Indentation properties of young and old osteons

3D Modular Bioceramic Scaffolds for the Investigation of the Interaction between Osteosarcoma Cells and MSCs

Recent advances in bone tissue engineering have shown promise for bone repair post osteosarcoma excision. However, conflicting research on mesenchymal stem cells (MSCs) has raised concerns about their potential to either promote or inhibit tumor cell proliferation. It is necessary to thoroughly understand the interactions between MSCs and tumor cells. Most previous studies only focused on the interactions between cells within the tumor tissues. It has been challenging to develop an in vitro model of osteosarcoma excision sites replicating the complexity of the bone microenvironment and cell distribution. In this work, we designed and fabricated modular bioceramic scaffolds to assemble into a co-culture model. Because of the bone-like composition and mechanical property, tricalcium phosphate bioceramic could mimic the bone microenvironment and recapitulate the cell-extracellular matrix interaction. Moreover, the properties for easy assembly enabled the modular units to mimic the spatial distribution of cells in the osteosarcoma excision site. Under this co-culture model, MSCs showed a noticeable tumor-stimulating effect with a potential risk of tumor recurrence. In addition, tumor cells also could inhibit the osteogenic ability of MSCs. To undermine the stimulating effects of MSCs on tumor cells, we present the methods of pre-differentiated MSCs, which had lower expression of IL-8 and higher expression of osteogenic proteins. Both in vitro and in vivo studies confirm that pre-differentiated MSCs could maintain high osteogenic capacity without promoting tumor growth, offering a promising approach for MSCs' application in bone regeneration. Overall, 3D modular scaffolds provide a valuable tool for constructing hard tissue in vitro models. STATEMENT OF SIGNIFICANCE: Bone tissue engineering using mesenchymal stem cells (MSCs) and biomaterials has shown promise for bone repair post osteosarcoma excision. However, conflicting researches on MSCs have raised concerns about their potential to either promote or inhibit tumor cell proliferation. It remains challenges to develop in vitro models to investigate cell interactions, especially of osteosarcoma with high hardness and special composition of bone tissue. In this work, modular bioceramic scaffolds were fabricated and assembled to co-culture models. The interactions between MSCs and MG-63 were manifested as tumor-stimulating and osteogenesis-inhibiting, which means potential risk of tumor recurrence. To undermine the stimulating effect, pre-differentiated method was proposed to maintain high osteogenic capacity without tumor-stimulating, offering a promising approach for MSCs' application in bone regeneration.

Effect of geometrical structure variations on strength and damage onset of cortical bone using multi-scale cohesive zone based finite element method

Three-dimensional multi-scale finite element models were designed to examine the effects of geometrical structure variations on the damage onset in cortical bone at multiple structural scales. A cohesive zone finite element approach, together with anisotropic damage initiation criteria, is used to predict the onset of damage. The finite element models are developed to account for the onset of microdamage from the microscopic length scales consisting of collagen fibres, to the macroscopic level consisting of osteons and the Haversian canals. Numerical results indicated that the yield strain at the initiation of microcracks is independent of variations in the local mineral volume fraction at each structural scale. Further, the yield strain and strength properties of cortical bone are dependent on its structural anisotropy and hierarchical structure. A positive correlation is observed between bone strength and mineral content at each length scale.

Effect of radiation-induced damage of trabecular bone tissue evaluated using indentation and digital volume correlation

Exposure to X-ray radiation for an extended amount of time can cause damage to the bone tissue and therefore affect its mechanical properties. Specifically, high-resolution X-ray Computed Tomography (XCT), in both synchrotron and lab-based systems, has been employed extensively for evaluating bone micro-to-nano architecture. However, to date, it is still unclear how long exposures to X-ray radiation affect the mechanical properties of trabecular bone, particularly in relation to lab-XCT systems. Indentation has been widely used to identify local mechanical properties such as hardness and elastic modulus of bone and other biological tissues. The purpose of this study is therefore, to use indentation and XCT-based investigative tools such as digital volume correlation (DVC) to assess the microdamage induced by long exposure of trabecular bone tissue to X-ray radiation and how this affects its local mechanical properties. Trabecular bone specimens were indented before and after X-ray exposures of 33 and 66 h, where variation of elastic modulus was evaluated at every stage. The resulting elastic modulus was decreased, and micro-cracks appeared in the specimens after the first long X-ray exposure and crack formation increased after the second exposure. High strain concentration around the damaged tissue exceeding 1% was also observed from DVC analysis. The outcomes of this study show the importance of designing appropriate XCT-based experiments in lab systems to avoid degradation of the bone tissue mechanical properties due to radiation and these results will help to inform future studies that require long X-ray exposure for in situ experiments or generation of reliable subject-specific computational models.

Pathophysiology of Demineralization, Part I: Attrition, Erosion, Abfraction, and Noncarious Cervical Lesions

PURPOSE OF THE REVIEW

Compare pathophysiology for infectious and noninfectious demineralization disease relative to mineral maintenance, physiologic fluoride levels, and mechanical degradation.

RECENT FINDINGS

Environmental acidity, biomechanics, and intercrystalline percolation of endemic fluoride regulate resistance to demineralization relative to osteopenia, noncarious cervical lesions, and dental caries. Demineralization is the most prevalent chronic disease in the world: osteoporosis (OP) >10%, dental caries ~100%. OP is severely debilitating while caries is potentially fatal. Mineralized tissues have a common physiology: cell-mediated apposition, protein matrix, fluid logistics (blood, saliva), intercrystalline ion percolation, cyclic demineralization/remineralization, and acid-based degradation (microbes, clastic cells). Etiology of demineralization involves fluid percolation, metabolism, homeostasis, biomechanics, mechanical wear (attrition or abrasion), and biofilm-related infections. Bone mineral density measurement assesses skeletal mass. Attrition, abrasion, erosion, and abfraction are diagnosed visually, but invisible subsurface caries <400μm cannot be detected. Controlling demineralization at all levels is an important horizon for cost-effective wellness worldwide.

