Fracture of bone tissue: The ‘hows’ and the ‘whys’

QCT-based computational bone strength assessment updated with MRI-derived 'hidden' microporosity

Microdamage accumulated through sustained periods of cyclic loading or single overloading events contributes to bone fragility through a reduction in stiffness and strength. Monitoring microdamage in vivo remains unattainable by clinical imaging modalities. As such, there are no established computational methods for clinical fracture risk assessment that account for microdamage that exists in vivo at any specific timepoint. We propose a method that combines multiple clinical imaging modalities to identify an indicative surrogate, which we term 'hidden porosity', that incorporates pre-existing bone microdamage in vivo. To do so, we use the third metacarpal bone of the equine athlete as an exemplary model for fatigue induced microdamage, which coalesces in the subchondral bone. N = 10 metacarpals were scanned by clinical quantitative computed tomography (QCT) and magnetic resonance imaging (MRI). We used a patch-based similarity method to quantify the signal intensity of a fluid sensitive MRI sequence in bone regions where microdamage coalesces. The method generated MRI-derived pseudoCT images which were then used to determine a pre-existing damage (Dpex) variable to quantify the proposed surrogate and which we incorporate into a nonlinear constitutive model for bone tissue. The minimum, median, and maximum detected Dpex of 0.059, 0.209, and 0.353 reduced material stiffness by 5.9%, 20.9%, and 35.3% as well as yield stress by 5.9%, 20.3%, and 35.3%. Limb-specific voxel-based finite element meshes were equipped with the updated material model. Lateral and medial condyles of each metacarpal were loaded to simulate physiological joint loading during gallop. The degree of detected Dpex correlated with a relative reduction in both condylar stiffness (p = 0.001, R2 > 0.74) and strength (p < 0.001, R2 > 0.80). Our results illustrate the complementary value of looking beyond clinical CT, which neglects the inclusion of microdamage due to partial volume effects. As we use clinically available imaging techniques, our results may aid research beyond the equine model on fracture risk assessment in human diseases such as osteoarthritis, bone cancer, or osteoporosis.

Relationship between CT-Derived Bone Mineral Density and UTE-MR-Derived Porosity Index in Equine Third Metacarpal and Metatarsal Bones

Fatigue-related subchondral bone injuries of the third metacarpal/metatarsal (McIII/MtIII) bones are common causes of wastage, and they are welfare concerns in racehorses. A better understanding of bone health and strength would improve animal welfare and be of benefit for the racing industry. The porosity index (PI) is an indirect measure of osseous pore size and number in bones, and it is therefore an interesting indicator of bone strength. MRI of compact bone using traditional methods, even with short echo times, fail to generate enough signal to assess bone architecture as water protons are tightly bound. Ultra-short echo time (UTE) sequences aim to increase the amount of signal detected in equine McIII/MtIII condyles. Cadaver specimens were imaged using a novel dual-echo UTE MRI technique, and PI was calculated and validated against quantitative CT-derived bone mineral density (BMD) measures. BMD and PI are inversely correlated in equine distal Mc/MtIII bone, with a weak mean r value of -0.29. There is a statistically significant difference in r values between the forelimbs and hindlimbs. Further work is needed to assess how correlation patterns behave in different areas of bone and to evaluate PI in horses with and without clinically relevant stress injuries.

Chitosan, Gelatin, and Collagen Hydrogels for Bone Regeneration

Hydrogels are versatile biomaterials characterized by three-dimensional, cross-linked, highly hydrated polymeric networks. These polymers exhibit a great variety of biochemical and biophysical properties, which allow for the diffusion of diverse molecules, such as drugs, active ingredients, growth factors, and nanoparticles. Meanwhile, these polymers can control chemical and molecular interactions at the cellular level. The polymeric network can be molded into different structures, imitating the structural characteristics of surrounding tissues and bone defects. Interestingly, the application of hydrogels in bone tissue engineering (BTE) has been gathering significant attention due to the beneficial bone improvement results that have been achieved. Moreover, essential clinical and osteoblastic fate-controlling advances have been achieved with the use of synthetic polymers in the production of hydrogels. However, current trends look towards fabricating hydrogels from biological precursors, such as biopolymers, due to the high biocompatibility, degradability, and mechanical control that can be regulated. Therefore, this review analyzes the concept of hydrogels and the characteristics of chitosan, collagen, and gelatin as excellent candidates for fabricating BTE scaffolds. The changes and opportunities brought on by these biopolymers in bone regeneration are discussed, considering the integration, synergy, and biocompatibility features.

Quantifying microcracks on fractured bone surfaces – Potential use in forensic anthropology

Bone fracture surface morphology (FSM) can provide valuable information on the cause of failure in forensic and archaeological applications and it depends primarily on three factors, the loading conditions (like strain rate), the ambient conditions (wet or dry bone material) and the quality of bone material itself. The quality of bone material evidently changes in taphonomy as a result of the decomposition process and that in turn is expected to affect FSM. Porcine bones were fractured by a standardised impact during the course of soft tissue decomposition, at 28-day intervals, over 140 days (equivalent to 638 cooling degree days). Measurements of the associated microcracks on the fractured cortical bone surfaces indicated a progressive increase in mean length during decomposition from around 180 μm-375 μm. The morphology of these microcracks also altered, from multiple intersecting microcracks emanating from a central point at 0-28 cumulative cooling degree days, to longer linear cracks appearing to track lamellae as soft tissue decomposition progressed. The implications of these findings are that taphonomic changes of bone may offer the real possibility of distinguishing perimortem and taphonomic damage and also provide a new surrogate parameter for estimation of post-mortem interval (PMI) in forensics.

Cortical bone continuum damage mechanics constitutive model with stress triaxiality criterion to predict fracture initiation and pattern

A primary objective of finite element human body models (HBMs) is to predict response and injury risk in impact scenarios, including cortical bone fracture initiation, fracture pattern, and the potential to simulate post-fracture injury to underlying soft tissues. Current HBMs have been challenged to predict the onset of failure and bone fracture patterns owing to the use of simplified failure criteria. In the present study, a continuum damage mechanics (CDM) model, incorporating observed mechanical response (orthotropy, asymmetry, damage), was coupled to a novel phenomenological effective strain fracture criterion based on stress triaxiality and investigated to predict cortical bone response under different modes of loading. Three loading cases were assessed: a coupon level notched shear test, whole bone femur three-point bending, and whole bone femur axial torsion. The proposed material model and fracture criterion were able to predict both the fracture initiation and location, and the fracture pattern for whole bone and specimen level tests, within the variability of the reported experiments. There was a dependence of fracture threshold on finite element mesh size, where higher mesh density produced similar but more refined fracture patterns compared to coarser meshes. Importantly, the model was functional, accurate, and numerically stable even for relatively coarse mesh sizes used in contemporary HBMs. The proposed model and novel fracture criterion enable prediction of fracture initiation and resulting fracture pattern in cortical bone such that post-fracture response can be investigated in HBMs.

