Fatigue Damage In Bone Research Articles

The characterization of bovine compact bone fatigue damage using terahertz spectroscopy

Fatigue can cause cracks to propagate from the micro- to the macroscale, which results in a decrease of Young’s modulus of the bone. Non-destructive measurements of bone fatigue damage are of great importance for bone quality assessment and fracture prevention. Unfortunately, there is still a lack of effective nondestructive methods sensitive to the initial deterioration during damage accumulation, particularly in the field of orthopedics and biomechanics. In this study, terahertz spectroscopy was adopted to evaluate microscale bone damage. Specifically, the refractive index and Young’s modulus of bone samples subjected to different degrees of fatigue damage were tested at a fixed area. Both parameters are found to decrease in two stages under cycled fatigue loading, which is attributed to the initial onset and subsequent development of microdamage during fatigue loading. The change in refractive index reflects the accumulation of fatigue damage as well as the decrease in Young’s modulus.

Tensile and compressive strain evolutions of bovine compact bone under four-point bending fatigue loading

Bones are biological composite materials with multiscale structures. Bone fatigue damage is commonly characterized by an increase in strain that is accompanied by microdamage at different scales. This study investigated the damage evolutions of bone specimens under four-point bending fatigue loading using neutral axis migration. Tensile and compressive strains during the fatigue process were simultaneously measured using a digital image correlation technique. The compressive strain of the bone specimen increased rapidly at first and then proceeded slowly while the tensile strain decreased during fatigue loading. Consequently, the neutral axis shifted downward as the damage accumulated. A positive correlation exists between the downward offset of the neutral axis and the number of cycles. The variation in compressive strain is larger than that in tensile strain in this situation.

Fatigue evaluation of long cortical bone using ultrasonic guided waves

Bone fatigue fracture is a progressive disease due to stress concentration. This study aims to evaluate the long bone fatigue damage using the ultrasonic guided waves. Two-dimensional finite-difference time-domain method was employed to simulate the ultrasonic guided wave propagation in the long bone under different elastic modulus. The experiment was conducted on a 3.8 mm-thick bovine bone plate. The phase velocities of two fundamental guided modes, A1 and S1, were measured by using the axial transmission technique. Simulation shows that the phase velocities of guided modes A1 and S1 decrease with the increasing of the fatigue damage. After 20,000 cycles of fatigue loading on the bone plate, the average phase velocities of A1 and S1 modes were 6.6% and 5.3% respectively, lower than those of the intact bone. The study suggests that ultrasonic guided waves can be potentially used to evaluate the fatigue damage in long bones.

Antagonism of cysteinyl leukot-riene receptor 1 (cysLT1R) by montelukast regulates differentiation of MC3T3-E1 cells under overloaded mechanical environment

Long-term exposure to overloaded mechanical environment induces bone fatigue damage symptoms and osteoblast damages. Montelukast is a selective cysteinyl leukot-riene receptor 1 (cysLT1R) antagonist, which has been used for the treatment of bronchial asthma in clinics. In the current study, we have identified a novel pharmacological role of montelukast by finding that it has protective properties against overload damage in osteoblastic MC3T3-E1 cells. Firstly, our results show that CysLT1R is expressed in MC3T3-E1 cells. Mechanical tensile strain of 5000–7000 με resulted in a significant upregulation of CysLT1R in osteoblastic MC3T3-E1 cells in an intensity dependent manner. Secondly, MTT assay indicates that loading with 5000 με mechanical strain inhibited cell proliferation, which was suppressed by montelukast treatment. Furthermore, montelukast promotes cell differentiation by increasing the expression of ALP and RUNX2. Alizarin Red S staining assay showed that montelukast abolished the inhibitory effects of overload mechanics on osteoblast mineralization. Mechanistically, the effect of montelukast on osteoblastic differentiation acted by activating the extracellular regulated protein kinases (ERK) pathway. The obtained results suggested that montelukast promotes proliferation and differentiation in osteoblasts exposed to overload mechanics.