Hardness, an Important Indicator of Bone Quality, and the Role of Collagen in Bone Hardness

Bone is a nanocomposite material where the hard inorganic (hydroxyapatite crystallites) and organic (collagen fibrils) components are hierarchically arranged in the nanometer scale. Bone quality is dependent on the spatial distributions in the shape, size and composition of bone constituents (mineral, collagen and water). Bone hardness is an important property of bone, which includes both elastic and plastic deformation. In this study, a microhardness test was performed on a deer bone samples. The deer tibia shaft (diaphysis) was divided into several cross-sections of equal thickness; samples were prepared in untreated, boiled water treatment (100 °C for 30 min) and sodium hypochlorite (NaOCl) treatment conditions. Microhardness tests were performed on various regions of the tibial diaphysis to study the heterogeneous characteristics of bone microhardness and highlight the role of the organic matrix in bone hardness. The results indicated that boiled water treatment has a strong negative correlation with bone hardness. The untreated bone was significantly (+20%) harder than the boiled-water-treated bone. In general, the hardness values near the periosteal surface was significantly (23 to 45%) higher than the ones near the endosteal surface. Samples treated with NaOCl showed a significant reduction in hardness.

Effect of geometrical structure variations on the viscoelastic and anisotropic behaviour of cortical bone using multi-scale finite element modelling

Multi-scale finite element analysis is performed to ascertain the effect of geometrical changes at multiple structural scales on the mechanical properties of cortical bone. Finite element models are developed, with reference to experimental data from existing literature, to account for bone's viscoelastic behaviour and anisotropic structure from the most fundamental level of bone consisting of mineralised collagen fibrils, up to the macroscopic level consisting of osteons and the Haversian canals. A statistical approach is incorporated to perform sensitivity analyses on the effects of different geometrical parameters on the effective material properties of cortical bone at each length scale. Numerical results indicate that there is an exponential correlation between the mineral volume fraction and the effective stiffness constants at each length scale. This contributes to the exponential behaviour of the instantaneous moduli describing cortical bone's two-phase stress relaxation process: a fast and slow response relaxation behaviour. Results indicate that the fast response relaxation time is independent of bone's structural anisotropy, whilst being dependent on variations in the global mineral volume fraction between length scales. However, the slow response relaxation time is independent of the changes in mineral volume fraction. It is also observed that the slow response relaxation time varies with bone's anisotropic structure, and therefore, contributes to the anisotropic properties of bone.

Regional variations of jaw bone characteristics in an ovariectomized rat model

Postmenopausal osteoporosis causes severe loss of bone quantity and quality in limb bone but has a lesser effect on jaw bone. Thus, the objective of this study was to examine whether ovariectomy (OVX) and mastication alter the regional variation of jaw bone characteristics. Sprague-Dawley female rats (6 months) were given a bilateral OVX or a sham operation (SHAM) (n = 10 for each group). After 2 months post-OVX, the hemi-mandible from each rat was dissected. A micro-computed tomography based mean, standard deviation (SD), the lower and upper 5th percentile (Low₅ and High₅) values of tissue mineral density (TMD) histograms were assessed for whole bone (WB), alveolar bone (AB), cortical bone (CB), and trabecular bone (TB) regions. Morphology of TB and periodontal ligament (PDL) was also obtained. Layers of AB were segmented up to 400 μm from the PDL. Mechanical properties at the tissue level were measured by nanoindentation at the same site by a single loading-unloading cycle of indentation in hydration. The AB and TB regions had significantly lower TMD Mean, Low₅, and High₅ but higher SD than the CB region for both sham and OVX groups (p < 0.01). TMD parameters of the OVX group rapidly increased up to 60 μm away from the PDL and were significantly higher than those of the sham group starting at 280 μm and farther in the CB region (p < 0.05). All values of morphological and nanoindentation parameters were not significantly different between sham and OVX groups (p > 0.06). Estrogen deficiency induced by OVX did not deteriorate bone characteristics including mineral density, morphology, and nanoindentation parameters in rat mandibles. Masticatory loading had an effect on the TMD parameters at the limited region of AB. These results provide insight into why osteoporosis-associated jaw bone fractures are extremely rare.

Clinical measurements of bone tissue mechanical behavior using reference point indentation

Over the last thirty years, it has become increasingly clear the amount of bone (e.g. 'bone quantity') and the quality of the bone matrix (e.g. 'bone quality') both critically contribute to bone's tissue-level mechanical behavior and the subsequent ability of bone to resist fracture. Although determining the tissue-level mechanical behavior of bone through mechanical testing is relatively straightforward in the laboratory, the destructive nature of such testing is unfeasible in humans and in animal models requiring longitudinal observation. Therefore, surrogate measurements are necessary for quantifying tissue-level mechanical behavior for the pre-clinical and clinical evaluation of bone strength and fracture risk in vivo. A specific implementation of indentation known as reference point indentation (RPI) enables the mechanical testing of bone tissue without the need to excise and prepare the bone surface. However, this compromises the ability to carefully control the specimen geometry that is required to define the bone tissue material properties. Yet the versatility of such measurements in clinical populations is provocative, and to date there are a number of promising studies that have utilized this tool to discern bone pathologies and to monitor the effects of therapeutics on bone quality. Concurrently, on-going efforts continue to investigate the aspects of bone material behavior measured by RPI, and the compositional factors that contribute to these measurements. There are currently two variants, cyclic- and impact- RPI, that have been utilized in pre-clinical and clinical studies. This review surveys clinical studies that utilize RPI, with particular emphasis on the clinical instrument, as well as the endeavors to understand the fundamental mechanisms of such measurements. Ultimately, an improved awareness in the tradeoffs and limitations of in vivo RPI is critical towards the effective and successful utilization of this tool for the overall improvement of fragility determination in the clinic.