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.

Biodegradable and Biocompatible Adhesives for the Effective Stabilisation, Repair and Regeneration of Bone

Bone defects and complex fractures present significant challenges for orthopaedic surgeons. Current surgical procedures involve the reconstruction and mechanical stabilisation of complex fractures using metal hardware (i.e., wires, plates and screws). However, these procedures often result in poor healing. An injectable, biocompatible, biodegradable bone adhesive that could glue bone fragments back together would present a highly attractive solution. A bone adhesive that meets the many clinical requirements for such an application has yet to be developed. While synthetic and biological polymer-based adhesives (e.g., cyanoacrylates, PMMA, fibrin, etc.) have been used effectively as bone void fillers, these materials lack biomechanical integrity and demonstrate poor injectability, which limits the clinical effectiveness and potential for minimally invasive delivery. This systematic review summarises conventional approaches and recent developments in the area of bone adhesives for orthopaedic applications. The required properties for successful bone repair adhesives, which include suitable injectability, setting characteristics, mechanical properties, biocompatibility and an ability to promote new bone formation, are highlighted. Finally, the potential to achieve repair of challenging bone voids and fractures as well as the potential of new bioinspired adhesives and the future directions relating to their clinical development are discussed.

Damage analysis of human cortical bone under compressive and tensile loadings

Developing advanced fracture tools can increase the understanding of crack growth trajectories in human cortical bone. The present study investigates fracture micromechanics of human cortical bone under compressive and tensile loadings utilizing a phase field method. We construct two-dimensional finite element models from cortical microstructure of a human tibia cross section. We apply compression on the cortical bone models to create compressive microcracks. Then, we simulate the fracture of these models under tension to discover influential parameters on microcracks formation and post-yielding behavior. The results show that cement lines are susceptible sites to damage nucleation under compression rather than tension. The findings of this study also indicate a higher accumulation of initial damage (induced by compression) can lead to a lower microscopic stiffness as well as a less resistant material to damage initiation under tension. The simulations further indicate that the post-yielding properties (e.g., toughness) can be dependent on different variables such as morphological information of the osteons, the initial accumulation of microcracks, and the total length of cement lines.

3D pull-out finite element simulation of the pedicle screw-trabecular bone interface at strain rates

Biomedical experimental studies such as pull-out (PO), screw loosening experience variability mechanical properties of fresh bone, legal procedures of cadaver bone samples and time-consuming problems. Finite Element Method (FEM) could overcome experimental problems in biomechanics. However, material modelling of bone is quite difficult, which has viscoelastic and viscoplastic properties. The study presents a bone material model which is constructed at the strain rates with the Johnson-Cook (JC) material model, one of the robust constitutive material models. The JC material constants of trabecular bone are determined by the curve fitting method at strain rates for the 3D PO finite element simulation, which defines the screw-bone interface relationship. The PO simulation is performed using the Abaqus/CAE software program. Bone fracture mechanisms are simulated with dynamic/explicit solutions during the PO phenomenon. The paper exposes whether the strain rate has effects on the PO performance. Moreover, simulation reveals the relationship between pedicle screw diameter and PO performance. The results obtained that the maximum pull-out force (POF) improves as both the screw diameter and the strain rate increase. For 5.5 mm diameter pedicle screw POFs were 487, 517 and 1708 N at strain rate 0.00015, 0.015 and 0.015 s^-1, respectively. The FOFs obtained from the simulation of the other screw were 730, 802 and 2008 N at strain rates 0.00015, 0.0015 and 0.015, respectively. PO phenomenon was also simulated realistically in the finite element analysis (FEA).

A computational framework for crack propagation in spatially heterogeneous materials

This paper presents a mathematical formulation and numerical modelling framework for brittle crack propagation in heterogeneous elastic solids. Such materials are present in both natural and engineered scenarios. The formulation is developed in the framework of configurational mechanics and solved numerically using the finite-element method. We show the methodology previously established for homogeneous materials without the need for any further assumptions. The proposed model is based on the assumption of maximal dissipation of energy and uses the Griffith criterion; we show that this is sufficient to predict crack propagation in brittle heterogeneous materials, with spatially varying Young's modulus and fracture energy. Furthermore, we show that the crack path trajectory orientates itself such that it is always subject to Mode-I. The configurational forces and fracture energy release rate are both expressed exclusively in terms of nodal quantities, avoiding the need for post-processing and enabling a fully implicit formulation for modelling the evolving crack front and creation of new crack surfaces. The proposed formulation is verified and validated by comparing numerical results with both analytical solutions and experimental results. Both the predicted crack path and load-displacement response show very good agreement with experiments where the crack path was independent of material heterogeneity for those cases. Finally, the model is successfully used to consider the real and challenging scenario of fracture of an equine bone, with spatially varying material properties obtained from CT scanning. This article is part of a discussion meeting issue 'A cracking approach to inventing new tough materials: fracture stranger than friction'.

Effect of ageing on microstructure and fracture behavior of cortical bone as determined by experiment and Extended Finite Element Method (XFEM)

Bone fracture is a severe health concern; therefore, understanding the causes of bone fracture are crucial. This paper investigates the microstructure and fracture behaviour of cadaveric cortical bone of two different groups (Young, n= 6; Aged, n=7). The microstructure is obtained from µ-CT images, and the material parameters are measured with nanoindentation. Fracture behaviour in transverse and longitudinal orientations is investigated experimentally and numerically. The results show that the Haversian canal (HC) size increases and the osteon wall thickness (OWT) decreases significantly in the aged group, whereas a nonsignificant difference is found in tissue properties. The crack initiation (J_ic) and crack growth (J_grow) toughness of the aged group are found to be significantly lower (p<0.01) than the young group in the transverse orientation; however, for the longitudinal orientation, only the value of J_ic in the aged group is found significantly lower. Further, a 4-phase XFEM (based on micro-CT image) model is developed to investigate the crack propagation behaviour in both orientations. For the transverse orientation, results show that in the aged group, the crack initially follows the cementline and then penetrates the osteon, whereas, in the young group, it propagates along the cementline. These results are in agreement with experimental results where the decrease in J_grow is more significant than the J_ic in the aged group. This study suggests that ageing leads to a larger HC and reduced OWT, which weakens the crack deflection ability and causes fragility fracture. Further, the XFEM results indicate that the presence of a small microcrack in the vicinity of a major crack tip causes an increase in the critical stress intensity factor.

