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

Exploring the hierarchical structure of lamellar bone and its impact on fracture behaviour: A computational study using a phase field damage model

Bone is a naturally occurring composite material composed of a stiff mineral phase and a compliant organic matrix of collagen and non-collagenous proteins (NCP). While diverse mineral morphologies such as platelets and grains have been documented, the precise role of individual constituents, and their morphology, remains poorly understood. To understand the role of constituent morphology on the fracture behaviour of lamellar bone, a damage based representative volume element (RVE) was developed, which considered various mineral morphologies and mineralised collagen fibril (MCF) configurations. This model framework incorporated a novel phase-field damage model to predict the onset and evolution of damage at mineral-mineral and mineral-MCF interfaces. It was found that platelet-based mineral morphologies had superior mechanical performance over their granular counterparts, owing to their higher load-bearing capacity, resulting from a higher aspect ratio. It was also found that MCFs had a remarkable capacity for energy dissipation under axial loading, with these fibrillar structures acting as barriers to crack propagation, thereby enhancing overall elongation and toughness. Interestingly, the presence of extrafibrillar platelet-based minerals also provided an additional toughening through a similar mechanism, whereby these structures also inhibited crack propagation. These findings demonstrate that the two primary constituent materials of lamellar bone play a key role in its toughening behaviour, with combined effect by both mineral and MCFs to inhibit crack propagation at this scale. These results have provided novel insight into the fracture behaviour of lamellar bone, enhancing our understanding of microstructure-property relationships at the sub-tissue level.

Prediction of the critical energy release rate for rat femoral cortical bone structure under different failure conditions

BACKGROUND AND OBJECTIVE

Critical energy release rate is a global fracture parameter that could be measured during the failing process, and its value may change under different failure conditions even in the same bone structure. The aim of this study was to propose an approach that combined the experimental test and finite element analysis to predict the critical energy release rates in the femoral cortical bone structures under compression and three-point bending loads.

METHODS

Three-point bending and compression experiments and the corresponding fracture simulations were performed on the rat femoral cortical bone structures. Different values of energy release rate were repeatedly assigned to the finite element models to perform fracture simulations, and then the load-displacement curves predicted in each simulation were compared with the experimental data to back-calculate the critical energy release rate.

RESULTS

The predicted data were similar to the experimental results when the calibrated energy release rate was suitable. The results showed that the cortical bone structure occurred shear open failure under compression load, and the predicted critical energy release rate was 0.12 N/mm. The same cortical bone structure occurred tensile open failure under three-point bending load, and the predicted critical energy release rate was 0.16 N/mm.

CONCLUSIONS

The critical energy release rates were different under various failure conditions in one cortical bone structure. A comprehensive analysis from the perspectives of material mechanical properties, failure mode, and damage fracture mechanism was conducted to reveal the reasons for the differences in the critical energy release rate in the cortical bone structure, which provided a theoretical basis for the measurement of the critical energy release rate and the accurate fracture simulation.

Relationship between mechanical and microstructural parameters of rat lumbar spine in different ages

Exploring the relationships between microstructure and mechanical properties of bones may provide effective suggestions for increasing bone strength and reducing osteoporotic fracture. In this research, the tissue-level mechanical parameters, microstructure parameters of cancellous bone, and apparent mechanical parameters of L6 vertebral body were calculated in female SD rats aged 1-, 3-, 5-, 7-, 9-, 11-, 13-, 15-, 16-, and 17-month-old. Data were processed with Kruskal-Wallis test, linear regression and Spearman's rank correlation analysis. Appropriately increasing the plate Tb.N could enhance mechanical properties of bone. Tb.Th and Tb.N were two key factors in determining the tissue-level mechanical properties of cancellous bone. The microstructure could significantly predict mechanical parameters. Our findings may help to further understand the mechanism of osteoporotic fractures.

Effects of different running intensities on the micro-level failure strain of rat femoral cortical bone structures: a finite element investigation

BACKGROUND

Running with the appropriate intensity may produce a positive influence on the mechanical properties of cortical bone structure. However, few studies have discussed the effects of different running intensities on the mechanical properties at different levels, especially at the micro-level, because the micromechanical parameters are difficult to measure experimentally.

METHODS

An approach that combines finite element analysis and experimental data was proposed to predict a micromechanical parameter in the rat femoral cortical bone structure, namely, the micro-level failure strain. Based on the previous three-point bending experimental information, fracture simulations were performed on the femur finite element models to predict their failure process under the same bending load, and the micro-level failure strains in tension and compression of these models were back-calculated by fitting the experimental load-displacement curves. Then, the effects of different running intensities on the micro-level failure strain of rat femoral cortical bone structure were investigated.

