Visualisation of three-dimensional microcracks in compact bone

On the role of bone damage in calcium homeostasis

Bone serves as the reservoir of some minerals including calcium. If calcium is needed anywhere in the body, it can be removed from the bone matrix by resorption and put back into the blood flow. During bone remodelling the resorbed tissue is replaced by osteoid which gets mineralized very slowly. Then, calcium homeostasis is controlled by bone remodelling, among other processes: the more intense is the remodelling activity, the lower is the mineral content of bone matrix. Bone remodelling is initiated by the presence of microstructural damage. Some experimental evidences show that the fatigue properties of bone are degraded and more microdamage is accumulated due to the external load as the mineral content increases. That damage initiates bone remodelling and the mineral content is so reduced. Therefore, this process prevents the mineral content of bone matrix to reach very high (non-physiological) values. A bone remodelling model has been used to simulate this regulatory process. In this model, damage is an initiation factor for bone remodelling and is estimated through a fatigue algorithm, depending on the macroscopic strain level. Mineral content depends on bone remodelling and mineralization rate. Finally, the bone fatigue properties are defined as dependent on the mineral content, closing the interconnection between damage and mineral content. The remodelling model was applied to a simplified example consisting of a bar under tension with an initially heterogeneous mineral distribution. Considering the fatigue properties as dependent on the mineral content, the mineral distribution tends to be homogeneous with an ash fraction within the physiological range. If such dependance is not considered and fatigue properties are assumed constant, the homogenization is not always achieved and the mineral content may rise up to high non-physiological values. Thus, the interconnection between mineral content and fatigue properties is essential for the maintenance of bone's structural integrity as well as for the calcium homeostasis.

Validity of serial milling-based imaging system for microdamage quantification

Understanding the three-dimensional distribution of microdamage within trabecular bone may help provide a better understanding of the mechanisms of bone failure. Toward that end, a novel serial milling-based fluorescent imaging system was developed for quantifying microscopic damage in three dimensions throughout cores of trabecular bone. The overall goal for this study was to compare two-dimensional (2D), surface-based measures of microdamage extracted from this new imaging system against those from more conventional histological section analyses. Human vertebral trabecular cores were isolated, stained en bloc with a series of chelating fluorochromes, monotonically loaded, and underwent microdamage quantification via the two methods. Bone area fraction measured by the new system was significantly correlated to that measured by histological point counting (p<0.001, R(2)=0.80). Additionally, the new system produced statistically equivalent (p=0.021) measures of damage fraction (mean+/-SD), Dx.AF=0.047+/-0.021, to that obtained from stereological point counting, Dx.AF=0.048+/-0.017, at a 10% difference level. These results demonstrate that this serial milling-based fluorescent imaging system provides a destructive yet practical alternative to more conventional histologic section analysis in addition to its ability to provide a better understanding of the three-dimensional nature of microdamage.

Detection of trabecular bone microdamage by micro-computed tomography

Microdamage is an important component of bone quality and affects bone remodeling. Improved techniques to assess microdamage without the need for histological sectioning would provide insight into the role of microdamage in trabecular bone strength by allowing the spatial distribution of damage within the trabecular microstructure to be measured. Nineteen cylindrical trabecular bone specimens were prepared and assigned to two groups. The specimens in group I were damaged to 3% compressive strain and labeled with BaSO(4). Group II was not loaded, but was labeled with BaSO(4). Micro-computed tomography (Micro-CT) images of the specimens were obtained at 10 microm resolution. The median intensity of the treated bone tissue was compared between groups. Thresholding was also used to measure the damaged area fraction in the micro-CT scans. The histologically measured damaged area fraction, the median CT intensity, and the micro-CT measured damaged area fraction were all higher in the loaded group than in the unloaded group, indicating that the micro-CT images could differentiate the damaged specimen group from the unloaded specimens. The histologically measured damaged area fraction was positively correlated with the micro-CT measured damaged area fraction and with the median CT intensity of the bone, indicating that the micro-CT images can detect microdamage in trabecular bone with sufficient accuracy to differentiate damage levels between samples. This technique provides a means to non-invasively assess the three-dimensional distribution of microdamage within trabecular bone test specimens and could be used to gain insight into the role of trabecular architecture in microdamage formation.

Fatigue of mineralized tissues: cortical bone and dentin

Gaining a mechanistic understanding of the mechanical properties of mineralized tissues, such as dentin and cortical bone, is important from the perspective of developing a framework for predicting and preventing failure of teeth and whole bones, particularly with regard to understanding the effects of microstructural modifications from factors such as aging, disease, or medical treatments. Accordingly, considerable research efforts have been made to determine the specific mechanisms involved in the fatigue and fracture of mineralized tissues, and to discover how these mechanisms relate to features within the respective microstructures. This article seeks to review the progress that has been made specifically in the area of fatigue, focusing on the research that moves our understanding beyond simple fatigue life (S/N) concepts and instead addresses the separate mechanisms for microdamage initiation, crack propagation, and in the case of bone, repair and remodeling.