Reproduction Differentially Affects Trabecular Bone Depending on Its Mechanical Versus Metabolic Role

During pregnancy and lactation, the maternal skeleton provides calcium for fetal/infant growth, resulting in substantial bone loss, which partially recovers after weaning. However, the amount of bone that is lost and the extent of post-weaning recovery are highly variable among different skeletal sites, and, despite persistent alterations in bone structure at some locations, reproductive history does not increase postmenopausal fracture risk. To explain this phenomenon, we hypothesized that the degree of reproductive bone loss/recovery at trabecular sites may vary depending on the extent to which the trabecular compartment is involved in the bone's load-bearing function. Using a rat model, we quantified the proportion of the load carried by the trabeculae, as well as the extent of reproductive bone loss and recovery, at two distinct skeletal sites: the tibia and lumbar vertebra. Both sites underwent significant bone loss during pregnancy and lactation, which was partially recovered post-weaning. However, the extent of the deterioration and the resumption of trabecular load-bearing capacity after weaning varied substantially. Tibial trabecular bone, which bore a low proportion of the total applied load, underwent dramatic and irreversible microstructural deterioration during reproduction. Meanwhile, vertebral trabecular bone bore a greater fraction of the load, underwent minimal deterioration in microarchitecture, and resumed its full load-bearing capacity after weaning. Because pregnancy and lactation are physiological processes, the distinctive responses to these natural events among different skeletal sites may help to elucidate the extent of the trabecular bone's structural versus metabolic functions.

Influences of Light-emitting Diode Illumination Bleaching Technique on Nanohardness of Computer-aided Design and Computer-aided Manufacturing Ceramic Restorative Materials

AIM

This study evaluated the effect of light-emitting diode (LED) illumination bleaching technique on the surface nanohardness of various computer-aided design and computer-aided manufacturing (CAD/CAM) ceramic materials.

MATERIALS AND METHODS

Twenty disk-shaped samples (width, length, and thickness = 10, 15, and 2 mm) were prepared from each of the ceramic materials for CAD/CAM, including Lava™ Ultimate (L_V), Vita Enamic® (E_n) IPS e.max® CAD (M_e), inCoris® TZI (I_C), and Prettau® zirconia (P_r). The samples from each type of ceramic were randomly divided into two groups based on the different bleaching techniques to be used on them, using 35% hydrogen peroxide with and without LED illumination. The ceramic disk samples were bleached according to the manufacturer's instruction. Surface hardness test was performed before and after bleaching using nanohardness tester with a Berkovich diamond indenter.

RESULTS

The respective Vickers hardness number upon no bleaching and bleaching without or with LED illumination [mean ± standard deviation (SD)] for each type of ceramic were as follows: 102.52 ± 2.09, 101.04 ± 1.18, and 98.17 ± 1.15 for L_V groups; 274.96 ± 5.41, 271.29 ± 5.94, and 268.20 ± 7.02 for E_n groups; 640.74 ± 31.02, 631.70 ± 22.38, and 582.32 ± 33.88 for M_e groups; 1,442.09 ± 35.07, 1,431.32 ± 28.80, and 1,336.51 ± 34.03 for I_C groups; and 1,383.82 ± 33.87, 1,343.51 ± 38.75, and 1,295.96 ± 31.29 for P_r groups. The results indicated surface hardness reduction following the bleaching procedure of varying degrees for different ceramic materials. Analysis of variance (ANOVA) revealed a significant reduction in surface hardness due to the effect of bleaching technique, ceramic material, and the interaction between bleaching technique and ceramic material (p < 0.05).

CONCLUSION

Bleaching resulted in a diminution of the surface hardness of dental ceramic for CAD/CAM. Using 35% hydrogen peroxide bleaching agent with LED illumination exhibited more reduction in surface hardness of dental ceramic than what was observed without LED illumination.

CLINICAL SIGNIFICANCE

Clinicians should consider protection of the existing restoration while bleaching.

