Age-related changes in the collagen network and toughness of bone

Novel Raman Spectroscopic Biomarkers Indicate That Postyield Damage Denatures Bone's Collagen

Raman spectroscopy has become a powerful tool in the assessment of bone quality. However, the use of Raman spectroscopy to assess collagen quality in bone is less established than mineral quality. Because postyield mechanical properties of bone are mostly determined by collagen rather than the mineral phase, it is essential to identify new spectroscopic biomarkers that help infer the status of collagen quality. Amide I and amide III bands are uniquely useful for collagen conformational analysis. Thus, the first aim of this work was to identify the regions of amide bands that are sensitive to thermally induced denaturation. Collagen sheets and bone were thermally denatured to identify spectral measures that change significantly following denaturation. The second aim was to assess whether mechanical damage denatures the collagen phase of bone, as reflected by the molecular spectroscopic biomarkers identified in the first aim. The third aim was to assess the correlation between these new spectroscopic biomarkers and postyield mechanical properties of cortical bone. Our results revealed five peaks whose intensities were sensitive to thermal and mechanical denaturation: ∼1245, ∼1270, and ∼1320 cm(-1) in the amide III band, and ∼1640 and ∼1670 cm(-1) in the amide I band. Four peak intensity ratios derived from these peaks were found to be sensitive to denaturation: 1670/1640, 1320/1454, 1245/1270, and 1245/1454. Among these four spectral biomarkers, only 1670/1640 displayed significant correlation with all postyield mechanical properties. The overall results showed that these peak intensity ratios can be used as novel spectroscopic biomarkers to assess collagen quality and integrity. The changes in these ratios with denaturation may reflect alterations in the collagen secondary structure, specifically a transition from ordered to less-ordered structure. The overall results clearly demonstrate that this new spectral information, specifically the ratio of 1670/1640, can be used to understand the involvement of collagen quality in the fragility of bone. © 2015 American Society for Bone and Mineral Research.

Structural and Ultrastructural Characteristics of Bone-Tendon Junction of the Calcaneal Tendon of Adult and Elderly Wistar Rats

Tendons are transition tissues that transfer the contractile forces generated by the muscles to the bones, allowing movement. The region where the tendon attaches to the bone is called bone-tendon junction or enthesis and may be classified as fibrous or fibrocartilaginous. This study aims to analyze the collagen fibers and the cells present in the bone-tendon junction using light microscopy and ultrastructural techniques as scanning electron microscopy and transmission electron microscopy. Forty male Wistar rats were used in the experiment, being 20 adult rats at 4 months-old and 20 elderly rats at 20 months-old. The hind limbs of the rats were removed, dissected and prepared to light microscopy, transmission electron microscopy and scanning electron microscopy. The aging process showed changes in the collagen fibrils, with a predominance of type III fibers in the elderly group, in addition to a decrease in the amount of the fibrocartilage cells, fewer and shorter cytoplasmic processes and a decreased synthetic capacity due to degradation of the organelles involved in synthesis.

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.

On the importance of aging to the crack growth resistance of human enamel

With improvements in oral health and an overall increase in quality of life, the percentage of fully or largely dentate seniors is increasing. Understanding the effects of aging on the mechanical properties of teeth is essential to the maintenance of lifelong oral health. In this investigation the effects of aging on the fracture toughness of human enamel were evaluated from incremental crack growth experiments performed on tissue of donor teeth representing "young" (17 ⩽ age ⩽ 25) and "old" (age ⩾ 55) age groups. Results showed that the old enamel exhibited significantly lower resistance to fracture than that of the young tissue in two orthogonal directions of crack growth. For crack growth transverse to the enamel rods, the fracture toughness of the old enamel (0.37 ± 0.15 MPa m(0.5)) was nearly 70% lower than that of tissue from the young teeth (1.23 ± 0.20 MPa m(0.5)). Based on results from a mechanistic analysis of crack growth, the reduction in fracture resistance is attributed to a decrease in the degree of extrinsic toughening. The practice of restorative dentistry should account for these changes in tooth tissues in the treatment of senior patients.

STATEMENT OF SIGNIFICANCE

The mechanical behavior of enamel has been studied for over 3 decades. Due to the limited volume of tissue available for evaluation, past work has been largely based on indentation methods. In this investigation we have evaluated the resistance to fracture of human enamel using a conventional fracture mechanics approach and incremental crack growth. We compared the fracture resistance of cuspal enamel obtained from the teeth of representative "young" and "old" donor groups. Our results show that there is a substantial reduction in the resistance to fracture with age, that it is anisotropic, and that the degradation is more severe than that which occurs to dentin. As such, we feel this work is a significant contribution to the field.

Material heterogeneity in cancellous bone promotes deformation recovery after mechanical failure

Many natural structures use a foam core and solid outer shell to achieve high strength and stiffness with relatively small amounts of mass. Biological foams, however, must also resist crack growth. The process of crack propagation within the struts of a foam is not well understood and is complicated by the foam microstructure. We demonstrate that in cancellous bone, the foam-like component of whole bones, damage propagation during cyclic loading is dictated not by local tissue stresses but by heterogeneity of material properties associated with increased ductility of strut surfaces. The increase in surface ductility is unexpected because it is the opposite pattern generated by surface treatments to increase fatigue life in man-made materials, which often result in reduced surface ductility. We show that the more ductile surfaces of cancellous bone are a result of reduced accumulation of advanced glycation end products compared with the strut interior. Damage is therefore likely to accumulate in strut centers making cancellous bone more tolerant of stress concentrations at strut surfaces. Hence, the structure is able to recover more deformation after failure and return to a closer approximation of its original shape. Increased recovery of deformation is a passive mechanism seen in biology for setting a broken bone that allows for a better approximation of initial shape during healing processes and is likely the most important mechanical function. Our findings suggest a previously unidentified biomimetic design strategy in which tissue level material heterogeneity in foams can be used to improve deformation recovery after failure.

Modelling of bone fracture and strength at different length scales: a review

In this paper, we review analytical and computational models of bone fracture and strength. Bone fracture is a complex phenomenon due to the composite, inhomogeneous and hierarchical structure of bone. First, we briefly summarize the hierarchical structure of bone, spanning from the nanoscale, sub-microscale, microscale, mesoscale to the macroscale, and discuss experimental observations on failure mechanisms in bone at these scales. Then, we highlight representative analytical and computational models of bone fracture and strength at different length scales and discuss the main findings in the context of experiments. We conclude by summarizing the challenges in modelling of bone fracture and strength and list open topics for scientific exploration. Modelling of bone, accounting for different scales, provides new and needed insights into the fracture and strength of bone, which, in turn, can lead to improved diagnostic tools and treatments of bone diseases such as osteoporosis.

