OPN, BSP, and Bone Quality-Structural, Biochemical, and Biomechanical Assessment in OPN-/-, BSP-/-, and DKO Mice

Osteopontin (OPN) and Bone Sialoprotein (BSP), abundantly expressed by osteoblasts and osteoclasts, appear to have important, partly overlapping functions in bone. In gene-knockout (KO, -/-) models of either protein and their double (D)KO in the same CD1/129sv genetic background, we analyzed the morphology, matrix characteristics, and biomechanical properties of femur bone in 2 and 4 month old, male and female mice. OPN-/- mice display inconsistent, perhaps localized hypermineralization, while the BSP-/- are hypomineralized throughout ages and sexes, and the low mineralization of young DKO mice recovers with age. The higher contribution of primary bone remnants in OPN-/- shafts suggests a slow turnover, while their lower percentage in BSP-/- indicates rapid remodeling, despite FTIR-based evidence in this genotype of a high maturity of the mineralized matrix. In 3-point bending assays, OPN-/- bones consistently display higher Maximal Load, Work to Max. Load and in young mice Ultimate Stress, an intrinsic characteristic of the matrix. Young male and old female BSP-/- also display high Work to Max. Load along with low Ultimate Stress. Principal Component Analysis confirms the major role of morphological traits in mechanical competence, and evidences a grouping of the WT phenotype with the OPN-/- and of BSP-/- with DKO, driven by both structural and matrix parameters, suggesting that the presence or absence of BSP has the most profound effects on skeletal properties. Single or double gene KO of OPN and BSP thus have multiple distinct effects on skeletal phenotypes, confirming their importance in bone biology and their interplay in its regulation.

Dietary supplementation with nacre reduces cortical bone loss in aged female mice

Aging is associated with detrimental bone loss leading to fragility fractures in both men and women. Notably, a majority of bone loss with aging is cortical, as well as a large number of fractures are non-vertebral and at the non-hip sites. Nacre is a product of mollusks composed of calcium carbonate embedded in organic components. As our previous study demonstrated the protective effect of nacre supplementation on trabecular bone loss in ovariectomized rats, we sought to evaluate the effect of dietary nacre on bone loss related to aging in female mice which do not suffer true menopause as observed in women. The current study compared the effect of a 90-day long nacre-supplemented diet to that of Standard or CaCO3 diets on both bone mass and strength in 16-month-old C57BL/6 female mice. Multiple approaches were performed to assess the microarchitecture and mechanical properties of long bones, analyze trabecular histomorphometry, and measure bone cell-related gene expressions, and bone turnover markers. In the cortex, dietary nacre improved cortical bone strength in line with lower expression levels of genes reflecting osteoclasts activity compared to Standard or CaCO3 diets (p < 0.05). In the trabeculae, nacre-fed mice were characterized by a bone remodeling process more active than the other groups as shown by greater histomorphometric parameters and osteoblast-related gene expressions (p < 0.05). But these differences were not exhibited at the level of the trabecular microarchitecture at this age. Collectively, these data suggest that dietary nacre should be a potential candidate for reducing aging-associated cortical bone loss in the elderly.

Prediction of osteoporotic degradation of tibia human bone at trabecular scale

A theoretical numerical model is proposed to predict patient dependent osteoporotic bone degradation. The model parameters are identified through a particle swarm optimization algorithm and based on individual patient high resolution peripherical quantitative computer tomography (HRpQCT) scan data. The degradation model is based on cellular activity initiated by the elastic strain energy developed in the bone microstructure through patient's body weight. The macro (organ scale) and meso (trabecular scale) scale analyses are carried out and predicted bone volume fraction and microstructure evolution are compared with in-vivo experimental bone degradation for four elderly women over a period of 10 years. A significant correlation (r > 0.9) is observed between the model predictions and in-vivo experiments in all cases with an average deviation error of 1.46%. The model can easily be extended to other patients and provide good predictions for different population categories such as ethnicity, gender, age, etc.

Dual-energy CT hybridation and kernel processing effects on the estimation of bone mineral mass and density: a calcination study on ex vivo human femur

INTRODUCTION

Recent technological advances with dual-energy quantitative computed tomography (DEQCT) allow to combine two images of different level of energy to obtain simulated mono-energetic images at 60 keV (_SIM60KeV-QCT) with improved image contrast in clinical practice. This study includes three topics: (1) compare bone mineral content (BMC), areal and volumetric bone mineral density (aBMD, vBMD) obtained with _SIM60KeV-QCT, single-energy QCT (SEQCT), and dual X-ray absorptiometry (DXA); (2) compare ash density and weight with respective vBMD and BMC assessed on _SIM60KeV-QCT, SEQCT, and DXA; and (3) compare the influence of reconstruction kernels on the accuracy of vBMD and BMC using ash density and ash weight as the reference values.

METHODS

DXA, SEQCT, and DEQCT acquisitions were performed ex vivo on 42 human femurs. Standard kernel (SK) and bone kernel (BK) were applied to each stack of images. Ten diaphyses and 10 femoral necks were cut, scanned, and reconstructed using the techniques described above. Finally, the bone specimens were calcined to obtain the ash weight.

RESULTS

QCT analysis (SEQCT, _SIM60KeV-QCT) underestimated BMC value compared to DXA. For femoral necks, all QCT analyses provided an unbiased estimate of ash weight but underestimated ash density regardless of the kernel used. For femoral diaphysis, SEQCT BK, _SIM60KeV-QCT BK, and SK underestimated ash weight but not ash density.