RESULTS

The micro-level failure strains of the cortical bone structures expressed statistical variations under different running intensities, which indicated that different mechanical stimuli of running had significant influences on the micromechanical properties. The greatest failure strain occurred in the cortical bone structure under low-intensity running, and the lowest failure strain occurred in the structure under high-intensity running.

CONCLUSIONS

Moderate and low-intensity running were effective in enhancing the micromechanical properties, whereas high-intensity running led to the weakening of the micromechanical properties of cortical bone. Based on these, the changing trends in the micromechanical properties were exhibited, and the effects of different running intensities on the fracture performance of rat cortical bone structures could be discussed in combination with the known mechanical parameters at the macro- and nano-levels, which provided the theoretical basis for reducing fracture incidence through running exercise.

Biomechanical evaluation of different strain judging criteria on the prediction precision of cortical bone fracture simulation under compression

Introduction: The principal strain or equivalent strain is mainly used in current numerical studies to determine the mechanical state of the element in the cortical bone finite element model and then perform fracture simulation. However, it is unclear which strain is more suitable for judging the element mechanical state under different loading conditions due to the lack of a general strain judging criterion for simulating the cortical bone fracture. Methods: This study aims to explore a suitable strain judging criterion to perform compressive fracture simulation on the rat femoral cortical bone based on continuum damage mechanics. The mechanical state of the element in the cortical bone finite element model was primarily assessed using the principal strain and equivalent strain separately to carry out fracture simulation. The prediction accuracy was then evaluated by comparing the simulated findings with different strain judging criteria to the corresponding experimental data. Results: The results showed that the fracture parameters predicted using the principal strain were closer to the experimental values than those predicted using the equivalent strain. Discussion: Therefore, the fracture simulation under compression was more accurate when the principal strain was applied to control the damage and failure state in the element. This finding has the potential to improve prediction accuracy in the cortical bone fracture simulation.

Investigation on the Differences in the Failure Processes of the Cortical Bone under Different Loading Conditions

Cortical bone is a transversely isotropic material, and the mechanical properties may be related to the loading direction on the osteon. Therefore, analyzing the differences in the failure processes of cortical bone under different loading conditions is necessary to explore the measures for reducing the incidence of fracture. In this study, to investigate the effects of different loading directions on the fracture performance in the cortical bone, a numerical method that could simultaneously simulate the failure processes in the cortical bone structure under compression and bending loads was established based on continuum damage mechanics theory. The prediction accuracy and feasibility of the numerical method were first verified by comparing with the corresponding experimental results. Then, the differences in the failure process and fracture performance of the same cortical bone structure under compression and bending loads were investigated. The simulation results indicated that for the same structure, the slip-open failure mode appeared under compression load, and the crack propagated along a certain angle to the loading direction; the tension-open failure mode appeared under bending load, and the crack propagated along the direction perpendicular to the loading direction. Meanwhile, the fracture load was greater and the fracture time was later in the compression than in the bending condition. These phenomena stated that discrepant failure processes and fracture patterns occurred in the same cortical bone structure under different loading conditions. The main reason may be related to the tension–compression asymmetry and transversely isotropic characteristics in the cortical bone material. The fracture simulations in the cortical bone under different loading conditions could improve the prediction accuracy in bone biomechanics and provide the prevention method for cortical bone damage and fracture.

Aging exacerbates the morphological and mechanical response of mineralized collagen fibrils in murine cortical bone to disuse

Mineralized collagen fibrils (MCFs) are the fundamental building blocks of bone tissue and contribute significantly to the mechanical behavior of bone. However, it is still largely unknown how the collagen network in bone responds to aging and the disuse normally accompanying it. Utilizing atomic force microscopy, nanoindentation and Raman spectroscopy, age-related alterations in the microstructure and mechanical properties of murine cortical tibia at multiple scales were investigated in this study. The potential difference in the responses of bone to disuse at different ages was studied. The results indicated that the age- and disuse-related alterations in bone initiate from MCFs in the bone matrix. The D-periodic spacing, radial elastic modulus of a single MCF and the mineral-to-matrix ratio on the cortical bone surface were larger in aged mice than in adult mice. Disuse, on the other hand, mainly has a major influence on aged mice, particularly on the morphology and mechanical properties of MCFs, but it only has modest effects on adult bone. These findings revealed insights into the morphological and mechanical adaptation of mineralized collagen fibrils in murine cortical bone to aging and disuse. STATEMENT OF SIGNIFICANCE: Bone is a complex structured composite material consisting of an interwoven framework of collagen fibrils reinforced by mineral particles and embedded in an extrafibrillar mineralized matrix. Utilizing atomic force microscopy, nanoindentation and Raman spectroscopy, this study suggests that the effects of aging, as well as the accompanying disuse, on the morphology and mechanical properties of bone initiate from the mineralized collagen fibril level. More interestingly, the MCF in the bone of aged mice seems to be more sensitive to disuse than that in adult mice. These findings significantly further the current understanding of the adaptation process of bone to aging at the mineralized collagen fibril level and provide direct insights into the physiological response of bone to aging and the abnormal mechanical environment.

