Long-term ovariectomy decreases ovine compact bone viscoelasticity

The Effect of Omega-9 on Bone Viscoelasticity and Strength in an Ovariectomized Diet-Fed Murine Model

Few studies have investigated the effect of a monosaturated diet high in ω-9 on osteoporosis. We hypothesized that omega-9 (ω-9) protects ovariectomized (OVX) mice from a decline in bone microarchitecture, tissue loss, and mechanical strength, thereby serving as a modifiable dietary intervention against osteoporotic deterioration. Female C57BL/6J mice were assigned to sham-ovariectomy, ovariectomy, or ovariectomy + estradiol treatment prior to switching their feed to a diet high in ω-9 for 12 weeks. Tibiae were evaluated using DMA, 3-point-bending, histomorphometry, and microCT. A significant decrease in lean mass (p = 0.05), tibial area (p = 0.009), and cross-sectional moment of inertia (p = 0.028) was measured in OVX mice compared to the control. A trend was seen where OVX bone displayed increased elastic modulus, ductility, storage modulus, and loss modulus, suggesting the ω-9 diet paradoxically increased both stiffness and viscosity. This implies beneficial alterations on the macro-structural, and micro-tissue level in OVX bone, potentially decreasing the fracture risk. Supporting this, no significant differences in ultimate, fracture, and yield stresses were measured. A diet high in ω-9 did not prevent microarchitectural deterioration, nevertheless, healthy tibial strength and resistance to fracture was maintained via mechanisms independent of bone structure/shape. Further investigation of ω-9 as a therapeutic in osteoporosis is warranted.

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

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

Effects of extracellular matrix viscoelasticity on cellular behaviour

Substantial research over the past two decades has established that extracellular matrix (ECM) elasticity, or stiffness, affects fundamental cellular processes, including spreading, growth, proliferation, migration, differentiation and organoid formation. Linearly elastic polyacrylamide hydrogels and polydimethylsiloxane (PDMS) elastomers coated with ECM proteins are widely used to assess the role of stiffness, and results from such experiments are often assumed to reproduce the effect of the mechanical environment experienced by cells in vivo. However, tissues and ECMs are not linearly elastic materials-they exhibit far more complex mechanical behaviours, including viscoelasticity (a time-dependent response to loading or deformation), as well as mechanical plasticity and nonlinear elasticity. Here we review the complex mechanical behaviours of tissues and ECMs, discuss the effect of ECM viscoelasticity on cells, and describe the potential use of viscoelastic biomaterials in regenerative medicine. Recent work has revealed that matrix viscoelasticity regulates these same fundamental cell processes, and can promote behaviours that are not observed with elastic hydrogels in both two- and three-dimensional culture microenvironments. These findings have provided insights into cell-matrix interactions and how these interactions differentially modulate mechano-sensitive molecular pathways in cells. Moreover, these results suggest design guidelines for the next generation of biomaterials, with the goal of matching tissue and ECM mechanics for in vitro tissue models and applications in regenerative medicine.

Genetic engineering a large animal model of human hypophosphatasia in sheep

The availability of tools to accurately replicate the clinical phenotype of rare human diseases is a key step toward improved understanding of disease progression and the development of more effective therapeutics. We successfully generated the first large animal model of a rare human bone disease, hypophosphatasia (HPP) using CRISPR/Cas9 to introduce a single point mutation in the tissue nonspecific alkaline phosphatase (TNSALP) gene (ALPL) (1077 C > G) in sheep. HPP is a rare inherited disorder of mineral metabolism that affects bone and tooth development, and is associated with muscle weakness. Compared to wild-type (WT) controls, HPP sheep have reduced serum alkaline phosphatase activity, decreased tail vertebral bone size, and metaphyseal flaring, consistent with the mineralization deficits observed in human HPP patients. Computed tomography revealed short roots and thin dentin in incisors, and reduced mandibular bone in HPP vs. WT sheep, accurately replicating odonto-HPP. Skeletal muscle biopsies revealed aberrant fiber size and disorganized mitochondrial cristae structure in HPP vs. WT sheep. These genetically engineered sheep accurately phenocopy human HPP and provide a novel large animal platform for the longitudinal study of HPP progression, as well as other rare human bone diseases.

