Fracture toughness and work of fracture of hydrated, dehydrated, and ashed bovine bone

Water absorption in artificial composites: Curse or blessing?

OBJECTIVES

This study evaluated the impact of mutable water uptake on the durability of mechanical properties and the long-term reliability of artificial composites.

METHODS

Three resin-based CAD/CAM restorative materials (CRMs) were investigated in three-point bending tests to calculate flexural strength (FS), modulus of elasticity (ME), modulus of resilience (MR), modulus of toughness (MT), and elastic recovery (ER). All specimens (n = 180) were stored under the same conditions and tested in four subsets (n = 15 per material) that were respectively withdrawn after repeated thermocycling (5000 cycles; 5-55 °C, H2O) and repetitive drying (7 d; 37 °C, air). For every specimen, weight differences were determined per storage condition. Likewise, loss tangent data were separately recorded via dynamic mechanical analysis to reliably assess damping characteristics.

RESULTS

Repeated thermocycling always induced weight increase and a concurrent significant loss in all mechanical properties except for MT and ER of a polymethylmethacrylate-based CRM. Drying consistently provoked weight loss and raised mechanical properties to initial values. Weight increase, however, enhanced loss tangent values and accordingly distinct damping characteristics, whereas weight decrease markedly lowered damping properties.

SIGNIFICANCE

Water uptake repeatedly induced a decrease in common mechanical properties but concurrently increased damping behavior. Invertible equilibrium processes were found with no evidence for permanent material degradation.

A numerical study of dehydration induced fracture toughness degradation in human cortical bone

A 2D plane strain extended finite element method (XFEM) model was developed to simulate three-point bending fracture toughness tests for human bone conducted in hydrated and dehydrated conditions. Bone microstructures and crack paths observed by micro-CT imaging were simulated using an XFEM damage model. Critical damage strains for the osteons, matrix, and cement lines were deduced for both hydrated and dehydrated conditions and it was found that dehydration decreases the critical damage strains by about 50%. Subsequent parametric studies using the various microstructural models were performed to understand the impact of individual critical damage strain variations on the fracture behavior. The study revealed the significant impact of the cement line critical damage strains on the crack paths and fracture toughness during the early stages of crack growth. Furthermore, a significant sensitivity of crack growth resistance and crack paths on critical strain values of the cement lines was found to exist for the hydrated environments where a small change in critical strain values of the cement lines can alter the crack path to give a significant reduction in fracture resistance. In contrast, in the dehydrated state where toughness is low, the sensitivity to changes in critical strain values of the cement lines is low. Overall, our XFEM model was able to provide new insights into how dehydration affects the micromechanisms of fracture in bone and this approach could be further extended to study the effects of aging, disease, and medical therapies on bone fracture.

A comprehensive set of ultrashort echo time magnetic resonance imaging biomarkers to assess cortical bone health: A feasibility study at clinical field strength

INTRODUCTION

Conventional bone imaging methods primarily use X-ray techniques to assess bone mineral density (BMD), focusing exclusively on the mineral phase. This approach lacks information about the organic phase and bone water content, resulting in an incomplete evaluation of bone health. Recent research highlights the potential of ultrashort echo time magnetic resonance imaging (UTE MRI) to measure cortical porosity and estimate BMD based on signal intensity. UTE MRI also provides insights into bone water distribution and matrix organization, enabling a comprehensive bone assessment with a single imaging technique. Our study aimed to establish quantifiable UTE MRI-based biomarkers at clinical field strength to estimate BMD and microarchitecture while quantifying bound water content and matrix organization.

METHODS

Femoral bones from 11 cadaveric specimens (n = 4 males 67-92 yrs of age, n = 7 females 70-95 yrs of age) underwent dual-echo UTE MRI (3.0 T, 0.45 mm resolution) with different echo times and high resolution peripheral quantitative computed tomography (HR-pQCT) imaging (60.7 μm voxel size). Following registration, a 4.5 mm HR-pQCT region of interest was divided into four quadrants and used across the multi-modal images. Statistical analysis involved Pearson correlation between UTE MRI porosity index and a signal-intensity technique used to estimate BMD with corresponding HR-pQCT measures. UTE MRI was used to calculate T1 relaxation time and a novel bound water index (BWI), compared across subregions using repeated measures ANOVA.

RESULTS

The UTE MRI-derived porosity index and signal-intensity-based estimated BMD correlated with the HR-pQCT variables (porosity: r = 0.73, p = 0.006; BMD: r = 0.79, p = 0.002). However, these correlations varied in strength when we examined each of the four quadrants (subregions, r = 0.11-0.71). T1 relaxometry and the BWI exhibited variations across the four subregions, though these differences were not statistically significant. Notably, we observed a strong negative correlation between T1 relaxation time and the BWI (r = -0.87, p = 0.0006).

CONCLUSION

UTE MRI shows promise for being an innocuous method for estimating cortical porosity and BMD parameters while also giving insight into bone hydration and matrix organization. This method offers the potential to equip clinicians with a more comprehensive array of imaging biomarkers to assess bone health without the need for invasive or ionizing procedures.

