Comparison of proximal femur and vertebral body strength improvements in the FREEDOM trial using an alternative finite element methodology

An integrated experimental-computational framework to assess the influence of microstructure and material properties on fracture toughness in clinical specimens of human femoral cortical bone

Microstructural and compositional changes that occur due to aging, pathological conditions, or pharmacological treatments alter cortical bone fracture resistance. However, the relative importance of these changes to the fracture resistance of cortical bone has not been quantified in detail. In this technical note, we developed an integrated experimental-computational framework utilizing human femoral cortical bone biopsies to advance the understanding of how fracture resistance of cortical bone is modulated due to modifications in its microstructure and material properties. Four human biopsy samples from individuals with varying fragility fracture history and osteoporosis treatment status were converted to finite element models incorporating specimen-specific material properties and were analyzed using fracture mechanics-based modeling. The results showed that cement line density and osteonal volume had a significant effect on crack volume. The removal of cement lines substantially increased the crack volume in the osteons and interstitial bone, representing straight crack growth, compared to models with cement lines due to the lack of crack deflection in the models without cement lines. Crack volume in the osteons and interstitial bone increased when mean elastic modulus and ultimate strength increased and mean fracture toughness decreased. Crack volume in the osteons and interstitial bone was reduced when material property heterogeneity was incorporated in the models. Although both the microstructure and the heterogeneity of the material properties of the cortical bone independently increased the fracture toughness, the relative contribution of the microstructure was more significant. The integrated experimental-computational framework developed here can identify the most critical microscale features of cortical bone modulated by pathological processes or pharmacological treatments that drive changes in fracture resistance and improve our understanding of the relative influence of microstructure and material properties on fracture resistance of cortical bone.

The elasto-plastic nano- and microscale compressive behaviour of rehydrated mineralised collagen fibres

The hierarchical design of bio-based nanostructured materials such as bone enables them to combine unique structure-mechanical properties. As one of its main components, water plays an important role in bone's material multiscale mechanical interplay. However, its influence has not been quantified at the length-scale of a mineralised collagen fibre. Here, we couple in situ micropillar compression, and simultaneous synchrotron small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) with a statistical constitutive model. Since the synchrotron data contain statistical information on the nanostructure, we establish a direct connection between experiment and model to identify the rehydrated elasto-plastic micro- and nanomechanical fibre behaviour. Rehydration led to a decrease of 65%-75% in fibre yield stress and compressive strength, and 70% in stiffness with a 3x higher effect on stresses than strains. While in agreement with bone extracellular matrix, the decrease is 1.5-3x higher compared to micro-indentation and macro-compression. Hydration influences mineral more than fibril strain with the highest difference to the macroscale when comparing mineral and tissue levels. The effect of hydration seems to be strongly mediated by ultrastructural interfaces while results provide insights towards mechanical consequences of reported water-mediated structuring of bone apatite. The missing reinforcing capacity of surrounding tissue for an excised fibril array is more pronounced in wet than dry conditions, mainly related to fibril swelling. Differences leading to higher compressive strength between mineralised tissues seem not to depend on rehydration while the lack of kink bands supports the role of water as an elastic embedding influencing energy-absorption mechanisms. STATEMENT OF SIGNIFICANCE: Characterising structure-property-function relationships in hierarchical biological materials helps us to elucidate mechanisms that enable their unique properties. Experimental and computational methods can advance our understanding of their complex behaviour with the potential to inform bio-inspired material development. In this study, we close a gap for bone's fundamental mechanical building block at micro- and nanometre length scales. We establish a direct connection between experiments and simulations by coupling in situ synchrotron tests with a statistical model and quantify the behaviour of rehydrated single mineralised collagen fibres. Results suggest a high influence of hydration on structural interfaces, and the role of water as an elastic embedding by outlining important differences between wet and dry elasto-plastic properties of mineral nanocrystals, fibrils and fibres.

Twelve Months of Denosumab and/or Alendronate Is Associated With Improved Bone Fatigue Life, Microarchitecture, and Density in Ovariectomized Cynomolgus Monkeys

Prolonged use of antiresorptives such as the bisphosphonate alendronate (ALN) and the RANKL inhibitor denosumab (DMAb) are associated with rare cases of atypical femoral fracture (AFF). The etiology of AFF is unclear, but it has been hypothesized that potent osteoclast inhibitors may reduce bone fatigue resistance. The purpose of this study was to quantify the relationship between antiresorptive treatment and fatigue life (cycles to failure) in bone from ovariectomized cynomolgus monkeys. We analyzed humeral bone from 30 animals across five treatment groups. Animals were treated for 12 months with subcutaneous (sc) vehicle (VEH), sc DMAb (25 mg/kg/month), or intravenous (iv) ALN (50 μg/kg/month). Another group received 6 months VEH followed by 6 months DMAb (VEH-DMAb), and the final group received 6 months ALN followed by 6 months DMAb (ALN-DMAb). A total of 240 cortical beam samples were cyclically tested in four-point bending at 80, 100, 120, or 140 MPa peak stress. High-resolution imaging and density measurements were performed to evaluate bone microstructure and composition. Samples from the ALN (p = 0.014), ALN-DMAb (p = 0.008), and DMAb (p < 0.001) groups illustrated higher fatigue-life measurements than VEH. For example, at 140 MPa the VEH group demonstrated a median ± interquartile range (IQR) fatigue life of 1987 ± 10593 cycles, while animals in the ALN, ALN-DMAb, and DMAb groups survived 9850 ± 13648 (+395% versus VEH), 10493 ± 16796 (+428%), and 14495 ± 49299 (+629%) cycles, respectively. All antiresorptive treatment groups demonstrated lower porosity, smaller pore size, greater pore spacing, and lower number of canals versus VEH (p < 0.001). Antiresorptive treatment was also associated with greater apparent density, dry density, and ash density (p ≤ 0.03). We did not detect detrimental changes following antiresorptive treatments that would explain their association with AFF. In contrast, 12 months of treatment may have a protective effect against fatigue fractures. © 2022 American Society for Bone and Mineral Research (ASBMR).