Age-related changes in the fracture resistance of male Fischer F344 rat bone

In addition to the loss in bone volume that occurs with age, there is a decline in material properties. To test new therapies or diagnostic tools that target such properties as material strength and toughness, a pre-clinical model of aging would be useful in which changes in bone are similar to those that occur with aging in humans. Toward that end, we hypothesized that similar to human bone, the estimated toughness and material strength of cortical bone at the apparent-level decreases with age in the male Fischer F344 rat. In addition, we tested whether the known decline in trabecular architecture in rats translated to an age-related decrease in vertebra (VB) strength and whether non-X-ray techniques could quantify tissue changes at micron and sub-micron length scales. Bones were harvested from 6-, 12-, and 24-month (mo.) old rats (n=12 per age). Despite a loss in trabecular bone with age, VB compressive strength was similar among the age groups. Similarly, whole-bone strength (peak force) in bending was maintained (femur) or increased (radius) with aging. There was though an age-related decrease in post-yield toughness (radius) and bending strength (femur). The ability to resist crack initiation was actually higher for the 12-mo. and 24-mo. than for 6-mo. rats (notch femur), but the estimated work to propagate the crack was less for the aged bone. For the femur diaphysis region, porosity increased while bound water decreased with age. For the radius diaphysis, there was an age-related increase in non-enzymatic and mature enzymatic collagen crosslinks. Raman spectroscopy analysis of embedded cross-sections of the tibia mid-shaft detected an increase in carbonate subsitution with advanced aging for both inner and outer tissue.

Effect of type 2 diabetes-related non-enzymatic glycation on bone biomechanical properties

There is clear evidence that patients with type 2 diabetes mellitus (T2D) have increased fracture risk, despite having high bone mineral density (BMD) and body mass index (BMI). Thus, poor bone quality has been implicated as a mechanism contributing to diabetic skeletal fragility. Poor bone quality in T2D may result from the accumulation of advanced glycation end-products (AGEs), which are post-translational modifications of collagen resulting from a spontaneous reaction between extracellular sugars and amino acid residues on collagen fibers. This review discusses what is known and what is not known regarding AGE accumulation and diabetic skeletal fragility, examining evidence from in vitro experiments to simulate a diabetic state, ex vivo studies in normal and diabetic human bone, and diabetic animal models. Key findings in the literature are that AGEs increase with age, affect bone cell behavior, and are altered with changes in bone turnover. Further, they affect bone mechanical properties and microdamage accumulation, and can be inhibited in vitro by various inhibitors and breakers (e.g. aminoguanidine, N-Phenacylthiazolium Bromide, vitamin B6). While a few studies report higher AGEs in diabetic animal models, there is little evidence of AGE accumulation in bone from diabetic patients. There are several limitations and inconsistencies in the literature that should be noted and studied in greater depth including understanding the discrepancies between glycation levels across reported studies, clarifying differences in AGEs in cortical versus cancellous bone, and improving the very limited data available regarding glycation content in diabetic animal and human bone, and its corresponding effect on bone material properties in T2D.

Fabrication of hydrogels with elasticity changed by alkaline phosphatase for stem cell culture

The objective of this study is to design hydrogels whose elasticity can be changed by alkaline phosphatase (ALP) in cell culture and evaluate the effect of hydrogel elasticity on an osteogenic gene expression of cells. Hydrogels were prepared by the radical polymerization of acrylamide (AAm), N,N'-methylenebisacrylamide (BIS), and Phosmer™M containing phosphate groups (PE-PAAm hydrogels). The storage modulus of PE-PAAm hydrogels prepared was changed by the preparation conditions. When human mesenchymal stem cells (hMSC) were cultured on the ALP-responsive PE-PAAm hydrogels in the presence or absence of ALP, the morphology of hMSC was observed and one of the osteogenic differentiation markers, Runx2, was evaluated. By ALP addition into the culture medium, the morphology of hMSC was changed into an elongated shape without cell damage. ALP addition modified the level of Runx2 gene expression, which was influenced by the modulus of PE-PAAm hydrogels. It is concluded that the elasticity change of hydrogel substrates in cell culture had an influence on the Runx2 gene expression of hMSC.

STATEMENT OF SIGNIFICANCE

Stem cells sense the surface elasticity of culture substrates, and their differentiation fate is biologically modified by substrate properties. Most of experiments have been performed in static conditions during cell culture, while the in vivo microenvironment is dynamically changed. In this study, we established to design an enzyme-responsive hydrogel whose elasticity can be changed by alkaline phosphatase (ALP) in cell culture to mimic in vivo conditions. As a result, the cells were deformed and the gene expression level of an osteogenic maker, Runx2, was modified by ALP treatment. This is the novel report describing to demonstrate that the dynamic alteration of hydrogel substrate elasticity could modulate the osteoblastic gene expression of human MSC in vitro.

Effects of 18-month treatment with bazedoxifene on enzymatic immature and mature cross-links and non-enzymatic advanced glycation end products, mineralization, and trabecular microarchitecture of vertebra in ovariectomized monkeys

Bazedoxifene (BZA) is used for the treatment of post-menopausal osteoporosis. To elucidate changes in collagen, mineralization, and structural properties and their relationship to bone strength after treatment with BZA in ovariectomized (OVX) monkeys, the levels of collagen and enzymatic immature, mature, and non-enzymatic cross-links were simultaneously examined, as well as trabecular architecture and mineralization of vertebrae. Adult female cynomolgus monkeys were divided into 4 groups (n=18 each) as follows: Sham group, OVX group, and OVX monkeys given either 0.2 or 0.5mg/kg BZA for 18 months. Collagen concentration, enzymatic and non-enzymatic pentosidine cross-links, whole fluorescent advanced glycation end products (AGEs), trabecular architecture, mineralization, and cancellous bone strength of vertebrae were analyzed. The levels of enzymatic immature and mature cross-links, bone volume (BV/TV), and trabecular thickness (Tb.Th) in BZA-treated groups were significantly higher than those in the OVX control group. In contrast, the trabecular bone pattern factor (TBPf), the structure model index (SMI), the enzymatic cross-link ratio, and the levels of pentosidine and whole AGEs in BZA-treated groups were significantly lower than those in the OVX control group. Stepwise logistic regression analysis revealed that BV/TV, Tb.Th, TbPf, and pentosidine or whole AGEs independently affected ultimate load (model R(2)=0.748, p<0.001) and breaking energy (model R(2)=0.702). Stiffness was affected by Tb.Th, enzymatic immature cross-link levels and their ratio (model R(2)=0.400). Treatment with BZA prevented OVX-induced deterioration in the total levels of immature enzymatic cross-links and AGEs accumulation and structural properties such as BV/TV, Tb.Th, and TbPf, which contribute significantly to vertebral cancellous bone strength.