Targeted bone remodeling involves BMU steering as well as activation

Microdamage removal is an important function of bone remodeling. Experiments have repeatedly shown that remodeling of cortical bone by Basic Multicellular Units (BMUs) is initiated in response to microdamage, and this has become known as "targeted remodeling". This paper considers the possibility that microdamage is not only able to activate new BMUs, but may also attract or "steer" existing BMUs as they continue to tunnel through the bone matrix. An initial analysis of the relationship of between mean microcrack length and BMU resorption space density in cortical bone indicates that BMUs have an effective area about 40 times greater than their actual cross-section. Interpreting this as evidence that the osteoclasts in a tunneling BMU are able to sense and steer toward microdamage, a model is developed for "BMU steering" based on the hypothesis that osteoclasts are guided not only in the principal stress direction, as proposed by Burger et al. (Burger, E.H., Klein-Nulend, J., Smit, T.H. Strain-derived canalicular fluid flow regulates osteoclast activity in a remodelling osteon-a proposal. J. Biomech 36 (2003) 1453-1459), but also toward microdamage, depending on its proximity.

Strain redistribution and cracking behavior of human bone during bending

Long bones often fail due to bending loads. Understanding the fracture process during bending is of great importance to the prevention and treatment of bone fractures. In this study, we investigated the origin of long bone's bending strength through the study of the dynamic strain redistribution happening during the post-yield stage of deformation and its relation to microdamage at the microstructural level. This was accomplished by comparing the behaviors of human long bones with standard cortical bone specimens in terms of strain redistribution, Poisson's ratios, microdamage morphologies, and macro-scale fracture patterns. It was found that human tibia failure in bending was very similar to that of standard beam cortical bone specimens with respect to the four previous aspects. Also, the examination of bone's Poisson's ratio indicated very different inelastic deformation mechanisms under tension and compression: bone volume expanded in tension but was nearly conserved in compression. Finally, as a result of strain redistribution, bone's bending strength mainly depended on its compressive strength, which was significantly influenced by the osteonal "porous" microstructure of human bone as compared to its tensile behavior. Thus, we concluded that bone microstructure at the Haversian system level plays an important role in bone deformation and fracture.

Automated high-resolution three-dimensional fluorescence imaging of large biological specimens

We describe a novel automated technique for visualizing the three-dimensional distribution of fluorochrome-labelled components, in which image resolution is uncoupled from specimen size. This method is based on computer numerically controlled milling technology and combines an arrayed imaging technique with fluorescence capabilities. Fluorescent signals are segmented by emission spectra such that multiple fluorochromes present within a single specimen may be reconstructed and visualized individually or as a group. The automated nature of the system minimizes the workload and time involved in image capture and volume reconstruction. As an application, the system was used to image zones of fluorochrome-labelled microdamage within an 8-mm diameter cylinder of trabecular bone at a voxel size of 3 x 3 x 8 microm3. Our reconstruction of this specimen provides a visual map and quantitative measures of the volume of damage present throughout the cylinder, clearly demonstrating the interpretive power afforded by three-dimensional visualization. The three-dimensional nature of this highly automated and adaptable system has the potential to facilitate new diagnostic tools and techniques with application to a wide range of biological and medical research fields.

Microcracks in compact bone: a three-dimensional view

Microcracks have been implicated in the loss of bone quality for osteoporosis. In order to detect and monitor their growth, and to use these data to predict fractures, it is essential to obtain quantitative data regarding their shape in three dimensions. Beam-shaped bone samples from sheep radii were prepared and stained with fluorochrome dyes and tested in cyclical fatigue under four-point bending in a servo-hydraulic fatigue-testing machine. Samples were tested at a frequency of 30 Hz under load control at a stress range of 100 MPa. Holes were drilled into bone samples and used as reference points for reconstructions. A series of thin longitudinal sections were cut using a sledge macrotome. A two-dimensional image of each section was examined using an epifluorescence microscope and images transferred to a PC via a CCD low-light colour video camera. A three-dimensional image of each microcrack was reconstructed using computer software, and its dimensions measured. Cracks were elliptical in shape, longer in the longitudinal direction and with a mean aspect ratio of 5.5 +/- 1.05. The mean (+/- SD) length and width of labelled microcracks were 488 +/- 151 and 88 +/- 21 microm, respectively.

The role of osteocytes and bone microstructure in preventing osteoporotic fractures

The skeleton alters its geometry following trauma, the introduction of artificial defects and of fatigue-induced microcracks. The precise mechanism by which the skeleton adapts remains unclear. Microcracks might directly affect the cell by damaging the osteocyte cell network or causing apoptosis. Bone microstructure may play an important role in these processes by diverting and arresting propagating microcracks and so prevent fracture failure. This paper discusses the effects of microstructure on propagating cracks, how microdamage may act as a stimulus for bone adaptation and its potential effects on bone biochemistry.