Assessment of bovine cortical bone fracture behavior using impact microindentation as a surrogate of fracture toughness

The fracture behavior of bone is critically important for evaluating its mechanical competence and ability to resist fractures. Fracture toughness is an intrinsic material property that quantifies a material's ability to withstand crack propagation under controlled conditions. However, properly conducting fracture toughness testing requires the access to calibrated mechanical load frames and the destructive testing of bone samples, and therefore fracture toughness tests are clinically impractical. Impact microindentation mimicks certain aspects of fracture toughness measurements, but its relationship with fracture toughness remains unknown. In this study, we aimed to compare measurements of notched fracture toughness and impact microindentation in fresh and boiled bovine bone. Skeletally mature bovine bone specimens (n = 48) were prepared, and half of them were boiled to denature the organic matrix, while the other half remained preserved in frozen conditions. All samples underwent a notched fracture toughness test to determine their resistance to crack initiation (KIC) and an impact microindentation test using the OsteoProbe to obtain the Bone Material Strength index (BMSi). Boiling the bone samples increased the denatured collagen content, while mineral density and porosity remained unaffected. The boiled bones also showed significant reduction in both KIC (P < .0001) and the average BMSi (P < .0001), leading to impaired resistance of bone to crack propagation. Remarkably, the average BMSi exhibited a high correlation with KIC (r = 0.86; P < .001). A ranked order difference analysis confirmed the excellent agreement between the 2 measures. This study provides the first evidence that impact microindentation could serve as a surrogate measure for bone fracture behavior. The potential of impact microindentation to assess bone fracture resistance with minimal sample disruption could offer valuable insights into bone health without the need for cumbersome testing equipment and sample destruction.

Ex vivo exposure to calcitonin or raloxifene improves mechanical properties of diseased bone through non-cell mediated mechanisms

Raloxifene (RAL) reduces clinical fracture risk despite modest effects on bone mass and density. This reduction in fracture risk may be due to improved material level-mechanical properties through a non-cell mediated increase in bone hydration. Synthetic salmon calcitonin (CAL) has also demonstrated efficacy in reducing fracture risk with only modest bone mass and density improvements. This study aimed to determine if CAL could modify healthy and diseased bone through cell-independent mechanisms that alter hydration similar to RAL. 26-week-old male C57BL/6 mice induced with chronic kidney disease (CKD) beginning at 16 weeks of age via 0.2 % adenine-laced casein-based (0.9 % P, 0.6 % C) chow, and their non-CKD control littermates (Con), were utilized. Upon sacrifice, right femora were randomly assigned to the following ex vivo experimental groups: RAL (2 μM, n = 10 CKD, n = 10 Con), CAL (100 nM, n = 10 CKD, n = 10 Con), or Vehicle (VEH; n = 9 CKD, n = 9 Con). Bones were incubated in PBS + drug solution at 37 °C for 14 days using an established ex vivo soaking methodology. Cortical geometry (μCT) was used to confirm a CKD bone phenotype, including porosity and cortical thinning, at sacrifice. Femora were assessed for mechanical properties (3-point bending) and bone hydration (via solid state nuclear magnetic resonance spectroscopy with magic angle spinning (ssNMR)). Data were analyzed by two-tailed t-tests (μCT) or 2-way ANOVA for main effects of disease, treatment, and their interaction. Tukey's post hoc analyses followed a significant main effect of treatment to determine the source of the effect. Imaging confirmed a cortical phenotype reflective of CKD, including lower cortical thickness (p < 0.0001) and increased cortical porosity (p = 0.02) compared to Con. In addition, CKD resulted in weaker, less deformable bones. In CKD bones, ex vivo exposure to RAL or CAL improved total work (+120 % and +107 %, respectively; p < 0.05), post-yield work (+143 % and +133 %), total displacement (+197 % and +229 %), total strain (+225 % and +243 %), and toughness (+158 % and +119 %) vs. CKD VEH soaked bones. Ex vivo exposure to RAL or CAL did not impact any mechanical properties in Con bone. Matrix-bound water by ssNMR showed CAL treated bones had significantly higher bound water compared to VEH treated bones in both CKD and Con cohorts (p = 0.001 and p = 0.01, respectively). RAL positively modulated bound water in CKD bone compared to VEH (p = 0.002) but not in Con bone. There were no significant differences between bones soaked with CAL vs. RAL for any outcomes measured. RAL and CAL improve important post-yield properties and toughness in a non-cell mediated manner in CKD bone but not in Con bones. While RAL treated CKD bones had higher matrix-bound water content in line with previous reports, both Con and CKD bones exposed to CAL had higher matrix-bound water. Therapeutic modulation of water, specifically the bound water fraction, represents a novel approach to improving mechanical properties and potentially reducing fracture risk.

