lesions with radiographic "dripping candle wax" appearance. Somatic activating mutations in MAP2K1 have recently been identified as a cause of melorheostosis. However, little is known about the development, composition, structure, and mechanical properties of the bone lesions. We performed a multi-method phenotype characterization of material properties in affected and unaffected bone biopsy samples from six melorheostosis patients with MAP2K1 mutations. On standard histology, lesions show a zone with intensively remodeled osteonal-like structure and prominent osteoid accumulation, covered by a shell formed through bone apposition, consisting of compact multi-layered lamellae oriented parallel to the periosteal surface and devoid of osteoid. Compared with unaffected bone, melorheostotic bone has lower average mineralization density measured by quantitative backscattered electron imaging (CaMean: -4.5%, p = 0.04). The lamellar portion of the lesion is even less mineralized, possibly because the newly deposited material has younger tissue age. Affected bone has higher porosity by micro-CT, due to increased tissue vascularity and elevated 2D-microporosity (osteocyte lacunar porosity: +39%, p = 0.01) determined on quantitative backscattered electron images. Furthermore, nano-indentation modulus characterizing material hardness and stiffness was strictly dependent on tissue mineralization (correlation with typical calcium concentration, CaPeak: r = 0.8984, p = 0.0150, and r = 0.9788, p = 0.0007, respectively) in both affected and unaffected bone, indicating that the surgical hardness of melorheostotic lesions results from their lamellar structure. The results suggest a model for pathophysiology of melorheostosis caused by somatic activating mutations in MAP2K1, in which the genetically induced gradual deterioration of bone microarchitecture triggers a periosteal reaction, similar to the process found to occur after bone infection or local trauma, and leads to an overall cortical outgrowth. The micromechanical properties of the lesions reflect their structural heterogeneity and correlate with local variations in mineral content, tissue age, and remodeling rates, in the same way as normal bone. © 2018 American Society for Bone and Mineral Research.

Hypermineralization in the femoral neck of the elderly

Hip fragility depends on the decline in bone mass as well as changes in bone microstructure and the properties of bone mineral and organic matrix. Although it is well-established that low bone mass or osteoporosis is a key factor in hip fracture risk, it is striking to observe that 92% of 24 patients who have sustained an intracapsular hip fracture showed hypermineralization at the superior-anterior quadrant, a critical region associated with increased hip fracture risk. In-depth material studies on a total of 12 human cadaver femurs revealed increased degree of mineralization in the hypermineralized tissue: calcium weight percentage as measured by quantitative backscattered electron imaging increased by approximately 15% compared with lamellar bone; mineral-to-matrix ratio obtained by Raman microspectroscopy imaging also increased. Immunohistochemistry revealed localized type II collagen in the hypermineralized region, implying its cartilaginous nature. At the ultrastructural level, X-ray scattering revealed significantly smaller (on average 2.3 nm thick and 15.6 nm long) and less ordered bone minerals in the hypermineralized tissue. Finally, the hypermineralized tissue was more brittle than lamellar bone under hydrated state - cracks propagated easily in the hypermineralized region but stopped at the lamellar boundary. This study demonstrates that hypermineralization of femoral neck cortical bone is a source of bone fragility which is worth considering in future fracture risk assessment when the origin of hip fracture is unclear based on current evaluation standards. STATEMENT OF SIGNIFICANCE: Hypermineralization of femoral cortical bone in older adults might occur in many more hip fracture cases than presently known. Yet, this tissue remains largely unknown to the orthopedic community possibly due to coarse resolution of clinical imaging. The current study showed the hypermineralized tissue had reduced fracture resistance which could be attributed to the material changes in mineral content, organic matrix, and mineral platelets properties. It thus could be a source for fracture initiation. Consequently, we believe hypermineralization of femoral neck cortical bone should be considered in hip fragility assessment, especially when low bone mass cannot be identified as a primary contributor to hip fracture.

Mechanical behavior of metastatic vertebrae are influenced by tissue architecture, mineral content, and organic feature alterations

Diminished vertebral mechanical behavior with metastatic involvement is typically attributed to modified architecture and trabecular bone content. Previous work has identified organic and mineral phase bone quality changes in the presence of metastases, yet limited work exists on the potential influence of such tissue level modifications on vertebral mechanical characteristics. This work seeks to determine correlations between features of bone (structural and tissue level) and mechanical behavior in metastatically involved vertebral bone. It is hypothesized that tissue level properties (mineral and organic) will improve these correlations beyond architectural properties and BMD alone. Twenty-four female athymic rats were inoculated with HeLa or Ace-1 cancer cells lines producing osteolytic (N = 8) or mixed (osteolytic/osteoblastic, N = 7) metastases, respectively. Twenty-one days post-inoculation L1-L3 pathologic vertebral motion segments were excised and μCT imaged. 3D morphometric parameters and axial rigidity of the L2 vertebrae were quantified. Sequential loading and μCT imaging measured progression of failure, stiffness and peak force. Relationships between mechanical testing (whole bone and tissue-level) and tissue-level material property modifications with metastatic involvement were evaluated utilizing linear regression models. Osteolytic involvement reduced vertebral trabecular bone volume, structure, CT-derived axial rigidity, stiffness and failure force compared to healthy controls (N = 9). Mixed metastases demonstrated similar trends. Previously assessed collagen cross-linking and proline-based residues were correlated to mechanical behavior and improved the predictive ability of the regression models. Similarly, collagen organization improved predictive regression models for metastatic bone hardness. This work highlights the importance of both bone content/architecture and organic tissue-level features in characterizing metastatic vertebral mechanics. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3013-3022, 2018.