Denosumab treatment is associated with decreased cortical porosity and increased bone density and strength at the proximal humerus of ovariectomized cynomolgus monkeys

Upper extremity fractures, including those at the humerus, are common among women with postmenopausal osteoporosis. Denosumab was shown to reduce humeral fractures in this population; however, no clinical or preclinical studies have quantified the effects of denosumab on humerus bone mineral density or bone microarchitecture changes. This study used micro-computed tomography (μCT) and computed tomography (CT), alongside image-based finite element (FE) models derived from both modalities, to quantify the effects of denosomab (DMAb) and alendronate (ALN) on humeral bone from acutely ovariectomized (OVX) cynomolgus monkeys. Animals were treated with 12 monthly injections of s.c. vehicle (VEH; n = 10), s.c. denosumab (DMAb; 25 mg/kg, n = 9), or i.v. alendronate (ALN; 50 μg/kg, n = 10). Two more groups received 6 months of VEH followed by 6 months of DMAb (VEH-DMAb; n = 7) or 6 months of ALN followed by 6 months of DMAb (ALN-DMAb; n = 9). After treatment, humeri were harvested and μCT was used to quantify tissue mineral density, trabecular morphology, and cortical porosity at the humeral head. Clinical CT imaging was also used to quantify trabecular and cortical bone mineral density (BMD) at the ultra-proximal, proximal, 1/5 proximal and midshaft of the bone. Finally, μCT-based FE models in compression, and CT-based FE models in compression, torsion, and bending, were developed to estimate differences in strength. Compared to VEH, groups that received DMAb at any time demonstrated lower cortical porosity and/or higher tissue mineral density via μCT; no effects on trabecular morphology were observed. FE estimated strength based on μCT was higher after 12-months DMAb (p = 0.020) and ALN-DMAb (p = 0.024) vs. VEH; respectively, FE predicted mean (SD) strength was 4649.88 (710.58) N, and 4621.10 (1050.16) N vs. 3309.4 (876.09) N. All antiresorptive treatments were associated with higher cortical BMD via CT at the 1/5 proximal and midshaft of the humerus; however, no differences in CT-based FE predicted strength were observed. Overall, these results help to explain the observed reductions in humeral fracture rate following DMAb treatment in women with postmenopausal osteoporosis.

Clinical observation of diminished bone quality and quantity through longitudinal HR-pQCT-derived remodeling and mechanoregulation

High resolution peripheral quantitative computed tomography (HR-pQCT) provides methods for quantifying volumetric bone mineral density and microarchitecture necessary for early diagnosis of bone disease. When combined with a longitudinal imaging protocol and finite element analysis, HR-pQCT can be used to assess bone formation and resorption (i.e., remodeling) and the relationship between this remodeling and mechanical loading (i.e., mechanoregulation) at the tissue level. Herein, 25 patients with a contralateral distal radius fracture were imaged with HR-pQCT at baseline and 9-12 months follow-up: 16 patients were prescribed vitamin D3 with/without calcium supplement based on a blood biomarker measures of bone metabolism and dual-energy X-ray absorptiometry image-based measures of normative bone quantity which indicated diminishing (n = 9) or poor (n = 7) bone quantity and 9 were not. To evaluate the sensitivity of this imaging protocol to microstructural changes, HR-pQCT images were registered for quantification of bone remodeling and image-based micro-finite element analysis was then used to predict local bone strains and derive rules for mechanoregulation. Remodeling volume fractions were predicted by both average values of trabecular and cortical thickness and bone mineral density (R² > 0.8), whereas mechanoregulation was affected by dominance of the arm and group classification (p < 0.05). Overall, longitudinal, extended HR-pQCT analysis enabled the identification of changes in bone quantity and quality too subtle for traditional measures.