Dehydration-Induced alterations to sharp force trauma on Sus domesticus radii

Dehydration is a taphonomic process that affects nearly all skeletal remains, yet there is a dearth of evidence on this process within the forensic taphonomy literature. When considering the forensic implications of skeletal dehydration, a particular area of concern is sharp force trauma due to its global prominence in forensic cases. In an attempt to address these literature gaps and quantify the effects that dehydration has on skeletal elements, a controlled experiment subjected Sus domesticus (i.e., domestic pig) radii samples (n = 36) to laboratory-induced dehydration after they were inflicted with knife trauma. All samples were photographed pre- and post-dehydration; bone section and kerf mark length, width, and area were then measured from these photographs using ImageJ. Statistical analysis of pre- and post-dehydration samples showed that all measurements experienced significant (p ≤ 0.001) shrinkage, with bone sample area shrinking an average of 8.8 % and kerf mark area an average of 29.7 %. Alterations in length, width and area between the kerf marks and bone samples showed a weak, moderate, and strong correlation, respectively. These findings suggest that anthropological analysis may be affected by dehydration-induced shrinkage, highlighting the necessity of continued research into the effects of dehydration on skeletal trauma.

Bone hydration: How we can evaluate it, what can it tell us, and is it an effective therapeutic target?

Water constitutes roughly a quarter of the cortical bone by volume yet can greatly influence mechanical properties and tissue quality. There is a growing appreciation for how water can dynamically change due to age, disease, and treatment. A key emerging area related to bone mechanical and tissue properties lies in differentiating the role of water in its four different compartments, including free/pore water, water loosely bound at the collagen/mineral interfaces, water tightly bound within collagen triple helices, and structural water within the mineral. This review summarizes our current knowledge of bone water across the four functional compartments and discusses how alterations in each compartment relate to mechanical changes. It provides an overview on the advent of- and improvements to- imaging and spectroscopic techniques able to probe nano-and molecular scales of bone water. These technical advances have led to an emerging understanding of how bone water changes in various conditions, of which aging, chronic kidney disease, diabetes, osteoporosis, and osteogenesis imperfecta are reviewed. Finally, it summarizes work focused on therapeutically targeting water to improve mechanical properties.

New method to differentiate surface damping behavior and stress absorption capacities of common CAD/CAM restorative materials

OBJECTIVES

To assess energy dissipation capacities and surface damping abilities of different CAD/CAM restorative materials (CRMs) to characterize stress resistance during load peaks.

METHODS

Using instrumented indentation testing (IIT), Martens hardness (HM) together with its elastic (η_IT) and plastic index (η_ITdis) and Leeb hardness (HLD) together with its deduced energy dissipation (HLD_dis) were determined for eight ceramic, eight composite, and four polymer-based materials as well as three metals. The results were compared to those of bovine enamel. Ten indentations per material were performed at room temperature (23 ± 1 °C) on two separate specimens (12.0 × 12.0 × 3.5 mm³) after water storage (24 h; 37.0 ± 1.0 °C). Hardness parameters were recorded, and data were analyzed with one-way MANOVA (Games-Howell post hoc tests, α = 0.05). Correlations between different parameters were tested (Pearson, α = 0.05).

RESULTS

Independently determined HLD_dis, and η_ITdis values substantiated different energy dissipation characteristics of CRM, whereby a strong correlation was observed for the two datasets (r = 0.956, p = 0.011). Ceramics had the significantly lowest values (p < 0.001) while both parameters revealed the highest surface damping effects for metals (p < 0.001), followed in both cases by bovine enamel. Energy dissipation of polymer and composite CRM was in between ceramics and bovine enamel (p < 0.001), whereas only for HLD_dis did both show no significant difference (p > 0.05).

SIGNIFICANCE

Promising new HLD_dis and η_ITdis data allow a reliable differentiation of energy dissipation and surface damping capacities of CRMs. Previously published rankings of edge chipping and loss tangent results were perfectly reproduced, especially by HLD_dis.

Fracture toughness of bone at the microscale

Bone's hierarchical arrangement of collagen and mineral generates a confluence of toughening mechanisms acting at every length scale from the molecular to the macroscopic level. Molecular defects, disease, and age alter bone structure at different levels and diminish its fracture resistance. However, the inability to isolate and quantify the influence of specific features hampers our understanding and the development of new therapies. Here, we combine in situ micromechanical testing, transmission electron microscopy and phase-field modelling to quantify intrinsic deformation and toughening at the fibrillar level and unveil the critical role of fibril orientation on crack deflection. At this level dry bone is highly anisotropic, with fracture energies ranging between 5 and 30 J/m² depending on the direction of crack propagation. These values are lower than previously calculated for dehydrated samples from large-scale tests. However, they still suggest a significant amount of energy dissipation. This approach provides a new tool to uncouple and quantify, from the bottom up, the roles played by the structural features and constituents of bone on fracture and how can they be affected by different pathologies. The methodology can be extended to support the rational development of new structural composites.

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.

A novel phase field method for modeling the fracture of long bones

A proximal humerus fracture is an injury to the shoulder joint that necessitates medical attention. While it is one of the most common fracture injuries impacting the elder community and those who suffer from traumatic falls or forceful collisions, there are almost no validated computational methods that can accurately model these fractures. This could be due to the complex, inhomogeneous bone microstructure, complex geometries, and the limitations of current fracture mechanics methods. In this paper, we develop a novel phase field method to investigate the proximal humerus fracture. To model the fracture in the inhomogeneous domain, we propose a power-law relationship between bone mineral density and critical energy release rate. The method is validated by an in vitro experiment, in which a human humerus is constrained on both ends while subjected to compressive loads on its head, in the longitudinal direction, that lead to fracture at the anatomical neck. CT scans are employed to acquire the bone geometry and material parameters, from which detailed finite element meshes with inhomogeneous Young modulus distributions are generated. The numerical method, implemented in a high performance computing environment, is used to quantitatively predict the complex 3D brittle fracture of the bone and is shown to be in good agreement with experimental observations. Furthermore, our findings show that the damage is initiated in the trabecular bone-head and propagates outward towards the bone cortex. We conclude that the proposed phase field method is a promising approach to model bone fracture.