Effects of Osteoporosis on Bone Morphometry and Material Properties of Individual Human Trabeculae in the Femoral Head

Osteoporosis is the most common bone disease and is conventionally classified as a decrease of total bone mass. Current diagnosis of osteoporosis is based on clinical risk factors and dual energy X‐ray absorptiometry (DEXA) scans, but changes in bone quantity (bone mass) and quality (trabecular structure, material properties, and tissue composition) are not distinguished. Yet, osteoporosis is known to cause a deterioration of the trabecular network, which might be related to changes at the tissue scale—the material properties. The goal of the current study was to use a previously established test method to perform a thorough characterization of the material properties of individual human trabeculae from femoral heads in cyclic tensile tests in a close to physiologic, wet environment. A previously developed rheological model was used to extract elastic, viscous, and plastic aspects of material behavior. Bone morphometry and tissue mineralization were determined with a density calibrated micro‐computed tomography (μCT) set‐up. Osteoporotic trabeculae neither showed a significantly changed material or mechanical behavior nor changes in tissue mineralization, compared with age‐matched healthy controls. However, donors with osteopenia indicated significantly reduced apparent yield strain and elastic work with respect to osteoporosis, suggesting possible initial differences at disease onset. Bone morphometry indicated a lower bone volume to total volume for osteoporotic donors, caused by a smaller trabecular number and a larger trabecular separation. A correlation of age with tissue properties and bone morphometry revealed a similar behavior as in osteoporotic bone. In the range studied, age does affect morphometry but not material properties, except for moderately increased tissue strength in healthy donors and moderately increased hardening exponent in osteoporotic donors. Taken together, the distinct changes of trabecular bone quality in the femoral head caused by osteoporosis and aging could not be linked to suspected relevant changes in material properties or tissue mineralization. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Highly aligned hierarchical intrafibrillar mineralization of collagen induced by periodic fluid shear stress

Periodic fluid shear stress (FSS) is one of the main mechanical microenvironments in mineralization of bone matrix. To elucidate the mechanism of periodic FSS in collagen mineralization, a mechanical loading induced mineralization system is developed and compared with traditional polyacrylic acid (PAA) induced mineralization. Fourier transform infrared (FTIR) spectroscopy, calcium-to-phosphorus molar ratio and transmission electron microscopy (TEM) demonstrate that both periodic FSS and PAA can control the size of amorphous calcium phosphate (ACP) to avoid aggregation and help the formation of intrafibrillar mineralization. Differently, periodic FSS under a proper cycle and range can accelerate the conversion of ACP to apatite crystals and alleviate the reduced transformation caused by PAA. Under the action of template analogues, periodic FSS can also promote the formation of highly oriented hierarchical intrafibrillar mineralized (HIM) collagen. These findings are helpful for understanding the mechanism of collagen mineralization in natural bone matrix and contribute to the design of novel bone substitute materials with hierarchical structures.

Age-Related Trends in the Trabecular Micro-Architecture of the Medial Clavicle: Is It of Use in Forensic Science?

The mechanical and structural properties of bone are known to change significantly with age. Within forensic and archaeological investigations, the medial end of the clavicle is typically used for estimating the age-at-death of an unknown individual. Although, this region of the skeleton is of interest to forensic and clinical domains, alterations beyond the macro-scale have not been fully explored. For this study, non-destructive micro-computed tomography (μ-CT) was employed to characterize structural alterations to the cancellous bone of the medial clavicle. Fresh human cadaveric specimens (12-59 years) obtained at autopsy were utilized for this study, and were scanned with a voxel size of ~83 μm. Morphometric properties were quantified and indicated that the bone volume, connectivity density, mineral density, and number of trabeculae decreased with age, while the spacing between the trabeculae increased with age. In contrast to other sub-regions of the skeleton, trabecular thickness, and degree of anisotropy did not correlate with age. Collectively, this could suggest that the network is becoming increasingly perforated with age rather than exhibiting trabecular thinning. These results are used in the context of deriving a potential protocol for forensic investigations by using this particular and largely unexplored region of the skeleton, and provide inspiration for future experiments concerning micro-architectural and small scale changes in other regions of the human skeleton.