Use of an Osteoblast Overload Damage Model to Probe the Effect of Icariin on the Proliferation, Differentiation and Mineralization of MC3T3-E1 Cells through the Wnt/β-Catenin Signalling Pathway

Background/Aims: Mechanical loading plays an important role in the regulation of bone mass. However, bone cells are not always under physiological stress. In some cases, bone tissue is subjected to an overloaded mechanical environment. For example, a person who is weight training and a stevedore often experience bone pain, inflammation and other bone fatigue damage symptoms. Icariin is the major ingredient of Herba epimedii, which has been widely used for the treatment of bone injury in traditional Chinese medicine, but its mechanism remains unknown. The aim of this study was to probe the effect of icariin on the proliferation and differentiation of osteoblasts exposed to overload and to determine whether the Wnt/β-catenin signalling pathway is involved in the drug response in osteoblasts. Methods: Mouse MC3T3-E1 cells were exposed to mechanical tensile strain using a four- point bending device to create an overload damage model. An MTT assay was performed to determine the effects of icariin on MC3T3-E1 cell proliferation. The mRNA and protein levels of ALP, COL-I, OCN, RUNX2 and β-catenin were assessed using RT-PCR and immunoblotting. The protein levels of β-catenin in the MC3T3-E1 cells were also determined using fluorescence microscopy. The mineralization of osteoblasts was assessed using Alizarin Red S staining. Results: We found that icariin enhanced the proliferation of osteoblasts exposed to overload and promoted MC3T3-E1 cell differentiation and mineralization. Furthermore, the gene and protein expression levels of β-catenin and RUNX2 all increased with icariin treatment compared with those in the damage group. Conclusion: Our study suggested that icariin promotes proliferation and differentiation in osteoblasts exposed to overload. The effect of icariin on osteoblastic differentiation acted by activating the RUNX2 promoter and the Wnt/β- catenin pathway.

Development of an in vivo bone fatigue damage model using axial compression of the rabbit forelimb

Many nontraumatic fractures seen clinically in patients with metabolic bone disorders or on antiresorptive treatment show an increased incidence of microdamage accumulation and impaired intracortical remodeling. However, the lack of basal remodeling and Haversian bone in rodents limits their translatability in studying bone damage repair mechanisms. The work presented here demonstrates the development of the forelimb loading model in rabbits, the smallest mammal with intracortical Haversian remodeling. The forelimbs of post-mortem female New Zealand white rabbits were loaded in axial end compression to determine their basic monotonic and fatigue properties. Following time zero characterization, stress fractures were created in vivo and animals were allowed to recover for a period of two to five weeks. The rabbit forelimb when loaded in axial compression demonstrates a consistent mid-diaphyseal fracture location characterized by a local mixed compression-bending loading environment. Forelimb apparent stiffness, when fatigue loaded, demonstrates a progressive increase until macrocrack formation, at which time apparent stiffness rapidly declines until failure. Stress fractures in the rabbit ulna display robust periosteal expansion and woven bone formation two weeks following fracture. Subsequent healing at five weeks post-fracture is marked by woven bone densification, resorption and intracortical remodeling along the stress fracture line. The rabbit forelimb fatigue model is a promising new platform by which bone׳s response to damage may be studied.

Osteocyte expression of caspase-3, COX-2, IL-6 and sclerostin are spatially and temporally associated following stress fracture initiation

Stress fractures (SFxs) are debilitating injuries and exact mechanisms that initiate their repair incompletely understood. We hypothesised that osteocyte apoptosis and expression of cytokines and proteins such as sclerostin, VEGF, TGF-β, COX-2 and IL-6 were early signalling events to facilitate the formation of periosteal woven bone and recruitment of osteoclast precursors to the site of remodelling. A SFx was created in the right ulna of mature female wistar rats using cyclic end loading. Rats were killed 1, 4 and 7 days after loading (n=5 per group). Standard histological staining was used to examine SFx morphology and immunohistochemistry to detect the localisation of these proteins and in situ hybridisation to detect mRNA along the SFx line or gene expression to quantify the target genes. Unloaded ulnae served as controls. The labelling index of caspase-3, COX-2 and IL-6 was significantly elevated in the region of SFxs at all time points compared with controls (P<0.001). In addition, the labelling index of sclerostin protein was significantly reduced in osteocytes adjacent to the SFx region when compared with controls at all three time points (P<0.001). Both VEGF and TGF-β expressions were only localised in the woven bone. These data reinforce the involvement of osteocyte apoptosis in the healing of fatigue damage in bone, and demonstrate that local regulation of sclerostin, COX-2 and IL-6 are important signalling events associated with new bone formation and SFx remodelling.