Altered topological blueprint of trabecular bone associates with skeletal pathology in humans

Bone is a hierarchically organized biological material, and its strength is usually attributed to overt factors such as mass, density, and composition. Here we investigate a covert factor – the topological blueprint, or the network organization pattern of trabecular bone. This generally conserved metric of an edge-and-node simplified presentation of trabecular bone relates to the average coordination/valence of nodes and the equiangular 3D offset of trabeculae emanating from these nodes. We compare the topological blueprint of trabecular bone in presumably normal, fractured osteoporotic, and osteoarthritic samples (all from human femoral head, cross-sectional study). We show that bone topology is altered similarly in both fragility fracture and in joint degeneration. Decoupled from the morphological descriptors, the topological blueprint subjected to simulated loading associates with an abnormal distribution of strain, local stress concentrations and lower resistance to the standardized load in pathological samples, in comparison with normal samples. These topological effects show no correlation with classic morphological descriptors of trabecular bone. The negative effect of the altered topological blueprint may, or may not, be partly compensated for by the morphological parameters. Thus, naturally occurring optimization of trabecular topology, or a lack thereof in skeletal disease, might be an additional, previously unaccounted for, contributor to the biomechanical performance of bone, and might be considered as a factor in the life-long pathophysiological trajectory of common bone ailments.

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Mechanical performance of the skeleton results from many factors and their interplay.

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Topological blueprint as a basic trabecular design plan is an understudied factor.

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Topological blueprint deviation undermines mechanical properties of trabecular bone.

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Higher bone mass or thicker trabeculae do not compensate for deviant topology.

Diffuse microdamage in bone activates anabolic response by osteoblasts via involvement of voltage-gated calcium channels

INTRODUCTION

Matrix damage sustained by bone tissue is repaired by the concerted action of bone cells. Previous studies have reported extracellular calcium ([Ca²⁺]_E) efflux to originate from regions of bone undergoing diffuse microdamage termed as "diffuse microdamage-induced calcium efflux" (DMICE). DMICE has also been shown to activate and increase intracellular calcium ([Ca²⁺]_I) signaling in osteoblasts via the involvement of voltage-gated calcium channels (VGCC). Past studies have assessed early stage (< 1 h) responses of osteoblasts to DMICE. The current study tested the hypothesis that DMICE has longer-term sustained effect such that it induces anabolic response of osteoblasts.

MATERIALS AND METHODS

Osteoblasts derived from mouse calvariae were seeded on devitalized bovine bone wafers. Localized diffuse damage was induced in the vicinity of cells by bending. The response of osteoblasts to DMICE was evaluated by testing gene expression, protein synthesis and mineralized nodule formation.

RESULTS

Cells on damaged bone wafers showed a significant increase in RUNX2 and Osterix expression compared to non-loaded control. Also, RUNX2 and Osterix expression were suppressed significantly when the cells were treated with bepridil, a non-selective VGCC inhibitor, prior to loading. Significantly higher amounts of osteocalcin and mineralized nodules were synthesized by osteoblasts on diffuse damaged bone wafers, while bepridil treatment resulted in a significant decrease in osteocalcin production and mineralized nodule formation.

CONCLUSION

In conclusion, this study demonstrated that DMICE activates anabolic responses of osteoblasts through activation of VGCC. Future studies of osteoblast response to DMICE in vivo will help to clarify how bone cells repair diffuse microdamage.

Strain rate dependence of work of fracture tests on bone and similar tissues: Reflections on testing methods and mineral content effects

This paper is concerned with the effect of different strain rate on the Work of Fracture (W_f) of various vertebrate mineralised tissues, controlling for the effect of mineral content and Young's modulus of elasticity. Using specimens of uniform shape and size values for the Work of Fracture of specimens tested at various deformation rates, and also the energy absorbed by notched specimens in impact, are reported. The results indicated that, of those tested, for most bone specimens the Work of Fracture measurements were constant like in the case for a 'material property'. Variations due to loading conditions (deformation rate) were small, with the exemption of antler, which is relatively poorly mineralised and in which the Work of Fracture values increased by a factor of 4 across the range from quasistatic loading to impact. The Tattersall and Tappin (1966) test has shown itself to offer some great advantages: if the quest is for a fracture toughness test for an unknown tissue it offers reliability, it is perhaps more forgiving to handling errors, it also suffers less of the influence of strain rate effects and uses relatively simple instrumentation. It is also able to demonstrate the remarkable toughness of antler bone which other more commonly used fracture toughness methods cannot do.

Force, impulse and energy during falling with and without knee protection: an in-vitro study

The mechanics of protective knee padding mitigating injury from a high-force fall have not been investigated in real-life scenarios to date. This study compares the effect of wearing knee pads to unprotected impact on a hard surface. We hypothesized that knee pads reduce the force and energy transmitted to the bony structures of the knee cap compared with unprotected conditions. Eight human knee cadaver specimens were embedded and fixed with a flexion angle of 100 degrees in a custom-made drop testing device (75 kg including the knee). The usage of a knee pad led to an average peak force attenuation on impact of 15% (no pad: 5932 N SD: 2472 N; pad: 4210 N SD: 2199 N; p < 0.001). Contact time on the plate was higher with a knee pad (no pad: 0.015 s SD: 0.009 s; pad: 0.028 s SD: 0.014 s; p < 0.001). Therefore, the observed impulse was also increased (no pad: 62.2 Ns SD: 17.8 Ns; pad: 74.6 Ns SD: 18.6 Ns; p < 0.001). This effect diminished as drop height was increased. Energy dissipation, defined as the difference between kinetic energy pre-impact and peak potential energy post-impact, was higher without a knee pad (no pad: 10.5 J SD: 6.2 J; pad: 4.2 J SD: 5.0 J; p < 0.001). The results from this study illustrate the magnitude of influence that knee pads have on peak forces, transmitted impulse, and energy transfer from a high-force impact in real-life scenarios. Contrary to expectations, the knee pad did not act as a mechanical damper. The mechanical behavior more closely resembled a spring that temporarily stores energy and consequentially reduces peak forces upon impact. Based on this study, future developments in padding might benefit from focusing on the aspect of energy storage and temporarily delayed energy dissipation.