An experimentally informed statistical elasto-plastic mineralised collagen fibre model at the micrometre and nanometre lengthscale

Bone is an intriguingly complex material. It combines high strength, toughness and lightweight via an elaborate hierarchical structure. This structure results from a biologically driven self-assembly and self-organisation, and leads to different deformation mechanisms along the length scales. Characterising multiscale bone mechanics is fundamental to better understand these mechanisms including changes due to bone-related diseases. It also guides us in the design of new bio-inspired materials. A key-gap in understanding bone's behaviour exists for its fundamental mechanical unit, the mineralised collagen fibre, a composite of organic collagen molecules and inorganic mineral nanocrystals. Here, we report an experimentally informed statistical elasto-plastic model to explain the fibre behaviour including the nanoscale interplay and load transfer with its main mechanical components. We utilise data from synchrotron nanoscale imaging, and combined micropillar compression and synchrotron X-ray scattering to develop the model. We see that a 10-15% micro- and nanomechanical heterogeneity in mechanical properties is essential to promote the ductile microscale behaviour preventing an abrupt overall failure even when individual fibrils have failed. We see that mineral particles take up 45% of strain compared to collagen molecules while interfibrillar shearing seems to enable the ductile post-yield behaviour. Our results suggest that a change in mineralisation and fibril-to-matrix interaction leads to different mechanical properties among mineralised tissues. Our model operates at crystalline-, molecular- and continuum-levels and sheds light on the micro- and nanoscale deformation of fibril-matrix reinforced composites.

Femoral strength can be predicted from 2D projections using a 3D statistical deformation and texture model with finite element analysis

Ultimate force of the proximal human femur can be predicted using Finite Element Analysis (FEA), but the models rely on 3D computed tomography images. Landmark-based statistical appearance models (SAM) and B-Spline transformation-based statistical deformation models (SDM) have been used to estimate 3D images from 2D projections, which facilitates model generation and reduces the radiation dose. However, there is no literature on the accuracy of SDM-based FEA models of bones with respect to experimental results. In this study, a methodology for an enhanced SDM with textural information is presented. The statistical deformation and texture models (SDTMs) are based on a set of 37 quantitative CT (QCT) images. They were used to estimate 3D images from two or one projections of the set in a leave-one-out setup. These estimations where then used to create FEA models. The ultimate force predicted by FEA models estimated from two or one projection using the SDTMs were compared to the experimental ultimate force from a previous study on the same femora and to the results of standard QCT-based FEA models. High correlations between predictions and experimental measurements were found for FEA models reconstructed from 2D projections with R²=0.835 when based on two projections and R²=0.724 when using one projection. The correlations were comparable to those reached with standard QCT-based FE-models with the experimental results (R²=0.795). This study shows the high potential of SDTM-based 3D image reconstruction and FEA modelling from 2D projections to predict femoral ultimate force.

Experimental testing and biomechanical CT analysis of Chinese cadaveric vertebrae with different modeling approaches

Osteoporosis is characterized by reduced bone strength predisposing to an increased risk of fracture. Biomechanical computed tomography (BCT), predicting bone strength via CT-based finite element analysis (FEA), is now clinically available in the USA for diagnosing osteoporosis or assessing fracture risk. However, it has not been previously validated using a cohort of only Chinese subjects. Additionally, the effect of various modeling approaches on BCT outcomes remains elusive. To address these issues, we performed DXA and QCT scanning, compression testing, and BCT analyses on thirteen vertebrae derived from Chinese donors. Three BCT models were created (voxBCT and tetBCT: voxel-based and tetrahedral element-based FE models generated by a commercial software; matBCT: tetrahedral element-based FE model generated by a custom MATLAB program). BCT-computed outcomes were compared with experimental measures or between different BCT models. Results showed that, DXA-measured areal bone mineral density (aBMD) showed weak correlations with experimentally-measured vertebral stiffness (R² = 0.28) and strength (R² = 0.34). Compared to DXA-aBMD, BCT-computed stiffness provided improved correlations with experimentally-measured stiffness (voxBCT: R² = 0.82; tetBCT: R² = 0.77; matBCT: R² = 0.76) and strength (voxBCT: R² = 0.55; tetBCT: R² = 0.57; matBCT: R² = 0.53); BCT-computed mechanical parameters (stiffness, stress and strain) of the three different models were highly correlated with each other, with coefficient of determination (R²) values of 0.89-0.98. These results, based on a cohort of Chinese vertebral cadavers, suggest that BCT is superior over aBMD to consistently predict vertebral mechanical characteristics, regardless of the modeling approaches of choice.

Finite element analysis informed variable selection for femoral fracture risk prediction

Logistic regression classification (LRC) is widely used to develop models to predict the risk of femoral fracture. LRC models based on areal bone mineral density (aBMD) alone are poor, with area under the receiver operator curve (AUROC) scores reported to be as low as 0.63. This has led to researchers investigating methods to extract further information from the image to increase performance. Recently, the use of active shape (ASM) and appearance models (AAM) have resulted in moderate improvements, but there is a risk that inclusion of too many modes will lead to overfitting. In addition, there are concerns that the effort required to extract the additional information does not justify the modest improvement in fracture risk prediction. This raises the question, are we reaching the limits of the information that can be extracted from an image? Finite element analysis was used in combination with active shape and appearance modelling to select variables to develop LRC models of fracture risk. Active shape and active appearance models were constructed based on a previously reported cohort of 94 post-menopausal Caucasian women (47 with and 47 without a fracture). T-tests were used to identify differences between the two groups for each mode of variation. Femur strength was predicted for two load cases, stance and a fall. Stepwise multi-variate linear regression was used to identify shape and appearance modes that were predictors of strength for the femurs in the training set. Femurs were also synthetically generated to explore the influence of the first 10 modes of the shape and appearance models. Identified modes of variation were then used to generate LRC models to predict fracture risk. Only 6 modes, 4 active appearance and 2 active shape modes, were identified that had a significant influence on predicted fracture strength. Of these, only two active appearance modes were needed to substantially improve the predictive mode performance (ΔAUROC = 0.080). The addition of 3 more modes (1 AAM and two ASM) further improved the performance of the classifier (ΔAUROC = 0.123). Further addition of modes did not result in any further substantial improvements. Based on these findings, it is suggested that we are reaching the limits of the information that can be extracted from an image to predict fracture risk.