CONCLUSION

BMC and vBMD quantifications with the _SIM60KeV-QCT gave similar results as the SEQCT. Further studies are needed to optimize the use of _SIM60KeV-QCT in clinical situations. SK should be used given the effect of kernels on QCT assessment.

Influence of loading conditions in finite element analysis assessed by HR-pQCT on ex vivo fracture prediction

Many fractures occur in individuals with normal areal Bone Mineral Density (aBMD) measured by Dual X-ray Absorptiometry (DXA). High Resolution peripheral Quantitative Computed Tomography (HR-pQCT) allows for non-invasive evaluation of bone stiffness and strength through micro finite element (μFE) analysis at the tibia and radius. These μFE outcomes are strongly associated with fragility fractures but do not provide clear enhancement compared with DXA measurements. The objective of this study was to establish whether a change in loading conditions in standard μFE analysis assessed by HR-pQCT enhance the discrimination of low-trauma fractured radii (n = 11) from non-fractured radii (n = 16) obtained experimentally throughout a mechanical test reproducing a forward fall. Micro finite element models were created using HR-pQCT images, and linear analyses were performed using four different types of loading conditions (axial, non-axial with two orientations and torsion). No significant differences were found between the failure load assessed with the axial and non-axial models. The different loading conditions tested presented the same area under the receiver operating characteristic (ROC) curves of 0.79 when classifying radius fractures with an accuracy of 81.5%. In comparison, the area under the curve (AUC) is 0.77 from DXA-derived ultra-distal aBMD of the forearm with an accuracy of 85.2%. These results suggest that the restricted HR-pQCT scanned region seems not sensitive to loading conditions for the prediction of radius fracture risk based on ex vivo experiments (n = 27).

Cortical bone viscoelastic damping assessed with resonant ultrasound spectroscopy reflects porosity and mineral content

Viscoelasticity is an essential property of bone related to fragility, which is altered in aging and bone disease. Bone viscoelastic behavior is attributed to several mechanisms involving collagen and mineral properties, porosities, and bone hierarchical tissue organization. We aimed to assess the relationships between cortical bone viscoelastic damping measured with Resonant Ultrasound Spectroscopy (RUS), microstructural and compositional characteristics. We measured 52 bone specimens from the femur of 26 elderly human donors. RUS provided a shear damping coefficient at a frequency of the order of 150 kHz. The characteristics of the structure of the vascular pore network and tissue mineral density were measured using synchrotron radiation high-resolution computed tomography (SR-μCT). Fourier transformed infrared microspectroscopy (FTIRM) was used to quantify mineral-to-organic phase ratio, mineral maturity, crystallinity, and collagen maturity. Cross-links were quantified from biochemistry. Viscoelastic damping was found to increase with vascular porosity (r=0.68), to decrease with the degree of mineralization of the extravascular matrix (r=-0.68), and was marginally affected by collagen. We built a multilinear model suggesting that when porosity is controlled, the variation of mineral content explains a small additional part of the variability of damping. The work supports the consideration of viscoelasticity measurement as a potential biomarker of fragility and provides a documentation of bone viscoelastic behavior and its determinants in a frequency range rarely investigated.

Variabilities in µQCT-based FEA of a tumoral bone mice model

A finite element analysis based on Micro-Quantitative Computed Tomography (µQCT) is a method with high potential to improve fracture risk prediction. However, the segmentation process and model generation are generally not automatized in their entirety. Even with a rigorous protocol, the operator might add uncertainties during the creation of the model. The aim of this study was to evaluate a µQCT-based model of mice tumoral and sham tibias in terms of the variabilities induced by the operator and sensitivity to operator-dependent variables (such as model orientation or length). Two different operators generated finite element (FE) models from µCT images of 8 female Balb/c nude mice tibias aged 10 weeks old with bone tumors induced in the right tibia and with sham injection in the left. From these models, predicted failure load was determined for two different boundary conditions: fixed support and spherical joints. The difference between the predicted and experimental failure load of both operators was large (-122% to 93%). The difference in the predicted failure load between operators was less for the spherical joints boundary conditions (9.8%) than for the fixed support (58.3%), p < 0.001, whereas varying the orientation of bone tibia caused more variability for the fixed support boundary condition (44.7%) than for the spherical joints (9.1%), p < 0.002. Varying tibia length had no significant effect, regardless of boundary conditions (<4%). When using the same mesh and same orientation, the difference between operators is non-significant (<6%) for each model. This study showed that the operator influences the failure load assessed by a µQCT-based finite element model of the tumoral and sham mice tibias. The results suggest that automation is needed for better reproducibility.

