Bone Toughness Research Articles

Time-lapsed assessment of microcrack initiation and propagation in murine cortical bone at submicrometer resolution

The strength of bone tissue is not only determined by its mass, but also by other properties usually referred to as bone quality, such as microarchitecture, distribution of bone cells, or microcracks and damage. It has been hypothesized that the bone ultrastructure affects microcrack initiation and propagation. Due to its high resolution, bone assessment by means of synchrotron radiation (SR)-based computed tomography (CT) allows unprecedented three-dimensional (3D) and non-invasive insights into ultrastructural bone phenotypes, such as the canal network and the osteocyte lacunar system. The aims of this study were to describe the initiation and propagation of microcracks and their relation with these ultrastructural phenotypes. To this end, femora from the two genetically distinct inbred mouse strains C3H/He (C3H) and C57BL/6 (B6) were loaded axially under compression, from 0% strain to failure, with 1% strain steps. Between each step, a high-resolution 3D image (700 nm nominal resolution) was acquired at the mid-diaphysis using SR CT for characterization and quantitative analysis of the intracortical porosity, namely the bone canal network, the osteocyte lacunar system and the emerging microcracks. For C3H mice, the canal, lacunar, and microcrack volume densities accounted typically for 1.91%, 2.11%, and 0.27% of the cortical total volume at 2% apparent strain, respectively. Due to its 3D nature, SR CT allowed to visualize and quantify also the volumetric extent of microcracks. At 2% apparent strain, the average microcrack thickness for both mouse strains was 2.0 μm for example. Microcracks initiated at canal and at bone surfaces, whereas osteocyte lacunae provided guidance to the microcracks. Moreover, we observed that microcracks could appear as linear cracks in one plane, but as diffuse cracks in a perpendicular plane. Finally, SR CT images permitted visualization of uncracked ligament bridging, which is thought to be of importance in bone toughening mechanisms. In conclusion, this study showed the power of SR CT for 3D visualization and quantification of the different ultrastructural phases of the intracortical bone porosity. We particularly postulate the necessity of 3D imaging techniques to unravel microcrack initiation and propagation and their effects on bone mechanics. We believe that this new investigation tool will be very useful to further enhance our understanding of bone failure mechanisms.

Changes in bone microstructure and toughness during the healing process of long bones

It is of great importance to understand how bone defects regain the microstructure and mechanical function of bone and how the microstructure affects the mechanical function during the bone healing process. In the present study on long bone defects, we investigated the relationship between the recovery process of fracture toughness and biological apatite (BAp)/collagen (Col) alignment as an index of the bone microstructure to clarify the bone toughening mechanisms. A 5-mm defect introduced in the rabbit ulna was allowed to heal naturally and a three-point bending test was conducted on the regenerated site to assess bone toughness. The bone toughness was quite low at the early stage of bone regeneration but increased during the postoperative period. The change in toughness agreed well with the characteristics of the fracture surface morphology, which reflected the history of the crack propagation. SEM and microbeam X-ray diffraction analyses indicated that the toughness was dominated by the degree and orientation of the preferred BAp/Col alignment, i.e. bundles aligned perpendicular to the crack propagation clearly contributed to the bone toughening owing to extra energy consumption for resistance to crack propagation. In conclusion, regenerated bone improves fracture toughness by reconstructing the preferred BAp/Col alignment along the bone longitudinal axis during the healing process of long bones.

Density and architecture have greater effects on the toughness of trabecular bone than damage