Bone remodelling algorithms incorporating both strain and microdamage stimuli

Biomechanical theories to predict bone remodelling have used either mechanical strain or microdamage as the stimulus driving cellular responses. Even though experimental data have implicated both stimuli in bone cell regulation, a mechano-regulatory system incorporating both stimuli has not yet been proposed. In this paper, we test the hypothesis that bone remodelling may be regulated by signals due to both strain and microdamage. Four mechano-regulation algorithms are studied where the stimulus is: strain, damage, combined strain/damage, and either strain or damage with damage-adaptive remodelling prioritised when damage is above a critical level. Each algorithm is implemented with both bone lining cell (surface) sensors and osteocyte cell (internal) sensors. Each algorithm is applied to prediction of a bone multicellular unit (BMU) remodelling on the surface of a bone trabecula. It is predicted that a regulatory system capable of responding to changes in either strain or microdamage but which prioritises removal of damaged bone when damage is above a critical level, is the only one that provides a plausible prediction of BMU behaviour. A mechanism for this may be that, below a certain damage threshold, osteocyte processes can sense changes in strain and fluid flow but above the threshold damage interferes with the signalling mechanism, or causes osteocyte apoptosis so that a remodelling response occurs to remove the dead osteocytes.

Strain patterns during tensile, compressive, and shear fatigue of human cortical bone and implications for bone biomechanics

It is a common theme in basic bone biomechanics and in biomechanical applications that much of the behavior can be determined and is dictated by the level of strain, whether this pertains to bone physiology, bone remodeling, osseoinduction, osseointegration, or the development of damage. The development of damage, demonstrated by stiffness loss measurements, has already been reported in detail in the literature. However, the systematic study of the development of "plastic" (residual) strains, which are associated with the inelastic mechanical behavior of bone tissue, has generally been overlooked. The present study compares the rates at which the elastic (e(a)) and plastic components (e(p)) of strain developed during tensile, compressive, and shear fatigue in human cortical bone of six individuals aged between 53 and 79 years. The overall hypothesis of this investigation is that there is a common underlying factor in the damage-related behavior of bone, which may allow us to link together the various aspects of the damage related behavior of bone. The rate of development of plastic strain (Deltae(p)/DeltaN) and the rate of growth in elastic strain amplitude (Deltae(a)/DeltaN) are described as a function of the stress (sigma), and/or stress normalized by the modulus of elasticity (sigma/E). The implications of our findings are discussed with respect to simple models/mechanisms, which may underlie the observed behavior.

New embedding medium for sectioning undecalcified bone

Methylmethacrylate (MMA) is the most commonly used embedding medium for sectioning undecalcified bone; however, a number of problems exist with its use in a research laboratory. MMA requires a long infiltration time and temperature control, and it reacts with many polymers. We used Kleer Set resin as an alternative embedding medium for sectioning undecalcified bone specimens. Fluorochrome labeled bone specimens were sectioned transversely using a ground section technique and longitudinally on a sledge macrotome. The slides were viewed using both transmitted light and epifluorescence microscopy. High quality sections were obtained using Kleer Set resin for both sectioning techniques. We have shown that this new embedding medium is simpler, safer, quicker to use and does not interfere with visualization of fluorochromes.

A novel scratching approach for measuring age-related changes in the in situ toughness of bone

A scratch test using a nanoindentation system was proposed in this study to assess the age-related changes in the in situ toughness of bone matrix at ultrastructural levels. A tissue removal energy density (u(r)) was defined and estimated as the work done by the scratch (U(T)) divided by the total volume of the scratch groove (u(s)). The value of u(s) was used as a relative measure of the in situ toughness of the tissue. Human cortical bone specimens obtained from middle-aged (between 49 and 59 years old) and elderly groups (over 69 years old) were tested using this technique. A significant difference in the estimated removal energy density (u(s)) in the secondary osteons was found between the middle-aged and elderly groups (5.49+/-0.696 vs. 4.09+/-1.30 N/mm(2), respectively).

Stress-concentrating effect of resorption lacunae in trabecular bone

Analyses of the distributions of stress and strain within individual bone trabeculae have not yet been reported. In this study, four trabeculae were imaged and finite elements models were generated in an attempt to quantify the variability of stress/strain in real trabeculae. In three of these trabeculae, cavities were identified with depths comparable to values reported for resorption lacunae ( approximately 50 microm)-although we cannot be certain, it is most probable that they are indeed resorption lacunae. A tensile load was applied to each trabeculum to simulate physiological loading and to ensure that bending was minimized. The force carried by each trabecula was calculated from this value using the average cross sectional area of each trabecula. The analyses predict that very high stresses (>100 MPa) existed within bone trabecular tissue. Stress and strain distributions were highly heterogeneous in all cases, more so in trabeculae with the presumptive resorption lacunae where at least 30% of the tissue had a strain greater than 4000 micoepsilon in all cases. Stresses were elevated at the pit of the lacunae, and peak stress concentrations were located in the longitudinal direction ahead of the lacunae. Given these high strains, we suggest that microdamage is inevitable around resorption lacunae in trabecular bone, and may cause the bone multicellular unit to proceed to resorb a packet of bone in the trabeculum rather than just resorb whatever localized area was initially targeted.