Intrinsic properties of osteomalacia bone evaluated by nanoindentation and FTIRM analysis

Osteomalacia is a pathological bone condition consisting in a deficient primary mineralization of the matrix, leading to an accumulation of osteoid tissue and reduced bone mechanical strength. The amounts, properties and organization of bone constituents at tissue level, are known to influence its mechanical properties. It is then important to investigate the relationship between mechanical behavior and tissue composition at this scale in order to provide a better understanding of bone fragility mechanisms associates with this pathology. Our purpose was to analyze the links between ultra-structural properties and the mechanical behavior of this pathological bone tissue (osteomalacia) at tissue level (mineral and osteoid separately, or global). Four bone biopsies were taken from patients with osteomalacia, and subsequently embedded, sectioned, and polished. Then nanoindentation tests were performed to determine local elastic modulus E, contact hardness Hc and true hardness H for both mineralized and organic bone phases and for the global bone. The creep of the bone was also studied using a special indentation procedure in order to assess visco-elasto-plastic (creep) bone behavior. This allowed a detailed study of the rheological models adapted to the bone and to calculate the parameters associated to a Burgers model. Ultra-structural parameters were measured by Fourier Transform InfraRed Microspectroscopy (FTIRM) on the same position as the indents. The use of rheological models confirmed a significant contribution from the organic phase on the viscous character of bone tissue. The elastic E and the elasto-plastic H_c deformation were correlated to both collagen maturity and Mineral/Matrix. The pure plastic deformation H was only correlated to the mineral phase. Our data show that mineral phase greatly affects mechanical variables (moduli and viscosities) and that organic phase (as illustrated in osteoid tissue) may play an important role in the creep behavior of bone. In conclusion, this study brings mechanical and physicochemical values for osteoid and mineral phases.

Ultrasounds could be considered as a future tool for probing growing bone properties

Juvenile bone growth is well described (physiological and anatomical) but there are still lacks of knowledge on intrinsic material properties. Our group has already published, on different samples, several studies on the assessment of intrinsic material properties of juvenile bone compared to material properties of adult bone. The purpose of this study was finally to combine different experimental modalities available (ultrasonic measurement, micro-Computed Tomography analysis, mechanical compression tests and biochemical measurements) applied on small cubic bone samples in order to gain insight into the multiparametric evaluation of bone quality. Differences were found between juvenile and adult groups in term of architectural parameters (Porosity Separation), Tissue Mineral Density (TMD), diagonal stiffness coefficients (C₃₃, C_44, C_55, C₆₆) and ratio between immature and mature cross-links (CX). Diagonal stiffness coefficients are more representative of the microstructural and biochemical parameters of child bone than of adult bone. We also found that compression modulus E was highly correlated with several microstructure parameters and CX in children group while it was not at all correlated in the adult group. Similar results were found for the CX which was linked to several microstructure parameters (TMD and E) only in the juvenile group. To our knowledge, this is the first time that, on a same sample, ultrasonic measurements have been combined with the assessment of mechanical and biochemical properties. It appears that ultrasonic measurements can provide relevant indicators of child bone quality (microstructural and biochemical parameters) which is promising for clinical application since, B-mode ultrasound is the preferred first-line modality over other more constraining imaging modalities (radiation, parent–child accessibility and access to the patient's bed) for pediatric patients.

What is the influence of two strain rates on the relationship between human cortical bone toughness and micro-structure?

Cortical bone fracture mechanisms are well studied under quasi-static loading. The influence of strain rate on crack propagation mechanisms needs to be better understood, however. We have previously shown that several aspects of the bone micro-structure are involved in crack propagation, such as the complete porosity network, including the Haversian system and the lacunar network, as well as biochemical aspects, such as the maturity of collagen cross-links. The aim of this study is to investigate the influence of strain rate on the toughness of human cortical bone with respect to its microstructure and organic non-collagenous composition. Two strain rates will be considered: quasi-static loading (10−4 s−1), a standard condition, and a higher loading rate (10−1 s−1), representative of a fall. Cortical bone samples were extracted from eight female donors (age 50–91 years). Three-point bending tests were performed until failure. Synchrotron radiation micro-computed tomography imaging was performed to assess bone microstructure including the Haversian system and the lacunar system. Collagen enzymatic cross-link maturation was measured using a high performance liquid chromatography column. Results showed that that under quasi-static loading, the elastic contribution of the fracture process is correlated to both the collagen cross-links maturation and the microstructure, while the plastic contribution is correlated only to the porosity network. Under fall-like loading, bone organization appears to be less linked to crack propagation.

Compositional and mechanical properties of growing cortical bone tissue: a study of the human fibula

Human cortical bone contains two types of tissue: osteonal and interstitial tissue. Growing bone is not well-known in terms of its intrinsic material properties. To date, distinctions between the mechanical properties of osteonal and interstitial regions have not been investigated in juvenile bone and compared to adult bone in a combined dataset. In this work, cortical bone samples obtained from fibulae of 13 juveniles patients (4 to 18 years old) during corrective surgery and from 17 adult donors (50 to 95 years old) were analyzed. Microindentation was used to assess the mechanical properties of the extracellular matrix, quantitative microradiography was used to measure the degree of bone mineralization (DMB), and Fourier transform infrared microspectroscopy was used to evaluate the physicochemical modifications of bone composition (organic versus mineral matrix). Juvenile and adult osteonal and interstitial regions were analyzed for DMB, crystallinity, mineral to organic matrix ratio, mineral maturity, collagen maturity, carbonation, indentation modulus, indicators of yield strain and tissue ductility using a mixed model. We found that the intrinsic properties of the juvenile bone were not all inferior to those of the adult bone. Mechanical properties were also differently explained in juvenile and adult groups. The study shows that different intrinsic properties should be used in case of juvenile bone investigation.