Glucocorticoids affect bone mineral density and bone remodelling in OVX sheep: A pilot study

The aim of this study was to validate the combination of ovariectomy and glucocorticoid treatment in sheep as a large animal model for osteoporosis by measuring the concentration of specific biomarkers in the blood of the sheep and measuring bone loss over five months. Aged Merino ewes were randomly allocated into four groups: control, ovariectomy (OVX), and two OVX groups receiving glucocorticoids-one group once-monthly for five months (OVXG), and the other for two months followed by no treatment for three months (OVXG2). Parameters measured were biochemical markers of bone turnover, areal bone mineral density, volumetric bone mineral density, and total and trabecular bone parameters. Ovariectomy increased the concentrations of bone resorption marker C-terminal telopeptides of type 1 collagen (CTx-1) and bone turnover marker serum osteocalcin (OC) concentrations in the OVX group compared to control sheep. The combination of ovariectomy and glucocorticoid treatment increased the concentrations of CTx-1 and decreased serum OC concentrations in the OVXG group compared to OVXG2. Femur and lumbar spine bone density were lower in experimentally treated groups when compared with the control group. Total and trabecular _vBMD in the proximal tibia were significantly lower in the treatment groups when compared with the control group. A significant negative correlation between femoral bone density and CTx-1 was found. The results of this study suggest that the combination of OVX and glucocorticoids induces bone loss in a short period of time in sheep.

Dynamic nanomechanical behaviour of healthy and OI human cortical bone

Viscoelasticity of bone has been of interest for many years because this time-dependent mechanical property relates to the fracture risk of bone under dynamic loading. Several factors have been claimed to contribute to this property including the nature of different constituents of bone and their interactions, as well as moisture content. In the present study, intact normal human cortical bone was demineralised, and molecular structures were identified using infrared spectroscopy. Osteogenesis imperfecta (OI) human cortical bone was also selected for comparison because OI bone has severe defects in collagen molecules, while its mineral phase is almost identical to that of normal bone. The dynamic nanomechanical behaviours of the intact, demineralised and OI human cortical bone specimens were examined using dynamic nanoindentation. Loss tangent, tan δ, was considered as a measure of the degree of the viscoelastic response. Variable dynamic load tests show that the viscoelastic responses of all bone specimens increase with frequency. With demineralisation, bone specimens show greater viscoelastic response than intact specimens. OI bone shows similar viscoelastic response as normal bone. Results suggest that the viscoelasticity of bone is mostly attributable to the mineral phase. The present study adds to the understanding of the viscoelastic response of bone material. In addition, the dynamic mechanical properties of OI bone are firstly reported here.

Long-term effects of ovariectomy on the properties of bone in goats

Large animal models of osteoporosis are essential for osteoporosis research. However, the time required to establish an accurate osteoporosis model is unknown. Therefore, the aim of the present study was to establish a large animal model of osteoporosis in goats. In total, 14 Chinese goats were divided into an ovariectomized (OVX, n=7) or sham-operated (SHAM, n=7) group. Vertebral bodies were used to measure the bone mineral density (BMD) prior to the ovariectomy and at 24 months after the ovariectomy. In addition, the BMD of the femoral neck, femoral diaphysis and tibial diaphysis were measured 24 months postoperatively. Bone samples from the vertebral body, femoral head and femoral neck were scanned by micro-computed tomography (CT) to visualize the trabecular and cortical microstructure. Furthermore, the vertebral body, femoral head, femoral neck and tibial diaphysis were analyzed for mechanical strength. The BMD of vertebral body of the OVX group decreased significantly (P<0.01) at 24 months after the ovariectomy when compared with the baseline measurements. Micro-CT scans of the vertebral body revealed that the bone volume fraction, trabecular number, trabecular thickness and the degree of anisotropy decreased by 37.1, 36.7, 10.5 and 16.5%, respectively (P<0.01) in the OVX group when compared with the SHAM group. Additionally, the specific bone surface and trabecular spacing significantly increased by 37.7 and 62%, respectively in the OVX group (P<0.001). Cortical bone porosity in the vertebral body and femoral neck was greater in the OVX group when compared with the SHAM group (P<0.05). In addition, mechanical testing revealed a statistically significant difference between the vertebral bodies of the OVX group and the SHAM group. In conclusion, the present study demonstrated that an ovariectomy was able to induce significant osteoporosis and deterioration of mechanical properties in the bones of goats.