Compressive behaviour of uniaxially aligned individual mineralised collagen fibres at the micro- and nanoscale

The increasing incidence of osteoporotic bone fractures makes fracture risk prediction an important clinical challenge. Computational models can be utilised to facilitate such analyses. However, they critically depend on bone's underlying hierarchical material description. To understand bone's irreversible behaviour at the micro- and nanoscale, we developed an in situ testing protocol that allows us to directly relate the experimental data to the mechanical behaviour of individual mineralised collagen fibres and its main constitutive phases, the mineralised collagen fibrils and the mineral nanocrystals, by combining micropillar compression of single fibres with small angle X-ray scattering (SAXS) and X-ray diffraction (XRD). Failure modes were assessed by SEM. Strain ratios in the elastic region at fibre, fibril and mineral levels were found to be approximately 22:5:2 with strain ratios at the point of compressive strength of 0.23 ± 0.11 for fibril-to-fibre and 0.07 ± 0.01 for mineral-to-fibre levels. Mineral-to-fibre levels showed highest strain ratios around the apparent yield point, fibril-to-fibre around apparent strength. The mineralised collagen fibrils showed a delayed mechanical response, contrary to the mineral phase, which points towards preceding deformations of mineral nanocrystals in the extrafibrillar matrix. No damage was measured at the level of the mineralised collagen fibre which indicates an incomplete separation of the mineral and collagen, and an extrafibrillar interface failure. The formation of kink bands and the gradual recruitment of fibrils upon compressive loading presumably led to localised strains. Our results from a well-controlled fibrillar architecture provide valuable input for micromechanical models and computational non-linear bone strength analyses that may provide further insights for personalised diagnosis and treatment as well as bio-inspired implants for patients with bone diseases. STATEMENT OF SIGNIFICANCE: Musculoskeletal diseases such as osteoporosis, osteoarthritis or bone cancer significantly challenge health care systems and make fracture risk prediction and treatment optimisation important clinical goals. Computational methods such as finite element models have the potential to optimise analyses but highly depend on underlying material descriptions. We developed an in situ testing set-up to directly relate experimental data to the mechanical behaviour of bone's fundamental building block, the individual mineralised collagen fibre and its main constituents. Low multilevel strain ratios suggest high deformations in the extrafibrillar matrix and energy dissipation at the interfaces, the absence of damage indicates both an incomplete separation between mineral and collagen and an extrafibrillar interface failure. The formation of kink bands in the fibril-reinforced composite presumably led to localised strains. The deformation behaviour of a well-controlled fibrillar architecture provides valuable input for non-linear bone strength analyses.

Fatigue and damage of porcine pars interarticularis during asymmetric loading

If the articular facets of the vertebra grow in an asymmetric manner, the developed bone geometry causes an asymmetry of loading. When the loading environment is altered by way of increased activity, the likelihood of acquiring a stress fracture may be increased. The combination of geometric asymmetry and increased activity is hypothesised to be the precursor to the stress fracture under investigation in this study, spondylolysis. This vertebral defect is an acquired fracture with 7% prevalence in the paediatric population. This value increases to 21% among athletes who participate in hyperextension sports. Tests were carried out on porcine lumbar vertebrae, on which the effect of facet angle asymmetry was simulated by offsetting the load laterally by 7mm from the mid-point. Strain in the vertebral laminae was recorded using six 3-element stacked rosette strain gauges placed bilaterally. Specimens were loaded cyclically at a rate of 2Hz. Fatigue cycles; strain, creep, secant modulus and hysteresis were measured. The principal conclusions of this paper are that differences in facet angle lead to an asymmetry of loading in the facet joints; this in turn leads to an initial increase in strain on the side with the more coronally orientated facet. The strain amplitude, which is the driving force for crack propagation, is greater on this side at all times up to fracture, the significance of this can be observed in the increased steady state creep rate (p = 0.036) and the increase in yielding and toughening mechanisms taking place, quantified by the force-displacement hysteresis (p = 0.026).

Nanoscale deformation mechanisms and yield properties of hydrated bone extracellular matrix

Bone features a hierarchical architecture combining antagonistic properties like toughness and strength. In order to better understand the mechanisms leading to this advantageous combination, its postyield and failure behaviour was analyzed on the length scale of a single lamella. Micropillars were compressed to large strains under hydrated conditions to measure their anisotropic yield and post-yield behaviour. An increase in strength compared to the macroscale by a factor of 1.55 and a strong influence of hydration with a decrease by 60% in yield stress compared to vacuum conditions were observed. Post-compression transmission electron microscopic analysis revealed anisotropic deformation mechanisms. In axial pillars, where fibrils were oriented along the loading axis, kink bands were observed and shear cracks emerged at the interface of ordered and disordered regions. Micromechanical analysis of fibril kinking allowed an estimate of the extrafibrillar matrix shear strength to be made: 120±40MPa. When two opposing shear planes met a wedge was formed, splitting the micropillar axially in a mode 1 crack. Making use of an analytical solution, the mode 1 fracture toughness of bone extracellular matrix for splitting along the fibril direction was estimated to be 0.07MPam. This is 1-2 orders of magnitude smaller than on the macroscale, which may be explained by the absence of extrinsic toughening mechanisms. In transverse pillars, where fibrils were oriented perpendicular to the loading axis, cracks formed in regions where adverse fibril orientation reduced the local fracture resistance. This study underlines the importance of bone's hierarchical microstructure for its macroscopic strength and fracture resistance and the need to study structure-property relationships as well as failure mechanisms under hydrated conditions on all length scales.

STATEMENT OF SIGNIFICANCE

Bone's hierarchical architecture combines toughness and strength. To understand the governing deformation mechanisms, its postyield behaviour was analyzed at the microscale. Micropillars were compressed in physiological solution; an increased strength compared to macroscale and an influence of hydration was found. Transmission electron microscopy revealed cracks forming in regions with adverse fibril orientation in transverse pillars. In axial pillars kink bands were observed and shear cracks emerged at the interface of ordered and disordered regions. It was estimated that bone's fracture toughness for splitting between fibrils is significantly smaller than on the macroscale. This study underlines the importance of bone's hierarchical microstructure and the need to study structure-property relationships on all length scales.

Acute and Stress-related Injuries of Bone and Cartilage: Pertinent Anatomy, Basic Biomechanics, and Imaging Perspective

Bone or cartilage, or both, are frequently injured related to either a single episode of trauma or repetitive overuse. The resulting structural damage is varied, governed by the complex macroscopic and microscopic composition of these tissues. Furthermore, the biomechanical properties of both cartilage and bone are not uniform, influenced by the precise age and activity level of the person and the specific anatomic location within the skeleton. Of the various histologic components that are found in cartilage and bone, the collagen fibers and bundles are most influential in transmitting the forces that are applied to them, explaining in large part the location and direction of the resulting internal stresses that develop within these tissues. Therefore, thorough knowledge of the anatomy, physiology, and biomechanics of normal bone and cartilage serves as a prerequisite to a full understanding of both the manner in which these tissues adapt to physiologic stresses and the patterns of tissue failure that develop under abnormal conditions. Such knowledge forms the basis for more accurate assessment of the diverse imaging features that are encountered following acute traumatic and stress-related injuries to the skeleton. (©) RSNA, 2016.

