Sequential labelling of microdamage in bone using chelating agents.

Basic fuchsin labels microcracks, but a series of stains is required to differentiate between preexisting and test-induced microcracks and to label their growth in vitro. Basic fuchsin and five chelating agents-alizarin complexone, calcein, calcein blue, oxytetracycline, and xylenol orange-were randomly assigned to label microcracks in sequential rib sections from 10 donors. The density, length, and location of the microcracks did not differ significantly between the six stains, suggesting that each was equally effective in detecting microcracks. Paired specimens of trabecular bone were machined from bovine tibiae, stained with oxytetracycline, and fatigued in compression. One specimen from each pair was then stained with xylenol orange. Preexisting microdamage was stained with oxytetracycline, propagating microcracks with both stains and new, test-initiated damage with xylenol orange. Chelating agents are site-specific markers of the initiation and growth of microcracks.     

Influence of the degradation of the organic matrix on the microscopic fracture behavior of trabecular bone

In recent years, the important role of the organic matrix for the mechanical properties of bone has become increasingly apparent. It is therefore of great interest to understand the interactions between the organic and inorganic constituents of bone and learn the mechanisms by which the organic matrix contributes to the remarkable properties of this complex biomaterial. In this paper, we present a multifaceted view of the changes of bone's properties due to heat-induced degradation of the organic matrix. We compare the microscopic fracture behavior (scanning electron microscopy; SEM), the topography of the surfaces (atomic force microscopy; AFM), the condition of bone constituents [X-ray diffraction (XRD), thermogravimetric analysis (TGA), and gel electrophoresis], and the macromechanical properties of healthy bovine trabecular bone with trabecular bone that has a heat-degraded organic matrix. We show that heat treatment changes the microfracture behavior of trabecular bone. The primary failure mode of untreated trabecular bone is fibril-guided delamination, with mineralized collagen filaments bridging the gap of the microcrack. In contrast, bone that has been baked at 200 degrees C fractures nondirectionally like a brittle material, with no fibers spanning the microcracks. Finally, bone that has been boiled for 2 h in PBS solution fractures by delamination with many small filaments spanning the microcracks, so that the edges of the microcracks become difficult to distinguish. Of the methods we used, baking most effectively weakens the mechanical strength of bone, creating the most brittle material. Boiled bone is stronger than baked bone, but weaker than untreated bone. Boiled bone is more elastic than untreated bone, which is in turn more elastic than baked bone. These studies clearly emphasize the importance of the organic matrix in affecting the fracture mechanics of bone.     

Changes in bone fatigue resistance due to collagen degradation

Clinical tools for evaluating fracture risk, such as dual energy X‐ray absorptiometry (DXA) and quantitative ultrasound (QUS), focus on bone mineral and cannot detect changes in the collagen matrix that affect bone mechanical properties. However, the mechanical response tissue analyzer (MRTA) directly measures a whole bone mechanical property. The aims of our study were to investigate the changes in fatigue resistance after collagen degradation and to determine if clinical tools can detect changes in bone mechanical properties due to fatigue. Male and female emu tibiae were endocortically treated with 1 M KOH for 1–14 days and then either fatigued to failure or fatigued to induce stiffness loss without fracture. Partial fatigue testing caused a decrease in modulus measured by mechanical testing even when not treated with KOH, which was detected by MRTA. At high stresses, only KOH‐treated samples had a lower fatigue resistance compared to untreated bones for both sexes. No differences were observed in fatigue behavior at low stresses for all groups. KOH treatment is hypothesized to have changed the collagen structure in situ and adversely affected the bone. Cyclic creep may be an important mechanism in the fast deterioration rate of KOH‐treated bones, as creep is the major cause of fatigue failure for bones loaded at high stresses. Therefore, collagen degradation caused by KOH treatment may be responsible for the observed altered fatigue behavior at high stresses, since collagen is responsible for the creep behavior in bone. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:197–203, 2011     