Study of bone fatigue evaluation with ultrasonic guide waves based on elastic modulus

How to diagnose the bone fatigue damage at early stages is one of the hot research subjects in bone evaluation field. In this study, the propagation characteristics of ultrasonic guide wave in a long bone under different elastic moduli are analyzed using theoretical calculation and simulation methods. First, theoretical solutions of guided wave in a long bone are calculated. Then, a finite difference time-domain (FDTD) numerical method of simulating the propagation of guide wave in a long bone is presented. The simulation results are in good agreement with the theoretical values. And, the relationship between elastic modulus and the propagation characteristics, including group velocity, central frequency and attenuation, is discussed. The results show that group velocity and central frequency increase with the increase of elastic modulus. However, attenuation decreases with the increase of elastic modulus. Those results demonstrate that the propagation of guide wave can reflect the variation of elastic modulus of long bone, which provides a theoretical basis for evaluating the early stages of fatigue damage in long bone.

Simulated evolution of the vertebral body based on basic multicellular unit activities

A numerical model based on the theory of bone remodeling is proposed to predict the evolution of trabecular bone architecture within the vertebral body and to investigate the process of degeneration in vertebral bone. In this study, particular attention is paid on the description of microstructure changes during the aging process. To take into account the effect of basic multicellular units (BMUs), a set of computational algorithms has been developed. It is assumed that BMU activation probability depends on the state of damaged bone tissue (damage accumulation, ω), which is evaluated according to previous research concerning bone fatigue damage. Combining these algorithms with the finite-element method (FEM), the microstructure of vertebral bone has been predicted for up to 8 simulated years. Moreover, biomechanical material properties have been monitored to investigate the changes of vertebral bone with age. This study shows that the simulation based on BMU activities has the potential to define and predict the morphological evolution of the vertebral body. It can be concluded that the novel algorithms incorporating the coupled effects of both adaptive remodeling and microdamage remodeling could be utilized to gain greater insight into the mechanism of bone loss in the elderly population.

Simulation of Damage in Human Cortical Bone with Nonlinear Finite Element Analysis in MATLAB

This paper examines bone fatigue damage using Finite Element Analysis (FEA) to predict changes in modulus, damage and cycles to failure.The MATLAB-coded material model has been previously implemented in a full FEA package.It is based on published models of damage verified with experimental data.The MATLAB program inputs a finite element mesh and simulates the damage iteratively.Four meshes are examined:a machined tensile specimen of the bone, a machined bending specimen, a single strut of cancellous bone, and a 2-D slice of cancellous bone.This model is a fast testing method for ideas, allows easy comparison of different test geometries and material damage models, and may help to bring the study of bone damage to a broader audience.

Sex differences in long bone fatigue using a rat model

Stress fractures can occur because of prolonged exercise and are associated with cyclic loading. Fatigue is the accumulated damage that results from cyclic loading and bone fatigue damage is of special concern for athletes and army recruits. Existing literature shows that the rates of stress fracture for female athletes and female army recruits are higher than their male counterparts. In this study, we used an ex vivo rat model to investigate the fatigue response of female and male bones. We determined the strain versus number of cycles to failure (S/N) for each sex and found that for a certain initial strain (5,000-7,000 microepsilon) female bones have shorter fatigue life. To further characterize the bone response to fatigue, we also determined the creep that occurred during the fatigue test. From the creep data, for a certain strain range, female bones accumulated greater residual strains and reached the critical strain at a faster rate. In summary, this study demonstrates that female rat bones have a lower resistance to fatigue in the absence of a physiological response such as muscle fatigue or osteogenic adaptation. From these results, we hypothesized that creep was the underlying mechanism that accounted for the fast deterioration of female bones during fatigue.