Environmentally-Controlled Near Infrared Spectroscopic Imaging of Bone Water

We have designed an environmentally-controlled chamber for near infrared spectroscopic imaging (NIRSI) to monitor changes in cortical bone water content, an emerging biomarker related to bone quality assessment. The chamber is required to ensure repeatable spectroscopic measurements of tissues without the influence of atmospheric moisture. A calibration curve to predict gravimetric water content from human cadaveric cortical bone was created using NIRSI data obtained at six different lyophilization time points. Partial least squares (PLS) models successfully predicted bone water content that ranged from 0–10% (R = 0.96, p < 0.05, root mean square error of prediction (RMSEP) = 7.39%), as well as in the physiologic range of 4–10% of wet tissue weight (R = 0.87, p < 0.05, RMSEP = 14.5%). Similar results were obtained with univariate and bivariate regression models for prediction of water in the 0–10% range. Further, we identified two new NIR bone absorbances, at 6560 cm⁻¹ and 6688 cm⁻¹, associated with water and collagen respectively. Such data will be useful in pre-clinical studies that investigate changes in bone quality with disease, aging and with therapeutic use.

Interfibril hydrogen bonding improves the strain-rate response of natural armour

Fish scales are laminated composites that consist of plies of unidirectional collagen fibrils with twisted-plywood stacking arrangement. Owing to their composition, the toughness of scales is dependent on the intermolecular bonding within and between the collagen fibrils. Adjusting the extent of this bonding with an appropriate stimulus has implications for the design of next-generation bioinspired flexible armours. In this investigation, scales were exposed to environments of water or a polar solvent (i.e. ethanol) to influence the extent of intermolecular bonding, and their mechanical behaviour was evaluated in uniaxial tension and transverse puncture. Results showed that the resistance to failure of the scales increased with loading rate in both tension and puncture and that the polar solvent treatment increased both the strength and toughness through interpeptide bonding; the largest increase occurred in the puncture resistance of scales from the tail region (a factor of nearly 7×). The increase in strength and damage tolerance with stronger intermolecular bonding is uncommon for structural materials and is a unique characteristic of the low mineral content. Scales from regions of the body with higher mineral content underwent less strengthening, which is most likely the result of interference posed by the mineral crystals to intermolecular bonding. Overall, the results showed that flexible bioinspired composite materials for puncture resistance should enrol constituents and complementary processing that capitalize on interfibril bonds.

Non-destructive NIR spectral imaging assessment of bone water: Comparison to MRI measurements

Bone fracture risk increases with age, disease states, and with use of certain therapeutics, such as acid-suppressive drugs, steroids and high-dose bisphosphonates. Historically, investigations into factors that underlie bone fracture risk have focused on evaluation of bone mineral density (BMD). However, numerous studies have pointed to factors other than BMD that contribute to fragility, including changes in bone collagen and water. The goal of this study is to investigate the feasibility of using near infrared spectral imaging (NIRSI) to determine the spatial distribution and relative amount of water and organic components in whole cross-sections of bone, and to compare those results to those obtained using magnetic resonance imaging (MRI) methods. Cadaver human whole-section tibiae samples harvested from 18 donors of ages 27-97years underwent NIRSI and ultrashort echo time (UTE) MRI. As NIRSI data is comprised of broad absorbances, second derivative processing was evaluated as a means to narrow peaks and obtain compositional information. The (inverted) second derivative peak heights of the NIRSI absorbances correlated significantly with the mean peak integration of the water, collagen and fat NIR absorbances, respectively, indicating that either processing method could be used for compositional assessment. The 5797cm^-1 absorbance was validated as arising from the fat present in bone marrow, as it completely disappeared after ultrasonication. The MRI UTE-determined bound water content in tibial cortical bone samples ranged from 62 to 91%. The NIRSI water peaks at 5152cm^-1 and at 7008cm^-1 correlated significantly with the UTE data, with r=0.735, p=0.016, and r=0.71, p=0.0096, respectively. There was also a strong correlation between the intensity of the NIRSI water peak at 7008cm^-1 and the intensity of the collagen peak at 4608cm^-1 (r=0.69, p=0.004). Since NIRSI requires minimal to no sample preparation, this approach has great potential to become a gold standard modality for the investigation of changes in water content, distribution, and environment in pre-clinical studies of bone pathology and therapeutics.

