Strain rate influence on human cortical bone toughness: A comparative study of four paired anatomical sites

Microcrack behavior in bone: Stress field analysis at osteon cement line tips

Bone microstructure governs microcrack propagation complexity. Current research, relying on linear elastic fracture mechanics, inadequately considers authentic multi-level structures, like cement lines and osteons, impacting stress intensity at cracks. This study, by constructing models encompassing single or multiple osteons, delves into the influence of factors like crack length, osteon radius, and modulus ratio on the stress intensity factor at the crack tip. Employing a fracture mechanics phase-field approach to simulate crack propagation paths, it particularly explores the role of cement lines as weak interfaces in crack extension. The aim is to comprehensively and systematically elucidate the critical factors of bone microstructure in the context of crack propagation.

3D quantification of the lacunocanalicular network on human femoral diaphysis through synchrotron radiation-based nanoCT

Osteocytes are the major actors in bone mechanobiology. Within bone matrix, they are trapped close together in a submicrometric interconnected network: the lacunocanalicular network (LCN). The interstitial fluid circulating within the LCN transmits the mechanical information to the osteocytes that convert it into a biochemical signal. Understanding the interstitial fluid dynamics is necessary to better understand the bone mechanobiology. Due to the submicrometric dimensions of the LCN, making it difficult to experimentally investigate fluid dynamics, numerical models appear as a relevant tool for such investigation. To develop such models, there is a need for geometrical and morphological data on the human LCN. This study aims at providing morphological data on the human LCN from measurement of 27 human femoral diaphysis bone samples using synchrotron radiation nano-computed tomography with an isotropic voxel size of 100 nm. Except from the canalicular diameter, the canalicular morphological parameters presented a high variability within one sample. Some differences in terms of both lacunar and canalicular morphology were observed between the male and female populations. But it has to be highlighted that all the canaliculi cannot be detected with a voxel size of 100 nm. Hence, in the current study, only a specific population of large canaliculi that could be characterize. Still, to the authors knowledge, this is the first time such a data set was introduced to the community. Further processing will be achieved in order to provide new insight on the LCN permeability.

Effect of ribose incubation on physical, chemical, and mechanical properties of human cortical bone

Raman spectroscopy (RS) is sensitive to the accumulation of advanced glycation end-products (AGEs), and it measures matrix-sensitive properties that correlate with the fracture toughness of human cortical bone. However, it is unclear whether sugar-mediated accumulation of AGEs affects the fracture toughness of human cortical bone in a manner that is consistent with the negative correlations between amide I sub-peak ratios and fracture toughness. Upon machining 64 single-edge notched beam (SENB) specimens from cadaveric femurs (8 male and 7 female donors between 46 years and 61 years of age), pairs of SENB specimens were incubated in 15 mL of phosphate buffered saline with or without 0.1 M ribose for 4 weeks at 37 °C. After acquiring 10 Raman spectra per bone specimen (n = 32 per incubation group), paired SENB specimens were loaded in three-point bending at a quasi-static or a high loading rate approximating 10-4 s-1 or 10-2 s-1, respectively (n = 16 per incubation group per loading rate). While 2 amide I sub-peak ratios, I1670/I1640 and I1670/I1610, decreased by 3-5% with a 100% increase in AGE content, as confirmed by fluorescence measurements, the ribose incubation to accumulate AGEs in bone did not affect linear elastic (KIc) nor non-linear elastic (KJc) measurements of bone's ability to resist crack growth. Moreover, AGE accumulation did not affect the change in these properties when the loading rate changed. Increasing the loading rate increased KIc but decreased KJc. Ribose incubation did not affect mineral-related RS properties such as mineral-to-matrix ratios, Type B carbonate substitutions, and crystallinity. It did however increase the thermal stability of demineralized bone (differential scanning calorimetry), without affecting the network connectivity of the organic matrix (i.e., maximum slope during a hydrothermal isometric tension test of demineralized bone). In conclusion, RS is sensitive to AGE accumulation via the amide I band (plus the hydroxyproline-to-proline ratio), but the increase in AGE content due to ribose incubation was not sufficient to affect the fracture toughness of human cortical bone.