Strain Distribution Evaluation of Rat Tibia under Axial Compressive Load by Combining Strain Gauge Measurement and Finite Element Analysis

This study is aimed at providing an effective method for determining strain-load relationship and at quantifying the strain distribution within the whole tibia under axial compressive load on rats. Rat tibial models with axial compressive load were designed. Strains in three directions (0°, 45°, and 90°) at the proximal shaft of the tibia were measured by using a strain gauge rosette, which was used to calculate the maximum and minimum principal strains. Moreover, the strain at the midshaft of the tibia was measured by a single-element strain gauge. The slopes of the strain-load curves with different peak loads were calculated to assess the stability of the strain gauge measurement. Mechanical environment in the whole tibia by the axial compressive load was quantified using finite element analysis (FEA) based on microcomputed tomography images. The von Mises elastic strain distributions of the whole tibiae were evaluated. Slopes of the strain-load curves showed no significant differences among different peak loads (ANOVA; P > 0.05), indicating that the strain-load relationship obtained from the strain gauge measurement was reasonable and stable. The FEA results corresponded to the experimental results with an error smaller than 15% (paired Student's t-test, P > 0.05), signifying that the FEA can simulate the experiment reasonably. FEA results showed that the von Mises elastic strain was the lowest in the middle and gradually increased to both sides along the lateral direction, with the maximal von Mises elastic strain being observed on the posterior side under the distal tibiofibular synostosis. The method of strain gauge measurements and FEA used in this study can provide a feasible way to obtain the mechanical environment of the tibiae under axial compressive load on the rats and serve as a reference for further exploring the mechanical response of the bone by axial compressive load.

Effects of treadmill with different intensities on bone quality and muscle properties in adult rats

BACKGROUND

Bone is a dynamically hierarchical material that can be divided into length scales of several orders of magnitude. Exercise can cause bone deformation, which in turn affects bone mass and structure. This study aimed to study the effects of treadmill running with different intensities on the long bone integrity and muscle biomechanical properties of adult male rats.

METHODS

Forty-eight 5-month-old male SD rats were randomly divided into 4 groups: i.e., sedentary group (SED), exercise with speed of 12 m/min group (EX12), 16 m/min group (EX16), and 20 m/min group (EX20). The exercise was carried out for 30 min every day, 5 days a week for 4 weeks. The femurs were examined using three-point bending test, microcomputer tomography scanning and nanoindentation test; the soleus muscle was dissected for tensile test; ALP and TRACP concentrations were measured by serum analysis.

RESULTS

The failure load was significantly increased by the EX12 group, whereas the elastic modulus was not significantly changed. The microstructure and mineral densities of the trabecular and cortical bone were significantly improved by the EX12 group. The mechanical properties of the soleus muscle were significantly increased by treadmill exercise. Bone formation showed significant increase by the EX12 group. Statistically higher nanomechanical properties of cortical bone were detected in the EX12 group.

CONCLUSION

The speed of 12 m/min resulted in significant changes in the microstructure and biomechanical properties of bone; besides, it significantly increased the ultimate load of the soleus muscle. The different intensities of treadmill running in this study provide an experimental basis for the selection of exercise intensity for adult male rats.

Apatite minerals derived from collagen phosphorylation modification induce the hierarchical intrafibrillar mineralization of collagen fibers

Collagen is the critical organic component of bone matrix, which is the template for bone biomineralization. Phosphorylation modification of collagen plays an important role in the process of biomineralization in vivo, but its mechanism on in vitro biomimetic mineralization of bone matrix remains unclear at the molecular level. Sodium tripolyphosphate is used to phosphorylate collagen in this study and new phosphate groups appear on collagen fibrils after phosphorylation modification. The chelating amount of calcium is improved linearly with increasing the phosphorylation degree of collagen fibrils, which demonstrates that the introduced phosphate groups serve as new nucleation sites and participate in the formation of apatite minerals inside the collagen fibers. Stabilized nanosized amorphous calcium phosphate by polyacrylic acid can also permeate into collagen fibers and further transform into another layer of hydroxyapatite minerals. Both layers of apatite minerals eventually induce the formation of hierarchical intrafibrillar mineralization structure within the phosphorylated collagen fibers. The present research enriches the previous biomineralization mechanism of bone matrix, provides a facile strategy for biomimetic mineralization of collagen, and offers the basis for future investigation of the advanced bone substitute materials.