3D analysis of the osteonal and interstitial tissue in human radii cortical bone

Human cortical bone has a complex hierarchical structure that is periodically remodelled throughout a lifetime. This microstructure dictates the mechanical response of the tissue under a critical load. If only some structural features, such as the different porosities observed in bone, are primarily studied, then investigations may not fully consider the osteonal systems in three-dimensions (3D). Currently, it is difficult to differentiate osteons from interstitial tissue using standard 3D characterization methods. Synchrotron radiation micro-computed tomography (SR-μCT) in the phase contrast mode is a promising method for the investigation of osteons. In the current study, SR-μCT imaging was performed on cortical bone samples harvested from eight human radii (female, 50-91 y.o.). The images were segmented to identify Haversian canals, osteocyte lacunae, micro-cracks, as well as osteons. The significant correlation between osteonal and Haversian canal volume fraction highlights the role of the canals as sites where bone remodelling is initiated. The results showed that osteocyte lacunae morphometric parameters depend on their distance to cement lines, strongly suggesting the evolution of biological activity from the beginning to the end of the remodelling process. Thus, the current study provides new data on 3D osteonal morphometric parameters and their relationships with other structural features in humans.

Anisotropic elastic properties of human femoral cortical bone and relationships with composition and microstructure in elderly

The strong dependence between cortical bone elasticity at the millimetre-scale (mesoscale) and cortical porosity has been evidenced by previous studies. However, bone is an anisotropic composite material made by mineral, proteins and water assembled in a hierarchical structure. Whether the variations of structural and compositional properties of bone affect the different elastic coefficients at the mesoscale is not clear. Aiming to understand the relationships between bone elastic properties and compositions and microstructure, we applied state-of-the-art experimental modalities to assess these aspects of bone characteristics. All elastic coefficients (stiffness tensor of the transverse isotropic bone material), structure of the vascular pore network, collagen and mineral properties were measured in 52 specimens from the femoral diaphysis of 26 elderly donors. Statistical analyses and micromechanical modeling showed that vascular pore volume fraction and the degree of mineralization of bone are the most important determinants of cortical bone anisotropic mesoscopic elasticity. Though significant correlations were observed between collagen properties and elasticity, their effects in bone mesoscopic elasticity were minor in our data. This work also provides a unique set of data exhibiting a range of variations of compositional and microstructural cortical bone properties in the elderly and gives strong experimental evidence and basis for further development of biomechanical models for human cortical bone. STATEMENT OF SIGNIFICANCE: This study reports the relationships between microstructure, composition and the mesoscale anisotropic elastic properties of human femoral cortical bone in elderly. For the first time, we provide data covering the complete anisotropic elastic tensor, the microstructure of cortical vascular porosity, mineral and collagen characteristics obtained from the same or adjacent samples in each donor. The results revealed that cortical vascular porosity and degree of mineralization of bone are the most important determinants of bone anisotropic stiffness at the mesoscale. The presented data gives strong experimental evidence and basis for further development of biomechanical models for human cortical bone.

Influence of loading condition and anatomical location on human cortical bone linear micro-cracks

Human cortical bone fracture toughness depends on the anatomical locations under quasi-static loading. Recent results also showed that under fall-like loading, cortical bone fracture toughness is similar at different anatomical locations in the same donor. While cortical bone toughening mechanisms are known to be dependent on the tissue architecture under quasi-static loading, the fracture mechanisms during a fall are less studied. In the current study, the structural parameters of eight paired femoral diaphyses, femoral necks and radial diaphyses were mechanically tested under quasi-static and fall-like loading conditions (female donors, 70 ± 14 y.o., [50-91 y.o.]). Synchrotron radiation micro-CT imaging was used to quantify the amount of micro-cracks formed during loading. The volume fraction of these micro-cracks was significantly higher within the specimens loaded under a quasi-static condition than under a loading representative of a fall. Under fall-like loading, there was no difference in crack volume fraction between the different paired anatomical locations. This result shows that the micro-cracking toughening mechanism depends both on the anatomical location and on the loading condition.

Deletion of OPN in BSP knockout mice does not correct bone hypomineralization but results in high bone turnover

The two SIBLING (Small Integrin Binding Ligand N-linked Glycoproteins), bone sialoprotein (BSP) and osteopontin (OPN) are expressed in osteoblasts and osteoclasts. In mature BSP knockout (KO, ^-/-) mice, both bone formation and resorption as well as mineralization are impaired. OPN^-/- mice display impaired resorption, and OPN is described as an inhibitor of mineralization. However, OPN is overexpressed in BSP^-/- mice, complicating the understanding of their phenotype. We have generated and characterized mice with a double KO (DKO) of OPN and BSP, to try and unravel their respective contributions. Despite the absence of OPN, DKO bones are still hypomineralized. The SIBLING, matrix extracellular phosphoglycoprotein with ASARM motif (MEPE) is highly overexpressed in both BSP^-/- and DKO and may impair mineralization through liberation of its ASARM (Acidic Serine-Aspartate Rich MEPE associated) peptides. DKO mice also display evidence of active formation of trabecular, secondary bone as well as primary bone in the marrow-ablation repair model. A higher number of osteoclasts form in DKO marrow cultures, with higher resorption activity, and DKO long bones display a localized and conspicuous cortical macroporosity. High bone formation and resorption parameters, and high cortical porosity in DKO mice suggest an active bone modeling/remodeling, in the absence of two key regulators of bone cell performance. This first double KO of SIBLING proteins thus results in a singular, non-trivial phenotype leading to reconsider the interpretation of each single KO, concerning in particular matrix mineralization and the regulation of bone cell activity.