Assessing bone quality through mechanical properties in postmenopausal trabecular bone

BACKGROUND

The inner structure of trabecular bone is a result of structural optimization provided by remodeling processes. Changes in hormonal status related to menopause cause bone tissue loss and micro-architectural deterioration with a consequent susceptibility to fracture. Accumulation of micro-damage in bone, as a function of the rate of production and rate of repair, underlies the development of stress fractures, increasing fragility associated to age and osteoporosis, especially in transmenopausal women.

PATIENTS AND METHODS

Quasi-static and nano-dynamic mechanical characterization were undertaken in trabecular bone from femoral neck biopsies of postmenopausal women. AFM (Atomic Force Microscopy) complementary studies were performed to determine nano-roughness (SRa) and the fibrils width of collagen. Nanoindentations were used to quantify transmenopausal changes in intrinsic mechanical properties of trabecular bone: hardness (Hi), modulus of Young (Ei), complex modulus (E*), tan delta (δ), storage modulus (E') and loss modulus (E").

RESULTS

As result of the quasi-static measurements, 0.149 (0.036) GPa and 2.95 (0.73) GPa of Hi and Ei were obtained, respectively. As result of the nano-dynamic measurements, 17.94 (3.15), 0.62 (0.10), 13.79 (3.21 and 6.39 (1.28) GPa of E*, tan (δ), E' and E" were achieved, respectively. 101.07 SRa and 831.28 nm of fibrils width were additionally obtained.

CONCLUSIONS

This study poses a first approach to the measurement of bone quality in postmenopausal trabecular bone by combining quasi-static, nano-DMA analysis and tribology of dentin surface through AFM characterization.

On the Relation of Bone Mineral Density and the Elastic Modulus in Healthy and Pathologic Bone

PURPOSE OF REVIEW

Osteoporosis could lead to the bone mechanical failure. To examine the bone health, mechanical properties are often estimated from the images of the bone density. Here, we review the relationships that have been experimentally determined between mineral density and the elastic modulus and factors that affect these relationships.

RECENT FINDINGS

Studies, which have investigated the relation between the elastic modulus and bone mineral at the bulk scale, have shown that approximately 70% of variations in the elastic modulus can be explained based on the amount of mineral in bone. At the tissue level, however, higher resolution techniques are used to characterize the density and modulus more locally, and this leads to the correlation of mineral with modulus to be not as strong as that of the bulk level and often times, insignificant. This observation indicates the importance of structural hierarchy and mineral crystal organization in determining the local stiffness of the bone tissue. At the bulk level in bone (cm scale), modulus (E) is related to density (ρ) through a power law relationship (E ∝ ρ^α). At the tissue level (μm-mm scale), the relationship between the modulus and density is weak, likely due to the effect of microstructural features at small length scales.

Effects of Moisture Content and Loading Profile on Changing Properties of Bone Micro-Biomechanical Characteristics

Background

Our study explored the influences of hydration conditions and loading methods on the mechanical properties of cortical bones and cancellous bones.

Material/Methods

Elastic modulus and hardness of human cortical bones and cancellous bones that contained different moisture levels (20%, 30%, 40%, 50%, and 60%) were measured with nanoindentation with different peak loads and loading rates. Cortical bones with 20% and 60% moisture were tested with 30 nm, 40 nm, and 50 nm peak loads at 6 nm/s, 8 nm/s, and 10 nm/s loading rates, respectively. Cancellous bones with 5% or 40% moisture percentages were tested with 600 μN, 750 μN, and 1000 μN peak loads at 200 μN/s, 250 μN/s, and 333 μN/s loading rates, respectively.

Results

Under the same loading condition, specimens with higher moisture contents showed decreased elastic modulus and hardness. Under different loading conditions, the loading modes had little influence on elastic modulus and hardness of cortical bone and cancellous bone with low moisture, but had significant influence on specimens with higher moistures.

Conclusions

The elastic modulus and bone hardness were affected by the moisture content and the loading conditions in cortical and cancellous bones with high hydration condition but not in those with low hydration condition.

Multiscale characterization of cortical bone composition, microstructure, and nanomechanical properties in experimentally induced osteoporosis

Cortical bone plays a vital role in determining overall bone strength. We investigate the structural, compositional, and nanomechanical properties of cortical bone following ovariectomy (OVX) of 12-week-old Sprague Dawley rats, since this animal model is frequently employed to evaluate the performance of implantable biomaterials in compromised bone healing conditions. Morphological parameters and material properties of bone in the geometrical center of the femoral cortex were investigated four and eight weeks post-OVX and in unoperated controls (Ctrl), using X-ray micro-computed tomography, backscattered electron scanning electron microscopy, Raman spectroscopy, and nanoindentation. The OVX animals showed increase in body weight, diminished bone mineral density, increased intracortical porosity, but increased bone mass through periosteal apposition (e.g., increases in periosteal perimeter, cortical cross-sectional thickness, and cross-sectional area). However, osteocyte densities, osteocyte lacunar dimensions, and the nanomechanical behavior on the single mineralized collagen fibril level remained unaffected. Our correlative multiscale investigation provides structural, chemical, and nanomechanical evidence substantiating earlier reports suggesting that rats ovariectomized at 12 weeks undergo simultaneous bone loss and growth, resulting in the effects of OVX being less obvious. Periosteal apposition contradicts the conventional view of bone loss in osteoporosis but appears advantageous for the greater functional demand imposed on the skeleton by increased body weight and fragility induced by increased intracortical porosity. Through a variety of morphological changes, it is likely that 12-week-old rats are able to adapt to OVX-related microstructural and compositional alterations. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 997-1007, 2018.