Bone damage has been cited as an important aspect of bone quality. As such, understanding the effects of damage on the toughness of trabecular bone should provide insight into trabecular bone behavior during energy-limiting cases, such as falls. The effects of damage on the toughness of 35 bovine trabecular bone specimens were studied. Damage was induced by compressing the on-axis specimens to either 1.5% or 2.5% strain, followed by compression to 7.5% strain. The overloads resulted in significant decreases in both modulus and elastic toughness, with significantly greater decreases for the high-damage group than the low-damage group. Following damage, the elastic toughness of the high-damage group was also lower than the undamaged elastic toughness of the control group. In contrast, there was no detectable effect of damage level on toughness measured to 7.5% strain. Toughness increased linearly with BMD (R2=0.50) and by a power law relationship with volume fraction (BV/TV) (R2=0.65). Microarchitectural parameters also predicted the toughness in the absence of BV/TV or BMD. Toughness decreased with increasing slenderness ratio (Tb.Sp/Tb.Th) and structure model index (SMI) (R2=0.68, multiple regression), again independent of damage level, suggesting that failure is influenced by trabecular buckling. Taken together, the results show that normal variations in toughness due to density and architecture dominate the changes due to damage at the levels induced in this study. Moreover, measuring toughness is sensitive to the final strain, as differences found in the elastic and initial plastic regions were undetectable at higher strains. The self-limiting nature of microcracks in trabecular bone, or the trabecular architecture itself, may inhibit microcracks from propagating to macroscopic trabecular fractures, thereby limiting the effect of damage on toughness and making it difficult to detect in comparison to normal population variability.

INFLUENCE OF RELATIVE STRENGTH OF CONSTITUENTS ON THE OVERALL STRENGTH AND TOUGHNESS OF BONE

Biocomposites such as bone exhibit synergistic superior mechanical properties compared to its constituents, protein (collagen) and mineral (hydroxyapatite). The importance of properties of constituents at the submicron scale with regard to the toughness and strength of bone is investigated employing a discrete lattice model. The results show that matrix failure as opposed to platelet breakage provides better toughness to the bone. There is a fairly sudden increase in the toughness of bone when the strength of mineral platelet to that of protein crosses a particular critical value. These could provide clues to the preparation of ultra-tough artificial composites and the treatment of diseases related to fragility of bone.

Toughness of Human Cortical Bone Measured with Realistically Short Cracks

Toughness of Human Cortical Bone Measured with Realistically Short Cracks

Microcracking damage and the fracture process in relation to strain rate in human cortical bone tensile failure

It is difficult to define the ‘physiological’ mechanical properties of bone. Traumatic failures in-vivo are more likely to be orders of magnitude faster than the quasistatic tests usually employed in-vitro. We have reported recently [Hansen, U., Zioupos, P., Simpson, R., Currey, J.D., Hynd, D., 2008. The effect of strain rate on the mechanical properties of human cortical bone. Journal of Biomechanical Engineering/Transactions of the ASME 130, 011011-1-8] results from tests on specimens of human femoral cortical bone loaded in tension at strain rates ( ε ˙ ) ranging from low (0.08 s −1) to high (18 s −1). Across this strain rate range the modulus of elasticity generally increased, stress at yield and failure and strain at failure decreased for rates higher than 1 s −1, while strain at yield was invariant for most strain rates and only decreased at rates higher than 10 s −1. The results showed that strain rate has a stronger effect on post-yield deformation than on initiation of macroscopic yielding. In general, specimens loaded at high strain rates were brittle, while those loaded at low strain rates were much tougher. Here, a post-test examination of the microcracking damage reveals that microcracking was inversely related to the strain rate. Specimens loaded at low strain rates showed considerable post-yield strain and also much more microcracking. Partial correlation and regression analysis suggested that the development of post-yield strain was a function of the amount of microcracking incurred (the cause), rather than being a direct result of the strain rate (the excitation). Presumably low strain rates allow time for microcracking to develop, which increases the compliance of the specimen, making them tougher. This behaviour confirms a more general rule that the degree to which bone is brittle or tough depends on the amount of microcracking damage it is able to sustain. More importantly, the key to bone toughness is its ability to avoid a ductile-to-brittle transition for as long as possible during the deformation. The key to bone's brittleness, on the other hand, is the strain and damage localisation early on in the process, which leads to low post-yield strains and low-energy absorption to failure.