Positive alterations of viscoelastic and geometric properties in ovariectomized rat femurs with concurrent administration of ibandronate and PTH

Besides bone mineral density (BMD), structural and nano-level viscoelastic properties of bone are also crucial determinants of bone strength. However, treatment induced viscosity changes in osteoporotic bone have seldom been characterized. In this study, the effects of anabolic, antiresorptive and concurrent treatments on ovariectomized rat bones were thoroughly analyzed using multiple bone strength parameters. A total of 52 female Sprague-Dawley rats of 3 months age were divided into 5 groups and subjected to sham (SHM group) or ovariectomy surgery (OVX, PTH, IBN and COM groups). Weekly low-dose parathyroid hormone (PTH) and/or ibandronate or its vehicle was administered subcutaneously to the respective groups starting from 4th week post-surgery. Four rats per group were euthanized every 4 weeks and their femurs were harvested. The BMD, micro-architectural parameters, cortical bone geometry and viscoelastic parameters were measured at the distal femoral metaphysis. Our results showed that PTH, ibandronate or its concurrent treatment can effectively reverse ovariectomy induced deteriorations in both trabecular and cortical bone. Different drugs had selective effects especially in preserving geometric and viscoelastic properties of the bone. The concurrent administration of PTH and ibandronate was shown to offer an added advantage in preserving mean BMD and had a positive effect on cortical bone geometry, resulting from an increased periosteal formation and a decreased endocortical resorption. Viscosity (η) was prominently restored in combined treatment group. It is in accordance with an observed denser alignment of collagen fibers and hydroxyapatite crystal matrix with fewer pores, which may play an important role in hindering fracture propagation.

Estrogen depletion results in nanoscale morphology changes in dermal collagen

Tissue cryo-sectioning combined with atomic force microscopy imaging reveals that the nanoscale morphology of dermal collagen fibrils, quantified using the metric of D-periodic spacing, changes under the condition of estrogen depletion. Specifically, a new subpopulation of fibrils with D-spacings in the region between 56 and 59 nm is present 2 years following ovariectomy in ovine dermal samples. In addition, the overall width of the distribution, both values above and below the mean, was found to be increased. The change in width due to an increase in lower values of D-spacings was previously reported for ovine bone; however, this report demonstrates that the effect is also present in non-mineralized collagen fibrils. A nonparametric Kolmogorov-Smirnov test of the cumulative density function indicates a statistical difference in the sham and OVX D-spacing distributions (P<0.01).

Intrinsic material properties of trabecular bone by nanoindentation testing of biopsies taken from healthy women before and after menopause

Postmenopausal osteoporosis in women is characterized by an increase in bone fragility and risk of fracture. In addition to transmenopausal decline in three-dimensional trabecular bone architecture, changes in intrinsic material properties (local stiffness, damping, and hardness) may contribute to increased bone fragility. In this study, nanoindentation was used to quantify transmenopausal changes in the intrinsic properties of trabecular bone. Paired transilial biopsy specimens were used from a previously reported study in which bone biopsies were obtained from women prior to menopause (premenopausal, age 49.0 ± 1.9) and at 12 months past the last menstrual period (postmenopausal, age 54.6 ± 2.2). Elastic and viscoelastic material properties of the trabecular bone were measured using quasi-static and dynamic nanoindentation techniques, respectively. Paired Student's t tests (n = 15) were performed to assess the significance of the measured intrinsic properties. Trabecular bone microarchitecture is compromised in postmenopausal women, and although this loss is associated with a trend toward reduction in some intrinsic properties (storage modulus), we found no statistically significant changes in bone intrinsic properties between healthy pre- and postmenopausal biopsies in the quasi-static results and frequency-averaged dynamic results.