Bone cortical thickness and porosity assessment using ultrasound guided waves: An ex vivo validation study

Several studies showed the ability of the cortex of long bones such as the radius and tibia to guide mechanical waves. Such experimental evidence has given rise to the emergence of a category of quantitative ultrasound techniques, referred to as the axial transmission, specifically developed to measure the propagation of ultrasound guided waves in the cortical shell along the axis of long bones. An ultrasound axial transmission technique, with an automated approach to quantify cortical thickness and porosity is described. The guided modes propagating in the cortex are recorded with a 1-MHz custom made linear transducer array. Measurement of the dispersion curves is achieved using a two-dimensional spatio-temporal Fourier transform combined with singular value decomposition. Automatic parameters identification is obtained through the solution of an inverse problem in which the dispersion curves are predicted with a two-dimensional transverse isotropic free plate model. Thirty-one radii and fifteen tibiae harvested from human cadavers underwent axial transmission measurements. Estimates of cortical thickness and porosity were obtained on 40 samples out of 46. The reproducibility, given by the root mean square error of the standard deviation of estimates, was 0.11 mm for thickness and 1.9% for porosity. To assess accuracy, site-matched micro-computed tomography images of the bone specimens imaged at 9 μm voxel size served as the gold standard. Agreement between micro-computed tomography and axial transmission for quantification of thickness and porosity at the radius and tibia ranged from R²=0.63 for porosity (root mean square error RMSE=1.8%) to 0.89 for thickness (RMSE=0.3 mm). Despite an overall good agreement for porosity, the method performs less well for porosities lower than 10%. The heterogeneity and general complexity of cortical bone structure, which are not fully accounted for by our model, are suspected to weaken the model approximation. This study presents the first validation study for assessing cortical thickness and porosity using the axial transmission technique. The automatic signal processing minimizes operator-dependent errors for parameters determination. Recovering the waveguide characteristics, that is to say cortical thickness and porosity, could provide reliable information about skeletal status and future fracture risk.

Relationships between human cortical bone toughness and collagen cross-links on paired anatomical locations

Human cortical bone fracture processes depend on the internal porosity network down to the lacunar length scale. Recent results show that at the collagen scale, the maturation of collagen cross-links may have a negative influence on bone mechanical behavior. While the effect of pentosidine on human cortical bone toughness has been studied, the influence of mature and immature enzymatic cross-links has only been studied in relation to strength and work of fracture. Moreover, these relationships have not been studied on different paired anatomical locations. Thus, the aim of the current study was to assess the relationships between both enzymatic and non-enzymatic collagen cross-links and human cortical bone toughness, on four human paired anatomical locations. Single Edge Notched Bending toughness tests were performed for two loading conditions: a quasi-static standard condition, and a condition representative of a fall. These tests were done with 32 paired femoral diaphyses, femoral necks and radial diaphyses (18 women, age 81 ± 12 y.o.; 14 men, age 79 ± 8 y.o.). Collagen enzymatic and non-enzymatic crosslinks were measured on the same bones. Maturation of collagen was defined as the ratio between immature and mature cross-links (CX). The results show that there was a significant correlation between collagen cross-link maturation and bone toughness when gathering femoral and radial diaphyses, but not when considering each anatomical location individually. These results show that the influence of collagen enzymatic and non-enzymatic cross-links is minor when considering human cortical bone crack propagation mechanisms.

An ex vivo experiment to reproduce a forward fall leading to fractured and non-fractured radii

Forward falls represent a risk of injury for the elderly. The risk is increased in elderly persons with bone diseases, such as osteoporosis. However, half of the patients with fracture were not considered at risk based on bone density measurement (current clinical technique). We assume that loading conditions are of high importance and should be considered. Real loading conditions in a fall can reach a loading speed of 2m/s on average. The current study aimed to apply more realistic loading conditions that simulate a forward fall on the radius ex vivo. Thirty radii from elderly donors (79y.o.±12y.o., 15 males, 15 females) were loaded at 2m/s using a servo-hydraulic testing machine to mimic impact that corresponds to a fall. Among the 30 radii, 14 had a fracture after the impact, leading to two groups (fractured and non-fractured). Surfacic strain fields were measured using stereovision and allow for visualization of fracture patterns. The average maximum load was 2963±1274N. These experimental data will be useful for assessing the predictive capability of fracture risk prediction methods such as finite element models.

One-month spaceflight compromises the bone microstructure, tissue-level mechanical properties, osteocyte survival and lacunae volume in mature mice skeletons

The weightless environment during spaceflight induces site-specific bone loss. The 30-day Bion-M1 mission offered a unique opportunity to characterize the skeletal changes after spaceflight and an 8-day recovery period in mature male C57/BL6 mice. In the femur metaphysis, spaceflight decreased the trabecular bone volume (−64% vs. Habitat Control), dramatically increased the bone resorption (+140% vs. Habitat Control) and induced marrow adiposity invasion. At the diaphysis, cortical thinning associated with periosteal resorption was observed. In the Flight animal group, the osteocyte lacunae displayed a reduced volume and a more spherical shape (synchrotron radiation analyses), and empty lacunae were highly increased (+344% vs. Habitat Control). Tissue-level mechanical cortical properties (i.e., hardness and modulus) were locally decreased by spaceflight, whereas the mineral characteristics and collagen maturity were unaffected. In the vertebrae, spaceflight decreased the overall bone volume and altered the modulus in the periphery of the trabecular struts. Despite normalized osteoclastic activity and an increased osteoblast number, bone recovery was not observed 8 days after landing. In conclusion, spaceflight induces osteocyte death, which may trigger bone resorption and result in bone mass and microstructural deterioration. Moreover, osteocyte cell death, lacunae mineralization and fatty marrow, which are hallmarks of ageing, may impede tissue maintenance and repair.