Bone Morphometry Strongly Predicts Cortical Bone Stiffness and Strength, but Not Toughness, in Inbred Mouse Models of High and Low Bone Mass

Inbred strains of mice make useful models to study bone properties. Our aim was to compare bone competence and cortical morphometric parameters of two inbred strains to better determine the role of bone structure and geometry in the process of bone failure. Morphometric analysis was performed on 20 murine femora with a low bone mass (C57BL/6J; B6) and 20 murine femora with a high bone mass (C3H/HeJ; C3H) using desktop microCT. The bones were tested under three-point bending to measure their mechanical properties. Results showed that the C3H strain is a more reproducible model regarding bone morphometric and mechanical phenotypes than the B6 strain. Bone strength, stiffness, yield force, yield displacement, and toughness, as well as morphometric traits, were all significantly different between the two strains, whereas postyield displacement was not. It was found that bone volume, cortical thickness, and cross-sectional area predicted almost 80% (p < 0.05) of bone stiffness, strength, and yield force. Nevertheless, cortical bone postyield properties such as bone toughness could not be explained by morphometry, but postyield whitening was observed in that phase. In conclusion, we found that morphometric parameters are strong predictors of preyield but not postyield properties. The lack of morphometric influence on bone competence in the postyield phase in combination with the observed postyield whitening confirmed the important contribution of ultrastructure and microdamage in the process of overall bone failure behavior, especially in the postyield phase.

EXERCISE HISTORY AND STRESS FRACTURE REMODELING ARE RELATED TO CORTICAL BONE ULTIMATE STRESS

Bone, a tri-phase composite, consists of nano-sized apatite minerals, an organic component, and water. Heat-treated bovine cortical bone has been proposed as a candidate for void-filling bone substitute. However, the toughness of heat-treated bone is not yet fully studied. Fracture toughness (Kc) and work of fracture (Wf) of hydrated, dehydrated, and ashed bovine bone were estimated using a single-edge V-notched beam method. Thermal gravimetric analysis and differential thermal analysis were used to determine the temperature at which the organics and water were removed. Dehydrated specimens were obtained by placing the samples in a 60 °C vacuum oven for 24 h or a 110 °C furnace for 2 h. Ashed specimens were obtained by heat-treating samples at 600 °C for 24 h. Kc of bovine specimens decreased from 5.5 MPa·m1/2 for hydrated bone, to 3.8 MPa·m1/2 for dehydrated specimens, and to 0.36 MPa·m1/2 for ashed specimens. Wf decreased from 7.1 to 1.1 kJ/m2 for dehydrated specimens, and to 0.04 kJ/m2 for ashed specimens. The main reasons for the significant decreases in Kc and Wf may be attributed to water's ability in stabilizing collagen structure and to the organics’ ability in making bone more ductile. Because of the large decrease in fracture toughness and work of fracture, we suggest that ashed bone is not appropriate for load-bearing bone substitute in areas where bone experiences loadings in flexure.

The true toughness of human cortical bone measured with realistically short cracks

Bone is more difficult to break than to split. Although this is well known, and many studies exist on the behaviour of long cracks in bone, there is a need for data on the orientation-dependent crack-growth resistance behaviour of human cortical bone that accurately assesses its toughness at appropriate size scales. Here, we use in situ mechanical testing to examine how physiologically pertinent short (<600 microm) cracks propagate in both the transverse and longitudinal orientations in cortical bone, using both crack-deflection/twist mechanics and nonlinear-elastic fracture mechanics to determine crack-resistance curves. We find that after only 500 microm of cracking, the driving force for crack propagation was more than five times higher in the transverse (breaking) direction than in the longitudinal (splitting) direction owing to major crack deflections/twists, principally at cement sheaths. Indeed, our results show that the true transverse toughness of cortical bone is far higher than previously reported. However, the toughness in the longitudinal orientation, where cracks tend to follow the cement lines, is quite low at these small crack sizes; it is only when cracks become several millimetres in length that bridging mechanisms can fully develop leading to the (larger-crack) toughnesses generally quoted for bone.

Alendronate Reduces Bone Toughness of Ribs without Significantly Increasing Microdamage Accumulation in Dogs Following 3 Years of Daily Treatment