Contribution of extrafibrillar matrix to the mechanical behavior of bone using a novel cohesive finite element model

The mechanical behavior of bone is determined at all hierarchical levels, including lamellae (the basic building block of bone) that are comprised of mineralized collagen fibrils and extrafibrillar matrix. The mechanical behavior of mineralized collagen fibrils has been investigated intensively using both experimental and computational approaches. Yet, the contribution of the extrafibrillar matrix to bone mechanical properties is poorly documented. In this study, we intended to address this issue using a novel cohesive finite element (FE) model, in conjunction with the experimental observations reported in the literature. In the FE model, the extrafibrillar matrix was considered as a nanocomposite of hydroxyapatite (HA) crystals bounded through a thin organic interface modeled as a cohesive interfacial zone. The parameters required by the cohesive FE model were defined based on the experimental data reported in the literature. This hybrid nanocomposite model was tested in two loading modes (i.e. tension and compression) and under two hydration conditions (i.e. wet and dry). The simulation results indicated that (1) the failure modes of the extrafibrillar matrix predicted using the cohesive FE model were closely coincided with those experimentally observed in tension and compression tests; (2) the pre-yield deformation (i.e. internal strain) of HA crystals with respect to the applied strain was consistent with that obtained from the synchrotron X-ray scattering measurements irrespective of the loading modes and hydration status; and (3) the mechanical behavior of the extrafibrillar matrix was dictated by the properties of the organic interface between the HA crystals. Taken together, we postulate that the extrafibrillar matrix plays a major role in the pre-yield deformation and the failure mode of bone, thus, giving rise to important insights in the ultrastructural origins of bone fragility.

Mechanical-chemical analyses and sub-chronic systemic toxicity of chemical treated organic bovine bone

Sequentially chemical-treated bovine bone was not only evaluated by mechanical and chemical analyses but also implanted into the gluteal muscles of rats for 12 weeks to investigate potential local pathological effects and systemic toxicities. The test (chemical treated bone) and control (heat treated bone) materials were compared using scanning electron microscope (SEM), x-ray diffraction pattern, inductively coupled plasma analysis, and bending strength test. In the SEM images, the micro-porous structure of heat-treated bone was changed to sintered ceramic-like structure. The structure of bone mineral from test and control materials was analyzed as100% hydroxyapatite. The ratio of calcium (Ca) to potassium (P), the main inorganic elements, was same even though the Ca and P percentages of the control material was relatively higher than the test material. No death or critical symptoms arose from implantation of the test (chemical treated bone) and control (physiological saline) materials during 12 weeks. The implanted sites were macroscopically examined, with all the groups showing non-irritant results. Our results indicate that chemical processed bovine bone has a better mechanical property than the heat treated bone and the implantation of this material does not produce systemic or pathological toxicity.

Effect of glucose on fatigue-induced changes in the microstructure and mechanical properties of demineralized bovine cortical bone

PURPOSE

The aim of this study was to test a hypothesis that fatigue-induced weakening of cortical bone was intensified in bone incubated in glucose and that this weakening is revealed in the microstructure and mechanical competence of the bone matrix.

METHODS

Cubic specimens of bovine femoral shaft were incubated in glucose solution (G) or in buffer (NG). One half of G samples and one half of NG were axially loaded in 300 cycles (30 mm/min) at constant deformation (F); the other half was a control (C). Samples from each group (GF, NGF, GC, NGC) were completely demineralized. Slices from demineralized samples were used for microscopic image analysis. A combined effect of glycation and fatigue on demineralized bone was tested in compression (10 mm/min). Damage of samples during the test was examined in terms of acoustic emission analysis (AE).

RESULTS

During the fatigue procedure, resistance to loading in glycated samples decreased by 14.5% but only by 8.1% in nonglycated samples. In glycated samples fatigue resulted in increased porosity with pores significantly larger than in the other groups. Under compression, strain at failure in demineralized bone was significantly affected by glucose and fatigue. AE from demineralized bone matrix was considerably related to the largest pores in the tissue.

CONCLUSIONS

The results confirm the hypothesis that the effect of fatigue on cortical bone tissue was intensified after incubation in glucose, both in the terms of the mechanical competence of bone tissue and the structural changes in the collagenous matrix of bone.

The Role of Water Compartments in the Material Properties of Cortical Bone

Comprising ~20% of the volume, water is a key determinant of the mechanical behavior of cortical bone. It essentially exists in two general compartments: within pores and bound to the matrix. The amount of pore water-residing in the vascular-lacunar-canalicular space-primarily reflects intracortical porosity (i.e., open spaces within the matrix largely due to Haversian canals and resorption sites) and as such is inversely proportional to most mechanical properties of bone. Movement of water according to pressure gradients generated during dynamic loading likely confers hydraulic stiffening to the bone as well. Nonetheless, bound water is a primary contributor to the mechanical behavior of bone in that it is responsible for giving collagen the ability to confer ductility or plasticity to bone (i.e., allows deformation to continue once permanent damage begins to form in the matrix) and decreases with age along with fracture resistance. Thus, dehydration by air-drying or by solvents with less hydrogen bonding capacity causes bone to become brittle, but interestingly, it also increases stiffness and strength across the hierarchical levels of organization. Despite the importance of matrix hydration to fracture resistance, little is known about why bound water decreases with age in hydrated human bone. Using (1)H nuclear magnetic resonance (NMR), both bound and pore water concentrations in bone can be measured ex vivo because the proton relaxation times differ between the two water compartments, giving rise to two distinct signals. There are also emerging techniques to measure bound and pore water in vivo with magnetic resonance imaging (MRI). The NMR/MRI-derived bound water concentration is positively correlated with both the strength and toughness of hydrated bone and may become a useful clinical marker of fracture risk.