Multiscale experimental study on the effects of different weight-bearing levels during moderate treadmill exercise on bone quality in growing female rats

Background

Bone tissue displays a hierarchical organization. Mechanical environments influence bone mass and structure. This study aimed to explore the effects of different mechanical stimuli on growing bone properties at macro–micro–nano scales.

Methods

Sixty five-week-old female Wistar rats were treadmill exercised at moderate intensity with the speed of 12 m/min, and then randomly divided into five groups according to weight-bearing level. After 8 weeks of experiment, femurs were harvested to perform multiscale tests.

Results

Bone formation was significantly increased by weight-bearing exercise, whereas bone resorption was not significantly inhibited. Trabecular and cortical bone mineral densities showed no significant increase by weight-bearing exercise. The microstructure of trabecular bone was significantly improved by 12% weight-bearing exercise. However, similar positive effects were not observed with further increase in weight-bearing levels. The nanomechanical properties of trabecular bone were not significantly changed by weight-bearing exercise. The macrostrength of whole femur and the nanomechanical properties of cortical bone significantly decreased in the 19% and 26% weight-bearing exercise groups.

Conclusion

When rats ran on the treadmill at moderate intensity during growth period, additional 12% weight-bearing level could significantly increase bone formation, improve microstructure of trabecular bone, as well as maintain the structure and mechanical properties of cortical bone. Excessive weight-bearing level caused no positive effects on the trabecular bone microstructure and properties of cortical bone at all scales. In addition, increased weight-bearing level exerted no significant influence on trabecular and cortical bone mineral densities.

Small-angle X-ray scattering (SAXS) and nitrogen porosimetry (NP): two novel techniques for the evaluation of urinary stone hardness

PURPOSE

To evaluate urinary stones using small-angle X-ray scattering (SAXS) and nitrogen porosimetry (NP). Traditionally, stones are categorized as hard or soft based on their chemical composition. We hypothesized that stone hardness is associated not only with its chemical composition but also with its internal architecture. SAXS and NP are well-known techniques in material sciences. We tested whether SAXS and NP are applicable for evaluating human urinary stones and whether they provide information at the nanoscale level that could be useful in clinical practice.

METHODS

Thirty endoscopically removed urinary stones were studied. Standard techniques for stone analysis were used to determine the stone composition. SAXS was used to evaluate the solid part of the stone by measuring the crystal thickness (T) and the fractal dimension (D_m/D_s), while NP was used to evaluate the porosity of the stone, i.e., the pore radius, pore volume, and specific surface area (SSA).

RESULTS

All stones were successfully analyzed with SAXS and NP. Each stone demonstrated unique characteristics regarding T, D_m/D_s, pore radius, pore volume, and SSA. Significant differences in those parameters were seen among the stones with almost identical chemical compositions. The combination of high T, high SSA, low D_m/D_s, low pore volume, and low pore radius is indicative of a hard material and vice versa.

CONCLUSIONS

SAXS and NP can be used to evaluate human urinary stones. They provide information on stone hardness based on their nanostructure characteristics, which may be different even among stones with similar compositions.

Effects of tofacitinib in early arthritis-induced bone loss in an adjuvant-induced arthritis rat model

Objectives

The main goal of this work was to analyse how treatment intervention with tofacitinib prevents the early disturbances of bone structure and mechanics in the rat model of adjuvant-induced arthritis. This is the first study to access the impact of tofacitinib on the skeletal bone effects of inflammation.

Methods

Fifty Wistar rats with adjuvant-induced arthritis were randomly housed in experimental groups, as follows: non-arthritic healthy group (n = 20); arthritic non-treated group (n = 20); and 10 animals undergoing tofacitinib treatment. Rats were monitored during 22 days after disease induction for the inflammatory score, ankle perimeter and body weight. Healthy non-arthritic rats were used as controls for comparison. After 22 days of disease progression, rats were killed and bone samples collected for histology, micro-CT, three-point bending and nanoindentation analysis. Blood samples were also collected for quantification of bone turnover markers and systemic cytokines.

Results

At the tissue level, measured by nanoindentation, tofacitinib increased bone cortical and trabecular hardness. However, micro-CT and three-point bending tests revealed that tofacitinib did not reverse the effects of arthritis on the cortical and trabecular bone structure and on mechanical properties.

Conclusion

Possible reasons for these observations might be related to the mechanism of action of tofacitinib, which leads to direct interactions with bone metabolism, and/or to the kinetics of its bone effects, which might need longer exposure.