Intermittent Parathyroid Hormone After Prolonged Alendronate Treatment Induces Substantial New Bone Formation and Increases Bone Tissue Heterogeneity in Ovariectomized Rats

Postmenopausal osteoporosis is often treated with bisphosphonates (eg, alendronate, [ALN]), but oversuppression of bone turnover by long-term bisphosphonate treatment may decrease bone tissue heterogeneity. Thus, alternate treatment strategies after long-term bisphosphonates are of great clinical interest. The objective of the current study was to determine the effect of intermittent parathyroid hormone (PTH) following 12 weeks of ALN (a bisphosphonate) treatment in 6-month-old, ovariectomized (OVX) rats on bone microarchitecture, bone remodeling dynamics, and bone mechanical properties at multiple length scales. By using in vivo μCT and 3D in vivo dynamic bone histomorphometry techniques, we demonstrated the efficacy of PTH following ALN therapy for stimulating new bone formation, and increasing trabecular thickness and bone volume fraction. In healthy bone, resorption and formation are coupled and balanced to sustain bone mass. OVX results in resorption outpacing formation, and subsequent bone loss and reduction in bone tissue modulus and tissue heterogeneity. We showed that ALN treatment effectively reduced bone resorption activity and regained the balance with bone formation, preventing additional bone loss. However, ALN treatment also resulted in significant reductions in the heterogeneity of bone tissue mineral density and tissue modulus. On the other hand, PTH treatment was able to shift the bone remodeling balance in favor of formation, with or without a prior treatment with ALN. Moreover, by altering the tissue mineralization, PTH alleviated the reduction in heterogeneity of tissue material properties induced by prolonged ALN treatment. Furthermore, switching to PTH treatment from ALN improved bone's postyield mechanical properties at both the whole bone and apparent level compared to ALN alone. The current findings suggest that intermittent PTH treatment should be considered as a viable treatment option for patients with prior treatment with bisphosphonates. © 2017 American Society for Bone and Mineral Research.

Regional variation of bone tissue properties at the human mandibular condyle

The temporomandibular joint (TMJ) bears different types of static and dynamic loading during occlusion and mastication. As such, characteristics of mandibular condylar bone tissue play an important role in determining the mechanical stability of the TMJ under the macro-level loading. Thus, the objective of this study was to examine regional variation of the elastic, plastic, and viscoelastic mechanical properties of human mandibular condylar bone tissue using nanoindentation. Cortical and trabecular bone were dissected from mandibular condyles of human cadavers (9 males, 54-96 years). These specimens were scanned using microcomputed tomography to obtain bone tissue mineral distribution. Then, nanoindentation was conducted on the surface of the same specimens in hydration. Plastic hardness (H) at a peak load, viscoelastic creep (Creep/Pmax), viscosity (η), and tangent delta (tan δ) during a 30 second hold period, and elastic modulus (E) during unloading were obtained by a cycle of indentation at the same site of bone tissue. The tissue mineral and nanoindentation parameters were analyzed for the periosteal and endosteal cortex, and trabecular bone regions of the mandibular condyle. The more mineralized periosteal cortex had higher mean values of elastic modulus, plastic hardness, and viscosity but lower viscoelastic creep and tan δ than the less mineralized trabecular bone of the mandibular condyle. These characteristics of bone tissue suggest that the periosteal cortex tissue may have more effective properties to resist elastic, plastic, and viscoelastic deformation under static loading, and the trabecular bone tissue to absorb and dissipate time-dependent viscoelastic loading energy at the TMJ during static occlusion and dynamic mastication.

Elastic Properties of Human Osteon and Osteonal Lamella Computed by a Bidirectional Micromechanical Model and Validated by Nanoindentation

Knowledge of the anisotropic elastic properties of osteon and osteonal lamellae provides a better understanding of various pathophysiological conditions, such as aging, osteoporosis, osteoarthritis, and other degenerative diseases. For this reason, it is important to investigate and understand the elasticity of cortical bone. We created a bidirectional micromechanical model based on inverse homogenization for predicting the elastic properties of osteon and osteonal lamellae of cortical bone. The shape, the dimensions, and the curvature of osteon and osteonal lamellae are described by appropriately chosen curvilinear coordinate systems, so that the model operates close to the real morphology of these bone components. The model was used to calculate nine orthotropic elastic constants of osteonal lamellae. The input values have the elastic properties of a single osteon. We also expressed the dependence of the elastic properties of the lamellae on the angle of orientation. To validate the model, we performed nanoindentation tests on several osteonal lamellae. We compared the experimental results with the calculated results, and there was good agreement between them. The inverted model was used to calculate the elastic properties of a single osteon, where the input values are the elastic constants of osteonal lamellae. These calculations reveal that the model can be used in both directions of homogenization, i.e., direct homogenization and also inverse homogenization. The model described here can provide either the unknown elastic properties of a single lamella from the known elastic properties at the level of a single osteon, or the unknown elastic properties of a single osteon from the known elastic properties at the level of a single lamella.

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

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

A comparative study of young and mature bovine cortical bone

The mechanical properties and microstructure of young and mature bovine femur bone were investigated by optical microscopy and compression testing in the longitudinal and transverse directions for untreated, deproteinized and demineralized cases. Optical microscopy revealed that mature bone has a more established and less porous microstructure compared to young bone. Mature bone was found to be stronger in both directions for the untreated and deproteinized cases. Mature untreated bone was also found to be stiffer and less tough compared to young bone in both directions. These results are related to the increase in mineralization of mature bone and significant microstructural differences. Young bone was found to be stronger in both directions for the demineralized case, which is attributed to alterations in the collagen network with age.