Reduced bone toughness, the energy absorption capacity of the tissue, has been consistently documented in vertebrae of animals treated with a wide range of bisphosphonate doses. Data regarding toughness changes in the rib are conflicting, with one report showing no effect and another showing a significant reduction following treatment of beagle dogs with high doses of bisphosphonates. The goal of this study was to evaluate changes in bone toughness and various other tissue-level properties of the rib following 3 years of bisphosphonate treatment with doses at and above those used to treat osteoporosis. Skeletally mature intact beagle dogs were treated daily for 3 years with vehicle (VEH), alendronate 0.2 mg/kg (ALN0.2), or alendronate 1.0 mg/kg (ALN1.0). The lower ALN dose approximates, on a milligram per kilogram basis, that used for treatment of postmenopausal osteoporosis, with the higher dose being five times higher. Ribs were assessed for biomechanical properties, bone turnover rate, microdamage, density, and geometry. Toughness was significantly lower with ALN1.0 (-33%) but not ALN0.2 (-19%) compared to VEH, while neither ultimate stress nor modulus differed among the groups. Bone density, geometry, and structural biomechanical properties were similar among the three groups. There was no significant difference in overall microdamage accumulation among the groups. Intracortical bone formation rate was significantly lower than VEH in both ALN groups (-69% to -90%). These data show that while rib cortical bone experiences significant reductions in turnover following bisphosphonate treatment, it is only in animals treated with doses above those used to treat osteoporosis that toughness is significantly compromised.

The toughness of bone and the influence of bisphosphonates on the resistance to fracture

The toughness of bone and the influence of bisphosphonates on the resistance to fracture

Enhanced Mechanism of Fracture Toughness in Cannon Bone

As a typical biological material, bone possesses high fracture strength and fracture toughness, which are closely related to its exquisite microstructure. SEM observation of a cannon bone shows that the bone is a kind of layered bioceramic composite consisting of hydroxyapatite sheets and collagen matrix. The hydroxyapatite sheets are of long and thin shape, distributing in parallel. The fracture toughness of the bone is analyzed with the representative model of the hydroxyapatite sheets and the concept of maximum pullout energy. It is shown that the lathy shape as well as the parallel distribution of the hydroxyapatite sheets increases the pullout energy and endows the bone with high fracture toughness.

Fracture toughness and work of fracture of hydrated, dehydrated, and ashed bovine bone

Bone, a tri-phase composite, consists of nano-sized apatite minerals, an organic component, and water. Heat-treated bovine cortical bone has been proposed as a candidate for void-filling bone substitute. However, the toughness of heat-treated bone is not yet fully studied. Fracture toughness ( K c) and work of fracture ( W f) of hydrated, dehydrated, and ashed bovine bone were estimated using a single-edge V-notched beam method. Thermal gravimetric analysis and differential thermal analysis were used to determine the temperature at which the organics and water were removed. Dehydrated specimens were obtained by placing the samples in a 60 °C vacuum oven for 24 h or a 110 °C furnace for 2 h. Ashed specimens were obtained by heat-treating samples at 600 °C for 24 h. K c of bovine specimens decreased from 5.5 MPa·m 1/2 for hydrated bone, to 3.8 MPa·m 1/2 for dehydrated specimens, and to 0.36 MPa·m 1/2 for ashed specimens. W f decreased from 7.1 to 1.1 kJ/m 2 for dehydrated specimens, and to 0.04 kJ/m 2 for ashed specimens. The main reasons for the significant decreases in K c and W f may be attributed to water's ability in stabilizing collagen structure and to the organics’ ability in making bone more ductile. Because of the large decrease in fracture toughness and work of fracture, we suggest that ashed bone is not appropriate for load-bearing bone substitute in areas where bone experiences loadings in flexure.

Failure of mineralized collagen fibrils: Modeling the role of collagen cross-linking

Experimental evidence demonstrates that collagen cross-linking in bone tissue significantly influences its deformation and failure behavior yet difficulties exist in determining the independent biomechanical effects of collagen cross-linking using in vitro and in vivo experiments. The aim of this study is to use a nano-scale composite material model of mineral and collagen to determine the independent roles of enzymatic and non-enzymatic cross-linking on the mechanical behavior of a mineralized collagen fibril. Stress–strain curves were obtained under tensile loading conditions without any collagen cross-links, with only enzymatic cross-links (modeled by cross-linking the end terminal position of each collagen domain), or with only non-enzymatic cross-links (modeled by random placement of cross-links within the collagen–collagen interfaces). Our results show enzymatic collagen cross-links have minimal effect on the predicted stress–strain curve and produce a ductile material that fails through debonding of the mineral–collagen interface. Conversely, non-enzymatic cross-links significantly alter the predicted stress–strain response by inhibiting collagen sliding. This inhibition leads to greater load transfer to the mineral, which minimally affects the predicted stress, increases modulus and decreases post-yield strain and toughness. As a consequence the toughness of bone that has more non-enzymatically mediated collagen cross-links will be drastically reduced.