Effects of different preservation on the mechanical properties of cortical bone under quasi-static and dynamic compression

Introduction: Mechanical properties of biological tissue are important for numerical simulations. Preservative treatments are necessary for disinfection and long-term storage when conducting biomechanical experimentation on materials. However, few studies have been focused on the effect of preservation on the mechanical properties of bone in a wide strain rate. The purpose of this study was to evaluate the influence of formalin and dehydration on the intrinsic mechanical properties of cortical bone from quasi-static to dynamic compression. Methods: Cube specimens were prepared from pig femur and divided into three groups (fresh, formalin, and dehydration). All samples underwent static and dynamic compression at a strain rate from 10^-3 s^-1 to 10³ s^-1. The ultimate stress, ultimate strain, elastic modulus, and strain-rate sensitivity exponent were calculated. A one-way ANOVA test was performed to determine if the preservation method showed significant differences in mechanical properties under at different strain rates. The morphology of the macroscopic and microscopic structure of bones was observed. Results: The results show that ultimate stress and ultimate strain increased as the strain rate increased, while the elastic modulus decreased. Formalin fixation and dehydration did not affect elastic modulus significantly whereas significantly increased the ultimate strain and ultimate stress. The strain-rate sensitivity exponent was the highest in the fresh group, followed by the formalin group and dehydration group. Different fracture mechanisms were observed on the fractured surface, with fresh and preserved bone tending to fracture along the oblique direction, and dried bone tending to fracture along the axial direction. Discussion: In conclusion, preservation with both formalin and dehydration showed an influence on mechanical properties. The influence of the preservation method on material properties should be fully considered in developing a numerical simulation model, especially for high strain rate simulation.

A Review on Three-Dimensional Printed Silicate-Based Bioactive Glass/Biodegradable Medical Synthetic Polymer Composite Scaffolds

In recent years, tissue engineering scaffolds have turned into the preferred option for the clinical treatment of pathological and traumatic bone defects. In this field, silicate-based bioactive glasses (SBGs) and biodegradable medical synthetic polymers (BMSPs) have attracted a great deal of attention owing to their shared exceptional advantages, like excellent biocompatibility, good biodegradability, and outstanding osteogenesis. Three-dimensional (3D) printed SBG/BMSP scaffolds can not only replicate the mechanical properties and microstructure of natural bone but also degrade in situ after service and end up being replaced by regenerated bone tissue in vivo. This review first consolidates the research efforts in 3D printed SBG/BMSP scaffolds, and then focuses on their composite mechanism. This review may help to provide a fresh perspective for SBG/BMSP composite system in bone regeneration.

The impact of fall-related loading rate on the formation of micro-damage in human cortical bone fracture

The quest for better predictive tools as well as new preventative and therapeutic measures for bone fragility and fracture has highlighted the need for greater mechanistic understanding of the bone fracture process. Cortical bone, the major load bearing part of the bone, employs different toughening mechanisms to either inhibit or slow down crack growth which leads to fracture. Among these toughening mechanisms, is the formation of a micro-damage process zone (MDPZ) around the region of the propagating crack. Investigations into the MDPZ to date have primarily been based on quasi-static or cyclic loading rate experiments which do not necessarily replicate physiological fracture rates. Consequently, the impact of fall-related loading rates on the formation of the micro-damage process zone was investigated comparing these to quasi-static loading rate equivalents. The size of MDPZ was found to be 42% smaller in the high-rate group compared to the quasi-static rate group. The smaller MDPZ size was associated with a brittle, unstable fracture behaviour and an overall smaller fracture resistance measure (J_max). This result points to the possibility of a strain rate hardening mechanism at the heart of micro-damage formation, which is hampered under high loading rates, resulting in lower overall fracture resistance.

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.

Experimental characterization and micromechanical modeling of the elastic response of the human humerus under bending impact

This paper investigates the characterization and numerical modeling of the elastic behavior of the human humerus bone using a recently developed micromechanical approach coupled to nanoindentation measurements. At first, standard three-point bending experiments were conducted under low static loading, using several humerus diaphysis in order to identify the apparent elastic modulus of the bone in static regime. Then, a drop tower impact experiment was used on the same set of humerus diaphysis specimens, in order to assess the elastic modulus in dynamic regime. These measurements will be used as reference bases for comparison purpose. The originality of this work, lies in the coupling between a two-phase micromechanical approach based on Mori-Tanaka homogenization scheme for cylindrical voids and nanoindentation measurements of the elastic modulus of the bone matrix phase. This model has been implemented using a user defined material subroutine VMAT in ABAQUS© Explicit code. The bone mechanical response prediction using the proposed methodology was validated against previous standard experimental data. Finally, it was shown that the numerical predictions are consistent with the physical measurements obtained on human humerus via the good estimation of the ultimate impact load.

What is the influence of two strain rates on the relationship between human cortical bone toughness and micro-structure?