Numerical investigation of mechanical behavior of human femoral diaphysis in normal and defective geometry: experimental evaluation

Failure and major reoperation after internal fixation (IF) in mature femoral bones are common and proper selection of fixation method may reduce the rate of reoperations. Investigating the mechanical behavior of the human femoral diaphysis, this article studies effect of mechanical properties and geometry of the bone on selection of IF method. To this aim, we calculated the bone mineral density in human femurs, and then, using computed tomography scan, we obtained geometry and nonhomogeneous properties of the bone. Finite element (FE) models of osteotomised femurs were reinforced using four types of screws with a locking compression plate (LCP). We performed buckling and 4-point bending simulations, and results of these simulations represent critical buckling loads, maximum von Mises stresses, and strains around the screws and the central defect. To evaluate FE analysis, we employed the compressive experiments and compared load vs. displacement curves with FE results. Results corresponding to intact, osteotomised, and reinforced states are compared together, and the effect of cortical and unicortical screws in LCPs is studied. The FE results showed that application of identical prophylactic IF for two persons with identical injuries in the same conditions bring quite inverse results. As a consequence, evaluation of osteoporosis, elastic modulus, and morphometric data are required before fixation and screw selection. Besides, for short diaphysis, unicortical screws have maximum strengthening factor in bending. While for long samples, these types of screws can be the worst option, application of cortical screws results to maximum strength in comparison with other types.

Optimization of the failure criterion in micro-Finite Element models of the mouse tibia for the non-invasive prediction of its failure load in preclinical applications

New treatments against osteoporosis require testing in animal models and the mouse tibia is among the most common studied anatomical sites. In vivo micro-Computed Tomography (microCT) based micro-Finite Element (microFE) models can be used for predicting the bone strength non-invasively, after proper validation against experiments. The aim of this study was to evaluate the ability of different microCT-based bone parameters and microFE models to predict tibial structural mechanical properties in compression. Twenty tibiae were scanned at 10.4 μm voxel size and subsequently tested in uniaxial compression at 0.03 mm/s until failure. Stiffness and failure load were measured from the load-displacement curves. Standard morphometric parameters were measured from the microCT images. The spatial distribution of bone mineral content (BMC) was evaluated by dividing the tibia into 40 regions. MicroFE models were generated by converting each microCT image into a voxel-based mesh with homogeneous isotropic material properties. Failure load was estimated by using different failure criteria, and the optimized parameters were selected by minimising the errors with respect to experimental measurements. Experimental and predicted stiffness were moderately correlated (R² = 0.65, error = 14% ± 8%). Normalized failure load was best predicted by microFE models (R² = 0.81, error = 9% ± 6%). Failure load was not correlated to the morphometric parameters and weakly correlated with some geometrical parameters (R² < 0.37). In conclusion, microFE models can improve the current estimation of the mouse tibia structural properties and in this study an optimal failure criterion has been defined. Since it is a non-invasive method, this approach can be applied longitudinally for evaluating temporal changes in the bone strength.

Biomechanical Computed Tomography analysis (BCT) for clinical assessment of osteoporosis

The surgeon general of the USA defines osteoporosis as "a skeletal disorder characterized by compromised bone strength, predisposing to an increased risk of fracture." Measuring bone strength, Biomechanical Computed Tomography analysis (BCT), namely, finite element analysis of a patient's clinical-resolution computed tomography (CT) scan, is now available in the USA as a Medicare screening benefit for osteoporosis diagnostic testing. Helping to address under-diagnosis of osteoporosis, BCT can be applied "opportunistically" to most existing CT scans that include the spine or hip regions and were previously obtained for an unrelated medical indication. For the BCT test, no modifications are required to standard clinical CT imaging protocols. The analysis provides measurements of bone strength as well as a dual-energy X-ray absorptiometry (DXA)-equivalent bone mineral density (BMD) T-score at the hip and a volumetric BMD of trabecular bone at the spine. Based on both the bone strength and BMD measurements, a physician can identify osteoporosis and assess fracture risk (high, increased, not increased), without needing confirmation by DXA. To help introduce BCT to clinicians and health care professionals, we describe in this review the currently available clinical implementation of the test (VirtuOst), its application for managing patients, and the underlying supporting evidence; we also discuss its main limitations and how its results can be interpreted clinically. Together, this body of evidence supports BCT as an accurate and convenient diagnostic test for osteoporosis in both sexes, particularly when used opportunistically for patients already with CT. Biomechanical Computed Tomography analysis (BCT) uses a patient's CT scan to measure both bone strength and bone mineral density at the hip or spine. Performing at least as well as DXA for both diagnosing osteoporosis and assessing fracture risk, BCT is particularly well-suited to "opportunistic" use for the patient without a recent DXA who is undergoing or has previously undergone CT testing (including hip or spine regions) for an unrelated medical condition.