Functional gradients in the pericarp of the green coconut inspire asymmetric fibre-composites with improved impact strength, and preserved flexural and tensile properties

Here we investigate the mechanical properties and structural design of the pericarp of the green coconut (Cocos nucifera L.). The pericarp showed excellent impact characteristics, and mechanical tests of its individual components revealed gradients in stiffness, strength and elongation at break from the outer to the inner layer of the pericarp. In order to understand more about the potential effect of such gradients on 'bulk' material properties, we designed simple, graded, cellulose fibre-reinforced polylactide (PLA) composites by stacking layers reinforced with fibres of different mechanical properties. Tensile properties of the graded composites were largely determined by the 'weakest' fibre, irrespective of the fibre distribution. However, a graded design led to pronounced asymmetric bending and impact properties. Bio-inspired, asymmetrically graded composites showed a flexural strength and modulus comparable to that of the strongest reference samples, but the elongation at maximum load was dependent on the specimen orientation. The impact strength of the graded composites showed a similar orientation-dependence, and peak values exceeded the impact strength of a non-graded reference composite containing identical fibre fractions by up to a factor of three. In combination, our results show that an asymmetric, systematic variation of fibre properties can successfully combine desirable properties of different fibre types, suggesting new routes for the development of high-performance composites, and improving our understanding of the structure-function relationship of the coconut pericarp.

Biomimetic Rotated Lamellar Plywood Motifs by Additive Manufacturing of Metal Alloy Scaffolds for Bone Tissue Engineering

Additive manufacturing presents opportunities to treat bone defects using biomimetic tissue scaffolds. Past investigations have explored modulating scaffold mechanical properties through varying materials and geometric motifs. Herein, we applied the rotated plywood structure of bone tissue to a 3D printed scaffold with the goal of improving mechanical performance compared to an orthogonal mesh design commonly used in tissue scaffold applications. The scaffolds were subjected to uniaxial compression followed by scanning electron microscopy and microcomputer tomography. The uniaxial compression test was characterized through elastic modulus (mean 1.32 GPa biomimetic, 0.196 GPa orthogonal, p < 0.001), ultimate compressive strength (mean 16.546 MPa biomimetic, 6.309 MPa orthogonal design, p < 0.001), and ultimate compressive strain values (4.867% biomimetic, 9.000% orthogonal, p < 0.005). Correlation of microfracture imaging to bulk scaffold mode of failure suggest that utilizing the biomimetic plywood design not only improved mechanical performance, but also reduced asymmetrtic buckling, plastic deformation, and fracture propagation similar to bone tissue.

Damage accumulation of bovine bone under variable amplitude loads

Stress fractures, a painful injury, are caused by excessive fatigue in bone. This study on damage accumulation in bone sought to determine if the Palmgren-Miner rule (PMR), a well-known linear damage accumulation hypothesis, is predictive of fatigue failure in bone. An electromagnetic shaker apparatus was constructed to conduct cyclic and variable amplitude tests on bovine bone specimens. Three distinct damage regimes were observed following fracture. Fractures due to a low cyclic amplitude loading appeared ductile ( 4000 μϵ), brittle due to high cyclic amplitude loading (> 9000 μϵ), and a combination of ductile and brittle from mid-range cyclic amplitude loading (6500 –6750 μϵ). Brittle and ductile fracture mechanisms were isolated and mixed, in a controlled way, into variable amplitude loading tests. PMR predictions of cycles to failure consistently over-predicted fatigue life when mixing isolated fracture mechanisms. However, PMR was not proven ineffective when used with a single damage mechanism.

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A novel apparatus is considered to conduct constant and variable amplitude loading tests.

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A wide range of forcing amplitudes are consider, such that damage mechanisms can be isolated.

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Active damage mechanisms are identified by fracture classification and SEM imaging.

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Palmgren-Miner Rule fails when multiple damage mechanisms are active, but does not fail when a single mechanism is active.

Does the relative density of periarticular bone influence the failure pattern of intra-articular fractures?

INTRODUCTION

The architecture of joints almost certainly influences the nature of intra-articular fractures, and the concavity is much more likely to fail than the associated convexity. However, local differences in periarticular bone density potentially also plays a critical role. The purpose of this study was to investigate if there was any difference in periarticular bone density in intra-articular fractures between the two opposing joint surfaces, comparing the convexity to the concavity.

MATERIALS AND METHODS

We retrospectively identified a series of 1003 intra-articular fractures of the hip, knee, and ankle; 129 of these patients had previously undergone CT scanning during their routine clinical assessment. Periarticular bone density was assessed using Hounsfield Units (HU) as a measure of the composite density of the adjacent bone. Bone density was compared between the opposite sides of each joint, to determine if a relationship exists between local bone density and the risk of articular surface fracture.

RESULTS

There was a statistically significant difference in density between the two opposing surfaces, with the convexity 19% more dense than the concavity (p=0.0001). The knee exhibited the largest difference (55%), followed by the hip (18%); in the ankle, an inverse relationship was observed, and the concave surface was paradoxically denser (5%). There was no significant difference between those cases where the concavity failed in isolation compared to those where the convexity also failed (p=0.28).

CONCLUSION

When the results were pooled for all three joints, there was a statistically significant higher local bone density demonstrated on the convex side of an intra-articular fracture. However, while this relationship was clearly exhibited in the knee, this was less evident in the other two joints; in the ankle the reverse was true, and the local bone adjacent to the concavity was found to have greater density. This suggests local bone density plays only a minor role in determining the nature of intra-articular fractures.

Bone Material Properties in Osteogenesis Imperfecta

Osteogenesis imperfecta entrains changes at every level in bone tissue, from the disorganization of the collagen molecules and mineral platelets within and between collagen fibrils to the macroarchitecture of the whole skeleton. Investigations using an array of sophisticated instruments at multiple scale levels have now determined many aspects of the effect of the disease on the material properties of bone tissue. The brittle nature of bone in osteogenesis imperfecta reflects both increased bone mineralization density-the quantity of mineral in relation to the quantity of matrix within a specific bone volume-and altered matrix-matrix and matrix mineral interactions. Contributions to fracture resistance at multiple scale lengths are discussed, comparing normal and brittle bone. Integrating the available information provides both a better understanding of the effect of current approaches to treatment-largely improved architecture and possibly some macroscale toughening-and indicates potential opportunities for alternative strategies that can influence fracture resistance at longer-length scales.

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.