Strain rate influence on human cortical bone toughness: A comparative study of four paired anatomical sites

Bone fracture is a major health issue worldwide and consequently there have been extensive investigations into the fracture behavior of human cortical bone. However, the fracture properties of human cortical bone under fall-like loading conditions remains poorly documented. Further, most published research has been performed on femoral diaphyseal bone, whereas it is known that the femoral neck and the radius are the most vulnerable sites to fracture. Hence, the aim of this study is to provide information on human cortical bone fracture behavior by comparing different anatomical sites including the radius and the femoral neck acquired from 32 elderly subjects (50 - 98 y.o.). In order to investigate the intrinsic fracture behavior of human cortical bone, toughness experiments were performed at two different strain rates: standard quasi-static conditions, and a higher strain rate representative of a fall from a standing position. The tests were performed on paired femoral neck, femoral, tibial and radius diaphyseal samples. Linear elastic fracture toughness and the non-linear J-integral method were used to take into account both the elastic and non-elastic behavior of cortical bone. Under quasi-static conditions, the radius presents a significantly higher toughness than the other sites. At the higher strain rate, all sites showed a significantly lower toughness. Also, at the high strain rate, there is no significant difference in fracture properties between the four anatomical sites. These results suggest that regardless of the anatomical site (femur, femoral neck, tibia and radius), the bone has the same fracture properties under fall loading conditions. This should be considered in biomechanical models under fall-like loading conditions.

Clinical cone beam computed tomography compared to high-resolution peripheral computed tomography in the assessment of distal radius bone

Clinical cone beam computed tomography (CBCT) was compared to high-resolution peripheral quantitative computed tomography (HR-pQCT) for the assessment of ex vivo radii. Strong correlations were found for geometry, volumetric density, and trabecular structure. Using CBCT, bone architecture assessment was feasible but compared to HR-pQCT, trabecular parameters were overestimated whereas cortical ones were underestimated.

INTRODUCTION

HR-pQCT is the most widely used technique to assess bone microarchitecture in vivo. Yet, this technology has been only applicable at peripheral sites, in only few research centers. Clinical CBCT is more widely available but quantitative assessment of the bone structure is usually not performed. We aimed to compare the assessment of bone structure with CBCT (NewTom 5G, QR, Verona, Italy) and HR-pQCT (XtremeCT, Scanco Medical AG, Brüttisellen, Switzerland).

METHODS

Twenty-four distal radius specimens were scanned with these two devices with a reconstructed voxel size of 75 μm for Newtom 5G and 82 μm for XtremeCT, respectively. A rescaling-registration scheme was used to define the common volume of interest. Cortical and trabecular compartments were separated using a semiautomated double contouring method. Density and microstructure were assessed with the HR-pQCT software on both modality images.

RESULTS

Strong correlations were found for geometry parameters (r = 0.98-0.99), volumetric density (r = 0.91-0.99), and trabecular structure (r = 0.94-0.99), all p < 0.001. Correlations were lower for cortical microstructure (r = 0.80-0.89), p < 0.001. However, absolute differences were observed between modalities for all parameters, with an overestimation of the trabecular structure (trabecular number, 1.62 ± 0.37 vs. 1.47 ± 0.36 mm(-1)) and an underestimation of the cortical microstructure (cortical porosity, 3.3 ± 1.3 vs. 4.4 ± 1.4 %) assessed on CBCT images compared to HR-pQCT images.

CONCLUSIONS

Clinical CBCT devices are able to analyze large portions of distal bones with good spatial resolution and limited irradiation. However, compared to dedicated HR-pQCT, the assessment of microarchitecture by NewTom 5G dental CBCT showed some discrepancies, for density measurements mainly. Further technical developments are required to reach optimal assessment of bone characteristics.

Ratio between mature and immature enzymatic cross-links correlates with post-yield cortical bone behavior: An insight into greenstick fractures of the child fibula

As a determinant of skeletal fragility, the organic matrix is responsible for the post-yield and creep behavior of bone and for its toughness, while the mineral apatite acts on stiffness. Specific to the fibula and ulna in children, greenstick fractures show a plastic in vivo mechanical behavior before bone fracture. During growth, the immature form of collagen enzymatic cross-links gradually decreases, to be replaced by the mature form until adolescence, subsequently remaining constant throughout adult life. However, the link between the cortical bone organic matrix and greenstick fractures in children remains to be explored. Here, we sought to determine: 1) whether plastic bending fractures can occur in vitro, by testing cortical bone samples from children's fibula and 2) whether the post-yield behavior (ωp plastic energy) of cortical bone before fracture is related to total quantity of the collagen matrix, or to the quantity of mature and immature enzymatic cross-links and the quantity of non-enzymatic cross-links. We used a two-step approach; first, a 3-point microbending device tested 22 fibula machined bone samples from 7 children and 3 elderly adults until fracture. Second, biochemical analysis by HPLC was performed on the sample fragments. When pooling two groups of donors, children and elderly adults, results show a rank correlation between total energy dissipated before fracture and age and a linear correlation between plastic energy dissipated before fracture and ratio of immature/mature cross-links. A collagen matrix with more immature cross-links (i.e. a higher immature/mature cross-link ratio) is more likely to plastically deform before fracture. We conclude that this ratio in the sub-nanostructure of the organic matrix in cortical bone from the fibula may go some way towards explaining the variance in post-yield behavior. From a clinical point of view, therefore, our results provide a potential explanation of the presence of greenstick fractures in children.