Intrinsic material property differences in bone tissue from patients suffering low-trauma osteoporotic fractures, compared to matched non-fracturing women

Osteoporotic (low-trauma) fractures are a significant public health problem. Over 50% of women over 50yrs. of age will suffer an osteoporotic fracture in their remaining lifetimes. While current therapies reduce skeletal fracture risk by maintaining or increasing bone density, additional information is needed that includes the intrinsic material strength properties of bone tissue to help develop better treatments, since measurements of bone density account for no more than ~50% of fracture risk. The hypothesis tested here is that postmenopausal women who have sustained osteoporotic fractures have reduced bone quality, as indicated with measures of intrinsic material properties compared to those who have not fractured. Transiliac biopsies (N=120) were collected from fracturing (N=60, Cases) and non-fracturing postmenopausal women (N=60, age- and BMD-matched Controls) to measure intrinsic material properties using the nano-indentation technique. Each biopsy specimen was embedded in epoxy resin and then ground, polished and used for the nano-indentation testing. After calibration, multiple indentations were made using quasi-static (hardness, modulus) and dynamic (storage and loss moduli) testing protocols. Multiple indentations allowed the median and variance to be computed for each type of measurement for each specimen. Cases were found to have significantly lower median values for cortical hardness and indentation modulus. In addition, cases showed significantly less within-specimen variability in cortical modulus, cortical hardness, cortical storage modulus and trabecular hardness, and more within-specimen variability in trabecular loss modulus. Multivariate modeling indicated the presence of significant independent mechanical effects of cortical loss modulus, along with variability of cortical storage modulus, cortical loss modulus, and trabecular hardness. These results suggest mechanical heterogeneity of bone tissue may contribute to fracture resistance. Although the magnitudes of differences in the intrinsic properties were not overwhelming, this is the first comprehensive study to investigate, and compare the intrinsic properties of bone tissue in fracturing and non-fracturing postmenopausal women.

Presence of pyrophosphate in bone from an atypical femoral fracture site: A case report

Long-term antiresorptives use has been linked to atypical subtrochanteric and diaphyseal femoral fractures (AFF), the pathogenesis of which is still unknown. In the present case report we present the results of analysis of bone chips from a 74-year old female patient that had been on alendronate, ibandronate and denosumab treatment, and who sustained an atypical femoral fracture, by histology, quantitative backscattered electron imaging, and Raman spectroscopic analysis.

The results indicate ongoing osteoclastic resorption, but also several abnormalities: 1) an altered arrangement of osteons; 2) impaired mineralization; 3) the presence of pyrophosphate, which might contribute to the impaired mineralization evident in the present case. Taken together, these changes may contribute to the focally reduced bone strength of this patient.

Role of cortical bone in hip fracture

In this review, I consider the varied mechanisms in cortical bone that help preserve its integrity and how they deteriorate with aging. Aging affects cortical bone in two ways: extrinsically through its effects on the individual that modify its mechanical loading experience and 'milieu interieur'; and intrinsically through the prolonged cycle of remodelling and renewal extending to an estimated 20 years in the proximal femur. Healthy femoral cortex incorporates multiple mechanisms that help prevent fracture. These have been described at multiple length scales from the individual bone mineral crystal to the scale of the femur itself and appear to operate hierarchically. Each cortical bone fracture begins as a sub-microscopic crack that enlarges under mechanical load, for example, that imposed by a fall. In these conditions, a crack will enlarge explosively unless the cortical bone is intrinsically tough (the opposite of brittle). Toughness leads to microscopic crack deflection and bridging and may be increased by adequate regulation of both mineral crystal size and the heterogeneity of mineral and matrix phases. The role of osteocytes in optimising toughness is beginning to be worked out; but many osteocytes die in situ without triggering bone renewal over a 20-year cycle, with potential for increasing brittleness. Furthermore, the superolateral cortex of the proximal femur thins progressively during life, so increasing the risk of buckling during a fall. Besides preserving or increasing hip BMD, pharmaceutical treatments have class-specific effects on the toughness of cortical bone, although dietary and exercise-based interventions show early promise.

Synchrotron radiation micro X-ray fluorescence spectroscopy of thin structures in bone samples: comparison of confocal and color X-ray camera setups

In the quest for finding the ideal synchrotron-radiation-induced imaging method for the investigation of trace element distributions in human bone samples, experiments were performed using both a scanning confocal synchrotron radiation micro X-ray fluorescence (SR-µXRF) (FLUO beamline at ANKA) setup and a full-field color X-ray camera (BAMline at BESSY-II) setup. As zinc is a trace element of special interest in bone, the setups were optimized for its detection. The setups were compared with respect to count rate, required measurement time and spatial resolution. It was demonstrated that the ideal method depends on the element of interest. Although for Ca (a major constituent of the bone with a low energy of 3.69 keV for its Kα XRF line) the color X-ray camera provided a higher resolution in the plane, for Zn (a trace element in bone) only the confocal SR-µXRF setup was able to sufficiently image the distribution.