Fragility of Bone Material Controlled by Internal Interfaces

Bone material is built in a complex multiscale arrangement of mineralized collagen fibrils containing water, proteoglycans and some noncollagenous proteins. This organization is not static as bone is constantly remodeled and thus able to repair damaged tissue and adapt to the loading situation. In preventing fractures, the most important mechanical property is toughness, which is the ability to absorb impact energy without reaching complete failure. There is no simple explanation for the origin of the toughness of bone material, and this property depends in a complex way on the internal architecture of the material on all scales from nanometers to millimeters. Hence, fragility may have different mechanical origins, depending on which toughening mechanism is not working properly. This article reviews the toughening mechanisms described for bone material and attempts to put them in a clinical context, with the hope that future analysis of bone fragility may be guided by this collection of possible mechanistic origins.

Microscopic Pattern of Bone Fractures as an Indicator of Blast Trauma: A Pilot Study

The assessment of fractures is a key issue in forensic anthropology; however, very few studies deal with the features of fractures due to explosion in comparison with other traumatic injuries. This study focuses on fractures resulting from blast trauma and two types of blunt force trauma (manual compression and running over), applied to corpses of pigs; 163 osteons were examined within forty fractures by the transmission light microscopy. Blast lesions showed a higher percentage of fracture lines through the Haversian canal, whereas in other types of trauma, the fractures went across the inner lamellae. Significant differences between samples hit by blast energy and those runover or manually compressed were observed (p<0.05). The frequency of pattern A is significantly higher in exploded bones than in runover and compressed. Microscopic analysis of the fracture line may provide information about the type of trauma, especially for what concerns blast trauma.

Does joint architecture influence the nature of intra-articular fractures?

INTRODUCTION

The architecture of joints has potentially the greatest influence on the nature of intra-articular fractures. We analysed a large number of intra-articular fractures with two aims: (1) to determine if the pattern of injuries observed supports our conjecture that the local skeletal architecture is an important factor and (2) to investigate whether associated dislocations further affect the fracture pattern.

METHODS

A retrospective study of intra-articular fractures over a 3.5-year period; 1003 joints met inclusion criteria and were analysed. Three independent investigators determined if fractures affected the convex dome, the concave socket, or if both joint surfaces were involved. Further review determined if a joint dislocation occurred with the initial injury. Statistical analysis was performed using a one-way frequency table, and the χ(2) test was used to compare the frequencies of concave and convex surface fractures. The odds ratios (ORs) were calculated to establish the association between the frequencies of concave and convex surface fractures, as well as between dislocation and either fracture surface involvement.

RESULTS

Of the 1003 fractures analysed, 956 (95.3%) involved only the concavity of the joint; in 21 fractures (2.1%) both joint surfaces were involved; and in 26 fractures (2.6%) only the convexity was involved (χ(2)=1654.9, df=2, p<0.0001). As expected, the concavity was 20.8 times more likely to fail than the convexity (11.2-36.6, 95% CI). However, the risk of fracturing the convex surface was 18.6 times higher (9.8-35.2, 95% CI) in association with a simultaneous joint dislocation, compared to those cases without a joint dislocation.

CONCLUSIONS

These results very strongly support the study hypotheses: the skeletal architecture of joints clearly plays a highly significant role in determining the nature of intra-articular fractures. Intra-articular fractures involving the convexity are much more likely to be associated with a concurrent joint dislocation.

Quantification of nonlinear elasticity for the evaluation of submillimeter crack length in cortical bone

The objective of this study was to investigate the sensitivity of the nonlinear elastic properties of cortical bone to the presence of a single submillimetric crack. Nonlinear elasticity was measured by nonlinear resonant ultrasound spectroscopy (NRUS) in 14 human cortical bone specimens. The specimens were parallelepiped beams (50×2×2 mm(3)). A central notch of 500 µm was made to control crack initiation and propagation during four-point bending. The nonlinear hysteretic elastic and dissipative parameters αf and αQ, and Young׳s modulus Eus were measured in dry condition for undamaged (control) specimens and in dry and wet conditions for damaged specimens. The length of the crack was assessed using synchrotron radiation micro-computed tomography (SR-μCT) with a voxel size of 1.4 μm. The initial values of αf, measured on the intact specimens, were remarkably similar for all the specimens (αf =-5.5±1.5). After crack propagation, the nonlinear elastic coefficient αf increased significantly (p<0.006), with values ranging from -4.0 to -296.7. Conversely, no significant variation was observed for αQ and Eus. A more pronounced nonlinear elastic behavior was observed in hydrated specimens compared to dry specimens (p<0.001) after propagation of a single submillimetric crack. The nonlinear elastic parameter αf was found to be significantly correlated to the crack length both in dry (R=0.79, p<0.01) and wet (R=0.84, p<0.005) conditions. Altogether these results show that nonlinear elasticity assessed by NRUS is sensitive to a single submillimetric crack induced mechanically and suggest that the humidity must be strictly controlled during measurements.

Influence of structural hierarchy on the fracture behaviour of tooth enamel

Tooth enamel has the critical role of enabling the mastication of food and also of protecting the underlying vital dentin and pulp structure. Unlike most vital tissue, enamel has no ability to repair or remodel and as such has had to develop robust damage tolerance to withstand contact fatigue events throughout the lifetime of a species. To achieve such behaviour, enamel has evolved a complex hierarchical structure that varies slightly between different species. The major component of enamel is apatite in the form of crystallite fibres with a nanometre-sized diameter that extend from the dentin-enamel junction to the oral surface. These crystallites are bound together by proteins and peptides into a range of hierarchical structures from micrometre diameter prisms to 50-100 μm diameter bundles of prisms known as Hunter-Schreger bands. As a consequence of such complex structural organization, the damage tolerance of enamel increases through various toughening mechanisms in the hierarchy but at the expense of fracture strength. This review critically evaluates the role of hierarchy on the development of the R-curve and the stress-strain behaviour. It attempts to identify and quantify the multiple mechanisms responsible for this behaviour as well as their impact on damage tolerance.

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.

Collagen modifications in postmenopausal osteoporosis: advanced glycation endproducts may affect bone volume, structure and quality

The classic model of postmenopausal osteoporosis (PM-OP) starts with the depletion of estrogen, which in turn stimulates imbalanced bone remodeling, resulting in loss of bone mass/volume. Clinically, this leads to fractures because of structural weakness. Recent work has begun to provide a more complete picture of the mechanisms of PM-OP involving oxidative stress and collagen modifications known as advanced glycation endproducts (AGEs). On one hand, AGEs may drive imbalanced bone remodeling through signaling mediated by the receptor for AGEs (RAGE), stimulating resorption and inhibiting formation. On the other hand, AGEs are associated with degraded bone material quality. Oxidative stress promotes the formation of AGEs, inhibits normal enzymatically derived crosslinking and can degrade collagen structure, thereby reducing fracture resistance. Notably, there are multiple positive feedback loops that can exacerbate the mechanisms of PM-OP associated with oxidative stress and AGEs. Anti-oxidant therapies may have the potential to inhibit the oxidative stress based mechanisms of this disease.