Cortical bone fracture mechanisms are well studied under quasi-static loading. The influence of strain rate on crack propagation mechanisms needs to be better understood, however. We have previously shown that several aspects of the bone micro-structure are involved in crack propagation, such as the complete porosity network, including the Haversian system and the lacunar network, as well as biochemical aspects, such as the maturity of collagen cross-links. The aim of this study is to investigate the influence of strain rate on the toughness of human cortical bone with respect to its microstructure and organic non-collagenous composition. Two strain rates will be considered: quasi-static loading (10−4 s−1), a standard condition, and a higher loading rate (10−1 s−1), representative of a fall. Cortical bone samples were extracted from eight female donors (age 50–91 years). Three-point bending tests were performed until failure. Synchrotron radiation micro-computed tomography imaging was performed to assess bone microstructure including the Haversian system and the lacunar system. Collagen enzymatic cross-link maturation was measured using a high performance liquid chromatography column. Results showed that that under quasi-static loading, the elastic contribution of the fracture process is correlated to both the collagen cross-links maturation and the microstructure, while the plastic contribution is correlated only to the porosity network. Under fall-like loading, bone organization appears to be less linked to crack propagation.

Ageing bone fractures: The case of a ductile to brittle transition that shifts with age

Human bone becomes increasingly brittle with ageing. Bones also fracture differently under slow and fast loadings, being ductile and brittle, respectively. The effects of a combination of these two factors have never been examined before. Here we show that cortical bone is most fracture-resistant at the physiologically prevalent intermediate strain rates of 10^-3 s^-1 to 10^-2 s^-1 such as they occur in walking or running, slightly weaker at slower quasistatic and much weaker at fast impact loading rates. In young cortical bone (15 years of age) the ductile-to-brittle transition (DBT) occurs at strain rates of 10^-2 s^-1, in old cortical bone (85 yrs) at speeds lower by a factor of 10 to 40. Other research has shown that the energy required to break bone (per unit of fracture surface) drops as much as 60% between these two ages. Therefore, DBT seems to compound the well-known phenomenon of 'brittle old bones'. Old bones can only cope with slow movement, young ones with both slow and fast movement. These observed material characteristics of (i) a shift of the DBT and (ii) a reduced energy absorption capacity appear to contribute at least as much to the loss of bone quality as the various quantity based (lowered bone density and mineral content) explanations of the past. They also provide a new powerful paradigm, which allows us to demonstrate mechanically, and uniquely, how human bone becomes increasingly brittle with age.

Anisotropic elastic properties of human femoral cortical bone and relationships with composition and microstructure in elderly

The strong dependence between cortical bone elasticity at the millimetre-scale (mesoscale) and cortical porosity has been evidenced by previous studies. However, bone is an anisotropic composite material made by mineral, proteins and water assembled in a hierarchical structure. Whether the variations of structural and compositional properties of bone affect the different elastic coefficients at the mesoscale is not clear. Aiming to understand the relationships between bone elastic properties and compositions and microstructure, we applied state-of-the-art experimental modalities to assess these aspects of bone characteristics. All elastic coefficients (stiffness tensor of the transverse isotropic bone material), structure of the vascular pore network, collagen and mineral properties were measured in 52 specimens from the femoral diaphysis of 26 elderly donors. Statistical analyses and micromechanical modeling showed that vascular pore volume fraction and the degree of mineralization of bone are the most important determinants of cortical bone anisotropic mesoscopic elasticity. Though significant correlations were observed between collagen properties and elasticity, their effects in bone mesoscopic elasticity were minor in our data. This work also provides a unique set of data exhibiting a range of variations of compositional and microstructural cortical bone properties in the elderly and gives strong experimental evidence and basis for further development of biomechanical models for human cortical bone. STATEMENT OF SIGNIFICANCE: This study reports the relationships between microstructure, composition and the mesoscale anisotropic elastic properties of human femoral cortical bone in elderly. For the first time, we provide data covering the complete anisotropic elastic tensor, the microstructure of cortical vascular porosity, mineral and collagen characteristics obtained from the same or adjacent samples in each donor. The results revealed that cortical vascular porosity and degree of mineralization of bone are the most important determinants of bone anisotropic stiffness at the mesoscale. The presented data gives strong experimental evidence and basis for further development of biomechanical models for human cortical bone.

Influence of loading condition and anatomical location on human cortical bone linear micro-cracks

Human cortical bone fracture toughness depends on the anatomical locations under quasi-static loading. Recent results also showed that under fall-like loading, cortical bone fracture toughness is similar at different anatomical locations in the same donor. While cortical bone toughening mechanisms are known to be dependent on the tissue architecture under quasi-static loading, the fracture mechanisms during a fall are less studied. In the current study, the structural parameters of eight paired femoral diaphyses, femoral necks and radial diaphyses were mechanically tested under quasi-static and fall-like loading conditions (female donors, 70 ± 14 y.o., [50-91 y.o.]). Synchrotron radiation micro-CT imaging was used to quantify the amount of micro-cracks formed during loading. The volume fraction of these micro-cracks was significantly higher within the specimens loaded under a quasi-static condition than under a loading representative of a fall. Under fall-like loading, there was no difference in crack volume fraction between the different paired anatomical locations. This result shows that the micro-cracking toughening mechanism depends both on the anatomical location and on the loading condition.