Limitations of Global Morphometry in Predicting Trabecular Bone Failure

Efforts in finding independent measures for accurate and reliable prediction of trabecular bone failure have led to the development of a number of morphometric indices characterizing trabecular bone microstructure. Generally, these indices assume a high homogeneity within the bone specimen. However, in the present study we found that the variance in bone volume fraction (BV/TV) in a single bone specimen can be relatively large (CV = 9.07% to 28.23%). To assess the limitations of morphometric indices in the prediction of bone failure for specimens in which the assumption of homogeneity is not met, we harvested 13 cadaveric samples from a single human spine. We tested these cylindrical samples using image‐guided failure assessment (IGFA), a technique combining stepwise microcompression and time‐lapsed micro–computed tomography (µCT). Additionally, we computed morphometric indices for the entire sample as well as for 10 equal subregions along the anatomical axis. We found that ultimate strength was equally well predicted by BV/TV of the entire sample (R² = 0.55) and BV/TV of the weakest subregion (R² = 0.57). Investigating three‐dimensional animations of structural bone failure, we showed that two main failure mechanisms determine the competence of trabecular bone samples; in homogeneous, isotropic trabecular bone samples, competence is determined by a whole set of trabecular elements, whereas in inhomogeneous, anisotropic bone samples a single or a missing trabeculae may induce catastrophic failure. The latter failure mechanism cannot be described by conventional morphometry, indicating the need for novel morphometric indices also applicable to the prediction of failure in inhomogeneous bone samples. © 2014 American Society for Bone and Mineral Research.     

Bone texture analysis is correlated with three-dimensional microarchitecture and mechanical properties of trabecular bone in osteoporotic femurs

Fracture of the proximal femur is a major public health problem in elderly persons. It has recently been suggested that combining texture analysis and bone mineral density measurement improves the failure load prediction in human femurs. In this study, we aimed to compare bone texture analysis with three-dimensional (3D) microarchitecture and mechanical properties of trabecular bone in osteoporotic femurs. Eight femoral heads from osteoporotic patients who fractured their femoral neck provided 31 bone cores. Bone samples were studied using a new high-resolution digital X-ray device (BMA?, D3A Medical Systems) allowing for texture analysis with fractal parameter H _mean, and were examined using micro-computed tomography (microCT) for 3D microarchitecture. Finally, uniaxial compression tests to failure were performed to estimate failure load and apparent modulus of bone samples. The fractal parameter H _mean was strongly correlated with bone volume fraction (BV/TV) (r = 0.84) and trabecular thickness (Tb.Th) (r = 0.91) (p < 0.01). H _mean was also markedly correlated with failure load (r = 0.84) and apparent modulus (r = 0.71) of core samples (p < 0.01). Bone volume fraction (BV/TV) and trabecular thickness (Tb.Th) demonstrated significant correlations with failure load (r = 0.85 and 0.72, respectively) and apparent modulus (r = 0.72 and 0.64, respectively) (p < 0.01). Overall, the best predictors of failure load were H _mean, bone volume fraction, and trabecular thickness, with r ² coefficients of 0.83, 0.76, and 0.80 respectively. This study shows that the fractal parameter H _mean is correlated with 3D microCT parameters and mechanical properties of femoral head bone samples, which suggests that radiographic texture analysis is a suitable approach for trabecular bone microarchitecture assessment in osteoporotic femurs.     

Microstructure Determines Apparent-Level Mechanics Despite Tissue-Level Anisotropy and Heterogeneity of Individual Plates and Rods in Normal Human Trabecular Bone

Trabecular plates and rods determine apparent elastic modulus and yield strength of trabecular bone, serving as important indicators of bone's mechanical integrity in health and disease. Although trabecular bone's apparent-level mechanical properties have been widely reported, tissue mechanical properties of individual trabeculae have not been fully characterized. We systematically measured tissue mineral density (TMD)–dependent elastic modulus of individual trabeculae using microindentation and characterized its anisotropy as a function of trabecular type (plate or rod), trabecular orientation in the global coordinate (longitudinal, oblique, or transverse along the anatomic loading axis), and indentation direction along the local trabecular coordinate (axial or lateral). Human trabecular bone samples were scanned by micro-computed tomography for TMD and microstructural measurements. Individual trabecula segmentation was used to decompose trabecular network into individual trabeculae, where trabecular type and orientation were determined. We performed precise, selective indentation of trabeculae in each category using a custom-built, microscope-coupled microindentation device. Co-localization of TMD at each indentation site was performed to obtain TMD-to-modulus correlations. We found significantly higher TMD and tissue modulus in trabecular plates than rods. Regardless of trabecular type and orientation, axial tissue modulus was consistently higher than lateral tissue modulus, with ratios ranging from 1.13 to 1.41. Correlations between TMD and tissue modulus measured from axial and lateral indentations were strong but distinct: axial correlation predicted higher tissue modulus than lateral correlation at the same TMD level. To assess the contribution of experimentally measured anisotropic tissue properties of individual trabeculae to apparent-level mechanics, we constructed non-linear micro-finite element models using a new set of trabecular bone samples and compared model predictions to mechanical testing measurements. Heterogeneous anisotropic models accurately predicted apparent elastic modulus but were no better than a simple homogeneous isotropic model. Variances in tissue-level properties may therefore contribute nominally to apparent-level mechanics in normal human trabecular bone. © 2021 American Society for Bone and Mineral Research (ASBMR).     