Mechanisms of short crack growth at constant stress in bone

This paper describes an experimental study of the growth of small (i.e. sub-millimetre) cracks in samples of cortical bone subjected to a constant tensile stress. Slow, stable crack growth occurred at a rate and angle which were dependent on the orientation of the sample: tests were conducted with the loading axis both parallel and perpendicular to the longitudinal axis of the bone. All cracks showed intermittent growth in which periods of relatively rapid propagation alternated with periods of temporary crack arrest or relatively slow growth. In some cases crack arrest could be clearly linked to microstructural features such as osteons or Volkmann's canals, which acted as barriers to crack growth. Crack-opening displacement increased over time during the arrest periods. These observations suggest a mechanism for the growth of small cracks in bone at constant stress, involving microstructural barriers, time-dependent deformation of material near the crack tip and strain-controlled propagation.

Microdamage: A cell transducing mechanism based on ruptured osteocyte processes

As a result of underlying pathological diseases, such as osteoporosis, osteopenia, or due to altered loading after joint replacements, bones become more susceptible to microdamage accumulation than those of normal human beings, as are those of athletes who undertake strenuous exercise [Stromsoe, 2004. Fracture fixation problems in osteoporosis. Injury 35, 107-113]. Experimental evidence has linked bone adaptation to microdamage, and to increased cell activity. In this work, we investigated whether microcrack detection is related to rupturing of the cellular material itself due to crack face displacements. Using specific cell staining techniques, it was confirmed that relative crack displacements are capable of tearing cell processes between neighbouring osteocytes. No ruptured cell processes were found near the crack tip where the displacements are less. Rupturing of cell processes due to crack opening and shear displacement is a feasible new mechanism by which bone can detect and estimate the size of a microcrack. Ruptured cell processes may directly secrete passive and active components in the extracellular matrix, triggering a repair response.

The behaviour of microcracks in compact bone

This paper summarises four separate studies carried out by our group over the past number of years in the area of bone microdamage. The first study investigated the manner by which microcracks accumulate and interact with bone microstructure during fatigue testing of compact bone specimens. In a series of fatigue tests carried out at four different stress ranges between 50 and 80 MPA, crack density increased with loading cycles at a rate determined by the applied stress. Variations in the patterns of microdamage accumulation suggest that that at low stress levels, larger amounts of damage can build up without failure occurring. In a second study using a series of four-pont bending tests carried out on ovine bone samples, it was shown that bone microstructure influenced the ability of microcracks to propagate, with secondary osteons acting as barriers to crack growth. In a third study, the manner by which crack growth disrupts the canalicular processes connecting osteocytes was investigated. Analysis of individual cracks showed that disruption of the canalicular processes connecting osteocytes occurred due to shear displacement at the face of propagating microcracks, suggesting that this may play some role in the mechanism that signals bone remodelling. In a fourth in vivo study, it was shown that altering the mechanical load applied to the long bones of growing rats causes microcrack formation. In vivo microdamage was present in rats subjected to hindlimb suspension with a higher microcrack density found in the humeri than the femora. Microdamage was also found in control animals. This is the first study to demonstrate in vivo microcracks in normally loaded bones in a rat model.

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.

A fatigue microcrack alters fluid velocities in a computational model of interstitial fluid flow in cortical bone

Targeted remodeling is activated by fatigue microcracks and plays an important role in maintaining bone integrity. It is widely believed that fluid flow-induced shear stress plays a major role in modulating the mechanotransduction process. Therefore, it is likely that fluid flow-induced shear stress plays a major role in the initiation of the repair of fatigue damage. Since no in vivo measurements of fluid flow within bone exist, computational and mathematical models must be employed to investigate the fluid flow field and the shear stress occurring within cortical bone. We developed a computational fluid dynamic model of cortical bone to examine the effect of a fatigue microcrack on the fluid flow field. Our results indicate that there are alterations in the fluid flow field as far as 150 microm away from the crack, and that at distances farther than this, the fluid flow field is similar to the fluid flow field of intact bone. Through the crack and immediately above and below it, the fluid velocity is higher, while at the lateral edges it is lower than that calculated for the intact model, with a maximum change of 29%. Our results suggest that the presence of a fatigue microcrack can alter the shear stress in regions near the crack. These alterations in shear stress have the potential to significantly alter mechanotransduction and may play a role in the initiation of the repair of fatigue microcracks.

Microcracks in cortical bone: how do they affect bone biology?