Tissue-Level Mechanical Properties of Bone Contributing to Fracture Risk

Tissue-level mechanical properties characterize mechanical behavior independently of microscopic porosity. Specifically, quasi-static nanoindentation provides measurements of modulus (stiffness) and hardness (resistance to yielding) of tissue at the length scale of the lamella, while dynamic nanoindentation assesses time-dependent behavior in the form of storage modulus (stiffness), loss modulus (dampening), and loss factor (ratio of the two). While these properties are useful in establishing how a gene, signaling pathway, or disease of interest affects bone tissue, they generally do not vary with aging after skeletal maturation or with osteoporosis. Heterogeneity in tissue-level mechanical properties or in compositional properties may contribute to fracture risk, but a consensus on whether the contribution is negative or positive has not emerged. In vivo indentation of bone tissue is now possible, and the mechanical resistance to microindentation has the potential for improving fracture risk assessment, though determinants are currently unknown.

Bone's Material Constituents and their Contribution to Bone Strength in Health, Disease, and Treatment

Type 1 collagen matrix volume, its degree of completeness of its mineralization, the extent of collagen crosslinking and water content, and the non-collagenous proteins like osteopontin and osteocalcin comprise the main constituents of bone's material composition. Each influences material strength and change in different ways during advancing age, health, disease, and drug therapy. These traits are not quantifiable using bone densitometry and their plurality is better captured by the term bone 'qualities' than 'quality'. These qualities are the subject of this manuscript.

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.

Multi-level femoral morphology and mechanical properties of rats of different ages

A macro-micro-nano-multi-level study was conducted to explore age-related structural and mechanical properties of bone, as well as the effects of aging on bone properties. A total of 70 male Wistar rats were used, ranging in the ages of 1, 3, 5, 7, 9, 11, 14, 15, 16, and 17 months (n = 7/age group). After micro-computed tomography (CT) scanning, longitudinal cortical bone specimens with a length of 5mm were cut along the femoral shaft axis from left femur shafts for mechanical testing, and the cross-sectional areas were measured. The macro-mechanical properties obtained in mechanical testing and microarchitecture parameters measured by micro-CT were significantly correlated with the animal age (r(2) = 0.96, p < 0.001). Scanning electron microscopy was used for detecting the microarchitecture features of the fractured surfaces, which exhibited age-related plate-fibrous-mixed fibrous-plate texture, resulting in changes in macro-mechanical properties (r(2) > 0.90, p < 0.001). The mineral phase of the left femoral shaft and head was analyzed by atomic force microscopy. Longitudinal and transverse trabecular bone tissues, as well as longitudinal cortical bone tissue, were used for nanoindentation test, and the chemical composition was evaluated by quantitative chemical analyses. The correlations between mineral content and bone material properties (i.e., elastic properties of the bone tissue and size and roughness of bone mineral grains) were highly significant (r > 0.95, p < 0.001). Multi-level femur morphology, mechanical property, and mineral content were significantly correlated with the animal age. The correlations between bone mineral content and bone material morphological and mechanical properties may partly explain the increase in bone fragility with aging, which will provide a theoretical basis for the investigation of age-related bone properties in clinics.

Distribution of mesoscale elastic properties and mass density in the human femoral shaft

Cortical bone properties are determined by tissue composition and structure at several hierarchical length scales. In this study, the spatial distribution of micro- and mesoscale elastic properties within a human femoral shaft has been investigated. Microscale tissue degree of mineralization (DMB), cortical vascular porosity Ct.Po and the average transverse isotropic stiffness tensor C(Micro) of cylindrical-shaped samples (diameter: 4.4 mm, N = 56) were obtained from cortical regions between 20 and 85% of the total femur length and around the periphery (anterior, medial, posterior and lateral quadrants) by means of synchrotron radiation µCT (SRµCT) and 50-MHz scanning acoustic microscopy (SAM). Within each cylinder, the volumetric bone mineral density (vBMD) and the mesoscale stiffness tensor C(Meso) were derived using a numerical homogenization approach. Moreover, microelastic maps of the axial elastic coefficient c33 measured by SAM at distinct cross-sectional locations along the femur were used to construct a 3-D multiscale elastic model of the femoral shaft. Variations of vBMD (6.1%) were much lower than the variations of mesoscale elastic coefficients (11.1-21.3%). The variation of DMB was only a minor predictor for variations of the mesoscale elastic properties (0.05 ≤ R(2) ≤ 0.34). Instead, variations of the mesoscale elastic properties could be explained by variations of cortical porosity and microscale elastic properties. These data were suitable inputs for numerical evaluations and may help to unravel the relations between structure and composition on the elastic function in cortical bone.