Effect of estrogen deficiency on regional variation of a viscoelastic tissue property of bone

Estrogen deficiency changes the regional distribution of tissue mineral density leading to alteration of the mechanical properties of bone at the tissue level. Direct measurement of the regional variation of elastic modulus and viscosity, which is the capacity to resist time-dependent viscoelastic deformation, will aid in our understanding of how estrogen deficiency alters bone quality. It was observed that, compared to bone from other anatomical sites, the jaw bone is less sensitive to estrogen deficiency. Thus, the objective of this study was to examine the effect of estrogen deficiency on (1) the regional variations of tissue modulus and viscosity of bone using nanoindentation, and (2) the modulus-viscosity relationships in jaw and vertebral bones for comparison between different anatomical sites. Mandibular and vertebral bone specimens of sham surgery and ovariectomized (OVX) rat groups were subject to nanoindentation in hydration. Indentation modulus and viscosity were measured at relatively new (less mineralized) tissue regions and at the corresponding pre-existing old (more mineralized) tissue regions of mandibular and vertebral bones. In the mandibular bones, significant regional variations of indentation modulus and viscosity were observed (p<0.039) and OVX increased the indentation viscosity. While significant positive correlations were found between indentation modulus and viscosity (p<0.001), the correlation slopes for the mandibular and vertebral bones were significant different (p<0.001). The current results indicated that changes in viscoelastic property and its regional variation should be examined to obtain a better understanding of estrogen deficiency-dependent alteration of bone quality.

Human bone hardness seems to depend on tissue type but not on anatomical site in the long bones of an old subject

It has been hypothesised that among different human subjects, the bone tissue quality varies as a function of the bone segment morphology. The aim of this study was to assess and compare the quality, evaluated in terms of hardness of packages of lamellae, of cortical and trabecular bones, at different anatomical sites within the human skeleton. The contralateral six long bones of an old human subject were indented at different levels along the diaphysis and at both epiphyses of each bone. Hardness value, which is correlated to the degree of mineralisation, of both cortical and trabecular bone tissues was calculated for each indentation location. It was found that the cortical bone tissue was harder (+18%) than the trabecular one. In general, the bone hardness was found to be locally highly heterogeneous. In fact, considering one single slice obtained for a bone segment, the coefficient of variation of the hardness values was up to 12% for cortical bone and up to 17% for trabecular bone. However, the tissue hardness was on average quite homogeneous within and among the long bones of the studied donor, although differences up to 9% among levels and up to 7% among bone segments were found. These findings seem not to support the mentioned hypothesis, at least not for the long bones of an old subject.

Age-dependence of power spectral density and fractal dimension of bone mineralized matrix in atomic force microscope topography images: potential correlates of bone tissue age and bone fragility in female femoral neck trabeculae

There is an increasing interest in bone nano-structure, the ultimate goal being to reveal the basis of age-related bone fragility. In this study, power spectral density (PSD) data and fractal dimensions of the mineralized bone matrix were extracted from atomic force microscope topography images of the femoral neck trabeculae. The aim was to evaluate age-dependent differences in the mineralized matrix of human bone and to consider whether these advanced nano-descriptors might be linked to decreased bone remodeling observed by some authors and age-related decline in bone mechanical competence. The investigated bone specimens belonged to a group of young adult women (n = 5, age: 20-40 years) and a group of elderly women (n = 5, age: 70-95 years) without bone diseases. PSD graphs showed the roughness density distribution in relation to spatial frequency. In all cases, there was a fairly linear decrease in magnitude of the power spectra with increasing spatial frequencies. The PSD slope was steeper in elderly individuals (-2.374 vs. -2.066), suggesting the dominance of larger surface morphological features. Fractal dimension of the mineralized bone matrix showed a significant negative trend with advanced age, declining from 2.467 in young individuals to 2.313 in the elderly (r = 0.65, P = 0.04). Higher fractal dimension in young women reflects domination of smaller mineral grains, which is compatible with the more freshly remodeled structure. In contrast, the surface patterns in elderly individuals were indicative of older tissue age. Lower roughness and reduced structural complexity (decreased fractal dimension) of the interfibrillar bone matrix in the elderly suggest a decline in bone toughness, which explains why aged bone is more brittle and prone to fractures.

Viscoelastic properties of human cortical bone tissue depend on gender and elastic modulus

Bone exhibits rate-dependent failure behavior, suggesting that viscoelasticity is a factor in the damage and fracture of bone. Microdamage initiates at scales below the macroscopic porosity in bone, and, as such, is affected by the intrinsic viscoelasticity of the bone tissue. The viscoelasticity of the bone tissue can be measured by nanoindentation and recording the creep behavior at constant load. The viscoelastic properties have been used to assess differences in tissue behavior with respect to fracture healing, aging, and mouse strains. In this study, we compared the viscoelastic behavior of human cortical bone between genders by using nanoindentation at a fixed load of 10 mN to measure the creep time constant. Bones from females had a significantly greater time constant, indicating slower creep and relaxation, than bones from males. The creep time constants decreased with increasing tissue modulus. The mineralization, collagen content, and collagen cross-link density, which were bulk measurements, were analyzed to determine if the differences in viscoelastic behavior were explained by compositional differences in the bone. However, none of the parameters differed between genders, nor were they correlated to the viscoelastic time constant. As such, the difference must depend on other matrix proteins that we did not assess or differences in the microstructural organization. This is one of the only intrinsic bone material properties that has been found to differ between males and females, and it may be important for assessing differences in fracture risk, since crack propagation is generally sensitive to viscoelastic properties.

Interaction of microstructure and microcrack growth in cortical bone: a finite element study

Microstructural features including osteons and cement lines are considered to play an important role in determining the crack growth behaviour in cortical bone. This study aims to develop a computational mechanics approach to evaluate microscale fracture mechanisms in bone. In this study, finite element models based on actual human cortical bone images that allow for arbitrary crack growth were utilised to determine the crack propagation behaviour. The simulations varied the cement line and osteon strength and fracture toughness in different bone microstructures to assess the crack propagation trajectory, stress-strain relationship and nonlinear strain energy density. The findings of this study provide additional insight into the individual influence of microstructural features and their properties on crack growth behaviour in bone using a computational approach.