Identifying Novel Clinical Surrogates to Assess Human Bone Fracture Toughness

Fracture risk does not solely depend on strength but also on fracture toughness; ie, the ability of bone material to resist crack initiation and propagation. Because resistance to crack growth largely depends on bone properties at the tissue level, including collagen characteristics, current X-ray based assessment tools may not be suitable to identify age-related, disease-related, or treatment-related changes in fracture toughness. To identify useful clinical surrogates that could improve the assessment of fracture resistance, we investigated the potential of (1)H nuclear magnetic resonance spectroscopy (NMR) and reference point indentation (RPI) to explain age-related variance in fracture toughness. Harvested from cadaveric femurs (62 human donors), single-edge notched beam (SENB) specimens of cortical bone underwent fracture toughness testing (R-curve method). NMR-derived bound water showed the strongest correlation with fracture toughness properties (r = 0.63 for crack initiation, r = 0.35 for crack growth, and r = 0.45 for overall fracture toughness; p < 0.01). Multivariate analyses indicated that the age-related decrease in different fracture toughness properties were best explained by a combination of NMR properties including pore water and RPI-derived tissue stiffness with age as a significant covariate (adjusted R(2)  = 53.3%, 23.9%, and 35.2% for crack initiation, crack growth, and overall toughness, respectively; p < 0.001). These findings reflect the existence of many contributors to fracture toughness and emphasize the utility of a multimodal assessment of fracture resistance. Exploring the mechanistic origin of fracture toughness, glycation-mediated nonenzymatic collagen crosslinks and intracortical porosity are possible determinants of bone fracture toughness and could explain the sensitivity of NMR to changes in fracture toughness. Assuming fracture toughness is clinically important to the ability of bone to resist fracture, our results suggest that improvements in fracture risk assessment could potentially be achieved by accounting for water distribution (quantitative ultrashort echo time magnetic resonance imaging) and by a local measure of tissue resistance to indentation, RPI.

The loss of activating transcription factor 4 (ATF4) reduces bone toughness and fracture toughness

Even though age-related changes to bone tissue affecting fracture risk are well characterized, only a few matrix-related factors have been identified as important to maintaining fracture resistance. As a gene critical to osteoblast differentiation, activating transcription factor 4 (ATF4) is possibly one of these important factors. To test the hypothesis that the loss of ATF4 affects the fracture resistance of bone beyond bone mass and structure, we harvested bones from Atf4+/+ and Atf4-/- littermates at 8 and 20 weeks of age (n≥9 per group) for bone assessment across several length scales. From whole bone mechanical tests in bending, femurs from Atf4-/- mice were found to be brittle with reduced toughness and fracture toughness compared to femurs from Atf4+/+ mice. However, there were no differences in material strength and in tissue hardness, as determined by nanoindentation, between the genotypes, irrespective of age. Tissue mineral density of the cortex at the point of loading as determined by micro-computed tomography was also not significantly different. However, by analyzing local composition by Raman Spectroscopy (RS), bone tissue of Atf4-/- mice was found to have higher mineral to collagen ratio compared to wild-type tissue, primarily at 20 weeks of age. From RS analysis of intact femurs at 2 orthogonal orientations relative to the polarization axis of the laser, we also found that the organizational-sensitive peak ratio, ν1Phosphate per Amide I, changed to a greater extent upon bone rotation for Atf4-deficient tissue, implying bone matrix organization may contribute to the brittleness phenotype. Target genes of ATF4 activity are not only important to osteoblast differentiation but also in maintaining bone toughness and fracture toughness.

Bone cell-independent benefits of raloxifene on the skeleton: a novel mechanism for improving bone material properties

Raloxifene is an FDA approved agent used to treat bone loss and decrease fracture risk. In clinical trials and animal studies, raloxifene reduces fracture risk and improves bone mechanical properties, but the mechanisms of action remain unclear because these benefits occur largely independent of changes to bone mass. Using a novel experimental approach, machined bone beams, both from mature male canine and human male donors, were depleted of living cells and then exposed to raloxifene ex vivo. Our data show that ex vivo exposure of non-viable bone to raloxifene improves intrinsic toughness, both in canine and human cortical bone beams tested by 4-point bending. These effects are cell-independent and appear to be mediated by an increase in matrix bound water, assessed using basic gravimetric weighing and sophisticated ultrashort echo time magnetic resonance imaging. The hydroxyl groups (OH) on raloxifene were shown to be important in both the water and toughness increases. Wide and small angle X-ray scattering patterns during 4-pt bending show that raloxifene alters the transfer of load between the collagen matrix and the mineral crystals, placing lower strains on the mineral, and allowing greater overall deformation prior to failure. Collectively, these findings provide a possible mechanistic explanation for the therapeutic effect of raloxifene and more importantly identify a cell-independent mechanism that can be utilized for novel pharmacological approaches for enhancing bone strength.