Perspectives on the non-invasive evaluation of femoral strength in the assessment of hip fracture risk

We reviewed the experimental and clinical evidence that hip bone strength estimated by BMD and/or finite element analysis (FEA) reflects the actual strength of the proximal femur and is associated with hip fracture risk and its changes upon treatment.

INTRODUCTION

The risk of hip fractures increases exponentially with age due to a progressive loss of bone mass, deterioration of bone structure, and increased incidence of falls. Areal bone mineral density (aBMD), measured by dual-energy X-ray absorptiometry (DXA), is the most used surrogate marker of bone strength. However, age-related declines in bone strength exceed those of aBMD, and the majority of fractures occur in those who are not identified as osteoporotic by BMD testing. With hip fracture incidence increasing worldwide, the development of accurate methods to estimate bone strength in vivo would be very useful to predict the risk of hip fracture and to monitor the effects of osteoporosis therapies.

METHODS

We reviewed experimental and clinical evidence regarding the association between aBMD and/orCT-finite element analysis (FEA) estimated femoral strength and hip fracture risk as well as their changes with treatment.

RESULTS

Femoral aBMD and bone strength estimates by CT-FEA explain a large proportion of femoral strength ex vivo and predict hip fracture risk in vivo. Changes in femoral aBMD are strongly associated with anti-fracture efficacy of osteoporosis treatments, though comparable data for FEA are currently not available.

CONCLUSIONS

Hip aBMD and estimated femoral strength are good predictors of fracture risk and could potentially be used as surrogate endpoints for fracture in clinical trials. Further improvements of FEA may be achieved by incorporating trabecular orientations, enhanced cortical modeling, effects of aging on bone tissue ductility, and multiple sideway fall loading conditions.

Patient-Specific Bone Multiscale Modelling, Fracture Simulation and Risk Analysis-A Survey

This paper provides a starting point for researchers and practitioners from biology, medicine, physics and engineering who can benefit from an up-to-date literature survey on patient-specific bone fracture modelling, simulation and risk analysis. This survey hints at a framework for devising realistic patient-specific bone fracture simulations. This paper has 18 sections: Section 1 presents the main interested parties; Section 2 explains the organzation of the text; Section 3 motivates further work on patient-specific bone fracture simulation; Section 4 motivates this survey; Section 5 concerns the collection of bibliographical references; Section 6 motivates the physico-mathematical approach to bone fracture; Section 7 presents the modelling of bone as a continuum; Section 8 categorizes the surveyed literature into a continuum mechanics framework; Section 9 concerns the computational modelling of bone geometry; Section 10 concerns the estimation of bone mechanical properties; Section 11 concerns the selection of boundary conditions representative of bone trauma; Section 12 concerns bone fracture simulation; Section 13 presents the multiscale structure of bone; Section 14 concerns the multiscale mathematical modelling of bone; Section 15 concerns the experimental validation of bone fracture simulations; Section 16 concerns bone fracture risk assessment. Lastly, glossaries for symbols, acronyms, and physico-mathematical terms are provided.

Predicting bone strength from CT data: Clinical applications

In this review we summarise over 15 years of research and development around the prediction of whole bones strength from Computed Tomography data, with particular reference to the prediction of the risk of hip fracture in osteoporotic patients. We briefly discuss the theoretical background, and then provide a summary of the laboratory and clinical validation of these modelling technologies. We then discuss the three current clinical applications: in clinical research, in clinical trials, and in clinical practice. On average the strength predicted with finite element models (QCT-FE) based on computed tomography is 7% more accurate that that predicted with areal bone mineral density from Dual X-ray Absorptiometry (DXA-aBMD), the current standard of care, both in term of laboratory validation on cadaver bones and in terms of stratification accuracy on clinical cohorts of fractured and non-fractured women. This improved accuracy makes QCT-FE superior to DXA-aBMD in clinical research and in clinical trials, where the its use can cut in half the number of patients to be enrolled to get the same statistical power. For routine clinical use to decide who to treat with antiresorptive drugs, QCT-FE is more accurate but less cost-effective than DXA-aBMD, at least when the decision is on first line treatment like bisphosphonates. But the ability to predict skeletal strength from medical imaging is now opening a number of other applications, for example in paediatrics and oncology.

The longitudinal effects of ovariectomy on the morphometric, densitometric and mechanical properties in the murine tibia: A comparison between two mouse strains