Direct comparison of nanoindentation and macroscopic measurements of bone viscoelasticity

Nanoindentation has become a standard method for measuring mechanical properties of bone, especially within microstructural units such as individual osteons or trabeculae. The use of nanoindentation to measure elastic properties has been thoroughly studied and validated. However, it is also possible to assess time dependent properties of bone by nanoindentation. The goal of this study was to compare time dependent mechanical properties of bone measured at the macroscopic level with those measured by nanoindentation. Twelve samples were prepared from the posterior distal femoral cortex of young cows. Initially, dogbone samples were prepared and subjected to torsional stress relaxation in a saline bath at 37 °C. A 5 mm thick disk was subsequently sectioned from the gage length, and subjected to nanoindentation. Nanoindentation was performed on hydrated samples using a standard protocol with 20 indents performed in 20 different osteons in each sample. Creep and stress relaxation data were fit to a Burgers four parameter rheological model, a five parameter generalized Maxwell model, and a three parameter standard linear solid. For Burgers viscoelastic model, the time constants measured by nanoindentation and torsion were weakly negatively correlated, while for the other two models the time constants were uncorrelated. The results support the notion that the viscoelastic behavior of bone at the macroscopic scale is primarily due to microstructural features, interfaces, or fluid flow, rather than viscous behavior of the bone tissue. As viscoelasticity affects the fatigue behavior of materials, the microscale properties may provide a measure of bone quality associated with initial damage formation.

Transplantation of human fetal blood stem cells in the osteogenesis imperfecta mouse leads to improvement in multiscale tissue properties

Osteogenesis imperfecta (OI or brittle bone disease) is a disorder of connective tissues caused by mutations in the collagen genes. We previously showed that intrauterine transplantation of human blood fetal stem/stromal cells in OI mice (oim) resulted in a significant reduction of bone fracture. This work examines the cellular mechanisms and mechanical bone modifications underlying these therapeutic effects, particularly examining the direct effects of donor collagen expression on bone material properties. In this study, we found an 84% reduction in femoral fractures in transplanted oim mice. Fetal blood stem/stromal cells engrafted in bones, differentiated into mature osteoblasts, expressed osteocalcin, and produced COL1a2 protein, which is absent in oim mice. The presence of normal collagen decreased hydroxyproline content in bones, altered the apatite crystal structure, increased the bone matrix stiffness, and reduced bone brittleness. In conclusion, expression of normal collagen from mature osteoblast of donor origin significantly decreased bone brittleness by improving the mechanical integrity of the bone at the molecular, tissue, and whole bone levels.

Reorganization of the femoral cortex due to age-, sex-, and endoprosthetic-related effects emphasized by osteonal dimensions and remodeling

Cortical remodeling is of particular importance to femoral strength. However, osteonal morphology and its quantitative changes are still poorly understood. Therefore, osteons as the cortexes basic structural units should serve for histomorphometrical assessment of reorganization mechanisms. Total hip replacement (THR) cases as well as relevant controls were investigated to gain insight into changes within stress shielded as well as age- and sex-associated bone with emphasis on the underlying osseous cell interactions. Control (n = 23) and THR (lifetime Ø14 months, n = 8) femora were prepared to grinding specimen. Osteon density (On.Dn.), osteon area (On.Ar.), haversian canal area (H.Ar.), osteon mean wall thickness (MWT), and cortical porosity (Ct.Po.) were evaluated periosteal and endosteal. Osteonal activity (OA) was determined by mineralization differentiation of active and quiescent osteons. Sex- and age-related differences in osteonal remodeling were evident in controls. Females showed age-dependent increases of H.Ar. during concomitant changes of On.Ar. and MWT resulting in elevated Ct.Po. at the entire cortex. Males showed this occurrence only periosteal. Increased OA and therefore higher remodeling rates were found in females above 70 years. THR led to decreased On.Dn. at the entire cortex. Periosteal increased OA represented increased periprosthetic remodeling resulting in elevated H.Ar., which enhanced Ct.Po.. Massive losses of endosteal circular osteonal structures following THR are reflected in a spongy reorganized cortex leading to increased Ct.Po.. Imbalances in osteoblast-osteoclast interactions whose intensity is related to the respective cortex regions occur in both stress shielded and aging bone and may thus contribute to implant failure and fracture risk.

Time sequence of secondary mineralization and microhardness in cortical and cancellous bone from ewes

Bone mineral is a major determinant of the mechanical resistance of bones. In bone structural units (BSUs), mineralization of osteoid tissue begins with a rapid primary mineralization followed by a secondary mineralization phase, i.e., a slow and gradual maturation of the mineral component leading to complete mineralization during an unknown period. The aim of this study was to determine the chronology of secondary bone mineralization in ewes, an animal model with a remodeling activity close to humans. Eighteen ewes received different fluorescent labels every 6 months to date the "age" of each labeled BSU. The degree of mineralization of bone (DMB) and Vickers microhardness were measured in labeled BSUs, while mineralization at the crystal level was assessed by Fourier transform infrared microspectroscopy (FTIRM). During the first 6 months of mineralization, degree of mineralization and microhardness significantly increased. They then increased more slowly until at 30 months they reach their maximal values. This progression during secondary mineralization was associated with an improvement of both the maturation and the crystal perfection of the mineral part of bone matrix. Finally, secondary mineralization in BSUs is completed after a period of 30 months. This observation should be taken into account for understanding the effects of long-term treatments of bone diseases.