Insights into the effects of tensile and compressive loadings on human femur bone

Background:

Fragile fractures are most likely manifestations of fatigue damage that develop under repetitive loading conditions. Numerous microcracks disperse throughout the bone with the tensile and compressive loads. In this study, tensile and compressive load tests are performed on specimens of both the genders within 19 to 83 years of age and the failure strength is estimated.

Materials and Methods:

Fifty five human femur cortical samples are tested. They are divided into various age groups ranging from 19-83 years. Mechanical tests are performed on an Instron 3366 universal testing machine, according to American Society for Testing and Materials International (ASTM) standards.

Results:

The results show that stress induced in the bone tissue depends on age and gender. It is observed that both tensile and compression strengths reduces as age advances. Compressive strength is more than tensile strength in both the genders.

Conclusion:

The compression and tensile strength of human femur cortical bone is estimated for both male and female subjecting in the age group of 19-83 years. The fracture toughness increases till 35 years in male and 30 years in female and reduces there after. Mechanical properties of bone are age and gender dependent.

Development and validation of a distal radius finite element model to simulate impact loading indicative of a forward fall

The purpose of this work was to develop and validate a finite element model of the distal radius to simulate impact loading. Eight-node hexahedral meshes of the bone and impactor components were created. Three separate impact events were simulated by altering the impact velocity assigned to the model projectile (pre-fracture, crack and fracture). Impact forces and maximum and minimum principal strains were calculated and used in the validation process by comparing with previously collected experimental data. Three measures of mesh quality (Jacobians, aspect ratios and orthogonality) and four validation methods (validation metric, error assessment, fracture comparisons and ensemble averages) assessed the model. The element Jacobians, aspect ratios and orthogonality measures ranged from 0.08 to 12, 1.1 to 26 and -70° to 80°, respectively. The force and strain validation metric ranged from 0.10 to 0.54 and 0.35 to 0.67, respectively. The estimated peak axial force was found to be a maximum of 28.5% greater than the experimental (crack) force, and all forces fell within ±2 standard deviation of the mean experimental fracture forces. The predicted strains were found to differ by a mean of 33% across all impact events, and the model was found to accurately predict the location and severity of bone damage. Overall, the model presented here is a valid representation of the distal radius subjected to impact.

Microstructural failure mechanisms in the human proximal femur for sideways fall loading

The etiology of hip fractures remains unclear but might be elucidated by an improved understanding of the microstructural failure mechanisms of the human proximal femur during a sideways fall impact. In this context, we biomechanically tested 12 cadaver proximal femurs (aged 76 ± 10 years; 8 female, 4 male) to directly measure strength for a sideways fall and also performed micro-computed tomography (CT)-based, nonlinear finite element analysis of the same bones (82-micron-sized elements, ∼120 million elements per model) to estimate the amount and location of internal tissue-level failure (by ductile yielding) at initial structural failure of the femur. We found that the correlation between the directly measured yield strength of the femur and the finite element prediction was high (R(2) = 0.94, p < 0.0001), supporting the validity of the finite element simulations of failure. In these simulations, the failure of just a tiny proportion of the bone tissue (1.5% to 6.4% across all bones) led to initial structural failure of the femur. The proportion of failed tissue, estimated by the finite element models, decreased with decreasing measured femoral strength (R(2) = 0.88, p < 0.0001) and was more highly correlated with measured strength than any measure of bone volume, mass, or density. Volume-wise, trabecular failure occurred earlier and was more prominent than cortical failure in all femurs and dominated in the very weakest femurs. Femurs with low measured strength relative to their areal bone mineral density (BMD) (by dual-energy X-ray absorptiometry [DXA]) had a low proportion of trabecular bone compared with cortical bone in the femoral neck (p < 0.001), less failed tissue (p < 0.05), and low structural redundancy (p < 0.005). We conclude that initial failure of the femur during a sideways fall is associated with failure of just a tiny proportion of the bone tissue, failure of the trabecular tissue dominating in the very weakest femurs owing in part to a lack of structural redundancy.

Non destructive characterization of cortical bone micro-damage by nonlinear resonant ultrasound spectroscopy

The objective of the study was to evaluate the ability of a nonlinear ultrasound technique, the so-called nonlinear resonant ultrasound spectroscopy (NRUS) technique, for detecting early microdamage accumulation in cortical bone induced by four-point bending fatigue. Small parallelepiped beam-shaped human cortical bone specimens were subjected to cyclic four-point bending fatigue in several steps. The specimens were prepared to control damage localization during four-point bending fatigue cycling and to unambiguously identify resonant modes for NRUS measurements. NRUS measurements were achieved to follow the evolution of the nonlinear hysteretic elastic behavior during fatigue-induced damage. After each fatigue step, a small number of specimens was removed from the protocol and set apart to quantitatively assess the microcrack number density and length using synchrotron radiation micro-computed tomography (SR-µCT). The results showed a significant effect of damage steps on the nonlinear hysteretic elastic behavior. No significant change in the overall length of microcracks was observed in damaged regions compared to the load-free control regions. Only an increased number of shortest microcracks, those in the lowest quartile, was noticed. This was suggestive of newly formed microcracks during the early phases of damage accumulation. The variation of nonlinear hysteretic elastic behavior was significantly correlated to the variation of the density of short microcracks. Our results suggest that the nonlinear hysteretic elastic behavior is sensitive to early bone microdamage. Therefore NRUS technique can be used to monitor fatigue microdamage progression in in vitro experiments.

Stress-whitening occurs in demineralized bone

INTRODUCTION

The incidence of age-related bone fracture is increasing with average population age. Bone scatters more light (stress-whitens) during loading, immediately prior to failure, in a manner visually similar to polymer crazing. We wish to understand the stress-whitening process because of its possible effect on bone toughness. The goals of this investigation were a) to establish that stress-whitening is a property of the demineralized organic matrix of bone rather than only a property of mineralized tissue and that stress whitening within the demineralized bone is dependent upon both b) hydrogen bonding and, c) the orientation of loading.

METHODS

Demineralized cortical bone specimens were loaded in tension to failure (0.08 strain/s). The effect of hydrogen bonding on mechanical properties and the stress-whitening process was probed by altering the Hansen's hydrogen bonding parameter (δh) of the immersing solution.

RESULTS

Stress-whitening occurred in the demineralized bone. Stress-whitening was negatively correlated with δh (R(2)=0.81, p<0.0001). Stress-whitening was significantly lower (p<0.0001) in specimens loaded orthogonally compared to those loaded parallel to the long (strong) axis.