Oestrogen deficiency-related bone loss in the ovariectomized (OVX) mouse is a common model for osteoporosis. However, a comprehensive in vivo assessment of intervention-related changes in multiple bone properties, and in multiple mouse strains, is required in order to identify an appropriate model for future evaluation of novel anti-osteoporotic therapies. The aim of this study was to evaluate the effect of OVX on the morphometric and densitometric properties measured in the microCT images and the mechanical properties estimated with finite element models of the tibia in two mouse strains, C57BL/6 and BALB/c. 14-weeks-old female C57BL/6 and BALB/c mice were divided into two groups per strain: (1) ovariectomized, (2) non-operated control. The right tibia was scanned at baseline (14 weeks) and then every two weeks thereafter, until 24-weeks-old, using in vivo microCT. Changes in trabecular and cortical bone morphometry, spatiotemporal changes in densitometric properties and in mechanical properties (from micro-finite element (μFE) analysis) were computed. Differences between OVX and non-operated controls were evaluated by ANCOVA, adjusted for 14-weeks baseline. In morphometry, trabecular bone mass was significantly reduced in both C57BL/6 and BALB/c from four weeks following surgery. Though the OVX-effect was transient in BALB/c as bone mass reached skeletal homeostasis. OVX inhibited the age-related thickening of cortical bone only in C57BL/6. In both strains, increments in bone mineral content were significantly lower with OVX only in the proximal tibia, with intervention-related differences increasing with time. OVX had no effect on μFE estimates of stiffness nor failure load in either strain. The results of this study show strain-, time- and region-(trabecular or cortical) dependent changes in morphometric and densitometric properties. These findings highlight the importance of choosing an appropriate mouse model and time points for research of treatments against accelerated bone resorption.

A round-robin finite element analysis of human femur mechanics between seven participating laboratories with experimental validation

Finite element analysis is a common tool that has been used for the past few decades to predict the mechanical behavior of bone. However, to our knowledge, there are no round-robin finite element analyses of long human bones with more than two participating biomechanics laboratories published yet, where the results of the experimental tests were not known in advance. We prepared a fresh-frozen human femur for a compression test in a universal testing machine measuring the strains at 10 bone locations as well as the deformation of the bone in terms of the displacement of the loading point at a load of 2 kN. The computed tomography data of the bone with a calibration phantom as well as the orientation of the bone in the testing machine with the according boundary conditions were delivered to seven participating laboratories. These were asked to perform a finite element analysis simulating the experimental setup and deliver their results to the coordinator without knowing the experimental results. Resultantly, four laboratories had deviations from the experimentally measured strains of less than 40%, and three laboratories had deviations of their numerically determined values compared to the experimental data of more than 120%. These deviations are thought to be based on different material laws and material data, as well as different material mapping methods. Investigations will be conducted to clarify and assess the reasons for the large deviations in the numerical data. It was shown that the precision of finite element models of the human femur is not yet as developed as desired by the biomechanics community.

Compressive behaviour of uniaxially aligned individual mineralised collagen fibres at the micro- and nanoscale

The increasing incidence of osteoporotic bone fractures makes fracture risk prediction an important clinical challenge. Computational models can be utilised to facilitate such analyses. However, they critically depend on bone's underlying hierarchical material description. To understand bone's irreversible behaviour at the micro- and nanoscale, we developed an in situ testing protocol that allows us to directly relate the experimental data to the mechanical behaviour of individual mineralised collagen fibres and its main constitutive phases, the mineralised collagen fibrils and the mineral nanocrystals, by combining micropillar compression of single fibres with small angle X-ray scattering (SAXS) and X-ray diffraction (XRD). Failure modes were assessed by SEM. Strain ratios in the elastic region at fibre, fibril and mineral levels were found to be approximately 22:5:2 with strain ratios at the point of compressive strength of 0.23 ± 0.11 for fibril-to-fibre and 0.07 ± 0.01 for mineral-to-fibre levels. Mineral-to-fibre levels showed highest strain ratios around the apparent yield point, fibril-to-fibre around apparent strength. The mineralised collagen fibrils showed a delayed mechanical response, contrary to the mineral phase, which points towards preceding deformations of mineral nanocrystals in the extrafibrillar matrix. No damage was measured at the level of the mineralised collagen fibre which indicates an incomplete separation of the mineral and collagen, and an extrafibrillar interface failure. The formation of kink bands and the gradual recruitment of fibrils upon compressive loading presumably led to localised strains. Our results from a well-controlled fibrillar architecture provide valuable input for micromechanical models and computational non-linear bone strength analyses that may provide further insights for personalised diagnosis and treatment as well as bio-inspired implants for patients with bone diseases. STATEMENT OF SIGNIFICANCE: Musculoskeletal diseases such as osteoporosis, osteoarthritis or bone cancer significantly challenge health care systems and make fracture risk prediction and treatment optimisation important clinical goals. Computational methods such as finite element models have the potential to optimise analyses but highly depend on underlying material descriptions. We developed an in situ testing set-up to directly relate experimental data to the mechanical behaviour of bone's fundamental building block, the individual mineralised collagen fibre and its main constituents. Low multilevel strain ratios suggest high deformations in the extrafibrillar matrix and energy dissipation at the interfaces, the absence of damage indicates both an incomplete separation between mineral and collagen and an extrafibrillar interface failure. The formation of kink bands in the fibril-reinforced composite presumably led to localised strains. The deformation behaviour of a well-controlled fibrillar architecture provides valuable input for non-linear bone strength analyses.

Biomechanics of Femoral Neck Fractures and Implications for Fixation

Fractures of the femoral neck can occur in young healthy individuals due to high loads occurring during motor vehicle accidents, impacts, or falls. Failure forces are lower if impacts occur sideways onto the greater trochanter as compared with vertical loading of the hip. Bone density, bone geometry, and thickness of cortical bone at the femoral neck contribute to its mechanical strength. Femoral neck fractures in young adults require accurate reduction and stable internal fixation. The available techniques for fracture fixation at the femoral neck (cannulated screws, hip screw systems, proximal femur plates, and cephallomedullary nails) are reviewed with respect to their competence to provide biomechanical stability. Mechanically unstable fractures require a load-bearing implant, such as hip screws, with antirotational screws or intramedullary nails. Subcapital or transcervical fracture patterns and noncomminuted fractures enable load sharing and can be securely fixed with cannulated screws or solitary hip screw systems without compromising fixation stability.