Evaluation of bound and pore water in cortical bone using ultrashort-TE MRI

Bone water exists in different states with the majority bound to the organic matrix and to mineral, and a smaller fraction in 'free' form in the pores of cortical bone. In this study, we aimed to develop and evaluate ultrashort-TE (UTE) MRI techniques for the assessment of T2*, T1 and concentration of collagen-bound and pore water in cortical bone using a 3-T clinical whole-body scanner. UTE MRI, together with an isotope study using tritiated and distilled water (THO-H2O) exchange, as well as gravimetric analysis, were performed on ten sectioned bovine bone samples. In addition, 32 human cortical bone samples were prepared for comparison between the pore water concentration measured with UTE MRI and the cortical porosity derived from micro-computed tomography (μCT). A short T2* of 0.27 ± 0.03 ms and T1 of 116 ± 6 ms were observed for collagen-bound water in bovine bone. A longer T2* of 1.84 ± 0.52 ms and T1 of 527 ± 28 ms were observed for pore water in bovine bone. UTE MRI measurements showed a pore water concentration of 4.7-5.3% by volume and collagen-bound water concentration of 15.7-17.9% in bovine bone. THO-H2O exchange studies showed a pore water concentration of 5.9 ± 0.6% and collagen-bound water concentration of 18.1 ± 2.1% in bovine bone. Gravimetric analysis showed a pore water concentration of 6.3 ± 0.8% and collagen-bound water concentration of 19.2 ± 3.6% in bovine bone. A mineral water concentration of 9.5 ± 0.6% was derived in bovine bone with the THO-H2O exchange study. UTE-measured pore water concentration is highly correlated (R(2) = 0.72, p < 0.0001) with μCT porosity in the human cortical bone study. Both bovine and human bone studies suggest that UTE sequences could reliably measure collagen-bound and pore water concentration in cortical bone using a clinical scanner.

Quantitative description of collagen fibre network on trabecular bone surfaces based on AFM imaging

The collagen fibre network is an important part of extracellular matrix (ECM) on trabecular bone surface. The geometry features of the network can provide us insights into its physical and physiological properties. However, previous researches have not focused on the geometry and the quantitative description of the collagen fibre network on trabecular bone surface. In this study,we developed a procedure to quantitatively describe the network and verified the validity of the procedure. The experiment proceeds as follow. Atomic force microscopy (AFM) was used to acquire submicron resolution images of the trabecular surface. Then, an image analysing procedure was built to extract important parameters, including, fibre orientation, fibre density, fibre width, fibre crossing numbers, the number of holes formed by fibre s, and the area of holes from AFM images. In order to verify the validity of the parameters extracted by image analysing methods, we adopted two other methods, which are statistical geometry model and computer simulation, to calculate those same parameters and check the consistency of the three methods' results. Statistical tests indicate that there is no significant difference between three groups. We conclude that, (a) the ECM on trabecular surface mainly consists of random collagen fibre network with oriented fibres; (b) our method based on image analysing can be used to characterize quantitative geometry features of the collagen fibre network effectively. This method may provide a basis for quantitative investigating the architecture and function of collagen fibre network.

Elastic modulus of the femoral trochanteric region measured by scanning acoustic microscopy in elderly women

PURPOSE

We have devised a method that can obtain continuous detailed distributions of the elastic modulus along the measurement line in a non-decalcified specimen of human bone tissue. The aim of this study was to determine whether local variations exist in the distribution of mechanical properties within the trochanteric region of the femur of elderly females using a newly developed form of scanning acoustic microscopy (SAM) technology.

METHODS

Human proximal femurs were harvested from seven female cadavers aged between 67 and 88 years at death. Using data collected with SAM, the elastic modulus of cortical and trabecular bone tissue of the lateral and medial trochanter was calculated and statistically analyzed.

RESULTS

The longitudinal and transverse elastic moduli in cortical bone tissue of the lateral trochanter were found to be significantly lower than those of the medial trochanter in all specimens from individuals over age 70 (p < 0.05). Compared to that of the distal region, the longitudinal and transverse elastic moduli of trabecular bone tissue of the proximal region of the lateral trochanter were significantly lower in all specimens from individuals over age 80 (p < 0.05).

CONCLUSION

Our new method allows obtaining detailed distributions of the elastic modulus of bone tissue.

Ratio between mature and immature enzymatic cross-links correlates with post-yield cortical bone behavior: An insight into greenstick fractures of the child fibula

As a determinant of skeletal fragility, the organic matrix is responsible for the post-yield and creep behavior of bone and for its toughness, while the mineral apatite acts on stiffness. Specific to the fibula and ulna in children, greenstick fractures show a plastic in vivo mechanical behavior before bone fracture. During growth, the immature form of collagen enzymatic cross-links gradually decreases, to be replaced by the mature form until adolescence, subsequently remaining constant throughout adult life. However, the link between the cortical bone organic matrix and greenstick fractures in children remains to be explored. Here, we sought to determine: 1) whether plastic bending fractures can occur in vitro, by testing cortical bone samples from children's fibula and 2) whether the post-yield behavior (ωp plastic energy) of cortical bone before fracture is related to total quantity of the collagen matrix, or to the quantity of mature and immature enzymatic cross-links and the quantity of non-enzymatic cross-links. We used a two-step approach; first, a 3-point microbending device tested 22 fibula machined bone samples from 7 children and 3 elderly adults until fracture. Second, biochemical analysis by HPLC was performed on the sample fragments. When pooling two groups of donors, children and elderly adults, results show a rank correlation between total energy dissipated before fracture and age and a linear correlation between plastic energy dissipated before fracture and ratio of immature/mature cross-links. A collagen matrix with more immature cross-links (i.e. a higher immature/mature cross-link ratio) is more likely to plastically deform before fracture. We conclude that this ratio in the sub-nanostructure of the organic matrix in cortical bone from the fibula may go some way towards explaining the variance in post-yield behavior. From a clinical point of view, therefore, our results provide a potential explanation of the presence of greenstick fractures in children.

The fracture mechanics of human bone: influence of disease and treatment

Aging and bone diseases are associated with increased fracture risk. It is therefore pertinent to seek an understanding of the origins of such disease-related deterioration in bone's mechanical properties. The mechanical integrity of bone derives from its hierarchical structure, which in healthy tissue is able to resist complex physiological loading patterns and tolerate damage. Indeed, the mechanisms through which bone derives its mechanical properties make fracture mechanics an ideal framework to study bone's mechanical resistance, where crack-growth resistance curves give a measure of the intrinsic resistance to the initiation of cracks and the extrinsic resistance to the growth of cracks. Recent research on healthy cortical bone has demonstrated how this hierarchical structure can develop intrinsic toughness at the collagen fibril scale mainly through sliding and sacrificial bonding mechanisms that promote plasticity. Furthermore, the bone-matrix structure develops extrinsic toughness at much larger micrometer length-scales, where the structural features are large enough to resist crack growth through crack-tip shielding mechanisms. Although healthy bone tissue can generally resist physiological loading environments, certain conditions such as aging and disease can significantly increase fracture risk. In simple terms, the reduced mechanical integrity originates from alterations to the hierarchical structure. Here, we review how human cortical bone resists fracture in healthy bone and how changes to the bone structure due to aging, osteoporosis, vitamin D deficiency and Paget's disease can affect the mechanical integrity of bone tissue.