New insights into the propagation of fatigue damage in cortical bone using confocal microscopy and chelating fluorochromes

Fatigue damage in bone occurs in the form of microcracks and plays an important role in the initiation of bone remodelling and in the occurrence of stress and fragility fractures. The process by which fatigue microcracks in bone initiate and grow remains poorly understood. The aim of this study was to investigate the microscopic tissue changes associated with microcracks during crack propagation in cortical bone and the influence of bone microstructure on this process. Cracks were mechanically initiated and extended longitudinally in a two-stage process, in six bovine tibial compact tension specimens. The sequential application of chelating fluorochromes, xylenol orange followed by calcein, allowed the nature of microcrack damage at different stages of propagation to be monitored by laser scanning confocal microscopy. Specimens were imaged at a focal plane 20 microm below the samples' surface, or as a series of z-plane images collected to a maximal depth of 200 microm and 35 microm for x 4 and x 40 objectives, respectively. Z-series image stacks were then reconstructed using Amira 3.0 software. Confocal images showed that xylenol orange localised to the crack surface and did not migrate into the crack's extension following further mechanical propagation. Similarly, calcein stained the extended crack's surface and displayed minimal incorporation within the original crack. High resolution confocal images provided a detailed visual description of the crack's 'process zone', and 'process zone wake'. Additionally, an 'interface region' was revealed, displaying a clear distinction between the end of the first crack and the commencement of its extension. Confocal images of the interface region demonstrated that the extended crack forms a continuum with the pre-existing crack and propagates through the former process zone. Upon viewing the three-dimensional reconstructed images, we found evidence suggesting a submicroscopic tissue involvement in fatigue damage, in addition to the potential influence of vascular canals and osteocyte lacunae on its propagation through the bone matrix. This study has provided new insights into the process of fatigue damage growth in bone and factors influencing its progression through the bone matrix. Confocal microscopy in combination with sequential chelating fluorochrome labelling is a valuable technique for monitoring microcrack growth in bone.

Aspects of in vitro fatigue in human cortical bone: time and cycle dependent crack growth

Although fatigue damage in bone induced by cyclic loading has been recognized as a problem of clinical significance, few fracture mechanics based studies have investigated how incipient cracks grow by fatigue in this material. In the present study, in vitro cyclic fatigue experiments were performed in order to quantify fatigue-crack growth behavior in human cortical bone. Crack-growth rates spanning five orders of magnitude were obtained for the extension of macroscopic cracks in the proximal-distal direction; growth-rate data could be well characterized by the linear-elastic stress-intensity range, using a simple (Paris) power law with exponents ranging from 4.4 to 9.5. Mechanistically, to discern whether such behavior results from “true” cyclic fatigue damage or is simply associated with a succession of quasi-static fracture events, cyclic crack-growth rates were compared to those measured under sustained (non-cyclic) loading. Measured fatigue-crack growth rates were found to exceed those “predicted” from the sustained load data at low growth rates (∼3×10−10 to 5×10−7 m/cycle), suggesting that a “true” cyclic fatigue mechanism, such as alternating blunting and re-sharpening of the crack tip, is active in bone. Conversely, at higher growth rates (∼5×10−7 to 3×10−5 m/cycle), the crack-growth data under sustained loads integrated over the loading cycle reasonably predicts the cyclic fatigue data, indicating that quasi-static fracture mechanisms predominate. The results are discussed in light of the occurrence of fatigue-related stress fractures in cortical bone.

Fatigue microdamage as an essential element of bone mechanics and biology.

The fossil record shows that bone remodeling has existed since the earliest large vertebrates became weight-bearing on land, but the functions of remodeling have long been debated. The principal protagonists in this debate have been those favoring a mechanical function and those asserting that remodeling serves to move calcium in and out of the skeleton. In recent years the arguments of the former school have included not only the adaptation of internal structure to specific kinds of stresses, but the need to remove fatigue damage. It has become clear that (1) physiologic strains continually produce fatigue damage in bone; (2) this damage weakens bone and is associated with both osteocyte apoptosis and the activation of remodeling; and (3) remodeling is the only means by which this damage can be removed. The significance of these observations is increased by the fact that fatigue failure is more likely in larger structures. This "volume effect," along with the advantages of enhanced mobility and metabolic efficiency, may have selected for bone remodeling as a means of controlling fatigue damage as it occurs, allowing larger vertebrates to maintain a relatively light skeleton over an extended lifetime. In this view, bone remodeling is not primarily a mechanism for calcium transport, but is intimately related to other inflammatory repair responses.