Whole body vibration with rest days could improve bone quality of distal femoral metaphysis by regulating trabecular arrangement

Low-magnitude, high-frequency vibration (LMHFV) with rest days (particularly seven rest days) was considerably effective in improving the morphological and mechanical properties of rat proximal femur. However, current knowledge is limited regarding the possible benefit of this mechanical regimen to other bone sites and whether the optimal rest days are the same. This study followed our previous experiment on LMHFV loading with rest days for three-month-old male Wistar rats. The experiment involved seven groups, namely, vibrational loading for X day followed with X day rest (X=1, 3, 5, 7), daily vibrational loading, tail suspension and baseline control. Micro-computed tomography (micro-CT) scanning was used to evaluate the microarchitecture of the distal femoral trabecular bone. Micro-CT image-based microfinite element analysis was performed for each distal femoral metaphysis. LMHFV with rest days substantially changed the trabecular arrangement from remarkably plate-like to rod-like. Vibrational loading with 1 day rest was substantially effective in improving the architecture and apparent- and tissuelevel mechanical properties of the rat distal femoral metaphysis. This study may provide an improved understanding of the sitespecific responses of bone tissue to LMHFV with rest days for a substantially effective therapy of a targeted bone site.

The Role of Matrix Composition in the Mechanical Behavior of Bone

PURPOSE OF REVIEW

While thinning of the cortices or trabeculae weakens bone, age-related changes in matrix composition also lower fracture resistance. This review summarizes how the organic matrix, mineral phase, and water compartments influence the mechanical behavior of bone, thereby identifying characteristics important to fracture risk.

RECENT FINDINGS

In the synthesis of the organic matrix, tropocollagen experiences various post-translational modifications that facilitate a highly organized fibril of collagen I with a preferred orientation giving bone extensibility and several toughening mechanisms. Being a ceramic, mineral is brittle but increases the strength of bone as its content within the organic matrix increases. With time, hydroxyapatite-like crystals experience carbonate substitutions, the consequence of which remains to be understood. Water participates in hydrogen bonding with organic matrix and in electrostatic attractions with mineral phase, thereby providing stability to collagen-mineral interface and ductility to bone. Clinical tools sensitive to age- and disease-related changes in matrix composition that the affect mechanical behavior of bone could potentially improve fracture risk assessment.

Early arthritis induces disturbances at bone nanostructural level reflected in decreased tissue hardness in an animal model of arthritis

Introduction

Arthritis induces joint erosions and skeletal bone fragility.

Objectives

The main goal of this work was to analyze the early arthritis induced events at bone architecture and mechanical properties at tissue level.

Methods

Eighty-eight Wistar rats were randomly housed in experimental groups, as follows: adjuvant induced arthritis (AIA) (N = 47) and a control healthy group (N = 41). Rats were monitored during 22 days for the inflammatory score, ankle perimeter and body weight and sacrificed at different time points (11 and 22 days post disease induction). Bone samples were collected for histology, micro computed tomography (micro-CT), 3-point bending and nanoindentation. Blood samples were also collected for bone turnover markers and systemic cytokine quantification.

Results

At bone tissue level, measured by nanoindentation, there was a reduction of hardness in the arthritic group, associated with an increase of the ratio of bone concentric to parallel lamellae and of the area of the osteocyte lacuna. In addition, increased bone turnover and changes in the microstructure and mechanical properties were observed in arthritic animals, since the early phase of arthritis, when compared with healthy controls.

Conclusion

We have shown in an AIA rat model that arthritis induces very early changes at bone turnover, structural degradation and mechanical weakness. Bone tissue level is also affected since the early phase of arthritis, characterized by decreased tissue hardness associated with changes in bone lamella organization and osteocyte lacuna surface. These observations highlight the pertinence of immediate control of inflammation in the initial stages of arthritis.