Effects of Collagen Crosslinking on Bone Material Properties in Health and Disease

Data have accumulated to show that various types of collagen crosslinking are implicated in the health of individuals, as well as in a number of disease states, such as osteoporosis, diabetes mellitus, chronic kidney disease, inflammatory bowel disease, or in conditions of mild hyperhomocysteinemia, or when glucocorticoid use is indicated. Collagen crosslinking is a posttranslational modification of collagen molecules and plays important roles in tissue differentiation and in the mechanical properties of collagenous tissue. The crosslinking of collagen in the body can form via two mechanisms: one is enzymatic crosslinking and the other is nonenzymatic crosslinking. Lysyl hydroxylases and lysyl oxidases regulate tissue-specific crosslinking patterns and quantities. Enzymatic crosslinks initially form via immature divalent crosslinking, and a portion of them convert into mature trivalent forms such as pyridinoline and pyrrole crosslinks. Nonenzymatic crosslinks form as a result of reactions which create advanced glycation end products (AGEs), such as pentosidine and glucosepane. These types of crosslinks differ in terms of their mechanisms of formation and function. Impaired enzymatic crosslinking and/or an increase of AGEs have been proposed as a major cause of bone fragility associated with aging and numerous disease states. This review focuses on the effects of collagen crosslinking on bone material properties in health and disease.

A direct role of collagen glycation in bone fracture

Non-enzymatic glycation (NEG) is an age-related process accelerated by diseases like diabetes, and causes the accumulation of advanced glycation end-products (AGEs). NEG-mediated modification of bone's organic matrix, principally collagen type-I, has been implicated in impairing skeletal physiology and mechanics. Here, we present evidence, from in vitro and in vivo models, and establish a causal relationship between collagen glycation and alterations in bone fracture at multiple length scales. Through atomic force spectroscopy, we established that NEG impairs collagen's ability to dissipate energy. Mechanical testing of in vitro glycated human bone specimen revealed that AGE accumulation due to NEG dramatically reduces the capacity of organic and mineralized matrix to creep and caused bone to fracture under impact at low levels of strain (3000-5000 μstrain) typically associated with fall. Fracture mechanics tests of NEG modified human cortical bone of varying ages, and their age-matched controls revealed that NEG disrupted microcracking based toughening mechanisms and reduced bone propagation and initiation fracture toughness across all age groups. A comprehensive mechanistic model, based on experimental and modeling data, was developed to explain how NEG and AGEs are causal to, and predictive of bone fragility. Furthermore, fracture mechanics and indentation testing on diabetic mice bones revealed that diabetes mediated NEG severely disrupts bone matrix quality in vivo. Finally, we show that AGEs are predictive of bone quality in aging humans and have diagnostic applications in fracture risk.

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

A macro-micro-nano-multi-level study was conducted to explore age-related structural and mechanical properties of bone, as well as the effects of aging on bone properties. A total of 70 male Wistar rats were used, ranging in the ages of 1, 3, 5, 7, 9, 11, 14, 15, 16, and 17 months (n = 7/age group). After micro-computed tomography (CT) scanning, longitudinal cortical bone specimens with a length of 5mm were cut along the femoral shaft axis from left femur shafts for mechanical testing, and the cross-sectional areas were measured. The macro-mechanical properties obtained in mechanical testing and microarchitecture parameters measured by micro-CT were significantly correlated with the animal age (r(2) = 0.96, p < 0.001). Scanning electron microscopy was used for detecting the microarchitecture features of the fractured surfaces, which exhibited age-related plate-fibrous-mixed fibrous-plate texture, resulting in changes in macro-mechanical properties (r(2) > 0.90, p < 0.001). The mineral phase of the left femoral shaft and head was analyzed by atomic force microscopy. Longitudinal and transverse trabecular bone tissues, as well as longitudinal cortical bone tissue, were used for nanoindentation test, and the chemical composition was evaluated by quantitative chemical analyses. The correlations between mineral content and bone material properties (i.e., elastic properties of the bone tissue and size and roughness of bone mineral grains) were highly significant (r > 0.95, p < 0.001). Multi-level femur morphology, mechanical property, and mineral content were significantly correlated with the animal age. The correlations between bone mineral content and bone material morphological and mechanical properties may partly explain the increase in bone fragility with aging, which will provide a theoretical basis for the investigation of age-related bone properties in clinics.

Identifying Novel Clinical Surrogates to Assess Human Bone Fracture Toughness

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

Immunology of Osteoporosis: A Mini-Review

Osteoporosis is a major cause of fractures and associated morbidity in the aged population. The pathogenesis of osteoporosis is multifactorial; whereas traditional pathophysiological concepts emphasize endocrine mechanisms, it has been recognized that also components of the immune system have a significant impact on bone. Since 2000, when the term 'osteoimmunology' was coined, novel insights into the role of inflammatory cytokines by influencing the fine-tuned balance between bone resorption and bone formation have helped to explain the occurrence of osteoporosis in conjunction with chronic inflammatory reactions. Moreover, the phenomenon of a low-grade, chronic, systemic inflammatory state associated with aging has been defined as 'inflamm-aging' by Claudio Franceschi and has been linked to age-related diseases such as osteoporosis. Given the tight anatomical and physiological coexistence of B cells and the bone-forming units in the bone marrow, a role of B cells in osteoimmunological interactions has long been suspected. Recent findings of B cells as active regulators of the RANK/RANKL/OPG axis, of altered RANKL/OPG production by B cells in HIV-associated bone loss or of a modulated expression of genes linked to B-cell biology in response to estrogen deficiency support this assumption. Furthermore, oxidative stress and the generation of advanced glycation end products have emerged as links between inflammation and bone destruction.

The materials science of collagen

Collagen is the principal biopolymer in the extracellular matrix of both vertebrates and invertebrates. It is produced in specialized cells (fibroblasts) and extracted into the body by a series of intra and extracellular steps. It is prevalent in connective tissues, and the arrangement of collagen determines the mechanical response. In biomineralized materials, its fraction and spatial distribution provide the necessary toughness and anisotropy. We review the structure of collagen, with emphasis on its hierarchical arrangement, and present constitutive equations that describe its mechanical response, classified into three groups: hyperelastic macroscopic models based on strain energy in which strain energy functions are developed; macroscopic mathematical fits with a nonlinear constitutive response; structurally and physically based models where a constitutive equation of a linear elastic material is modified by geometric characteristics. Viscoelasticity is incorporated into the existing constitutive models and the effect of hydration is discussed. We illustrate the importance of collagen with descriptions of its organization and properties in skin, fish scales, and bone, focusing on the findings of our group.