Multi-level characterization of human femoral cortices and their underlying osteocyte network reveal trends in quality of young, aged, osteoporotic and antiresorptive-treated bone

Characterization of bone's hierarchical structure in aging, disease and treatment conditions is imperative to understand the architectural and compositional modifications to the material and its mechanical integrity. Here, cortical bone sections from 30 female proximal femurs - a frequent fracture site - were rigorously assessed to characterize the osteocyte lacunar network, osteon density and patterns of bone matrix mineralization by backscatter-electron imaging and Fourier-transform infrared spectroscopy in relation to mechanical properties obtained by reference-point indentation. We show that young, healthy bone revealed the highest resistance to mechanical loading (indentation) along with higher mineralization and preserved osteocyte-lacunar characteristics. In contrast, aging and osteoporosis significantly alter bone material properties, where impairment of the osteocyte-lacunar network was evident through accumulation of hypermineralized osteocyte lacunae with aging and even more in osteoporosis, highlighting increased osteocyte apoptosis and reduced mechanical competence. But antiresorptive treatment led to fewer mineralized lacunae and fewer but larger osteons signifying rejuvenated bone. In summary, multiple structural and compositional changes to the bone material were identified leading to decay or maintenance of bone quality in disease, health and treatment conditions. Clearly, antiresorptive treatment reflected favorable effects on the multifunctional osteocytic cells that are a prerequisite for bone's structural, metabolic and mechanosensory integrity.

Sclerostin deficiency is linked to altered bone composition

High bone mass in animals and humans with sclerostin deficiency is associated with increased bone strength, which is not the case for all disorders with high bone mineral density, some of which are even associated with fragility fractures owing to unfavorable bone composition. In the current study we investigated whether alterations in bone composition may contribute to the bone strength characteristics associated with lack of sclerostin. We examined cortical bone of Sost-knockout (KO) mice (n = 9, 16 weeks old) and sclerosteosis patients (young [4 to 14 years], n = 4 and adults [24 and 43 years], n = 2) by quantitative backscattered electron imaging and Raman microspectroscopy and compared it to bone from wild-type mice and healthy subjects, respectively. In Sost-KO mice endocortical bone exhibited altered bone composition, whereas subperiosteal bone was unchanged. When comparing endocortical bone tissue of identical tissue age as defined by sequential dual fluorochrome labeling the average bone matrix mineralization was reduced -1.9% (p < 0.0001, younger tissue age) and -1.5% (p < 0.05, older tissue age), and the relative proteoglycan content was significantly increased. Similarly, bone matrix mineralization density distribution was also shifted toward lower matrix mineralization in surgical samples of compact bone of sclerosteosis patients. This was associated with an increase in mineralization heterogeneity in the young population. In addition, and consistently, the relative proteoglycan content was increased. In conclusion, we observed decreased matrix mineralization and increased relative proteoglycan content in bone subcompartments of Sost-KO mice-a finding that translated into sclerosteosis patients. We hypothesize that the altered bone composition contributes to the increased bone strength of patients with sclerostin deficiency.

Relationship of bone mineralization density distribution (BMDD) in cortical and cancellous bone within the iliac crest of healthy premenopausal women

Bone mineralization density distribution (BMDD) is an important determinant of bone mechanical properties. The most available skeletal site for access to the BMDD is the iliac crest. Compared to cancellous bone much less information on BMDD is available for cortical bone. Hence, we analyzed complete transiliac crest bone biopsy samples from premenopausal women (n = 73) aged 25-48 years, clinically classified as healthy, by quantitative backscattered electron imaging for cortical (Ct.) and cancellous (Cn.) BMDD. The Ct.BMDD was characterized by the arithmetic mean of the BMDD of the cortical plates. We found correlations between Ct. and Cn. BMDD variables with correlation coefficients r between 0.42 and 0.73 (all p < 0.001). Additionally to this synchronous behavior of cortical and cancellous compartments, we found that the heterogeneity of mineralization densities (Ct.Ca(Width)), as well as the cortical porosity (Ct.Po) was larger for a lower average degree of mineralization (Ct.Ca(Mean)). Moreover, Ct.Po correlated negatively with the percentage of highly mineralized bone areas (Ct.Ca(High)) and positively with the percentage of lowly mineralized bone areas (Ct.Ca(Low)). In conclusion, the correlation of cortical with cancellous BMDD in the iliac crest of the study cohort suggests coordinated regulation of bone turnover between both bone compartments. Only in a few cases, there was a difference in the degree of mineralization of >1wt % between both cortices suggesting a possible modeling situation. This normative dataset of healthy premenopausal women will provide a reference standard by which disease- and treatment-specific effects can be assessed at the level of cortical bone BMDD.

Reduced tissue-level stiffness and mineralization in osteoporotic cancellous bone

Osteoporosis alters bone mass and composition ultimately increasing the fragility of primarily cancellous skeletal sites; however, effects of osteoporosis on tissue-level mechanical properties of cancellous bone are unknown. Dual-energy X-ray absorptiometry (DXA) scans are the clinical standard for diagnosing osteoporosis though changes in cancellous bone mass and mineralization are difficult to separate using this method. The goal of this study was to investigate possible difference in tissue-level properties with osteoporosis as defined by donor T scores. Spine segments from Caucasian female cadavers (58-92 years) were used. A T score for each donor was calculated from DXA scans to determine osteoporotic status. Tissue-level composition and mechanical properties of vertebrae adjacent to the scan region were measured using nanoindentation and Raman spectroscopy. Based on T scores, six samples were in the Osteoporotic group (58-74 years) and four samples were in the Not Osteoporotic group (65-92 years). The indentation modulus and mineral to matrix ratio (mineral:matrix) were lower in the Osteoporotic group than the Not Osteoporotic group. Mineral:matrix ratio decreased with age (r (2) = 0.35, p = 0.05), and the indentation modulus increased with areal bone mineral density (r (2) = 0.41, p = 0.04). This study is the first to examine cancellous bone composition and mechanical properties from a fracture prone location with osteoporosis. We found differences in tissue composition and mechanical properties with osteoporosis that could contribute to increased fragility in addition to changes in trabecular architecture and bone volume.