CONCLUSION

The stress-whitening observed was consistent with increased Mie scattering. We suggest that the change in Mie scattering was due to collagen fibril dehydration driven by the externally applied stress. The presence of stress-whitening in demineralized bone suggests that this process may be a property of the collagenous matrix and hence may be present in other collagenous tissues rather than an emergent property of the bone composite.

Nanomechanics insights into the performance of healthy and osteoporotic bones

In situ nanoscopic observations of healthy and osteoporotic bone nanopillars under compression were performed. The structural-mechanical property relationship at the atomic scale suggests that cortical bone performance is correlated to the feature, arrangement, movement, distortion, and fracture of hydroxyapatite nanocrystals. Healthy bone comprising tightly bound mineral nanocrystals shows high structural stability with nanoscopic lattice distortions and dislocation activities. On the other hand, osteoporotic bone exhibits brittleness owing to the movements of dispersed minerals in and intergranular fracture along a weak organic matrix.

Investigation of the three-dimensional orientation of mineralized collagen fibrils in human lamellar bone using synchrotron X-ray phase nano-tomography

We investigate the three-dimensional (3-D) organization of mineralized collagen fibrils in human cortical bone based on synchrotron X-ray phase nano-tomography images. In lamellar bone the collagen fibrils are assumed to have a plywood-like arrangement, but due to experimental limitations the 3-D fibril structure has only been deduced from section surfaces so far and the findings have been controversial. Breakthroughs in synchrotron tomographic imaging have given access to direct 3-D information on the bone structure at the nanoscale level. Using an autocorrelation-based orientation measure we confirm that the fibrils are unidirectional in quasi-planes of sub-lamellae and find two specific dominant patterns, oscillating and twisted plywoods coexisting in a single osteon. Both patterns exhibit smooth orientation changes between adjacent quasi-planes. Moreover, we find that the periodic changes in collagen fibril orientation are independent of fluctuations in local mass density. These data improve our understanding of the lamellar arrangement in bone and allow more detailed investigations of structure-function relationships at this scale, providing templates for bio-inspired materials. The presented methodology can be applied to non-destructive 3-D characterization of the sub-micron scale structure of other natural and artificial mineralized biomaterials.

Variation in type I collagen fibril nanomorphology: the significance and origin

Although the axial D-periodic spacing is a well-recognized nanomorphological feature of type I collagen fibrils, the existence of a distribution of values has been largely overlooked since its discovery seven decades ago. Studies based on single fibril measurements occasionally noted variation in D-spacing values, but accredited it with no biological significance. Recent quantitative characterizations supported that a 10-nm collagen D-spacing distribution is intrinsic to collagen fibrils in various tissues as well as in vitro self-assembly of reconstituted collagen. In addition, the distribution is altered in Osteogenesis Imperfecta and long-term estrogen deprivation. Bone collagen is organized into lamellar sheets of bundles at the micro-scale, and D-spacings within a bundle of a lamella are mostly identical, whereas variations among different bundles contribute to the full-scale distribution. This seems to be a very general phenomenon for the protein as the same type of D-spacing/bundle organization is observed for dermal and tendon collagen. More research investigation of collagen nanomorphology in connection to bone biology is required to fully understand these new observations. Here we review the data demonstrating the existence of a D-spacing distribution, the impact of disease on the distribution and possible explanations for the origin of D-spacing variations based on various collagen fibrillogenesis models.

Micro-structure and mechanical properties of the turtle carapace as a biological composite shield

Turtle shell is a multi-scale bio-composite in which the components are arranged in various spatial patterns, leading to an unusually strong and durable structure. The keratin-coated dorsal shell, termed the carapace, exhibits a flat bone, sandwich-like structure made up of two exterior cortices enclosing a cancellous interior. This unique structure was developed by nature to protect the reptile from predator attacks by sustaining impact loads and dissipating energy. In the present study we attempt to correlate the micro-scale architecture with the mechanical properties of the carapace sub-regions of the red-eared slider turtle. The microscopic structural features were examined by scanning electron microscopy and micro-computed tomography. Nanoindentation tests were performed under dry and wet conditions on orthogonal anatomical planes to evaluate the elastic modulus and hardness of the various carapace sub-regions. The mineral content was also measured in the different regions of the carapace. Consequently, we discuss the influence of hydration on the carapace sub-regions and the contribution of each sub-region to the overall mechanical resistance of the assemblage.

Towards patient-specific material modeling of trabecular bone post-yield behavior

Bone diseases such as osteoporosis are one of the main causes of bone fracture and often result in hospitalization and long recovery periods. Researchers are aiming to develop new tools that consider the multiple determinants acting at the different scales of bone, and which can be used to clinically estimate patient-specific fracture risk and also assess the efficacy of new therapies. The main step towards this goal is a deep understanding of the bone organ, and is achieved by modeling the complexity of the structure and the high variability of the mechanical outcome. This review uses a hierarchical approach to evaluate bone mechanics at the macroscale, microscale, and nanoscale levels and the interactions between scales. The first section analyzes the experimental evidence of bone mechanics in the elastic and inelastic regions, microdamage generation, and post-yield toughening mechanisms from the organ level to the ultrastructural level. On the basis of these observations, the second section provides an overview of the constitutive models available to describe bone mechanics and predict patient-specific outcomes. Overall, the role of the hierarchical structure of bone and the interplay between each level is highlighted, and their effect is evaluated in terms of modeling biological variability and patient specificity.

Type I collagen D-spacing in fibril bundles of dermis, tendon, and bone: bridging between nano- and micro-level tissue hierarchy

Fibrillar collagens in connective tissues are organized into complex and diverse hierarchical networks. In dermis, bone, and tendon, one common phenomenon at the micrometer scale is the organization of fibrils into bundles. Previously, we have reported that collagen fibrils in these tissues exhibit a 10 nm width distribution of D-spacing values. This study expands the observation to a higher hierarchical level by examining fibril D-spacing distribution in relation to the bundle organization. We used atomic force microscopy imaging and two-dimensional fast Fourier transform analysis to investigate dermis, tendon, and bone tissues. We found that, in each tissue type, collagen fibril D-spacings within a single bundle were nearly identical and frequently differ by less than 1 nm. The full 10 nm range in D-spacing values arises from different values found in different bundles. The similarity in D-spacing was observed to persist for up to 40 μm in bundle length and width. A nested mixed model analysis of variance examining 107 bundles and 1710 fibrils from dermis, tendon, and bone indicated that fibril D-spacing differences arise primarily at the bundle level (∼76%), independent of species or tissue types.