Glucose-dependent insulinotropic polypeptide (GIP) directly affects collagen fibril diameter and collagen cross-linking in osteoblast cultures

Glucose-dependent insulinotropic polypeptide (GIP) is absolutely crucial in order to obtain optimal bone strength and collagen quality. However, as the GIPR is expressed in several tissues other than bone, it is difficult to ascertain whether the observed modifications of collagen maturity, reported in animal studies, were due to direct effects on osteoblasts or indirect through regulation of signals originating from other tissues. The aims of the present study were to investigate whether GIP can directly affect collagen biosynthesis and processing in osteoblast cultures and to decipher which molecular pathways were necessary for such effects. MC3T3-E1 cells were cultured in the presence of GIP ranged between 10 and 100pM. Collagen fibril diameter was investigated by electron microscopy whilst collagen maturity was determined by Fourier transform infra-red microspectroscopy (FTIRM). GIP treatment resulted in dose-dependent increases in lysyl oxidase activity and collagen maturity. Furthermore, GIP treatment shifted the collagen fiber diameter towards lower value but did not significantly affect collagen heterogeneity. GIP acted directly on osteoblasts by activating the adenylyl cyclase-cAMP pathway. This study provides evidences that GIP acts directly on osteoblasts and is capable of improving collagen maturity and fibril diameter.

A Novel Methodology for the Simulation of Athletic Tasks on Cadaveric Knee Joints with Respect to In Vivo Kinematics

Six degree of freedom (6-DOF) robotic manipulators have simulated clinical tests and gait on cadaveric knees to examine knee biomechanics. However, these activities do not necessarily emulate the kinematics and kinetics that lead to anterior cruciate ligament (ACL) rupture. The purpose of this study was to determine the techniques needed to derive reproducible, in vitro simulations from in vivo skin-marker kinematics recorded during simulated athletic tasks. Input of raw, in vivo, skin-marker-derived motion capture kinematics consistently resulted in specimen failure. The protocol described in this study developed an in-depth methodology to adapt in vivo kinematic recordings into 6-DOF knee motion simulations for drop vertical jumps and sidestep cutting. Our simulation method repeatably produced kinetics consistent with vertical ground reaction patterns while preserving specimen integrity. Athletic task simulation represents an advancement that allows investigators to examine ACL-intact and graft biomechanics during motions that generate greater kinetics, and the athletic tasks are more representative of documented cases of ligament rupture. Establishment of baseline functional mechanics within the knee joint during athletic tasks will serve to advance the prevention, repair and rehabilitation of ACL injuries.

The prevention of fragility fractures in diabetic patients

Patients with diabetes mellitus (DM) are at greater risk of fractures mostly due to not only extraskeletal factors, such as propensity to falls, but also to bone quality alteration, which reduces bone strength. In people with DM, insulin deficit and hyperglycemia seem to play a role in determining bone formation alteration by AGE accumulation which directly influences osteoblast activity. Although there are conflicting data in the literature, adequate glycemic control with hypoglycemic treatment may be an important element in preventing bone tissue alterations in both type 1 and type 2 DM. Diabetes status is a predictive of future hip and major osteoporosis fractures independently of BMD and FRAX probability. Attention should be paid to the use of thiazolidinediones, especially in older women, because the direct negative effect on bone could exceed the positive effect of glycemic control. Systematic screening for complications and fall prevention efforts, along with calcium and vitamin D repletion and adequate physical activity, represents the mainstay of fracture prevention in DM patients. All anticatabolic drugs (raloxifene, bisphosphonates, denosumab) seem to be effective in DM patients. On the basis of pathophysiological evidence that suggests low bone formation in DM patients, osteoanabolic therapies such as teriparatide might represent an important therapeutic option for DM patients with severe osteoporosis and/or multiple fractures. The search for better methods for the identification of fragility fracture risk in the growing population of adult and elderly subjects with DM might be considered a clinical priority which could improve the prevention of fracture in DM patients.

Diabetes, diabetic complications, and fracture risk

Diabetes and osteoporosis are both common diseases with increasing prevalences in the aging population. There is increasing evidence corroborating an association between diabetes mellitus and bone. This review will discuss the disease complications of diabetes on the skeleton, highlighting findings from epidemiological, molecular, and imaging studies in animal models and humans. Compared to control subjects, decreased bone mineral density (BMD) has been observed in type 1 diabetes mellitus, while on average, higher BMD has been found in type 2 diabetes; nonetheless, patients with both types of diabetes are seemingly at increased risk of fractures. Conventional diagnostics such as DXA measurements and the current fracture risk assessment tool (FRAX) risk prediction algorithm for estimating risk of osteoporotic fractures are not sufficient in the case of diabetes. A deterioration in bone microarchitecture and an inefficient distribution of bone mass with insufficiency of repair and adaptation mechanisms appear to be factors of relevance. A highly complex and heterogeneous molecular pathophysiology underlies diabetes-related bone disease, involving hormonal, immune, and perhaps genetic pathways. The detrimental effects of chronically elevated glucose levels on bone should be added to the more well-known complications of diabetes.

Treatment with eldecalcitol positively affects mineralization, microdamage, and collagen crosslinks in primate bone

Eldecalcitol (ELD), an active form of vitamin D analog approved for the treatment of osteoporosis in Japan, increases lumbar spine bone mineral density (BMD), suppresses bone turnover markers, and reduces fracture risk in patients with osteoporosis. We have previously reported that treatment with ELD for 6 months improved the mechanical properties of the lumbar spine in ovariectomized (OVX) cynomolgus monkeys. ELD treatment increased lumbar BMD, suppressed bone turnover markers, and reduced histomorphometric parameters of both bone formation and resorption in vertebral trabecular bone. In this study, we elucidated the effects of ELD on bone quality (namely, mineralization, microarchitecture, microdamage, and bone collagen crosslinks) in OVX cynomolgus monkeys in comparison with OVX-vehicle control monkeys. Density fractionation of bone powder prepared from lumbar vertebrae revealed that ELD treatment shifted the distribution profile of bone mineralization to a higher density, and backscattered electron microscopic imaging showed improved trabecular bone connectivity in the ELD-treated groups. Higher doses of ELD more significantly reduced the amount of microdamage compared to OVX-vehicle controls. The fractionated bone powder samples were divided according to their density, and analyzed for collagen crosslinks. Enzymatic crosslinks were higher in both the high-density (≥2.0 mg/mL) and low-density (<2.0 mg/mL) fractions from the ELD-treated groups than in the corresponding fractions in the OVX-vehicle control groups. On the other hand, non-enzymatic crosslinks were lower in both the high- and low-density fractions. These observations indicated that ELD treatment stimulated the enzymatic reaction of collagen crosslinks and bone mineralization, but prevented non-enzymatic reaction of collagen crosslinks and accumulation of bone microdamage. Bone anti-resorptive agents such as bisphosphonates slow down bone remodeling so that bone mineralization, bone microdamage, and non-enzymatic collagen crosslinks all increase. Bone anabolic agents such as parathyroid hormone decrease bone mineralization and bone microdamage by stimulating bone remodeling. ELD did not fit into either category. Histological analysis indicated that the ELD treatment strongly suppressed bone resorption by reducing the number of osteoclasts, while also stimulating focal bone formation without prior bone resorption (bone minimodeling). These bidirectional activities of ELD may account for its unique effects on bone quality.

Association between non-enzymatic glycation, resorption, and microdamage in human tibial cortices

To better understand the association between different components of bone quality, we investigated the relationship among in vivo generated non-enzymatic glycation, resorption, and microdamage. The results showed negative correlation between advanced glycation end-products (AGEs) and resorption independent of age highlighting the interaction between these parameters that may lead to bone fragility.