Effects of osteoporosis drug treatments on cortical and trabecular bone in the femur using DXA-based 3D modeling

Effects of osteoporosis drugs on proximal femur cortical and trabecular bone were studied using dual-energy x-ray absorptiometry (DXA)-based 3D modeling method. Changes observed in this head-to-head study were consistent with those obtained using computed tomography in the literature.

INTRODUCTION

The aim of the present study was to assess the effects of osteoporosis drugs on cortical and trabecular bone at the proximal femur using DXA-based 3D modeling.

METHODS

We retrospectively analyzed 155 patients stratified by treatments: naive of treatment (NAIVE), alendronate (AL), denosumab (DMAB), and teriparatide (TPTD). DXA scans were performed at baseline and after treatment, and areal bone mineral density at spine and femur were measured. A software algorithm (3D-SHAPER) was used to derive 3D models from hip DXA scans and compute: trabecular and cortical volumetric BMD (vBMD), cortical thickness (Cth), and cortical surface BMD (cortical sBMD). Changes from baseline were normalized at 24 months and evaluated in terms or percentage.

RESULTS

After 24 months, a non-significant decrease was observed for trabecular vBMD, Cortical sBMD, Cth, and cortical vBMD (- 2.3, - 0.8, - 0.3, and - 0.5%) in the NAIVE group. Under AL and DMAB, significant increases were observed in trabecular vBMD (3.8 and 7.3%), cortical vBMD (1.4 and 2.0%), and cortical sBMD (1.5 and 3.6%). An increase in Cth was observed in patients under DMAB (1.8%). Under TPTD, a significant increase in Trabecular vBMD was observed (5.9%) associated with a non-significant increase of Cth (+ 1%) concomitant with a decrease in cortical vBMD (- 1.1%).

CONCLUSION

Results obtained in this head-to-head study are consistent with those obtained using computed tomography in the literature. DXA-based modeling techniques could complement standard DXA examination to monitor treatment effects on trabecular and cortical compartments.

Denosumab: A Review in Postmenopausal Osteoporosis

Denosumab (Prolia^®; Pralia^®) is a human monoclonal antibody targeting the key bone resorption mediator RANKL. The drug is administered via subcutaneous injection once every 6 months and is approved for various indications, including the treatment of postmenopausal (PM) women with osteoporosis at increased/high risk of fracture or failure/intolerance of other osteoporosis therapies (indications featured in this review). Denosumab showed benefit in several phase 3 or 4 studies in PM women with osteoporosis or low bone mineral density (BMD), including the pivotal 3-year double-blind FREEDOM trial and its 7-year open-label extension. Denosumab reduced the risk of vertebral, nonvertebral and hip fractures and increased BMD across skeletal sites versus placebo in FREEDOM, with these benefits maintained over up to 10 years' therapy in the extension. The drug was also more effective in improving BMD than bisphosphonates, including in women switched from a bisphosphonate regimen, in 1-year trials; however, whether these differences translate into differences in anti-fracture efficacy is unclear. Denosumab was generally well tolerated over up to 10 years' treatment, although an increased risk of multiple vertebral fractures was observed after discontinuation of the drug. Thus, denosumab is a key treatment option for PM women with osteoporosis who have an increased/high risk of fracture or failure/intolerance of other osteoporosis therapies, although the potential for multiple vertebral fractures to occur after discontinuation of the drug requires consideration of subsequent management options.

Mapping anisotropy improves QCT-based finite element estimation of hip strength in pooled stance and side-fall load configurations

Hip fractures are one of the most severe consequences of osteoporosis. Compared to the clinical standard of DXA-based aBMD at the femoral neck, QCT-based FEA delivers a better surrogate of femoral strength and gains acceptance for the calculation of hip fracture risk when a CT reconstruction is available. Isotropic, homogenised voxel-based, finite element (hvFE) models are widely used to estimate femoral strength in cross-sectional and longitudinal clinical studies. However, fabric anisotropy is a classical feature of the architecture of the proximal femur and the second determinant of the homogenised mechanical properties of trabecular bone. Due to the limited resolution, fabric anisotropy cannot be derived from clinical CT reconstructions. Alternatively, fabric anisotropy can be extracted from HR-pQCT images of cadaveric femora. In this study, fabric anisotropy from HR-pQCT images was mapped onto QCT-based hvFE models of 71 human proximal femora for which both HR-pQCT and QCT images were available. Stiffness and ultimate load computed from anisotropic hvFE models were compared with previous biomechanical tests in both stance and side-fall configurations. The influence of using the femur-specific versus a mean fabric distribution on the hvFE predictions was assessed. Femur-specific and mean fabric enhance the prediction of experimental ultimate force for the pooled, i.e. stance and side-fall, (isotropic: r²=0.81, femur-specific fabric: r²=0.88, mean fabric: r²=0.86,p<0.001) but not for the individual configurations. Fabric anisotropy significantly improves bone strength prediction for the pooled configurations, and mapped fabric provides a comparable prediction to true fabric. The mapping of fabric anisotropy is therefore expected to help generate more accurate QCT-based hvFE models of the proximal femur for personalised or multiple load configurations.