The importance of intrinsic damage properties to bone fragility: a finite element study

As the average age of the population has increased, the incidence of age-related bone fracture has also increased. While some of the increase of fracture incidence with age is related to loss of bone mass, a significant part of the risk is unexplained and may be caused by changes in intrinsic material properties of the hard tissue. This investigation focused on understanding how changes to the intrinsic damage properties affect bone fragility. We hypothesized that the intrinsic (μm) damage properties of bone tissue strongly and nonlinearly affect mechanical behavior at the apparent (whole tissue, cm) level. The importance of intrinsic properties on the apparent level behavior of trabecular bone tissue was investigated using voxel based finite element analysis. Trabecular bone cores from human T12 vertebrae were scanned using microcomputed tomography (μCT) and the images used to build nonlinear finite element models. Isotropic and initially homogenous material properties were used for all elements. The elastic modulus (E(i)) of individual elements was reduced with a secant damage rule relating only principal tensile tissue strain to modulus damage. Apparent level resistance to fracture as a function of changes in the intrinsic damage properties was measured using the mechanical energy to failure per unit volume (apparent toughness modulus, W(a)) and the apparent yield strength (σ(ay), calculated using the 0.2% offset). Intrinsic damage properties had a profound nonlinear effect on the apparent tissue level mechanical response. Intrinsic level failure occurs prior to apparent yield strength (σ(ay)). Apparent yield strength (σ(ay)) and toughness vary strongly (1200% and 400%, respectively) with relatively small changes in the intrinsic damage behavior. The range of apparent maximum stresses predicted by the models was consistent with those measured experimentally for these trabecular bone cores from the experimental axial compressive loading (experimental: σ(max) = 3.0-4.3 MPa; modeling: σ(max) = 2-16 MPa). This finding differs significantly from previous studies based on nondamaging intrinsic material models. Further observations were that this intrinsic damage model reproduced important experimental apparent level behaviors including softening after peak load, microdamage accumulation before apparent yield (0.2% offset), unload softening, and sensitivity of the apparent level mechanical properties to variability of the intrinsic properties.

Strain-induced optical changes in demineralized bone

Bone "stress-whitens," becoming visibly white during mechanical loading, immediately prior to failure. Stress-whitening is known to make materials tougher by dissipating mechanical energy. A greater understanding of stress-whitening, both an optical and mechanical phenomenon, may help explain age-related increases in fracture risk that occur without changes in bone mineralization. In this work, we directly measure the optical properties of demineralized bone as a function of deformation and immersing fluid (with different hydrogen-bonding potentials, water, and ethanol). The change in refractive index of demineralized bone was linear: with deformation and not applied force. Changes in refractive index were likely due to pushing low-refractive-index fluid out of specimens and secondarily due to changes in the refractive index of the collagenous phase. Results were consistent with stress-whitening of demineralized bone previously observed. In ethanol, the refractive index values were lower and less sensitive to deformation compared with deionized water, corroborating the sensitivity to fluid hydration. Differences in refractive index were consistent with structural changes in the collagenous phase such as densification that may also occur under mechanical loading. Understanding bone quality, particularly stress-whitening investigated here, may lead to new therapeutic targets and noninvasive methods to assess bone quality.

The role of water and mineral-collagen interfacial bonding on microdamage progression in bone

Microdamage would be accumulated in bone due to high-intensity training or even normal daily activity, which may consequently cause fragility fracture or stress fracture. On the other hand, microdamage formation serves as a toughening mechanism in bone. However, the mechanisms that control microdamage initiation and accumulation in bone are still poorly understood. Our previous finite element model indicated that different interfacial properties between mineral and collagen in bone may lead to distinct patterns of microdamage accumulation. Therefore, the current study was designed to examine such prediction and to investigate the role of water and mineral-collagen interactions on microdamage accumulation in bone. To address these issues, 48 mice femurs were divided randomly into four groups. These groups were dehydrated or treated with perfluorotripropylamine (PFTA) or NaF solution to change water distribution and mineral-collagen interfacial bonding in bone. After three-point bending fatigue tests, the types of microdamage (i.e., linear microcracks or diffuse damage) formed in bone were compared between different groups. The results suggested that (1) bone tissues with strong mineral-collagen interfacial bonding facilitate the formation of linear microcraks, and (2) water has little contribution to the growth of microcracks.

Stress-whitening occurs in demineralized bone

INTRODUCTION

The incidence of age-related bone fracture is increasing with average population age. Bone scatters more light (stress-whitens) during loading, immediately prior to failure, in a manner visually similar to polymer crazing. We wish to understand the stress-whitening process because of its possible effect on bone toughness. The goals of this investigation were a) to establish that stress-whitening is a property of the demineralized organic matrix of bone rather than only a property of mineralized tissue and that stress whitening within the demineralized bone is dependent upon both b) hydrogen bonding and, c) the orientation of loading.

METHODS

Demineralized cortical bone specimens were loaded in tension to failure (0.08 strain/s). The effect of hydrogen bonding on mechanical properties and the stress-whitening process was probed by altering the Hansen's hydrogen bonding parameter (δh) of the immersing solution.

RESULTS

Stress-whitening occurred in the demineralized bone. Stress-whitening was negatively correlated with δh (R(2)=0.81, p<0.0001). Stress-whitening was significantly lower (p<0.0001) in specimens loaded orthogonally compared to those loaded parallel to the long (strong) axis.