Biomechanics of osteoporotic fractures

The objective of this review article is to present biomechanics concepts and data relevant to osteoporotic fractures. Fractures are mechanical events that occur when the applied load exceeds the fracture load (bone strength); both loading and strength must be considered to understand fracture risk. Hip fractures are almost always due to a fall, but only 5% of falls result in fracture. Hip fracture risk is greatest for a sideways fall that impacts on the greater trochanter. The loading events that cause vertebral fractures are poorly understood but include falls and heavy lifting. Activities that involve forward flexion and lifting generate the largest forces on the spine. Factors that affect bone strength include bone size (geometry) and bone density (vBMD). Men have larger bones at all ages compared to women and this is the main factor in the gender difference in whole-bone strength. Both men and women lose trabecular bone density and thus bone strength with aging, which is the main reason for the age-related loss of bone strength at the hip and spine, although dramatic decreases in the toughness of cortical bone may also contribute to osteoporotic fragility. The factor of risk (applied force/fracture force) is a useful concept for considering both the injury and the disease component of osteoporotic fractures. Within this article, I will review data on age-related changes in factors that affect fracture risk. Advanced engineering concepts will not be presented, but a familiarity with basic mechanical principles is assumed.

Fracture and repair of bone: a multiscale problem

This article reviews current work on the strength and toughness of bone, its mechanisms of fracture and its ability to repair and adapt its structure. These properties are affected at all size levels, from the nanostructure of collagen molecules and mineral crystals, through the microstructure of osteons and trabeculae, up to the macroscopic shape and density variations that occur at the level of a whole bone.

Osteonectin-derived peptide increases the modulus of a bone-mimetic nanocomposite

Many factors contribute to the toughness of bone including the presence of nano-size apatite crystals, a dense network of collagen fibers, and acidic proteins with the ability to link the mineral phase to the gelatinous collagen phase. We investigated the effect of a glutamic acid (negatively charged) peptide (Glu6), which mimics the terminal region of the osteonectin glycoprotein of bone, on the shear modulus of a synthetic hydrogel/apatite nanocomposite. One end of the synthesized peptide was functionalized with an acrylate group (Ac-Glu6) to covalently attach the peptide to the hydrogel phase of the composite matrix. When microapatite crystals (5 microm diameter) were used, addition of Ac-Glu6 peptide did not affect the modulus of the microcomposite. However, when nanoapatite crystals (100 nm diameter) were used, addition of Ac-Glu6 resulted in significant reinforcement of the shear modulus of the nanocomposite ( approximately 100% in elastic shear modulus). Furthermore, addition of Ac-Gly6 (a neutral glycine sequence) or Ac-Lys6 (a positively charged sequence) did not reinforce the nanocomposite. These results demonstrate that the reinforcement effect of the Glu6 peptide, a sequence in the terminal region of osteonectin, is modulated by the size of the apatite crystals. The findings of this work can be used to develop advanced biomimetic composites for skeletal tissue regeneration.

The use of fractography to supplement analysis of bone mechanical properties in different strains of mice