Microcrack accumulation in cortical bone has been implicated in skeletal fragility and stress fractures. These cracks have also been shown to affect the mechanical and material properties of cortical bone. Their growth has been linked to osteocyte apoptosis and the initiation of the remodeling process, which also has a role in their repair. Clinically, osteoporosis is diagnosed using dual energy x-ray absorptiometry. However, evidence now indicates that bone mass alone is insufficient to satisfactorily explain the skeletal fragility of osteoporosis and consideration needs to be given to bone quality in the diagnosis and treatment of the disease. Bone quality includes parameters such as trabecular and cortical microarchitecture, morphology, bone turnover, degree of mineralization of the bone matrix, and significantly, the amount of microdamage present in the bone. Current clinical treatments concentrate on the inhibition of osteoclast activity to maintain bone mass in osteoporotic patients. However, these cells have a major role in removing existing microcracks from the bone matrix, and hence the use of bone resorption- inhibiting drugs may lead to insufficient bone repair and therefore an increase in microdamage accumulation and loss of bone quality.

Confocal laser scanning microscopy in orthopaedic research

Confocal laser scanning microscopy (CLSM) is a type of high-resolution fluorescence microscopy that overcomes the limitations of conventional widefield microscopy and facilitates the generation of high-resolution 3D images from relatively thick sections of tissue. As a comparatively non-destructive imaging technique, CLSM facilitates the in situ characterization of tissue microstructure. Images generated by CLSM have been utilized for the study of articular cartilage, bone, muscle, tendon, ligament and menisci by the foremost research groups in the field of orthopaedics including those teams headed by Bush, Errington, Guilak, Hall, Hunziker, Knight, Mow, Poole, Ratcliffe and White. Recent evolutions in techniques and technologies have facilitated a relatively widespread adoption of this imaging modality, with increased "user friendliness" and flexibility. Applications of CLSM also exist in the rapidly advancing field of orthopaedic implants and in the investigation of joint lubrication.

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 ( approximately 3 x 10(-10) to 5 x 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 ( approximately 5 x 10(-7) to 3 x 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.

Trabecular bone microdamage and microstructural stresses under uniaxial compression

The balance between local remodeling and accumulation of trabecular bone microdamage is believed to play an important role in the maintenance of skeletal integrity. However, the local mechanical parameters associated with microdamage initiation are not well understood. Using histological damage labeling, micro-CT imaging, and image-based finite element analysis, regions of trabecular bone microdamage were detected and registered to estimated microstructural von Mises effective stresses and strains, maximum principal stresses and strains, and strain energy density (SED). Bovine tibial trabecular bone cores underwent a stepwise uniaxial compression routine in which specimens were micro-CT imaged following each compression step. The results indicate that the mode of trabecular failure observed by micro-CT imaging agreed well with the polarity and distribution of stresses within an individual trabecula. Analysis of on-axis subsections within specimens provided significant positive relationships between microdamage and each estimated tissue stress, strain and SED parameter. In a more localized analysis, individual microdamaged and undamaged trabeculae were extracted from specimens loaded within the elastic region and to the apparent yield point. As expected, damaged trabeculae in both groups possessed significantly higher local stresses and strains than undamaged trabeculae. The results also indicated that microdamage initiation occurred prior to apparent yield at local principal stresses in the range of 88-121 MPa for compression and 35-43 MPa for tension and local principal strains of 0.46-0.63% in compression and 0.18-0.24% in tension. These data provide an important step towards understanding factors contributing to microdamage initiation and establishing local failure criteria for normal and diseased trabecular bone.

Perforation of cancellous bone trabeculae by damage-stimulated remodelling at resorption pits: A computational analysis

Loss of trabeculae in cancellous bone is often attributed to a general decline in the bone mass leading to fracture of the thin trabeculae. It has never been investigated whether trabecular perforation may have any other biomechanical mechanism. In this paper, an alternative hypothesis is proposed and tested using a computational model. Taking it as given that osteoclastic resorption is targeted to microdamage, it is hypothesised that the creation of a resorption cavity during normal bone remodelling could cause a stress-concentration in the bone tissue. If the resorption cavities were excessively deep, as is seen during osteoporosis, then this stress concentration may be sufficient to generate more microdamage so that osteoclasts "chase" newly formed damage leading to perforation. If this were true then we should find that, for a given trabecular thickness, there is a critical depth of resorption cavity such that smaller cavities refill whereas deeper cavities cause microdamage accumulation, continued osteoclast activity, and eventual trabecular perforation. Computer simulation is used to test this hypothesis. Using a remodelling stimulus calculated from both strain and damage and a simplified finite element model of a trabeculum with cavities of different sizes, it is predicted that such a critical depth of resorption cavity does indeed exist. Therefore we suggest that an increase in resorption depth relative to the thickness of trabeculae may be responsible for trabecular perforation during osteoporosis, rather than simply trabecular fracture due to insufficient strength.