Determination of a tissue-level failure evaluation standard for rat femoral cortical bone utilizing a hybrid computational-experimental method

Macro-level failure in bone structure could be diagnosed by pain or physical examination. However, diagnosing tissue-level failure in a timely manner is challenging due to the difficulty in observing the interior mechanical environment of bone tissue. Because most fractures begin with tissue-level failure in bone tissue caused by continually applied loading, people attempt to monitor the tissue-level failure of bone and provide corresponding measures to prevent fracture. Many tissue-level mechanical parameters of bone could be predicted or measured; however, the value of the parameter may vary among different specimens belonging to a kind of bone structure even at the same age and anatomical site. These variations cause difficulty in representing tissue-level bone failure. Therefore, determining an appropriate tissue-level failure evaluation standard is necessary to represent tissue-level bone failure. In this study, the yield and failure processes of rat femoral cortical bones were primarily simulated through a hybrid computational-experimental method. Subsequently, the tissue-level strains and the ratio between tissue-level failure and yield strains in cortical bones were predicted. The results indicated that certain differences existed in tissue-level strains; however, slight variations in the ratio were observed among different cortical bones. Therefore, the ratio between tissue-level failure and yield strains for a kind of bone structure could be determined. This ratio may then be regarded as an appropriate tissue-level failure evaluation standard to represent the mechanical status of bone tissue.

Effect of celastrol on bone structure and mechanics in arthritic rats

OBJECTIVE

Rheumatoid arthritis (RA) is characterised by chronic inflammation leading to articular bone and cartilage damage. Despite recent progress in RA management, adverse effects, lack of efficacy and economic barriers to treatment access still limit therapeutic success. Therefore, safer and less expensive treatments that control inflammation and bone resorption are needed. We have previously shown that celastrol is a candidate for RA treatment. We have observed that it inhibits both interleukin (IL)-1β and tumor necrosis factor (TNF) in vitro, and that it has anti-inflammatory properties and ability to decrease synovial CD68+ macrophages in vivo. Herein our goal was to evaluate the effect of celastrol in local and systemic bone loss.

METHODS

Celastrol was administrated intraperitoneally at a dose of 1 µg/g/day to female Wistar adjuvant-induced arthritic rats. Rats were sacrificed after 22 days of disease progression, and blood, femurs, tibiae and paw samples were collected for bone remodelling markers quantification, 3-point bending test, micro-CT analysis, nanoindentation and Fourier transform infrared spectroscopy measurements, and immunohistochemical evaluation.

RESULTS

We have observed that celastrol preserved articular structures and decreased the number of osteoclasts and osteoblasts present in arthritic joints. Moreover, celastrol reduced tartrate-resistant acid phosphatase 5b, procollagen type 1 amino-terminal propeptide and C terminal crosslinked telopeptide of type II collagen serum levels. Importantly, celastrol prevented bone loss and bone microarchitecture degradation. Celastrol also preserved bone nanoproperties and mineral content. Additionally, animals treated with celastrol had less fragile bones, as depicted by an increase in maximum load and yield displacement.

CONCLUSIONS

These results suggest that celastrol reduces both bone resorption and cartilage degradation, and preserves bone structural properties.

Substrain-specific differences in bone parameters, alpha-2-macroglobulin circulating levels, and osteonecrosis incidence in a rat model

Osteonecrosis of the femoral head (ONFH) is a potentially devastating complication that occurs in up to 40% of young adults receiving chronic glucocorticoid (GC) therapy. Through a validated GC therapy rat model, we have previously shown that Wistar Kyoto (WK) rats exhibit a genetic susceptibility to GC-induced ONFH compared to Sasco Fischer (F344) rats. We have undertaken this study in order to investigate differences between these two strains for their bone parameters, alpha-2-macroglobulin (A2M) circulating levels and incidence of GC-induced osteonecrosis of the femoral head. WK and F344 rats were treated either with 1.5 mg/kg/day of prednisone or placebo for 6 months. Blood was taken every month. The femoral heads were harvested for histological examination to detect ONFH and analyzed with micro-computed tomography. After 3 months of GC-therapy, plasma A2M was elevated in treated rats only. GC-treated WK rats exhibited histological evidence of early ONFH through higher rates of cellular apoptosis and empty osteocyte lacunae in the subchondral bone compared to placebos and to F344 rats. Furthermore, micro-CT analysis exhibited femoral head collapse only in GC-treated WK rats. Interestingly, GC-treated F344 rats exhibited significant micro-CT changes, but such changes were less concentrated in the articular region and were accompanied histologically with increased marrow fat. These µCT and histological findings suggest that elevated A2M serum level is not predictive and suitable as an indicative biomarker for early GC-induced ONFH in rodents. Elevated A2M levels observed during GC treatment suggests that it plays role in the host reparative response to GC-associated effects. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1183-1194, 2017.