Insights into reference point indentation involving human cortical bone: sensitivity to tissue anisotropy and mechanical behavior

Reference point indentation (RPI) is a microindentation technique involving 20 cycles of loading in "force-control" that can directly assess a patient׳s bone tissue properties. Even though preliminary clinical studies indicate a capability for fracture discrimination, little is known about what mechanical behavior the various RPI properties characterize and how these properties relate to traditional mechanical properties of bone. To address this, the present study investigated the sensitivity of RPI properties to anatomical location and tissue organization as well as examined to what extent RPI measurements explain the intrinsic mechanical properties of human cortical bone. Multiple indents with a target force of 10N were done in 2 orthogonal directions (longitudinal and transverse) per quadrant (anterior, medial, posterior, and lateral) of the femoral mid-shaft acquired from 26 donors (25-101 years old). Additional RPI measurements were acquired for 3 orthogonal directions (medial only). Independent of age, most RPI properties did not vary among these locations, but they did exhibit transverse isotropy such that resistance to indentation is greater in the longitudinal (axial) direction than in the transverse direction (radial or circumferential). Next, beam specimens (~2mm×5mm×40mm) were extracted from the medial cortex of femoral mid-shafts, acquired from 34 donors (21-99 years old). After monotonically loading the specimens in three-point bending to failure, RPI properties were acquired from an adjacent region outside the span. Indent direction was orthogonal to the bending axis. A significant inverse relationship was found between resistance to indentation and the apparent-level mechanical properties. Indentation distance increase (IDI) and a linear combination of IDI and the loading slope, averaged over cycles 3 through 20, provided the best explanation of the variance in ultimate stress (r(2)=0.25, p=0.003) and toughness (r(2)=0.35, p=0.004), respectively. With a transverse isotropic behavior akin to tissue hardness and modulus as determined by micro- and nano-indentation and a significant association with toughness, RPI properties are likely influenced by both elastic and plastic behavior of bone tissue.

Modeling of femoral neck cortical bone for the numerical simulation of ultrasound propagation

Quantitative ultrasound assessment of the cortical compartment of the femur neck (FN) is investigated with the goal of achieving enhanced fracture risk prediction. Measurements at the FN are influenced by bone size, shape and material properties. The work described here was aimed at determining which FN material properties have a significant impact on ultrasound propagation around 0.5 MHz and assessing the relevancy of different models. A methodology for the modeling of ultrasound propagation in the FN, with a focus on the modeling of bone elastic properties based on scanning acoustic microscopy data, is introduced. It is found that the first-arriving ultrasound signal measured in through-transmission at the FN is not influenced by trabecular bone properties or by the heterogeneities of the cortical bone mineralized matrix. In contrast, the signal is sensitive to variations in cortical porosity, which can, to a certain extent, be accounted for by effective properties calculated with the Mori-Tanaka method.

The fragile elderly hip: mechanisms associated with age-related loss of strength and toughness

Every hip fracture begins with a microscopic crack, which enlarges explosively over microseconds. Most hip fractures in the elderly occur on falling from standing height, usually sideways or backwards. The typically moderate level of trauma very rarely causes fracture in younger people. Here, this paradox is traced to the decline of multiple protective mechanisms at many length scales from nanometres to that of the whole femur. With normal ageing, the femoral neck asymmetrically and progressively loses bone tissue precisely where the cortex is already thinnest and is also compressed in a sideways fall. At the microscopic scale of the basic remodelling unit (BMU) that renews bone tissue, increased numbers of actively remodelling BMUs associated with the reduced mechanical loading in a typically inactive old age augments the numbers of mechanical flaws in the structure potentially capable of initiating cracking. Menopause and over-deep osteoclastic resorption are associated with incomplete BMU refilling leading to excessive porosity, cortical thinning and disconnection of trabeculae. In the femoral cortex, replacement of damaged bone or bone containing dead osteocytes is inefficient, impeding the homeostatic mechanisms that match strength to habitual mechanical usage. In consequence the participation of healthy osteocytes in crack-impeding mechanisms is impaired. Observational studies demonstrate that protective crack deflection in the elderly is reduced. At the most microscopic levels attention now centres on the role of tissue ageing, which may alter the relationship between mineral and matrix that optimises the inhibition of crack progression and on the role of osteocyte ageing and death that impedes tissue maintenance and repair. This review examines recent developments in the understanding of why the elderly hip becomes fragile. This growing understanding is suggesting novel testable approaches for reducing risk of hip fracture that might translate into control of the growing worldwide impact of hip fractures on our ageing populations.