Rabbit cortical bone tissue increases its elastic stiffness but becomes less viscoelastic with age

Bone is dynamic tissue undergoing changes in its composition, structure and functional properties during growth. It has been proposed that especially changes in the collagen phase of bone are responsible for making the bone more fragile, and potentially less viscoelastic with age. Hence, robust methods to measure viscoelasticitiy are needed. This study aimed to characterize the development of the elastic and viscoelastic mechanical properties of rabbit bone during maturation and growth, as assessed by nanoindentation. The humeri from female New Zealand white rabbits of varying age (newborn, 11 days, 4 weeks, 3 and 6 months old, n=8 per group) were investigated. Mid-diaphyseal cortical bone samples were cut, dehydrated, embedded and polished. Nanoindentation probing, semi-dynamic testing with a frequency of 20 Hz and creep with a dwell time of 60 s were performed under load control to quantify the elastic and the time-dependent viscoelastic mechanical properties of bone. The elastic moduli were evaluated with all three methods and the viscoelastic parameters were assessed using the phase-shift and the creep time constant. The elastic stiffness of bone increased significantly with each consecutive age group, from 11 days to 6 months of age, based on the reduced modulus from the indentation probing, the storage modulus from the semi-dynamic test, and the first elastic parameter from the creep test. These elastic parameters correlated significantly (R=0.88-0.94, p<0.01). The values of viscoelastic parameters, the phase-shift and time creep constant, decreased significantly with age. The viscous properties determined by the creep and the semi-dynamic testing correlated significantly (R=0.90, p<0.01), however, no correlation was found between the phase-shift and the creep time constant. Additionally, the present results showed specific associations with tissue composition, as measured with Fourier Transform Infrared spectroscopy of the same samples. In summary, the present results reveal significant changes in material properties of rabbit cortical bone with age. The elastic modulus of bone tissue increased by approximately 60%, whereas the viscoelastic parameters decreased by 10% to 25% during the first 6 months of the rabbit's life. Together, this indicates significant structural and functional maturation of the bone matrix during growth of the rabbit.

Precision of nanoindentation protocols for measurement of viscoelasticity in cortical and trabecular bone

Nanoindentation has recently gained attention as a characterization technique for mechanical properties of biological tissues, such as bone, on the sub-micron level. However, optimal methods to characterize viscoelastic properties of bones are yet to be established. This study aimed to compare the time-dependent viscoelastic properties of bone tissue obtained with different nanoindentation methods. Bovine cortical and trabecular bone samples (n=8) from the distal femur and proximal tibia were dehydrated, embedded and polished. The material properties determined using nanoindentation were hardness and reduced modulus, as well as time-dependent parameters based on creep, loading-rate, dissipated energy and semi-dynamic testing under load control. Each loading protocol was repeated 160 times and the reproducibility was assessed based on the coefficient of variation (CV). Additionally, three well-characterized polymers were tested and CV values were calculated for reference. The employed methods were able to characterize time-dependent viscoelastic properties of bone. However, their reproducibility varied highly (CV 9-40%). The creep constant increased with increasing dwell time. The reproducibility was best with a 30s creep period (CV 18%). The dissipated energy was stable after three repeated load cycles, and the reproducibility improved with each cycle (CV 23%). The viscoelastic properties determined with semi-dynamic test increased with increase in frequency. These measurements were most reproducible at high frequencies (CV 9-10%). Our results indicate that several methods are feasible for the determination of viscoelastic properties of bone material. The high frequency semi-dynamic test showed the highest precision within the tested nanoindentation protocols.

The effect of yield damage on the viscoelastic properties of cortical bone tissue as measured by dynamic mechanical analysis

We have previously shown, using Dynamic Mechanical Analysis (DMA), that the presence of a defect in cortical bone tissue affects the apparent viscoelastic properties of that bone. However, mechanically induced damage is more complex than a machined defect making it difficult to predict its effect on bone viscoelasticity. We performed DMA measurements before and after introduction of yield damage into cortical bone beams from sheep radii. The specimens were placed in a DMA machine and baseline measurements of storage modulus (E1) and loss factor (tandelta) were performed using a 3-point bending configuration for a frequency range of 1-10 Hz. Measurements were done in all four bending directions (cranial, caudal, medial, and lateral) in random order. After subjecting the specimens to monotonic yield damage in a servohydraulic testing machine with the load applied to the cranial surface, oscillatory tests were repeated. To supplement results from the current experiment, additional analyses were performed on data from experiments where bone was either cut or fatigue-loaded between viscoelasticity measurements. Introduction of mechanical damage increased tan delta and frequency sensitivity of E1, consistent with the assertion that increased energy dissipation in damaged bone might contribute to its increased resistance to fatigue and fracture.