Validation of a novel microradiography device for characterization of bone mineralization

In order to simplify bone mineralization measurements, a system using radiographic films has been updated with a digital detector. The objective of this paper was to validate this new device. Technologies and physical phenomena involved in both systems (radiographic films and digital detector) are different. The methodology used to compare the two systems was based on image quality and assessed on two main parameters: contrast to noise ratio and spatial resolution. Results showed that the contrast to noise ratio was similar between the two systems, provided that acquisition parameters were optimized. With regard to spatial resolution, a magnification factor of at least 4 or more was required to achieve the same resolution as films. A final validation was also shown on a real image of a bone sample. The results showed that both systems have similar image quality performances, and the system using digital detector has several advantages (easier to use than films, no consumables and faster acquisition time).

The role of bone intrinsic properties measured by infrared spectroscopy in whole lumbar vertebra mechanics: organic rather than inorganic bone matrix?

Whole bone strength is determined by bone mass, microarchitecture and intrinsic properties of the bone matrix. However, few studies have directly investigated the contribution of bone tissue material properties to whole bone strength in humans. This study assessed the role of bone matrix composition on whole lumbar vertebra mechanics. We obtained 17 fresh frozen human lumbar spines (8 W, 9 M, aged 76±11years). L3 bone mass was measured by DXA and microarchitecture by μ-CT with a 35 μm-isotropic resolution. Microarchitectural parameters were directly measured: Tb.BV/TV, SMI, Tb.Th, DA, Ct.Th, Ct.Po and radius of anterior cortical curvature. Failure load (N), stiffness (N/mm) and work to failure (N.mm) were extracted from quasi-static uniaxial compressive testing performed on L3 vertebral bodies. FTIRM analysis was performed on 2 μm-thick sections from L2 trabecular cores, with a Perkin-Elmer GXII Auto-image Microscope equipped with a wide band detector. Twenty measurements per sample were performed at 30∗100 μm of spatial resolution. Each spectrum was collected at 4 cm(-1) resolution and 50 scans in transmission mode. Mineral and collagen maturity, and mineralization and crystallinity index were measured. There was no association between the bone matrix characteristics and bone mass or microarchitecture. Mineral maturity, mineralization and crystallinity index were not related to whole vertebra mechanics. However, collagen maturity was positively correlated with whole vertebra failure load and stiffness (r=0.64, p=0.005 and r=0.54, p=0.025, respectively). The collagen maturity (3rd step) in combination with bone mass (i.e., BMC, 1st step) and microarchitecture (i.e., Tb.Th, 2nd step) improved the prediction of whole vertebra mechanical properties in forward stepwise multiple regression models, together explaining 71% of the variability in whole vertebra stiffness (p=0.001). In conclusion, we demonstrated a substantial contribution of collagen maturity, but not mineralization parameters, to whole bone strength of human lumbar vertebrae that was independent of bone mass and microarchitecture.

A new murine model of osteoblastic/osteolytic lesions from human androgen-resistant prostate cancer

Background

Up to 80% of patients dying from prostate carcinoma have developed bone metastases that are incurable. Castration is commonly used to treat prostate cancer. Although the disease initially responds to androgen blockade strategies, it often becomes castration-resistant (CRPC for Castration Resistant Prostate Cancer). Most of the murine models of mixed lesions derived from prostate cancer cells are androgen sensitive. Thus, we established a new model of CRPC (androgen receptor (AR) negative) that causes mixed lesions in bone.

Methods

PC3 and its derived new cell clone PC3c cells were directly injected into the tibiae of SCID male mice. Tumor growth was analyzed by radiography and histology. Direct effects of conditioned medium of both cell lines were tested on osteoclasts, osteoblasts and osteocytes.

Results

We found that PC3c cells induced mixed lesions 10 weeks after intratibial injection. Invitro, PC3c conditioned medium was able to stimulate tartrate resistant acid phosphatase (TRAP)-positive osteoclasts. Osteoprotegerin (OPG) and endothelin-1 (ET1) were highly expressed by PC3c while dikkopf-1 (DKK1) expression was decreased. Finally, PC3c highly expressed bone associated markers osteopontin (OPN), Runx2, alkaline phosphatase (ALP), bone sialoprotein (BSP) and produced mineralized matrix invitro in osteogenic conditions.

Conclusions

We have established a new CRPC cell line as a useful system for modeling human metastatic prostate cancer which presents the mixed phenotype of bone metastases that is commonly observed in prostate cancer patients with advanced disease. This model will help to understand androgen-independent mechanisms involved in the progression of prostate cancer in bone and provides a preclinical model for testing the effects of new treatments for bone metastases.

Methodological approach for the detection of both microdamage and fluorochrome labels in ewe bone and human trabecular bone

The purpose of this study was to adapt various staining methods for the detection of microdamage in human bone, while preserving tetracycline labels. We describe two staining methods using calcein green and xylenol orange, first developed in ewe bone samples and validated in human trabecular bone samples. In ewe bones, we found that calcein green at 0.5 mM concentration diluted in 100% ethanol as well as xylenol orange at 5 mM were the most adequate fluorochromes both to detect microdamage and preserve the double tetracycline labeling. These results were verified in human trabecular bone (iliac crest for the tetracycline label, and vertebral bone for the double labeling). Results obtained in human bone samples were identical to those in ewes, so this combination of fluorochromes is now used in our laboratory.