INTRODUCTION

Changes in the quality of bone material contribute significantly to bone fragility. In order to establish a better understanding of the interaction of the different components of bone quality and their influence on bone fragility, we investigated the relationship between non-enzymatic glycation, resorption, and microdamage generated in vivo in cortical bone using bone specimens from the same donors.

METHODS

Total fluorescent advanced glycation end-products (AGEs) were measured in 96 human cortical bone samples from 83 donors. Resorption pit density, average resorption pit area, and percent resorption area were quantified in samples from 48 common donors with AGE measurements. Linear microcrack density and diffuse damage were measured in 21 common donors with AGE and resorption measurements. Correlation analyses were performed between all measured variables to establish the relationships among them and their variation with age.

RESULTS

We found that average resorption pit area and percent resorption area decreased with increasing AGEs independently of age. Resorption pit density and percent resorption area demonstrated negative age-adjusted correlation with diffuse damage. Furthermore, average resorption pit area, resorption pit density, and percent resorption area were found to decrease significantly with age.

CONCLUSIONS

The current study demonstrated the in vivo interrelationship between the organic constituents, remodeling, and damage formation in cortical bone. In addition to the age-related reduction in resorption, there is a negative correlation between AGEs and resorption independent of age. This inverse relationship indicates that AGEs alter the resorption process and/or accumulate in the tissue as a result of reduced resorption and may lead to bone fragility by adversely affecting fracture resistance through altered bone matrix properties.

Bone fracture toughness and strength correlate with collagen cross-link maturity in a dose-controlled lathyrism mouse model

Collagen cross-linking is altered in many diseases of bone, and enzymatic collagen cross-links are important to bone quality, as evidenced by losses of strength after lysyl oxidase inhibition (lathyrism). We hypothesized that cross-links also contribute directly to bone fracture toughness. A mouse model of lathyrism using subcutaneous injection of up to 500 mg/kg β-aminopropionitrile (BAPN) was developed and characterized (60 animals across 4 dosage groups). Three weeks of 150 or 350 mg/kg BAPN treatment in young, growing mice significantly reduced cortical bone fracture toughness, strength, and pyridinoline cross-link content. Ratios reflecting relative cross-link maturity were positive regressors of fracture toughness (HP/[DHLNL + HLNL] r(2)  = 0.208, p < 0.05; [HP + LP]/[DHNL + HLNL] r(2)  = 0.196, p < 0.1), whereas quantities of mature pyridinoline cross-links were significant positive regressors of tissue strength (lysyl pyridinoline r(2)  = 0.159, p = 0.014; hydroxylysyl pyridinoline r(2)  = 0.112, p < 0.05). Immature and pyrrole cross-links, which were not significantly reduced by BAPN, did not correlate with mechanical properties. The effect of BAPN treatment on mechanical properties was dose specific, with the greatest impact found at the intermediate (350 mg/kg) dose. Calcein labeling was used to define locations of new bone formation, allowing for the identification of regions of normally cross-linked (preexisting) and BAPN-treated (newly formed, cross-link-deficient) bone. Raman spectroscopy revealed spatial differences attributable to relative tissue age and effects of cross-link inhibition. Newly deposited tissues had lower mineral/matrix, carbonate/phosphate, and Amide I cross-link (matrix maturity) ratios compared with preexisting tissues. BAPN treatment did not affect mineral measures but significantly increased the cross-link (matrix maturity) ratio compared with newly formed control tissue. Our study reveals that spatially localized effects of short-term BAPN cross-link inhibition can alter the whole-bone collagen cross-link profile to a measureable degree, and this cross-link profile correlates with bone fracture toughness and strength. Thus, cross-link profile perturbations associated with bone disease may provide insight into bone mechanical quality and fracture risk.

Pathogenesis, management and prevention of atypical femoral fractures

Much attention has been paid to the relationship between atypical femoral fractures (AFF) and use of bisphosphonates (BPs). While a significant cause-effect relationship was not established in earlier studies, more recent data shows a growing relationship between AFF and BPs use. The definition of an 'AFF' has also undergone significant changes. This review briefly summarizes the definition, pathogenesis, and management of AFF.

The application of backscattered ultrasound and photoacoustic signals for assessment of bone collagen and mineral contents

BACKGROUND

This study examines the backscattered ultrasound (US) and back-propagating photoacoustic (PA) signals from trabecular bones, and their variations with reduction in bone minerals and collagen content. While the collagen status is directly related to the strength of the bone, diagnosis of its condition using US remains a challenge.

METHODS

For both PA and US methods, coded-excitation signals and matched filtering were utilized to provide high sensitivity of the detected signal. The optical source was a 805-nm CW laser and signals were detected employing a 2.2-MHz ultrasonic transducer. Bone decalcification and decollagenization were induced with mild ethylenediaminetetraacetic acid (EDTA) and sodium hypochlorite solutions, respectively.

RESULTS

The PA and US signals were measured on cattle bones, and apparent integrated backscatter/back-propagating (AIB) parameters were compared before and after demineralization and decollagenization.

CONCLUSIONS

The results show that both PA and US are sensitive to mineral changes. In addition, PA is also sensitive to changes in the collagen content of the bone, but US is not significantly sensitive to these changes.

Glycation of human cortical and cancellous bone captures differences in the formation of Maillard reaction products between glucose and ribose

To better understand some aspects of bone matrix glycation, we used an in vitro glycation approach. Within two weeks, our glycation procedures led to the formation of advanced glycation end products (AGEs) at the levels that corresponded to approx. 25-30 years of the natural in vivo glycation. Cortical and cancellous bones from human tibias were glycated in vitro using either glucose (glucosylation) or ribose (ribosylation). Both glucosylation and ribosylation led to the formation of higher levels of AGEs and pentosidine (PEN) in cancellous than cortical bone dissected from all tested donors (young, middle-age and elderly men and women). More efficient glycation of bone matrix proteins in cancellous bone most likely depended on the higher porosity of this tissue, which facilitated better accessibility of the sugars to the matrix proteins. Notably, glycation of cortical bone from older donors led to much higher AGEs levels as compared to young donors. Such efficient in vitro glycation of older cortical bone could result from aging-related increase in porosity caused by the loss of mineral content. In addition, more pronounced glycation in vivo would be driven by elevated oxidation processes. Interestingly, the levels of PEN formation differed pronouncedly between glucosylation and ribosylation. Ribosylation generated very high levels of PEN (approx. 6- vs. 2.5-fold higher PEN level than in glucosylated samples). Kinetic studies of AGEs and PEN formation in human cortical and cancellous bone matrix confirmed higher accumulation of fluorescent crosslinks for ribosylation. Our results suggest that in vitro glycation of bone using glucose leads to the formation of lower levels of AGEs including PEN, whereas ribosylation appears to support a pathway toward PEN formation. Our studies may help to understand differences in the progression of bone pathologies related to protein glycation by different sugars, and raise awareness for excessive sugar supplementation in food and drinks.