Nano-structural, compositional and micro-architectural signs of cortical bone fragility at the superolateral femoral neck in elderly hip fracture patients vs. healthy aged controls

To unravel the origins of decreased bone strength in the superolateral femoral neck, we assessed bone structural features across multiple length scales at this cortical fracture initiating region in postmenopausal women with hip fracture and in aged-matched controls. Our combined methodological approach encompassed atomic force microscopy (AFM) characterization of cortical bone nano-structure, assessment of mineral content/distribution via quantitative backscattered electron imaging (qBEI), measurement of bone material properties by reference point indentation, as well as evaluation of cortical micro-architecture and osteocyte lacunar density. Our findings revealed a wide range of differences between the fracture group and the controls, suggesting a number of detrimental changes at various levels of cortical bone hierarchical organization that may render bone fragile. Namely, mineral crystals at external cortical bone surfaces of the fracture group were larger (65.22nm±41.21nm vs. 36.75nm±18.49nm, p<0.001), and a shift to a higher mineral content and more homogenous mineralization profile as revealed via qBEI were found in the bone matrix of the fracture group. Fracture cases showed nearly 35% higher cortical porosity and showed significantly reduced osteocyte lacunar density compared to controls (226±27 vs. 247±32#/mm(2), p=0.05). Along with increased crystal size, a shift towards higher mineralization and a tendency to increased cortical porosity and reduced osteocyte lacunar number delineate that cortical bone of the superolateral femoral neck bears distinct signs of fragility at various levels of its structural organization. These results contribute to the understanding of hierarchical bone structure changes in age-related fragility.

Differential accumulation of lead and zinc in double-tidemarks of articular cartilage

INTRODUCTION

Long-term exposure to increased lead (Pb) concentrations is associated with several chronic diseases. The divalent cation zinc (Zn) is essential for numerous enzymes. In a recent study we found remarkably elevated concentrations of Pb and Zn in the tidemark (TM), which is the mineralization front of human articular cartilage.

OBJECTIVE

Duplication or multiplication of TMs occurs with advancing age or degeneration. We hypothesized that trace elements accumulate in TMs as a function of time. Thus, in cases of double TMs, the deep (older) TM should contain higher Pb and Zn concentrations than the superficial (younger) TM.

DESIGN

Undecalcified tissue from articular cartilage and subchondral bone of femoral heads and patellae was examined by synchrotron radiation induced confocal micro X-ray fluorescence analysis and by quantitative backscattered electron imaging to determine the local distribution of Ca, Zn, and Pb in this tissue.

RESULTS

The evaluation of X-ray fluorescence intensities in double TMs revealed in average a 2.6-fold higher Pb level in the deep TM compared to the superficial TM while Zn concentrations were similar. Pb and Zn contents were significantly enhanced in the deep TM (Pb: 35-fold, Zn: five-fold) and in the superficial TM (Pb: 12-fold, Zn: five-fold) compared to the bone level.

CONCLUSION

For the first time a differential accumulation of Pb and Zn is documented in regions with double TMs revealing various timescales for the accumulation of these elements. Increased amounts of Pb are present in the TMs (up to the 62-fold of the bone level) featuring a potential source of internal Pb release if the TM region is destroyed.

Spatial distribution of the trace elements zinc, strontium and lead in human bone tissue

Trace elements are chemical elements in minute quantities, which are known to accumulate in the bone. Cortical and trabecular bones consist of bone structural units (BSUs) such as osteons and bone packets of different mineral content and are separated by cement lines. Previous studies investigating trace elements in bone lacked resolution and therefore very little is known about the local concentration of zinc (Zn), strontium (Sr) and lead (Pb) in BSUs of human bone. We used synchrotron radiation induced micro X-ray fluorescence analysis (SR μ-XRF) in combination with quantitative backscattered electron imaging (qBEI) to determine the distribution and accumulation of Zn, Sr, and Pb in human bone tissue. Fourteen human bone samples (10 femoral necks and 4 femoral heads) from individuals with osteoporotic femoral neck fractures as well as from healthy individuals were analyzed. Fluorescence intensity maps were matched with BE images and correlated with calcium (Ca) content. We found that Zn and Pb had significantly increased levels in the cement lines of all samples compared to the surrounding mineralized bone matrix. Pb and Sr levels were found to be correlated with the degree of mineralization. Interestingly, Zn intensities had no correlation with Ca levels. We have shown for the first time that there is a differential accumulation of the trace elements Zn, Pb and Sr in BSUs of human bone indicating different mechanisms of accumulation.

At the moment of occurrence of a fragility hip fracture, men have higher mechanical properties values in comparison with women

Background

It is well established that males have lower fracture risk in comparison with females, which suggests a higher bone resistance in men. The aim of our study was to find out if in older patients with hip fragility fractures, gender has also an impact on trabecular bone material behaviour, specifically to determine whether trabecular mechanical properties under compressive loading differ between men and women who suffered a fragility hip fracture.

Methods

Femoral epiphyses were consecutively collected during hip replacement surgery due to proximal femur fragility fracture. Trabecular bone cylinders were drilled and submitted to uniaxial compression tests and mechanical properties were assessed.

Results

Seventy-three patients, 55 women (mean age 81 years and standard deviation of 7 years) and 18 men (mean age 81 years and standard deviation of 8 years) were evaluated. The ultimate stress of trabecular bone was significantly higher in men than in women: the median values and the interquartile range (IQR) were respectively 8.04(5.35-10.90) MPa vs. 4.46(3.02-7.73) MPa, (p-value = 0.005). The same difference between male and female was observed in the Young’s modulus: 293.68(166.67-538.18) MPa vs. 174.26(73.07-322.28) MPa, (p-value = 0.028), and also in the energy to failure: 0.25(0.07-0.42) MJ/m³ vs. 0.11(0.05-0.25) MJ/m³, (p-value = 0.058). These differences were also verified after adjusting the analysis for age in a multivariate model analysis.