Fractography has not been fully developed as a useful technique in assessing failure mechanisms of bone. While fracture surfaces of osteonal bone have been explored, this may not apply to conventional mechanical testing of mouse bone. Thus, the focus of this work was to develop and evaluate the efficacy of a fractography protocol for use in supplementing the interpretation of failure mechanisms in mouse bone. Micro-computed tomography and three-point bending were performed on femora of two groups of 6-month-old mice (C57BL/6 and a mixed strain background of 129SV/C57BL6). SEM images of fracture surfaces were collected, and areas of “tension”, “compression” and “transition” were identified. Percent areas of roughness were identified and estimated within areas of “tension” and “compression” and subsequently compared to surface roughness measurements generated from an optical profiler. Porosity parameters were determined on the tensile side. Linear regression analysis was performed to evaluate correlations between certain parameters. Results show that 129 mice exhibit significantly increased bone mineral density (BMD), number of “large” pores, failure strength, elastic modulus and energy to failure compared to B6 mice ( p < 0.001). Both 129 and B6 mice exhibit significantly ( p < 0.01) more percent areas of tension (49 ± 1%, 42 ± 2%; respectively) compared to compression (26 ± 2%, 31 ± 1%; respectively). In terms of “roughness”, B6 mice exhibit significantly less “rough” areas (30 ± 4%) compared to “smooth” areas (70 ± 4%) on the tensile side only ( p < 0.001). Qualitatively, 129 mice demonstrate more evidence of bone toughening through fiber bridging and loosely connected fiber bundles. The number of large pores is positively correlated with failure strength ( p = 0.004), elastic modulus ( p = 0.002) and energy to failure ( p = 0.041). Percent area of tensile surfaces is positively correlated with failure strength ( p < 0.001), elastic modulus ( p = 0.016) and BMD ( p = 0.037). Percent area of rough compressive surfaces is positively correlated with energy to failure ( p = 0.039). Evaluation of fracture surfaces has helped to explain why 129 mice have increased mechanical properties compared to B6 mice, namely via toughening mechanisms on the compressive side of failure. Several correlations exist between fractography parameters and mechanical behavior, supporting the utility of fractography with skeletal mouse models.

Effects of Intracortical Porosity on Fracture Toughness in Aging Human Bone: A μCT-Based Cohesive Finite Element Study

The extent to which increased intracortical porosity affects the fracture properties of aging and osteoporotic bone is unknown. Here, we report the development and application of a microcomputed tomography based finite element approach that allows determining the effects of intracortical porosity on bone fracture by blocking all other age-related changes in bone. Previously tested compact tension specimens from human tibiae were scanned using microcomputed tomography and converted to finite element meshes containing three-dimensional cohesive finite elements in the direction of the crack growth. Simulations were run incorporating age-related increase in intracortical porosity but keeping cohesive parameters representing other age-related effects constant. Additional simulations were performed with reduced cohesive parameters. The results showed a 6% decrease in initiation toughness and a 62% decrease in propagation toughness with a 4% increase in porosity. The reduction in toughnesses became even more pronounced when other age-related effects in addition to porosity were introduced. The initiation and propagation toughness decreased by 51% and 83%, respectively, with the combined effect of 4% increase in porosity and decrease in the cohesive properties reflecting other age-related changes in bone. These results show that intracortical porosity is a significant contributor to the fracture toughness of the cortical bone and that the combination of computational modeling with advanced imaging improves the prediction of the fracture properties of the aged and the osteoporotic cortical bone.

Assessment of simulated and functional disuse on cortical bone by nuclear magnetic resonance

A nuclear magnetic resonance (NMR) spin ( T 2) relaxation technique has been described for determining water distribution changes on turkey cortical bone tissue of simulated weightlessness (disuse) in vitro. The advantages of using NMR T 2 relaxation techniques for bone water distribution are illustrated. The Carr–Purcell–Meiboom–Gill (CPMG) T 2 relaxation data can be used to determine the porosity of bone, and can be inverted to T 2 relaxation distribution, and this distribution then can be transformed to a pore size distribution with the longer relaxation times corresponding to larger pores. The free induction delay (FID) T 2 relaxation data can be inverted to T 2 relaxation distribution and this distribution then can be transformed to bound and mobile water distribution with the longest relaxation time corresponding to mobile water and the middle relaxation time corresponding to bound water. The technique is applied to quantify apparent changes in porosity, bound and mobile water in normal and disuse cortical bone. Overall bone porosity is determined using the calibrated NMR fluid volume from the proton relaxation data divided by overall bone volume. The NMR bound and mobile water changes were determined from cortical bone specimens obtained from normal and disuse turkey bones. Differences in porosity and water distribution were found between specimens from normal and disuse. Our results show that the ratio of the average bound to mobile water in bone from normal turkey is higher than in bone from disuse turkey, and the porosity is lower in normal than in disuse turkey. We also show that the average bone porosity multiplied by the ratio of bound to mobile water may be constant for both normal and disuse bone groups. Currently, the influence of water removal on the strength and toughness of cortical bone has been studied by some researchers. Therefore, the porosity, bound and mobile water distribution changes could provide further information directly related to bone quality and be used for further bone mechanical studies.