Effect of in vivo loading on bone composition varies with animal age

Loading can increase bone mass and size and this response is reduced with aging. It is unclear, however how loading affects bone mineral and matrix properties. Fourier Transform Infrared Imaging and high resolution synchrotron scanning small angle X-ray scattering were used to study how bone’s microscale and nanoscale compositional properties were altered in the tibial midshaft of young, adult, and elderly female C57Bl/6J mice after two weeks of controlled in vivo compressive loading in comparison to physiological loading. The effect of controlled loading on bone composition varied with animal age, since it predominantly influenced the bone composition of elderly mice. Interestingly, controlled loading led to enhanced collagen maturity in elderly mice. In addition, although the rate of bone formation was increased by controlled loading based on histomorphometry, the newly formed tissue had similar material quality to new bone tissue formed during physiological loading. Similar to previous studies, our data showed that bone composition was animal and tissue age dependent during physiological loading. The findings that the new tissue formed in response to controlled loading and physiological loading had similar bone composition and that controlled loading enhanced bone composition in elderly mice further supports the use of physical activity as a noninvasive treatment to enhance bone quality as well as maintain bone mass in individuals suffering from age-related bone loss.

Interaction between bone and muscle in older persons with mobility limitations

Aging is associated with a progressive loss of bone-muscle mass and strength. When the decline in mass and strength reaches critical thresholds associated with adverse health outcomes, they are operationally considered geriatric conditions and named, respectively, osteoporosis and sarcopenia. Osteoporosis and sarcopenia share many of the same risk factors and both directly or indirectly cause higher risk of mobility limitations, falls, fractures and disability in activities of daily living. This is not surprising since bones adapt their morphology and strength to the long-term loads exerted by muscle during anti-gravitational and physical activities. Non-mechanical systemic and local factors also modulate the mechanostat effect of muscle on bone by affecting the bidirectional osteocyte-muscle crosstalk, but the specific pathways that regulate these homeostatic mechanisms are not fully understood. More research is required to reach a consensus on cut points in bone and muscle parameters that identify individuals at high risk for adverse health outcomes, including falls, fractures and disability. A better understanding of the muscle-bone physiological interaction may help to develop preventive strategies that reduce the burden of musculoskeletal diseases, the consequent disability in older persons and to limit the financial burden associated with such conditions. In this review, we summarize age-related bone-muscle changes focusing on the biomechanical and homeostatic mechanisms that explain bone-muscle interaction and we speculate about possible pathological events that occur when these mechanisms become impaired. We also report some recent definitions of osteoporosis and sarcopenia that have emerged in the literature and their implications in clinical practice. Finally, we outline the current evidence for the efficacy of available anti-osteoporotic and proposed antisarcopenic interventions in older persons.

Serum pentosidine levels after 3 years of bisphosphonate treatment in post-menopausal osteoporotic women

The present study measured changes in plasma pentosidine and bone turnover markers in elderly patients with osteoporosis treated using bisphosphonate. The relationship between pentosidine and bone turnover markers and bone mineral density (BMD) was investigated. This study consisted of post-menopausal osteoporotic women who could be treated using bisphosphonate for 3 years were included in the present analysis. The study population consisted of 58 cases, all women, ranging in age from 53 to 86 years (mean, 67.1 years). Bisphosphonate treatment significantly increased BMD of the lumbar spine to 0.914 ± 0.141 g/cm(2) and BMD of the femoral neck to 0.708 ± 0.086 g/cm(2) after 3 years (p < 0.001 versus baseline). The mean BAP level was 27.3 ± 8.3 U/L in patients at baseline. After bisphosphonate treatment, BAP significantly decreased to 18.1 ± 7.2 U/L at 3 years (p < 0.001). Urinary NTX also decreased after bisphosphonate treatment. After 3 years of treatment, urinary NTX significantly decreased from 50.0 ± 19.0 nmol BCE/mmol Cr to 24.6 ± 10.2 nmol BCE/mmol Cr at 3 years (p < 0.001). Serum pentosidine levels were 0.0413 ± 0.0094 μg/mL at baseline and 0.0413 ± 0.0122 μg/mL after 3 years. They were not significantly changed by bisphosphonate treatment. Serum pentosidine levels were not changed by treatment with bisphosphonates. Thus, serum pentosidine may not be suitable as a marker of bone quality after 3 years of bisphosphonate treatment.

In situ quasi-static and dynamic nanoindentation tests on calcified nodules formed by osteoblasts: Implication of glucocorticoids responsible for osteoblast calcification

The functional requirements of regenerated calcified tissues are that they enable the tissues to bear a variety of imposed stress and consequent contact-induced strain without substantial fracture. Here we demonstrate the effects of glucocorticoid hormones such as dexamethasone and hydrocortisone on the nanomechanical properties of calcified nodules formed by mouse osteoblastic MC3T3-E1 cells in differentiation-inducing medium containing ascorbic acid and β-glycerophosphate. Neither cell proliferation nor calcium deposition, evaluated using alizarin red and von Kossa staining, was affected by dexamethasone. On the other hand, calcified nodules formed in the presence of dexamethasone were significantly harder and stiffer than those formed in their absence. In particular, a series of nanoindentation tests revealed that the calcified nodules formed in the presence of dexamethasone showed enhanced stiffness against dynamic strain as compared to a quasi-static load. Furthermore, Raman spectroscopy revealed that dexamethasone and hydrocortisone increased the apatite/matrix ratio and lowered that of carbonate in the nodules. Our results suggest that glucocorticoids are required for in vitro formation by osteoblasts of more mature calcified nodules containing apatite/phosphate.

Predominant role of water in native collagen assembly inside the bone matrix

Bone is one of the most intriguing biomaterials found in nature consisting of bundles of collagen helixes, hydroxyapatite, and water, forming an exceptionally tough, yet lightweight material. We present here an experimental tool to map water-dependent subtle changes in triple helical assembly of collagen protein in its absolute native environment. Collagen being the most abundant animal protein has been subject of several structural studies in last few decades, mostly on an extracted, overexpressed, and synthesized form of collagen protein. Our method is based on a (1)H detected solid-state nuclear magnetic resonance (ssNMR) experiment performed on native collagen protein inside intact bone matrix. Recent development in (1)H homonuclear decoupling sequences has made it possible to observe specific atomic resolution in a large complex system. The method consists of observing a natural-abundance two-dimensional (2D) (1)H/(13)C heteronuclear correlation (HETCOR) and(1)H double quantum-single quantum (DQ-SQ) correlation ssNMR experiment. The 2D NMR experiment maps three-dimensional assembly of native collagen protein and shows that extracted form of collagen protein is significantly different from protein in the native state. The method also captures native collagen subtle changes (of the order of ∼1.0 Å) due to dehydration and H/D exchange, giving an experimental tool to map small changes. The method has the potential to be of wide applicability to other collagen containing biomaterials.