Impact of P2RX7 ablation on the morphological, mechanical and tissue properties of bones in a murine model of duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is an inherited, lethal disorder characterised by progressive muscle degeneration and associated bone abnormalities. We have previously demonstrated that P2RX7 purinergic receptors contribute to the pathogenesis of DMD, and found that P2RX7 ablation alleviated the severity of the disease. In this work we have used a dystrophic mdx mouse crossed with the global P2RX7 receptor to generate a knockout mouse (mdx/P2X7^-/-), and compared its morphometric, mechanical and tissue properties against those of mdx, as well as the wild type (WT) and the P2RX7 knockout (P2X7^-/-). Micro-computed tomography (µCT), three-point bending testing, scanning electron microscopy (SEM) and nano-indentation were utilised in the study. The bones were analysed at approximately 4 weeks of age to examine the impact of P2RX7 ablation on the bone properties during the acute disease phase, before muscle wasting is fully developed. The results show that P2RX7 purinoceptor ablation has produced improvement or significant improvement in some of the morphological, the mechanical and the tissue properties of the dystrophic bones examined. Specifically, although the ablation produced smaller bones with significantly lower total cross-section area (Tt.Ar) and Second Moment of Area (SMA), significantly higher cortical bone area (Ct.Ar), cortical area fraction (Ct.Ar/Tt.Ar) and trabecular bone volume fraction (BV/TV) are found in the mdx/P2X7^-/- mice than in any other types. Further, the mdx/P2X7^-/- bones have relatively higher average flexural strength, work-to-fracture and significantly higher strain to failure compared with those of mdx, suggesting greater resistance to fracture. Indentation modulus, elasticity and creep are also significantly improved in the knockout cortical bones over those of mdx. These findings seem to suggest that specific pharmacological blockade of P2RX7 may improve dystrophic bones, with a potential for therapeutic application in the treatment of the disease.

Tissue-Level Mechanical Properties of Bone Contributing to Fracture Risk

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

Quantification of Age-Related Tissue-Level Failure Strains of Rat Femoral Cortical Bones Using an Approach Combining Macrocompressive Test and Microfinite Element Analysis

Bone mechanical properties vary with age; meanwhile, a close relationship exists among bone mechanical properties at different levels. Therefore, conducting multilevel analyses for bone structures with different ages are necessary to elucidate the effects of aging on bone mechanical properties at different levels. In this study, an approach that combined microfinite element (micro-FE) analysis and macrocompressive test was established to simulate the failure of male rat femoral cortical bone. Micro-FE analyses were primarily performed for rat cortical bones with different ages to simulate their failure processes under compressive load. Tissue-level failure strains in tension and compression of these cortical bones were then back-calculated by fitting the experimental stress–strain curves. Thus, tissue-level failure strains of rat femoral cortical bones with different ages were quantified. The tissue-level failure strain exhibited a biphasic behavior with age: in the period of skeletal maturity (1–7 months of age), the failure strain gradually increased; when the rat exceeded 7 months of age, the failure strain sharply decreased. In the period of skeletal maturity, both the macro- and tissue-levels mechanical properties showed a large promotion. In the period of skeletal aging (9–15 months of age), the tissue-level mechanical properties sharply deteriorated; however, the macromechanical properties only slightly deteriorated. The age-related changes in tissue-level failure strain were revealed through the analysis of male rat femoral cortical bones with different ages, which provided a theoretical basis to understand the relationship between rat cortical bone mechanical properties at macro- and tissue-levels and decrease of bone strength with age.

Organ and tissue level properties are more sensitive to age than osteocyte lacunar characteristics in rat cortical bone

Modeling and remodeling induce significant changes of bone structure and mechanical properties with age. Therefore, it is important to gain knowledge of the processes taking place in bone over time. The rat is a widely used animal model, where much data has been accumulated on age-related changes of bone on the organ and tissue level, whereas features on the nano- and micrometer scale are much less explored. We investigated the age-related development of organ and tissue level bone properties such as bone volume, bone mineral density, and load to fracture and correlated these with osteocyte lacunar properties in rat cortical bone. Femora of 14 to 42-week-old female Wistar rats were investigated using multiple complementary techniques including X-ray micro-computed tomography and biomechanical testing. The body weight, femoral length, aBMD, load to fracture, tissue volume, bone volume, and tissue density were found to increase rapidly with age at 14–30 weeks. At the age of 30–42 weeks, the growth rate appeared to decrease. However, no accompanying changes were found in osteocyte lacunar properties such as lacunar volume, ellipsoidal radii, lacunar stretch, lacunar oblateness, or lacunar orientation with animal age. Hence, the evolution of organ and tissue level properties with age in rat cortical bone is not accompanied by related changes in osteocyte lacunar properties. This suggests that bone microstructure and bone matrix material properties and not the geometric properties of the osteocyte lacunar network are main determinants of the properties of the bone on larger length scales.