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.     

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.     

Age-related changes in trabecular bone microdamage initiation

With age, alterations occurring in bone quality, quantity, and microarchitecture affect the resistance of trabecular bone to local failure. The clinical implications of these changes are evident by the observed exponential increase in fracture incidence with age. Although age-related development of skeletal fragility is well established, it is unclear how the local failure properties of bone change with age. We previously reported a specimen-specific technique to assess microstructural stresses and strains associated with microdamage initiation but did not assess age-related changes. In this study, we compared younger (average age 2 years) and older (average age 10 years) bovine trabecular bone to evaluate how alterations in bovine bone quantity and quality with age affect the local mechanical environment associated with microdamage formation. The results show strong positive correlations between microdamage and local stresses and strains for both younger and older bovine trabecular bone. Correlation strength was slightly improved (<8%) for some parameters by incorporating heterogeneous local material properties based on mineral density into the finite element models. Within individual trabeculae, average stresses and strains were significantly higher in microdamaged trabeculae compared to randomly selected undamaged trabeculae, regardless of age. However, damaged trabeculae in older bone were found to have higher stresses and lower strains than those from younger bone. Corresponding differences in mineral density, microarchitecture, and FEM-determined local material properties were also observed between the two groups. Taken together, these data suggest marked age-related changes in the mechanics of microdamage initiation at the trabecular level. The combined experimental, computational, and histochemical approaches used in this study provide an improved understanding of microdamage initiation and bone quality.     

Experimental hip fracture load can be predicted from plain radiography by combined analysis of trabecular bone structure and bone geometry

Summary Computerized analysis of the trabecular structure was used to test whether femur failure load can be estimated from radiographs. The study showed that combined analysis of trabecular bone structure and geometry predicts in vitro failure load with similar accuracy as DXA. Introduction Since conventional radiography is widely available with low imaging cost, it is of considerable interest to discover how well bone mechanical competence can be determined using this technology. We tested the hypothesis that the mechanical strength of the femur can be estimated by the combined analysis of the bone trabecular structure and geometry. Methods The sample consisted of 62 cadaver femurs (34 females, 28 males). After radiography and DXA, femora were mechanically tested in side impact configuration. Fracture patterns were classified as being cervical or trochanteric. Computerized image analysis was applied to obtain structure-related trabecular parameters (trabecular bone area, Euler number, homogeneity index, and trabecular main orientation), and set of geometrical variables (neck-shaft angle, medial calcar and femoral shaft cortex thicknesses, and femoral neck axis length). Multiple linear regression analysis was performed to identify the variables that best explain variation in BMD and failure load between subjects. Results In cervical fracture cases, trabecular bone area and femoral neck axis length explained 64% of the variability in failure loads, while femoral neck BMD also explained 64%. In trochanteric fracture cases, Euler number and femoral cortex thickness explained 66% of the variability in failure load, while trochanteric BMD explained 72%. Conclusions Structural parameters of trabecular bone and bone geometry predict in vitro failure loads of the proximal femur with similar accuracy as DXA, when using appropriate image analysis technology.     

Cancellous bone adaptation to in vivo loading in a rabbit model

Biophysical stimuli are important to the development and maintenance of cancellous bone, but the regulatory mechanisms need to be understood. We investigated the effects of mechanical loading applied in vivo to native cancellous bone in the rabbit on bone formation and trabecular realignment. A novel device was developed to apply controlled compressive loads to cancellous bone in situ. The effect of loading on cancellous bone volume fraction and architecture was quantified. A 4-week experiment was performed in rabbits with devices implanted bilaterally. Cyclic 1 MPa pressures were applied daily to the right limb for 10, 25, or 50 cycles at 0.5 Hz, and the left limb served as the control without any applied loading. Microcomputed tomography and histomorphometry were used to characterize the cancellous tissue within a 4-mm spherical volume located below the loading core. In vivo cyclic loading significantly increased the bone volume fraction, direct trabecular thickness, mean intercept length, and mineral apposition rate in the loaded limbs compared with contralateral limbs. Insufficient evidence was found to demonstrate an effect of number of cycles on the cancellous adaptation between loaded and control limbs. Using a rabbit model, we demonstrated that mechanical loading applied to cancellous bone in situ increased bone formation and altered trabecular morphology. This in vivo model will allow further investigation of cancellous functional adaptation to controlled mechanical stimuli and the influence of mechanical loading parameters, metabolic status, and therapeutic agents.     