Dual function of ERRα in breast cancer and bone metastasis formation: implication of VEGF and osteoprotegerin

Bone metastasis is a complication occurring in up to 70% of advanced breast cancer patients. The estrogen receptor-related receptor alpha (ERRα) has been implicated in breast cancer and bone development, prompting us to examine whether ERRα may function in promoting the osteolytic growth of breast cancer cells in bone. In a mouse xenograft model of metastatic human breast cancer, overexpression of wild-type ERRα reduced metastasis, whereas overexpression of a dominant negative mutant promoted metastasis. Osteoclasts were directly affected and ERRα upregulated the osteoclastogenesis inhibitor, osteoprotegerin (OPG), providing a direct mechanistic basis for understanding how ERRα reduced breast cancer cell growth in bone. In contrast, ERRα overexpression increased breast cancer cell growth in the mammary gland. ERRα-overexpressing primary tumors were highly vascularized, consistent with an observed upregulation of angiogenic growth factor, the VEGF. In support of these findings, we documented that elevated expression of ERRα mRNA in breast carcinomas was associated with high expression of OPG and VEGF and with disease progression. In conclusion, our results show that ERRα plays a dual role in breast cancer progression in promoting the local growth of tumor cells, but decreasing metastatic growth of osteolytic lesions in bone.

Association between collagen cross-links and trabecular microarchitecture properties of human vertebral bone

It has been suggested that age-related deterioration in trabecular microarchitecture and changes in collagen cross-link concentrations may contribute to skeletal fragility. To further explore this hypothesis, we determined the relationships among trabecular bone volume fraction (BV/TV), microarchitecture, collagen cross-link content, and bone turnover in human vertebral trabecular bone. Trabecular bone specimens from L2 vertebrae were collected from 51 recently deceased donors (54-95 years of age; 20 men and 30 women). Trabecular bone volume and microarchitecture was assessed by microCT and bone formation, reflected by osteoid surface (OS/BS, %), was measured by 2D histomorphometry. Pyridinoline (PYD), deoxypyridinoline (DPD), pentosidine (PEN) and collagen content in the cancellous bone were analysed by high-performance liquid chromatography. Associations between variables were investigated by Pearson correlations and multiple regression models, which were constructed with BV/TV and collagen cross-links as explanatory variables and microarchitecture parameters as the dependent variables.

RESULTS

Microarchitecture parameters were modestly to strongly correlated with BV/TV (r(2)=0.10-0.71). The amount of mature enzymatic PYD and DPD cross-links were not associated with the microarchitecture, either before or after adjustment for BV/TV. However, there was a positive correlation between PEN content and trabecular number (r=0.45, p=0.001) and connectivity density (r=0.40, p=0.004), and a negative correlation between PEN content and trabecular separation (r=-0.29, p=0.04). In the multiple regression models including BV/TV, age and PEN content was still significantly associated with several of the microarchitecture variables. In summary, this study suggests a link between trabecular microarchitecture and the collagen cross-link profile. As PEN reflects non-enzymatic glycation of collagen and generally increases with bone age, the association between PEN and trabecular architecture suggests that the preserved trabeculae may contain mainly old bone and have undergone little remodeling. Thus, vertebral fragility may not only be due to alterations in bone architecture but also to modification of collagen cross-link patterns thereby influencing bone's mechanical behavior.

Intrinsic mechanical properties of trabecular calcaneus determined by finite-element models using 3D synchrotron microtomography

To determine intrinsic mechanical properties (elastic and failure) of trabecular calcaneus bone, chosen as a good predictor of hip fracture, we looked for the influence of image's size on a numerical simulation. One cubic sample of cancellous bone (9 x 9 x 9 mm(3)) was removed from the body of the calcaneus (6 females, 6 males, 79+/-9 yr). These samples were tested under compressive loading. Before compressive testing, these samples were imaged at 10.13 microm resolution using a 3D microcomputed tomography (muCT) (ESRF, France). The muCT images were converted to finite-element models. Depending on the bone density values (BV/TV), we compared two different finite element models: a linear hexahedral and a linear beam finite element models. Apparent experimental Young's modulus (E(app)(exp)) and maximum apparent experimental compressive stress (sigma(max)(exp)) were significantly correlated with bone density obtained by Archimedes's test (E(app)(exp)=236+/-231 MPa [19-742 MPa], sigma(max)(exp)=2.61+/-1.97 MPa [0.28-5.81 MPa], r>0.80, p<0.001). Under threshold at 40 microm, the size of the numerical samples (5.18(3) and 6.68(3)mm(3)) seems to be an important parameter on the accuracy of the results. The numerical trabecular Young's modulus was widely higher (E(trabecular)(num)=34,182+/-22,830 MPa [9700-87,211 MPa]) for the larger numerical samples and high BV/TV than those found classically by other techniques (4700-15,000 MPa). For rod-like bone samples (BV/TV<12%, n=7), Young's modulus, using linear beam element (E(trabecular)(num-skeleton): 10,305+/-5500 MPa), were closer to the Young's modulus found by other techniques. Those results show the limitation of hexahedral finite elements at 40 microm, mostly used, for thin trabecular structures.