3D micro structural analysis of human cortical bone in paired femoral diaphysis, femoral neck and radial diaphysis

Human bone is known to adapt to its mechanical environment in a living body. Both its architecture and microstructure may differ between weight-bearing and non-weight-bearing bones. The aim of the current study was to analyze in three dimensions, the morphology of the multi-scale porosities on human cortical bone at different locations. Eight paired femoral diaphyses, femoral necks, and radial diaphyses were imaged using Synchrotron Radiation µCT with a 0.7 µm isotropic voxel size. The spatial resolution facilitates the investigation of the multiscale porosities of cortical bone, from the osteonal canals system down to the osteocyte lacunar system. Our results showed significant differences in the microstructural properties, regarding both osteonal canals and osteocytes lacunae, between the different anatomical locations. The radius presents significantly lower osteonal canal volume fraction and smaller osteonal canals than the femoral diaphysis or neck. Osteocytes lacunae observed in the radius are significantly different in shape than in the femur, and lacunar density is higher in the femoral neck. These results show that the radius, a non-weight-bearing bone, is significantly different in terms of its microstructure from a weight-bearing bone such as the femur. This implies that the cortical bone properties evaluated on the femoral diaphysis, the main location studied within the literature, cannot be generalized to other anatomical locations.

Relationships between human cortical bone toughness and collagen cross-links on paired anatomical locations

Human cortical bone fracture processes depend on the internal porosity network down to the lacunar length scale. Recent results show that at the collagen scale, the maturation of collagen cross-links may have a negative influence on bone mechanical behavior. While the effect of pentosidine on human cortical bone toughness has been studied, the influence of mature and immature enzymatic cross-links has only been studied in relation to strength and work of fracture. Moreover, these relationships have not been studied on different paired anatomical locations. Thus, the aim of the current study was to assess the relationships between both enzymatic and non-enzymatic collagen cross-links and human cortical bone toughness, on four human paired anatomical locations. Single Edge Notched Bending toughness tests were performed for two loading conditions: a quasi-static standard condition, and a condition representative of a fall. These tests were done with 32 paired femoral diaphyses, femoral necks and radial diaphyses (18 women, age 81 ± 12 y.o.; 14 men, age 79 ± 8 y.o.). Collagen enzymatic and non-enzymatic crosslinks were measured on the same bones. Maturation of collagen was defined as the ratio between immature and mature cross-links (CX). The results show that there was a significant correlation between collagen cross-link maturation and bone toughness when gathering femoral and radial diaphyses, but not when considering each anatomical location individually. These results show that the influence of collagen enzymatic and non-enzymatic cross-links is minor when considering human cortical bone crack propagation mechanisms.

Bone Mechanical Properties in Healthy and Diseased States

The mechanical properties of bone are fundamental to the ability of our skeletons to support movement and to provide protection to our vital organs. As such, deterioration in mechanical behavior with aging and/or diseases such as osteoporosis and diabetes can have profound consequences for individuals' quality of life. This article reviews current knowledge of the basic mechanical behavior of bone at length scales ranging from hundreds of nanometers to tens of centimeters. We present the basic tenets of bone mechanics and connect them to some of the arcs of research that have brought the field to recent advances. We also discuss cortical bone, trabecular bone, and whole bones, as well as multiple aspects of material behavior, including elasticity, yield, fracture, fatigue, and damage. We describe the roles of bone quantity (e.g., density, porosity) and bone quality (e.g., cross-linking, protein composition), along with several avenues of future research.

Scaling effect on the fracture toughness of bone materials using MMTS criterion

The aim of this study is to present a stress based approach for investigating the effect of specimen size on the fracture toughness of bone materials. The proposed approach is a modified form of the classical fracture criterion called maximum tangential stress (MTS). The mechanical properties of bone are different in longitudinal and transverse directions and hence the tangential stress component in the proposed approach should be determined in the orthotropic media. Since only the singular terms of series expansions were obtained in the previous studies, the tangential stress is measured from finite element analysis. In this study, the critical distance is also assumed to be size dependent and a semi-empirical formulation is used for describing the size dependency of the critical distance. By comparing the results predicted by the proposed approach and those reported in the previous studies, it is shown that the proposed approach can predict the fracture resistance of cracked bone by taking into account the effect of specimen size.