CONCLUSION

The stress-whitening observed was consistent with increased Mie scattering. We suggest that the change in Mie scattering was due to collagen fibril dehydration driven by the externally applied stress. The presence of stress-whitening in demineralized bone suggests that this process may be a property of the collagenous matrix and hence may be present in other collagenous tissues rather than an emergent property of the bone composite.

Effects of fatigue on microstructure and mechanical properties of bone organic matrix under compression

The aim of the study was to investigate whether a fatigue induced weakening of cortical bone was revealed in microstructure and mechanical competence of demineralized bone matrix. Two types of cortical bone samples (plexiform and Haversian) were use. Bone slabs from the midshaft of bovine femora were subjected to cyclical bending. Fatigued and adjacent control samples were cut into cubes and demineralized in ethylenediaminetetraacetic acid. Demineralized samples were either subjected to microscopic quantitative image analysis, or compressed to failure (in longitudinal or transverse direction) with a simultaneous analysis of acoustic emission (AE). In fatigued samples porosity of organic matrix and average area of pores have risen, along with a change in the pores shape. The effect of fatigue depended on the type of the bone, being more pronounced in the plexiform than in Haversian tissue. Demineralized bone matrix was anisotropic under compressive loads in both types of cortical structure. The main result of fatigue pretreatment on mechanical parameters was a significant decrease of ultimate strain in the transverse direction in plexiform samples. The decrease of strain in this group was accompanied by a considerable increase of the fraction of large pores and a significant change in AE energy.

Partial removal of pore and loosely bound water by low-energy drying decreases cortical bone toughness in young and old donors

With an ability to quantify matrix-bound and pore water in bone, (1)H nuclear magnetic resonance (NMR) relaxometry can potentially be implemented in clinical imaging to assess the fracture resistance of bone in a way that is independent of current X-ray techniques, which assess bone mineral density as a correlate of bone strength. Working towards that goal, we quantified the effect of partial dehydration in air on the mechanical and NMR properties of human cortical bone in order to understand whether NMR is sensitive to water-bone interactions at low energy and whether such interactions contribute to the age-related difference in the toughness of bone. Cadaveric femurs were collected from male and female donors falling into two age groups: 21-60 years of age (young) and 74-99 years of age (old). After extracting two samples from the medial cortex of the mid-shaft, tensile tests were conducted on Wet specimens and paired, Partially Dry (PtlD) specimens (prepared by low-energy drying in air to remove ∼3% of original mass before testing). Prior analysis by micro-computed tomography found that there were no differences in intra-cortical porosity between the Wet and PtlD specimens nor did an age-related difference in porosity exist. PtlD specimens from young and old donors had significantly less toughness than Wet specimens, primarily due to a dehydration-related decrease in post-yield strain. The low-energy drying protocol did not affect the modulus and yield strength of bone. Subsequent dehydration of the PtlD specimens in a vacuum oven at 62°C and then 103°C, with quantification of water loss at each temperature, revealed an age-related shift from more loosely bound water to more tightly bound water. NMR detected a change in both bound and pore water pools with low-energy air-drying, and both pools were effectively removed when bone was oven-dried at 62°C, irrespective of donor age. Although not strictly significant due to variability in the drying and testing conditions, the absolute difference in toughness between Wet and PtlD tended to be greater for the younger donors that had higher bone toughness and more bound water for the wet condition than did the older donors. With sensitivity to low-energy bone-water interactions, NMR, which underpins magnetic resonance imaging, has potential to assess fracture resistance of bone as it relates to bone toughness.

Bone mineral crystallisation kinetics

The kinetics of bone apatite crystallisation are examined using a novel approach to obtain quantitative, direction dependence features such as growth rate and activation energy. X-ray diffraction was employed for analysis of bovine, porcine and 'anorganic' bone specimens. Apatite coherence length was utilised as the independent variable within a Johnson-Mehl-Avrami (JMA) model. A direction averaged crystallisation activation energy of 183 ± 8 kJ mol(-1) was observed for the three bone groups. The Johnson-Mehl-Avrami 'n' exponent decreased with increasing temperature for all bone groups, indicating that apatite crystallisation changes to a diffusion limited process at higher temperatures. The results revealed little evidence to support any organic component 'protective' effect, and, on the contrary indicated that the organic matrix promotes apatite crystallisation.

Characterization of 1H NMR signal in human cortical bone for magnetic resonance imaging

Recent advancements in MRI have enabled clinical imaging of human cortical bone, providing a potentially powerful new means for assessing bone health with molecular-scale sensitivities unavailable to conventional X-ray-based diagnostics. In human cortical bone, MRI is sensitive to populations of protons ((1)H) partitioned among water and protein sources, which may be differentiated according to intrinsic NMR properties such as chemical shift and transverse and longitudinal relaxation rates. Herein, these NMR properties were assessed in human cortical bone donors from a broad age range, and four distinct (1)H populations were consistently identified and attributed to five microanatomical sources. These findings show that modern human cortical bone MRI contrast will be dominated by collagen-bound water, which can also be exploited to study human cortical bone collagen via magnetization transfer.