Microcracks colocalize within highly mineralized regions of cortical bone tissue

While much work has been performed to quantify the extent of bone damage, its effects on the mechanical integrity of the tissue and its biological impact, the set of factors which gives forth to microdamage are nebulous, particularly the compositional properties local to microdamage. In this context, the current study tested the hypothesis that microcracks initiate within more mineralized regions of bone. Cortical bone specimens were taken from human male donors aged 31, 38, 53, 64, 71, and 84 years at the mid femoral diaphysis in a plane parallel to the osteonal orientation. The mineralization was assessed in a spatially resolved manner using Raman microspectroscopy. Arrays of measurements were taken over the entire area (i.e. global scans) of each sample followed by measurements in the vicinity of microcracks (i.e. local scans). Histograms of mineralization were constructed for global and local scans to determine whether the mineralization of damaged loci differed from the mean overall mineralization. Statistical analysis of this data revealed that the mean mineralization of damaged loci was significantly greater (P < 0.05) than the overall mineralization for each donor, indicating that there exists a highly-mineralized 'brittle volume' in bone. The presence of this damage prone 'brittle volume' has future implications for the assessment of fracture susceptibility.

The effect of bone microstructure on the initiation and growth of microcracks

Osteonal bone is often compared to a composite material and to metals as discontinuities within the material may provide sites of stress concentration for crack initiation and serve as barriers to crack growth. However, little experimental data exist to back up these hypotheses. Fluorescent chelating agents were applied at specific intervals to bone specimens fatigue tested in cyclic compression at a stress range of 80 MPa. The failed specimens were sectioned and labelled microcracks identified using UV epifluorescence microscopy. Microcrack lengths were measured and their relationship to cement lines surrounding secondary osteons recorded. Microcrack length at the time of encountering a cement line was also measured. Microcracks of less than 100mum stopped growing when they encountered a cement line. Microcracks of greater than 100mum in length continued to grow after encountering a cement line surrounding an osteon. Only microcracks greater than 300mum in length were capable of penetrating osteons and these microcracks were the only ones which were observed to cause failure in the specimen. These experimental data support the hypothesis that secondary osteons act as barriers to crack propagation in compact bone. However, it shows that this microstructural barrier effect is dependent on the crack length at the time of encountering an osteon. For the vast majority of cracks, osteons act as barriers to growth but for the minority of cracks that are long enough and do break through the cement line, an osteon may actually act as a weakness in the bone and facilitate crack propagation.

The cellular transducer in damage-stimulated bone remodelling: a theoretical investigation using fracture mechanics

This paper reports on some theoretical work which used fracture mechanics concepts to draw conclusions about the nature of the so-called 'cellular transducer': the means by which bone cells detect the presence of damage and thus initiate remodelling and adaptation activities. Using analytical and numerical methods, we estimated the strains and displacements around cracks of the typical size, shape and orientation that normally occur in compact bone. We predicted that it is not possible for osteocytes or their processes to be fractured as a result of direct tensile strains, because the strains generated are much less than the expected failure strains of cellular material. We proposed a new failure mechanism by which osteocyte processes spanning the crack are cut by shearing motions between the crack faces. We predicted that failures of this type can occur. Failures begin to occur if crack lengths become greater than normal (100 microm), so this could act as a signal to initiate repair processes for individual cracks. Very large numbers of cell processes (greater than 1000) will fail if the crack length and/or applied stress reach dangerous levels (300 microm and 60 Mpa, respectively) at which point bone deposition may be required to prevent stress fractures. Similar results also occurred if we proposed a different mechanism of damage detection, involving cells' ability to detect the high levels of strain that occur near crack tips. This work, though based on theoretical mechanics considerations, suggests some biological experiments which might confirm our findings.

Tensile damage and its effects on cortical bone

Plexiform bovine bone samples are repeatedly loaded in tension along their longitudinal axis. In order to induce damage in the bone tissue, bone samples are loaded past their yield point. Half of the bone samples from the damaged group were stored in saline to allow for viscoelastic recovery while the others were decalcified. Tensile tests were conducted on these samples to characterize the effects of damage on the mechanical behavior of the organic matrix (decalcified samples) as well as on bone tissue (stored in saline). The ultimate strain of the damaged decalcified bone is 29% higher compared to that of non-damaged decalcified (control) bone. The ultimate stresses as well as the elastic moduli are similar in both decalcified groups. This phenomenon is also observed in other collagenous tissue (tendon and ligament). This may suggest that damage in bone is caused by shear failure of the organic matrix; transverse separation of the collagen molecules or microfibrils from each other. In contrast, there is a trend towards lowered ultimate strains in damaged bone, which is soaked in saline, with respect to control bone samples (not damaged). The damaged bone tissue exhibits a bi-linear behavior in contrast to the mechanical behavior of non-damaged bone. The initial elastic modulus (below 55 MPa) and ultimate strength of damaged bone are similar to that in non-damaged bone.