Quasi-static and harmonic indentation of osteonal bone

The purpose of the study was to compare Quasi-Static (QS) and harmonic (CSM) methods of indentation testing. Bone sections were obtained from mid-femoral diaphyses of dogs which received a pair of calcein labels. Labeled (n = 35) and unlabeled (n = 112) osteons were identified. Indentation modulus (IM) and hardness (H) for the CSM method were collected during the entire loading cycle to peak depth, while IM and H for QS method were calculated at a peak depth of 500 nm. Results: The mean (SD) of the IM and H for labeled osteons were as follows: QS IM = 15.3 GPa (3.85) versus CSM IM = 14.7 GPa (3.58); P = .52 and QS H = .39 GPa (.171) versus CSM H = .42 GPa (.146); P = .32. The mean (SD) of the IM and H for unlabeled osteons were as follows: QS IM = 21.5 GPa (2.80) versus CSM IM = 20.6 GPa (2.53); P = .054 and QS H = .64 GPa (.117) versus CSM H = .70 GPa (.120); P = .017. There was no difference in IM and H for the two methods, except for H of the unlabeled osteons. In addition, for the CSM method, IM at 100 nm, 200 nm, 300 nm, 400 nm and 500 nm were not statistically significant different (P = .06). Bone is viscoelastic at an organ level. However, this component of its behavior was not detected at the length scale examined.

Relationships of Viscosity With Contact Hardness and Modulus of Bone Matrix Measured by Nanoindentation

Creep is an active form of time-dependent viscoelastic deformation that occurs in bone tissue during daily life. Recent findings indicate bone mineralization, which is involved in determining the elastic and plastic properties of bone matrix, can also contribute in controlling its viscoelastic property. Nanoindentation viscosity was used as a direct measure for the capacity of a material to resist viscous-like flow under loading. The objectives of this study were to examine (1) whether the nanoindentation viscosity obtained using the traditional viscoelastic Voigt model can describe creep response of bone matrix and (2) how the nanoindentation viscosity is related to contact hardness and elastic modulus. The Voigt model accurately described the creep behavior of bone matrix (r2>0.96, p<0.001). The nanoindentation viscosity had strong relationships with nanoindentation contact hardness (r2=0.94, p<0.001) and modulus (r2=0.83, p<0.001) independent of tissue ages of osteonal bone matrix. The strong positive relationships of nanoindentation viscosity with contact hardness and modulus can be interpreted as increases in the mineral portion of bone matrix may limit the interfibril motion of collagen while enhancing the mechanical stability of bone. We suggest that previous nanoindentation results can be reanalyzed to characterize the viscoelastic creep using the Voigt model.

Experimental determination of the permeability in the lacunar-canalicular porosity of bone

Permeability of the mineralized bone tissue is a critical element in understanding fluid flow occurring in the lacunar-canalicular porosity (PLC) compartment of bone and its role in bone nutrition and mechanotransduction. However, the estimation of bone permeability at the tissue level is affected by the influence of the vascular porosity in macroscopic samples containing several osteons. In this communication, both analytical and experimental approaches are proposed to estimate the lacunar-canalicular permeability in a single osteon. Data from an experimental stress-relaxation test in a single osteon are used to derive the PLC permeability by curve fitting to theoretical results from a compressible transverse isotropic poroelastic model of a porous annular disk under a ramp loading history (2007, "Compressible and Incompressible Constituents in Anisotropic Poroelasticity: The Problem of Unconfined Compression of a Disk," J. Mech. Phys. Solids, 55, pp. 161-193; 2008, "The Unconfined Compression of a Poroelastic Annular Cylindrical Disk," Mech. Mater., 40(6), pp. 507-523). The PLC tissue intrinsic permeability in the radial direction of the osteon was found to be dependent on the strain rate used and within the range of O(10(-24))-O(10(-25)). The reported values of PLC permeability are in reasonable agreement with previously reported values derived using finite element analysis (FEA) and nanoindentation approaches.

Probabilistic failure analysis of bone using a finite element model of mineral-collagen composites

Microdamage accumulation is a major pathway for energy dissipation during the post-yield deformation of bone. In this study, a two-dimensional probabilistic finite element model of a mineral-collagen composite was developed to investigate the influence of the tissue and ultrastructural properties of bone on the evolution of microdamage from an initial defect in tension. The probabilistic failure analyses indicated that the microdamage progression would be along the plane of the initial defect when the debonding at mineral-collagen interfaces was either absent or limited in the vicinity of the defect. In this case, the formation of a linear microcrack would be facilitated. However, the microdamage progression would be scattered away from the initial defect plane if interfacial debonding takes place at a large scale. This would suggest the possible formation of diffuse damage. In addition to interfacial debonding, the sensitivity analyses indicated that the microdamage progression was also dependent on the other material and ultrastructural properties of bone. The intensity of stress concentration accompanied with microdamage progression was more sensitive to the elastic modulus of the mineral phase and the nonlinearity of the collagen phase, whereas the scattering of failure location was largely dependent on the mineral to collagen ratio and the nonlinearity of the collagen phase. The findings of this study may help understanding the post-yield behavior of bone at the ultrastructural level and shed light on the underlying mechanism of bone fractures.