Are CT-Based Finite Element Model Predictions of Femoral Bone Strength Clinically Useful?

PURPOSE OF REVIEW

This study reviews the available literature to compare the accuracy of areal bone mineral density derived from dual X-ray absorptiometry (DXA-aBMD) and of subject-specific finite element models derived from quantitative computed tomography (QCT-SSFE) in predicting bone strength measured experimentally on cadaver bones, as well as their clinical accuracy both in terms of discrimination and prediction. Based on this information, some basic cost-effectiveness calculations are performed to explore the use of QCT-SSFE instead of DXA-aBMD in (a) clinical studies with femoral strength as endpoint, (b) predictor of the risk of hip fracture in low bone mass patients.

RECENT FINDINGS

Recent improvements involving the use of smooth-boundary meshes, better anatomical referencing for proximal-only scans, multiple side-fall directions, and refined boundary conditions increase the predictive accuracy of QCT-SSFE. If these improvements are adopted, QCT-SSFE is always preferable over DXA-aBMD in clinical studies with femoral strength as the endpoint, while it is not yet cost-effective as a hip fracture risk predictor, although pathways that combine both QCT-SSFE and DXA-aBMD are promising.

Denosumab for the treatment of osteoporosis

Denosumab, a specific inhibitor of RANK ligand, is a novel therapy for postmenopausal osteoporosis and related disorders. An extensive clinical development program has evaluated the clinical efficacy and safety of denosumab with several thousand patients being followed for up to 10 years. Combined with more than six years of postmarketing experience, these studies provide substantial confidence that denosumab is a convenient and appropriate treatment for patients, including Asians, at high risk for fracture. This review will summarize the clinical development of denosumab and lessons learned since its approval for clinical use in 2010.

Finite Element-Based Mechanical Assessment of Bone Quality on the Basis of In Vivo Images

Beyond bone mineral density (BMD), bone quality designates the mechanical integrity of bone tissue. In vivo images based on X-ray attenuation, such as CT reconstructions, provide size, shape, and local BMD distribution and may be exploited as input for finite element analysis (FEA) to assess bone fragility. Further key input parameters of FEA are the material properties of bone tissue. This review discusses the main determinants of bone mechanical properties and emphasizes the added value, as well as the important assumptions underlying finite element analysis. Bone tissue is a sophisticated, multiscale composite material that undergoes remodeling but exhibits a rather narrow band of tissue mineralization. Mechanically, bone tissue behaves elastically under physiologic loads and yields by cracking beyond critical strain levels. Through adequate cell-orchestrated modeling, trabecular bone tunes its mechanical properties by volume fraction and fabric. With proper calibration, these mechanical properties may be incorporated in quantitative CT-based finite element analysis that has been validated extensively with ex vivo experiments and has been applied increasingly in clinical trials to assess treatment efficacy against osteoporosis.

Finite Element Analysis of Denosumab Treatment Effects on Vertebral Strength in Ovariectomized Cynomolgus Monkeys

Finite element analysis has not yet been validated for measuring changes in whole-bone strength at the hip or spine in people after treatment with an osteoporosis agent. Toward that end, we assessed the ability of a clinically approved implementation of finite element analysis to correctly quantify treatment effects on vertebral strength, comparing against direct mechanical testing, in cynomolgus monkeys randomly assigned to one of three 16-month-long treatments: sham surgery with vehicle (Sham-Vehicle), ovariectomy with vehicle (OVX-Vehicle), or ovariectomy with denosumab (OVX-DMAb). After treatment, T12 vertebrae were retrieved, scanned with micro-CT, and mechanically tested to measure compressive strength. Blinded to the strength data and treatment codes, the micro-CT images were coarsened and homogenized to create continuum-type finite element models, without explicit porosity. With clinical translation in mind, these models were then analyzed for strength using the U.S. Food and Drug Administration (FDA)-cleared VirtuOst software application (O.N. Diagnostics, Berkeley, CA, USA), developed for analysis of human bones. We found that vertebral strength by finite element analysis was highly correlated (R(2)  = 0.97; n = 52) with mechanical testing, independent of treatment (p = 0.12). Further, the size of the treatment effect on strength (ratio of mean OVX-DMAb to mean OVX-Vehicle, as a percentage) was large and did not differ (p = 0.79) between mechanical testing (+57%; 95% CI [26%, 95%]) and finite element analysis (+51% [20%, 88%]). The micro-CT analysis revealed increases in cortical thickness (+45% [19%, 73%]) and trabecular bone volume fraction (+24% [8%, 42%]). These results show that a preestablished clinical finite element analysis implementation-developed for human bone and clinically validated in fracture-outcome studies-correctly quantified the observed treatment effects of denosumab on vertebral strength in cynomolgus monkeys. One implication is that the treatment effects in this study are well explained by the features contained within these finite element models, namely, the bone geometry and mass and the spatial distribution of bone mass. © 2016 American Society for Bone and Mineral Research.