A rate-dependent microcrack-bridging model that can explain the strain rate dependency of cortical bone apparent yield strength

Although there are empirical correlations between strain rate, cortical and cancellous bone apparent stiffness, apparent yield strength, apparent ultimate strength and cortical bone fracture toughness, a mechanistic description for these phenomena is lacking. Microcracking is a major mechanism in cortical and cancellous bone failure, however, microdamage content alone cannot explain the strain rate dependence of bone strength without considering time-dependent behavior of the crack. Using a rate-dependent model of a fiber-bridged microcrack and data from the literature, we demonstrate that the experimental apparent yield strength of bone can be predicted directly from measurements of apparent moduli of elasticity of bone constituents and failure strain of the collagenous matrix. Yield strength predictions for estrogen depleted bone were made using the model and data from ovariectomized sheep. It was predicted that the yield strength of estrogen-deficient bone is comparable to that of normal bone within strain rates associated with physiological activities. For high strain rates, however, the strength of estrogen-depleted bone was predicted to be much weaker than normals suggesting a higher fracture risk due to impact from falls, for individuals with estrogen-depleted bones such as in post-menopausal osteoporosis.

Detecting microdamage in bone

Fatigue-induced microdamage in bone contributes to stress and fragility fractures and acts as a stimulus for bone remodelling. Detecting such microdamage is difficult as pre-existing microdamage sustained in vivo must be differentiated from artefactual damage incurred during specimen preparation. This was addressed by bulk staining specimens in alcohol-soluble basic fuchsin dye, but cutting and grinding them in an aqueous medium. Nonetheless, some artefactual cracks are partially stained and careful observation under transmitted light, or epifluorescence microscopy, is required. Fuchsin lodges in cracks, but is not site-specific. Cracks are discontinuities in the calcium-rich bone matrix and chelating agents, which bind calcium, can selectively label them. Oxytetracycline, alizarin complexone, calcein, calcein blue and xylenol orange all selectively bind microcracks and, as they fluoresce at different wavelengths and colours, can be used in sequence to label microcrack growth. New agents that only fluoresce when involved in a chelate are currently being developed--fluorescent photoinduced electron transfer (PET) sensors. Such agents enable microdamage to be quantified and crack growth to be measured and are useful histological tools in providing data for modelling the material behaviour of bone. However, a non-invasive method is needed to measure microdamage in patients. Micro-CT is being studied and initial work with iodine dyes linked to a chelating group has shown some promise. In the long term, it is hoped that repeated measurements can be made at critical sites and microdamage accumulation monitored. Quantification of microdamage, together with bone mass measurements, will help in predicting and preventing bone fracture failure in patients with osteoporosis.

The real response of bone to exercise

This review presents findings made in studies of large mammalian bones, especially from racehorse training experiments (2-8 years old, third metacarpal, tarsal) and human autopsy orthopaedic femoral implant retrievals and other human biopsy and autopsy cases. Samples were cleaned to analyse mineralized matrix in three dimensions, or poly methyl-methacrylate embedded and micromilled to delete topography and study the superficial c. 0.5-microm two-dimensional section using quantitative backscattered electron imaging. With experimental implant studies in rabbits, observations were also made in vivo using confocal microscopy. Cracks in both calcified cartilage and bone may be removed by infilling with calcified matrix. This may be a general repair mechanism for calcified connective tissue crack repair. The fraction of the organ volume occupied by any form of bone tissue in equine distal third metacarpal extremities was increased in the more exercised groups by bone deposited within former marrow adipocytic space. Where deposited upon prior lamellar bone surfaces, this occurred without the intervention of prior resorption and without the formation of a hypermineralized cement line. Exercise inhibited osteoclastic resorption at external anatomical growth modelling sites where it normally occurs. Addition is not coupled to time-wasting resorption: both internally and externally, it occurs both by layering on existing cancellous surfaces and by creation of new immature scaffold, with de novo incorporation of a rich, capillary blood vessel supply. The real response within bone organs subjected to mechanical overload exercise within normal physiological limits is to make more, and to lose less, bone.

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 80MPa. 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.

Bone adaptation to load: microdamage as a stimulus for bone remodelling

Mechanical loading in the proximal radius was increased by ulnar osteotomy (Group O), altered by Steinmann pinning (Group P) or unaltered in sham operated controls (Group C) in skeletally mature female sheep, aged 2-4 years. A series of intravenous fluorochromes were given to label bone formation and fuchsin-stained microdamage assessed at intervals of up to 24 weeks. Microcracks were present in all groups and were found in the original cortex near the periosteal surface. No microcracks were found in the new, fibrolamellar bone laid down at periosteal or endosteal surfaces. Mean microcrack length (49 microm, SD 10 microm) did not differ between groups or overtime. Microcrack numerical and surface densities and resorption cavity density peaked in all groups at 6 weeks, consistent with a regional acceleratory phenomenon (RAP), but the peaks were significantly greater in Group O. The density of refilling or secondary osteons peaked at 10 weeks and the mean time required for the formation of an osteon was 7.51 +/- 0.59 weeks. Fatigue-induced microdamage is normally present in bone and is increased due to repetitive loading of the mechanically overloaded radius. The location and timing of microcracks, resorption cavities and secondary osteons are consistent with the activation-resorption-formation remodelling cycle and suggest that microdamage is a stimulus for bone remodelling.