Microcrack accumulation at different intervals during fatigue testing of compact bone

Fatigue damage in bone occurs in the form of microcracks. This microdamage contributes to the formation of stress fractures and acts as a stimulus for bone remodelling. A technique has been developed, which allows microcrack growth to be monitored during the course of a fatigue test by the application of a series of fluorescent chelating agents. Specimens were taken from bovine tibiae and fatigue tested in cyclic compression at a stress range of 80 MPa. The specimens were stained before testing with alizarin and up to three other chelating agents were applied during testing to label microcracks formed at different times. Microcracks initiated in interstitial bone in the early part of a specimen's life. Further accumulation of microcracks is then suppressed until the period late in the specimen's life. Microcracks were found to be longer in the longitudinal than in the transverse direction. Only a small proportion of cracks are actively propagating; these are longer than non-propagating cracks. These results support the concept of a microstructural barrier effect existing in bone, whereby cracks initiate easily but slow down or stop at barriers such as cement lines.

The nature of fatigue damage in bone

Bone is unusual among structural materials as it is alive and capable of self-repair. Fatigue-induced microdamage is repaired by bone remodelling, but if damage accumulates too quickly, or remodelling is deficient, fatigue failure may result. Fatigue is thought to contribute to both stress and fragility fractures which are of major clinical importance. Despite this, we do not fully understand the nature of fatigue damage in bone. Human rib sections, containing microcracks stained with basic fuchsin, were serially sectioned and microcracks identified and reconstructed in three dimensions using computer software. Microcracks were elliptical in shape, 400 μm long and 100 μm wide, typical of a transversely isotropic material. Chelating agents which bind Ca 2+ were found to label microcracks in rib, as well as mineralising bone surfaces and resorption sites, suggesting that microcracks are Ca 2+ ion-lined discontinuities in the hydroxyapatite matrix. Ca 2+ ions were exposed by scratching the surface of bovine bone specimens and labelled with chelating agents in sequence. The optimal four agent sequence was: alizarin, xylenol orange, calcein and calcein blue. Two dye sequences were used to differentiate between pre-existing and test-induced microdamage in bovine samples fatigue tested in compression and longer sequences labelled microcrack growth. Microcrack dimensions can be used to calculate stress intensity values and, together with fatigue test data, can aid theoretical models to predict fatigue failure in bone.

A paradigm for skeletal strength homeostasis.

This article integrates engineering principles with skeletal biology to describe skeletal strength homeostasis. Skeletal strength revolves around its perceived mechanical usage. Mass, geometric properties, and fatigue damage burden are the principle determinants of structural strength. Bone cells form sensor and effector systems that monitor usage and adjust strength and stiffness by changing mass, geometric properties, and fatigue damage burden. The bone lining cell-osteocyte complex is the sensor; the bone modeling and remodeling systems are the effectors. Deformation and fatigue damage in bone are the signals received by the sensor. Accumulated energy in the sensor's cytoskeleton determines the rate at which the sensor sends messages to the effectors. The activity of both effector systems is proportional to the rate of incoming messages. Modeling raises bone strength and stiffness by improving geometric properties as it adds bone where customary deformation is greatest. Remodeling improves bone strength by replacing fatigue-damaged areas without mass changes. Bone removed during modeling and remodeling comes from sites where the impact on bone strength and stiffness is least. Hormones and agents alter the rigidity of the cytoskeleton and, thus, its capacity to deform and store energy. Osteopenic agents make it more rigid, causing detection of fewer deformations and transmission of fewer loading signals to the effector. Osteotropic agents decrease the rigidity of the cytoskeleton, causing detection of more strain events and transmission of more loading signals to the effector. Agent treatment thus establishes false conditions of disuse or hyperuse.

Compact bone fatigue damage—I. Residual strength and stiffness

Residual strength and stiffness of compact bovine bone specimens during fatigue were investigated and the fracture surfaces were examined by scanning electron microscopy. Residual strength tests consisted of rotating bending fatigue followed by tensile tests to failure. Specimens loaded to 50% of their fatigue lives had approximately a 13% loss of tensile strength ( p < 0.01). Specimens tested in zero to tension axial fatigue showed a gradual loss of stiffness which in some specimens was greater than 20% at failure. These results indicate that repeated loading of bone causes a progressive loss of strength and stiffness. A similar fatigue behavior in composite materials is attributable to cumulative microcracking, debonding, void growth, and fiber breakage.