The role of mineralization and organic matrix in the microhardness of bone tissue from controls and osteoporotic patients

Degree of mineralization of bone (DMB) is a major intrinsic determinant of bone strength at the tissue level but its contribution to the microhardness (Vickers indentation) at the intermediary level of organization of bone tissue, i.e., Bone Structural Units (BSUs), has never been assessed. The purpose of this study was to analyze the relationship between the microhardness, the DMB and the organic matrix, measured in BSUs from human iliac bone biopsies. Iliac bone samples from controls and osteoporotic patients (men and women), embedded in methyl methacrylate, were used. Using a Vickers indenter, microhardness (kg/mm2) was measured, either globally on surfaced blocks or focally on 100 microm-thick sections from bone samples (load of 25 g applied during 10 sec; CV=5%). The Vickers indenter was more suited than the Knoop indenter for a tissue like bone in which components are diversely oriented. Quantitative microradiography performed on 100 microm-thick sections, allowed measurement of parameters reflecting the DMB (g/cm3). Assessed on the whole bone sample, both microhardness and DMB were significantly lower (-10% and -7%, respectively) in osteoporotic patients versus controls (p<0.001). When measured separately at the BSU level, there were significant positive correlations between microhardness and DMB in controls (r2=0.36, p<0.0001) and osteoporotic patients (r2=0.43, p<0.0001). Mineralization is an important determinant of the microhardness, but did not explain all of its variance. To highlight the role of the organic matrix in bone quality, microhardness of both osteoid and adjacent calcified matrix were measured in iliac samples from subjects with osteomalacia. Microhardness of organic matrix is 3-fold lower than the microhardness of calcified tissue. In human calcanei, microhardness was significantly correlated with DMB (r2=0.33, p=0.02) and apparent Young's modulus (r2=0.26, p=0.03). In conclusion, bone microhardness measured by Vickers indentation is an interesting methodology for the evaluation of bone strength and its determinants at the BSU level. Bone microhardness is linked to Young's modulus of bone and is strongly correlated to mineralization, but the organic matrix accounts for about one third of its variance.

The associations between mineral crystallinity and the mechanical properties of human cortical bone

It is well known that the amount of mineralization renders bone its stiffness. However, besides the mere amount of the mineral phase, size and shape of carbonated apatite crystals are postulated to affect the mechanical properties of bone tissue as predicted by composite mechanics models. Despite this predictive evidence, there is little experimental insight on the relation between the characteristics of mineral crystals and hard tissue mechanics. In this study, Raman spectroscopy was used to provide information on the crystallinity of bone's mineral phase, a parameter which is an overall indicator of mineral crystal size and stoichiometric perfection. Raman scans and mechanical tests (monotonic and fatigue; n=64 each) were performed on the anterior, medial, lateral and posterior quadrant sections of 16 human cadaveric femurs (52 y.o.-85 y.o.). The reported coefficient of determination values (R(2)) were adjusted for the effects of age to bring out the unbiased contribution of crystallinity. Crystallinity was able to explain 6.7% to 48.3% of the variation in monotonic mechanical properties. Results indicated that the tissue-level strength and stiffness increased with increasing crystallinity while the ductility reduced. Crystallinity explained 11.3% to 63.5% of the variation in fatigue properties. Moduli of specimens with greater crystallinity degraded at a slower rate and, also, they had longer fatigue lives. However, not every anatomical quadrant displayed these relationships. In conclusion, these results acknowledge crystal properties as an important bone quality factor and raise the possibility that aberrations in these properties may contribute to senile osteoporotic fractures.

Adaptations in the mandible and appendicular skeleton of high and low bone density inbred mice

The appendicular skeletons of high [C3H/HeJ (C3H)] and low [C57BL/6J (B6)] density inbred mice have been shown to differ in morphology, mechanical properties, and cellular activity. The focus of the current study was to (1) characterize the mandibular bone formation rate (BFR/BS), bone mass, indentation modulus (IM), and hardness of C3H and B6 mice and (2) investigate the relationship of the mechanical properties in three skeletal sites: mandible, femur, and tibia. Specimens from 17-week-old female C3H and B6 (n = 15/group) mice were obtained. Mandibular bone mass was estimated from the lateral-view area (LVA) and transverse cross sections. BFR/BS was measured in the mandibular section distal to the third molar. In addition, bone blocks from the distal surface of the third molar and the femoral and tibial midshaft were obtained for mechanical testing. BFR/BS, cortical area, and LVA were greater (P < 0.001) in C3H mandibles. IM was approximately 2 GPa higher in the C3H mandible (P > 0.05), femur (P < 0.001), and tibia (P < 0.01). Mandibular IM was lower (P < 0.05) than the femoral and tibial IM within each inbred mouse. IM was not significant between C3H and B6 mandibles. However, the magnitude of the difference ( approximately 12%) in the mandible was similar to the difference in the appendicular skeleton. This mandibular bone phenotype is similar to that observed in the appendicular skeleton of these distinct inbred mice.

Indentation modulus of the alveolar process in dogs

One mechanism of bone adaptation is alteration in tissue level material properties. We hypothesized that alteration in the indentation modulus of the alveolar process is an adaptive response to the localized mechanical environment. Forty-eight specimens representing anterior and posterior regions of the maxilla and mandible were obtained from 6 mature male beagle dogs. The indentation properties of the alveolar bone proper and more distant osteonal cortical bone were estimated. The bone types were further divided into 3 regions (coronal, middle, and apical), with 27 indents being made in each region of tooth-supporting bone. There was a significant difference (p < 0.001) in the indentation moduli of the jaws (maxilla/mandible), location (anterior/posterior), and bone type (alveolar bone proper vs. cortical bone). However, statistical interactions exist which preclude the simple interpretation of results. The distribution of relative stiffness provides a better understanding of bone adaptations in the alveolar process.