Cancellous bone microdamage in the proximal femur: influence of age and osteoarthritis on damage morphology and regional distribution

This study describes the in vivo distribution of cancellous bone microdamage in the proximal femur of an autopsy control sample. In addition, in vivo microdamage in the region medial to the greater trochanter of the proximal femur is compared between patients with severe osteoarthritis and controls. Taken at autopsy, the control group comprised normal right proximal femora that were then cut in the coronal plane with an Exakt saw (n = 12; aged 20-83 years). Cancellous bone samples were taken from the subchondral principal compressive region, the medial principal compressive region, and medial to the greater trochanter. A cancellous bone core biopsy was taken of the region medial to the greater trochanter (of the proximal femur) from patients with primary osteoarthritis undergoing total hip replacement surgery (n = 33; aged 37-85 years). Samples were embedded in resin, and in vivo microdamage identified in 70-microm-thick sections using the basic fuchsin en bloc staining technique. Microdamage was similar in all proximal femur sites in controls, except in the subchondral principal compressive region, where a significantly smaller crack length (microm) was identified (p < 0.05). In the region medial to the greater trochanter, osteoarthritic vs. control group comparisons showed that the crack density (#/mm(2)) and crack surface density (mm/mm(2)) were not significantly different, but crack length was significantly less (p < 0.03) and damage volume fraction was significantly increased for osteoarthritics (p < 0.005). The osteoarthritic and control data for crack density, and the osteoarthritic data for damage volume fraction, showed a nonlinear increase with age. Furthermore, crack length was not dependent on damage volume fraction or age for either the osteoarthritic or control group. This study identified differences in microdamage between osteoarthritic and autopsy control cases. We hypothesize that these results are consistent with the reported bone material property differences for osteoarthritis. In addition, the relatively uniform distribution of microdamage in the control group suggests that the principal components of the femoral cancellous bone network are equally exposed to deformations resulting in microdamage. Further study into the factors that influence the accumulation and skeletal distribution of microdamage is fundamental to understanding skeletal health.

Analysis of crack growth in a 3D Voronoi structure: a model for fatigue in low density trabecular bone

Both creep and crack growth contribute to the reduction in modulus associated with fatigue loading in bone. Here we simulate crack growth and subsequent strut failure in fatigue in an open-cell, three-dimensional Voronoi structure which is similar to that of low density, osteoporotic bone. The model indicates that sequential failure of struts leads to a precipitous drop in modulus: the failure of 1% of the struts leads to about a 10% decrease in modulus. A parametric study is performed to assess the influence of normalized stress range, relative density, initial crack size, crack shape and cell geometry on the fatigue life. The fatigue life is most sensitive to the relative density and the initial crack length. The results lead to a quantitative expression for the fatigue life associated with crack growth. Data for the fatigue life of trabecular bone are compared with the crack growth model described in this paper as well as with a previous model for creep of a three-dimensional Voronoi structure. In our models, creep dominates the fatigue behavior in low cycle fatigue while crack growth dominates in high cycle fatigue, consistent with previous observations on cortical bone. The large scatter in the trabecular bone fatigue data make it impossible to identify a transition between creep dominated fatigue and crack growth dominated fatigue. The parametric study of the crack growth model indicates that variations in relative density among specimens, initial crack size within trabeculae and crack shape could easily produce such variability in the test results.

Is all cortical bone remodeling initiated by microdamage?

There is increasing interest in the degree to which bone remodeling, particularly in cortical bone, is "targeted" at fatigue microdamage. The theory that microdamage initiates remodeling in close proximity to microcracks, thereby removing them, and that this accounts for a significant fraction of the overall remodeling activity, has been gaining acceptance. However, the association between the initial, resorptive stage of remodeling and microcracks in histologic sections of cortical bone is far from complete; indeed, the great majority of resorption spaces are not spatially associated with microcracks. This observation has maintained support for the older concept that most remodeling occurs primarily for such metabolic purposes as calcium homeostasis. To gain further insight regarding the degree to which microdamage governs remodeling, this study presents a mathematical analysis based on the unorthodox hypothesis that all cortical bone remodeling is initiated by, and in close proximity to, microcracks. Equations are derived showing that, because remodeling basic multicellular units (BMUs) travel several millimeters beyond their point of initiation, the relative numbers of resorption spaces and microcracks found in close spatial proximity or isolated from one another are consistent with the hypothesis. The results also predict the degree to which the spatial association between resorption spaces and microcracks should exceed that due to chance alone. There are as yet very limited experimental data suitable for testing this model, but the existing data closely correspond to the model's predictions.