Conclusions

Our observations demonstrated that, even in a population who suffered a fragility hip fracture, men still have higher trabecular bone mechanical properties in comparison with women.

Atomic force microscopy characterization of the external cortical bone surface in young and elderly women: potential nanostructural traces of periosteal bone apposition during aging

On the basis of the suggestion that bone nanostructure bears “tissue age” information and may reflect surface deposition/modification processes, we performed nanoscale characterization of the external cortical bone surface at the femoral neck in women using atomic force microscopy (AFM). The specific aims were to assess age-related differences in bone nanostructure and explore the existence of nanostructural traces of potential bone apposition at this surface. Our findings revealed that the external cortical surface represents a continuous phase composed of densely packed mineral grains. Although the grains varied in size and shape, there was a domination of small grains indicative of freshly deposited bone (mean grain size: young, 35 nm; old, 37 nm; p > 0.05). Advanced quantitative analysis of surface morphological patterns revealed comparable roughness and complexity of the surface, suggesting a similar rate of mineral particle deposition at the surface in both groups. Calcium/phosphorus ratio, a measure of bone tissue age, was within the same range in both groups. In summary, our AFM analyses showed consistent nanostructural and compositional bone features, suggesting existence of new bone at the periosteal bone surface in both young and elderly women. Considering observed age-related increase in the neck diameter, AFM findings may support the theory of continuous bone apposition at the periosteal surface.

Bone mineralization: from tissue to crystal in normal and pathological contexts

Bone is a complex and structured material; its mechanical behavior results from an interaction between the properties of each level of its structural hierarchy. The degree of mineralization of bone (bone density measured at tissue level) and the characteristics of the mineral deposited (apatite crystals) are major determinants of bone strength. Bone remodeling activity acts as a regulator of the degree of mineralization and of the distribution of mineral at the tissue level, directly impacting bone mechanical properties. Recent findings have highlighted the need to understand the underlying process occurring at the nanostructure level that may be independent of bone remodeling itself. A more global comprehension of bone qualities will need further works designed to characterize what are the consequences on whole bone strength of changes at nano- or microstructure levels relative to each other.

Computational and experimental methodology for site-matched investigations of the influence of mineral mass fraction and collagen orientation on the axial indentation modulus of lamellar bone

Relationships between mineralization, collagen orientation and indentation modulus were investigated in bone structural units from the mid-shaft of human femora using a site-matched design. Mineral mass fraction, collagen fibril angle and indentation moduli were measured in registered anatomical sites using backscattered electron imaging, polarized light microscopy and nano-indentation, respectively. Theoretical indentation moduli were calculated with a homogenization model from the quantified mineral densities and mean collagen fibril orientations. The average indentation moduli predicted based on local mineralization and collagen fibers arrangement were not significantly different from the average measured experimentally with nanoindentation (p=0.9). Surprisingly, no substantial correlation of the measured indentation moduli with tissue mineralization and/or collagen fiber arrangement was found. Nano-porosity, micro-damage, collagen cross-links, non-collagenous proteins or other parameters affect the indentation measurements. Additional testing/simulation methods need to be considered to properly understand the variability of indentation moduli, beyond the mineralization and collagen arrangement in bone structural units.

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Site-matched assessment of nanoindentation modulus, mineral mass fraction and collagen fibers orientation in human cortical bone sections.

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Comparison of experimental nanoindentation modulus with its computed equivalent based on the site-matched morphological data.

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While mean experimental and computed nanoindentation moduli match well, their variations exhibit very weak correlations.

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Considering factors like nano-porosity and damage may be necessary to understand variability of lamellar stiffness of bone structural units.

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This is not in conflict with the well known anisotropy associated with the rotated plywood model at the sublamellar scale.

Microfibril orientation dominates the microelastic properties of human bone tissue at the lamellar length scale

The elastic properties of bone tissue determine the biomechanical behavior of bone at the organ level. It is now widely accepted that the nanoscale structure of bone plays an important role to determine the elastic properties at the tissue level. Hence, in addition to the mineral density, the structure and organization of the mineral nanoparticles and of the collagen microfibrils appear as potential key factors governing the elasticity. Many studies exist on the role of the organization of collagen microfibril and mineral nanocrystals in strongly remodeled bone. However, there is no direct experimental proof to support the theoretical calculations. Here, we provide such evidence through a novel approach combining several high resolution imaging techniques: scanning acoustic microscopy, quantitative scanning small-Angle X-ray scattering imaging and synchrotron radiation computed microtomography. We find that the periodic modulations of elasticity across osteonal bone are essentially determined by the orientation of the mineral nanoparticles and to a lesser extent only by the particle size and density. Based on the strong correlation between the orientation of the mineral nanoparticles and the collagen molecules, we conclude that the microfibril orientation is the main determinant of the observed undulations of microelastic properties in regions of constant mineralization in osteonal lamellar bone. This multimodal approach could be applied to a much broader range of fibrous biological materials for the purpose of biomimetic technologies.