Strain-rate stiffening of cortical bone: observations and implications from nanoindentation experiments

While bone mineralization is considered to be responsible for its stiffness, bone durability partially associated with the time-dependent viscoelasticity of matrix proteins is still poorly elucidated. Here we demonstrate a novel mechanism of highly mineralized bone durability almost independent of inherent viscoelastic behaviour along with a protocol for measuring the mechanical properties of mineralized tissues. Strain-rate nanoindentation tests showed substantial stiffening of the highly mineralized calvarial bone, whereas large creep or stress relaxation was observed during constant load or displacement tests, respectively. Based on the lower viscoelasticity of the highly mineralized structure, such large time-dependent response appears to be associated with nanoscale dimensional recovery, rather than viscoelastic behaviour, implying the inverse namely strain-rate dependent dilatant behaviour. This dilatant expansion increased the indenter penetration resistance into the surface, enhancing instantaneous stiffness. The associated stiffening and higher effective elastic modulus were highly strain-rate dependent and more readily observed in more highly mineralized tissues such as the calvarial bone. Such strain-rate stiffening and consequent dimensional recovery may be vital responses of bone tissues against excessive deformation to maintain tissue integrity.

Importance of age on the dynamic mechanical behavior of intertubular and peritubular dentin

An experimental evaluation of human coronal dentin was performed using nanoscopic dynamic mechanical analysis (nanoDMA). The primary objectives were to quantify any unique changes in mechanical behavior of intertubular and peritubular dentin with age, and to evaluate the microstructure and mechanical behavior of the mineral deposited within the lumens. Specimens of coronal dentin were evaluated by nanoDMA using single indents and in scanning mode via scanning probe microscopy. Results showed that there were no significant differences in the storage modulus or complex modulus between the two age groups (18-25 versus 54-83 yrs) for either the intertubular or peritubular tissue. However, there were significant differences in the dampening behavior between the young and old dentin, as represented in the loss modulus and tanδ responses. For both the intertubular and peritubular components, the capacity for dampening was significantly lower in the old group. Scanning based nanoDMA showed that the tubules of old dentin exhibit a gradient in elastic behavior, with decrease in elastic modulus from the cuff to the center of tubules filled with newly deposited mineral.

Bone quality: the determinants of bone strength and fragility

Bone fragility is a major health concern, as the increased risk of bone fractures has devastating outcomes in terms of mortality, decreased autonomy, and healthcare costs. Efforts made to address this problem have considerably increased our knowledge about the mechanisms that regulate bone formation and resorption. In particular, we now have a much better understanding of the cellular events that are triggered when bones are mechanically stimulated and how these events can lead to improvements in bone mass. Despite these findings at the molecular level, most exercise intervention studies reveal either no effects or only minor benefits of exercise programs in improving bone mineral density (BMD) in osteoporotic patients. Nevertheless, and despite that BMD is the gold standard for diagnosing osteoporosis, this measure is only able to provide insights regarding the quantity of bone tissue. In this article, we review the complex structure of bone tissue and highlight the concept that its mechanical strength stems from the interaction of several different features. We revisited the available data showing that bone mineralization degree, hydroxyapatite crystal size and heterogeneity, collagen properties, osteocyte density, trabecular and cortical microarchitecture, as well as whole bone geometry, are determinants of bone strength and that each one of these properties may independently contribute to the increased or decreased risk of fracture, even without meaningful changes in aBMD. Based on these findings, we emphasize that while osteoporosis (almost) always causes bone fragility, bone fragility is not always caused just by osteoporosis, as other important variables also play a major role in this etiology. Furthermore, the results of several studies showing compelling data that physical exercise has the potential to improve bone quality and to decrease fracture risk by influencing each one of these determinants are also reviewed. These findings have meaningful clinical repercussions as they emphasize the fact that, even without leading to improvements in BMD, exercise interventions in patients with osteoporosis may be beneficial by improving other determinants of bone strength.

Tibia bone properties at different time course of ovariectomized rats

BACKGROUND

The model of bilaterally ovariectomized rats mimics the accelerated bone loss observed in postmenopausal women due to estrogen deficiency. Although calcium is main mineral in bone, previous study in human showed there is hypermineralization and higher calcium level in hydroxyapatite crystal structure from osteoporosis patients. This study was aimed to investigate the effect of time course ovariectomized on tibia bone turn over markers, mineral elements, hydroxyapatite crystale, mesostructure, and histomorphometry.

METHODS

A total of 30 Wistar female rats were randomly assigned into three groups (n = 10 each): control group, ovariectomy group follow up for one month and two month. All animals procedures was according to Animal Ethics Guidelines and approval by ethic committee of the Medical Faculty, Lambung Mangkurat University which obtained prior the study. Expression of osteocalcin (OC) and C-telopeptyde collagen type I (CTX) was analyzed by ELISA method. Tibia bone mineral element was measured using X-Ray Fluorescence. Hydroxyapatite crystale structure was analyzed using X-Ray Diffracttion. Mesostructure was determined using Scanning Electron Microscope. Histomorphometry was analyzed using BoneJ software analyzer. ANOVA test was used to analyze the different level of serum bone turnover markers and bone mineral elements.

RESULTS

Serum OC and CTX were significantly decrease in one month and two month after ovariectomized groups compared to sham-operated group (P < 0.05). The levels Ca, P, Fe, Cu, Zn, Ni, Ca/P, and Cu/Zn were not significantly different in all groups (P > 0.05). The structure of hydroxyapatite crystal in one month and two month after ovariectomized groups were different compared with sham-operated control group. Mesostructure of tibia bone after one and two month ovariectomized procedure significantly different than that in sham-operated rats. The level of trabecular volume were lower significantly on OVX-1 and OVX-2 groups compared with sham group (P < 0.05). The trabecular thickness and spacing were increase significantly on OVX-1 and OVX-2 groups compared with sham group (P < 0.05). The trabecular number were significantly decrease OVX-1 and OVX-2 groups than that sham group (P < 0.05).

CONCLUSION

We found that two month after ovariectomized decrease serum osteocalcin but not change bone mineral elements in rats. Also, we found the difference of lattice parameter of hydroxyapatite crystale structure and trabecular properties which determined bone mesostructure.

The role of nanoscale toughening mechanisms in osteoporosis

Strength is the most widely reported parameter with regards to bone failure. However, bone contains pre-existing damage and stress concentration sites, perhaps making measures of fracture toughness more indicative of the resistance of the tissue to withstand fracture. Several toughening mechanisms have been identified in bone, prominently, at the microscale. More recently, nanoscale toughness mechanisms, such as sacrificial-bonds and hidden-length or dilatational band formation, mediated by noncollagenous proteins, have been reported. Absence of specific noncollagenous proteins results in lowered fracture toughness in animal models. Further, roles of several other, putative influencing, factors such as closely bound water, collagen cross-linking and citrate bonds in bone mineral have also been proposed. Yet, it is still not clear if and which mechanisms are hallmarks of osteoporosis disease and how they influence fracture risk. Further insights on the workings of such influencing factors are of high importance for developing complementary diagnostics and therapeutics strategies.