Relationship between compressive properties of human os calcis cancellous bone and microarchitecture assessed from 2D and 3D synchrotron microtomography

The aim of this study was to determine the contribution of 2D and 3D microarchitectural characteristics in the assessment of the mechanical strength of os calcis cancellous bone. A sample of cancellous bone was removed in a medio-lateral direction from the posterior body of calcaneus, taken at autopsy in 17 subjects aged 61-91 years. The sample was first used for the assessment of morphological parameters from 2D morphometry and 3D synchrotron microtomography (microCT) (spatial resolution=10 microm). The 2D morphometry was obtained from three slices extracted from the 3D microCT images. Very good concordance was shown between 3D microCT slices and the corresponding physical histologic slices. In 2D, the standard histomorphometric parameters, fractal dimension, mean intercept length, and connectivity were computed. In 3D, histomorphometric parameters were computed using both the 3D mean intercept length method and model-independent techniques. The 3D fractal dimension and the 3D connectivity, assessed by Euler density, were also evaluated. The cubic samples were subjected to elastic compressive tests in three orthogonal directions (X, Y, Z) close to the main natural trabecular network directions. A test was performed until collapse of trabecular network in the main direction (Z). The mechanical properties were significantly correlated to most morphological parameters resulting from 2D and 3D analysis. In 2D, the correlation between the mechanical strength and bone volume/tissue volume was not significantly improved by adding structural parameters or connectivity parameter (nodes number/tissue volume). In 3D, one architectural parameter (the trabecular thickness, Tb.Th) permitted to improve the estimation of the compressive strength from the bone volume/tissue volume alone. However, this improvement was minor since the correlation with the BV/TV alone was high (r=0.96). In conclusion, which is in agreement with the statistic's rules, we found, in this study, that the determination of the os calcis bone compressive strength using the 3D bone volume fraction cannot be improved by adding 3D architectural parameters.     

Fatigue is associated with high prevalence and severity of physical and emotional symptoms in patients on chronic hemodialysis

Purpose

The symptom burden of fatigued hemodialysis patients is poorly known. We aimed to investigate possible differences in the prevalence and severity of symptoms between fatigued and not fatigued patients on chronic hemodialysis.

Methods

All prevalent patients on chronic hemodialysis referring to the Hemodialysis Service between January 2016 and June 2017 were considered eligible. The Dialysis Symptom Index (DSI) questionnaire was performed during the dialysis treatment. Patients underwent assessment of fatigue using the Italian version of the vitality scale of the SF-36 (SF-36VS).

Results

We studied 137 patients: 107 (78.1%) were fatigued and 30 (31.9%) were non-fatigued. The median [95% CI] number of symptoms was 15 [14–16] for patients who reported fatigue and 9 [8–19] for the non-fatigued (P?<?0.0001). In fatigued patients, with respect to non-fatigued ones, the prevalence of dry skin, itching, muscle soreness, bone or joint pain, restless legs, shortness of breath, feeling sad, feeling anxious, difficulty concentrating, and difficulty becoming sex aroused was significantly higher. Restless legs, feeling sad, difficulty concentrating, and difficulty becoming sex aroused were symptoms independently associated with fatigue. The severity of dry skin, trouble staying asleep, and bone/joint pain was higher in fatigued patients.

Conclusion

Fatigued hemodialysis patients report suffering from physical and emotional symptoms more frequently than non-fatigued patients. This finding suggests the need to accurately and routinely define the symptom burden of chronic hemodialysis patients and may help to investigate eventually common underlying pathogenic mechanisms of symptoms in these patients.

     

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.     

Effects of intermittent administration of pamidronate on the mechanical properties of canine cortical and trabecular bone

The mechanical properties of cortical and trabecular bones from beagles treated with the bisphosphonate pamidronate (administered intermittently 1 week every month for 3 months, at a dosage of 0.45 μmol/kg/day) were assessed. The mechanical properties of cortical bone were measured by four-point bending tests on femoral quadrants, in order to measure their elastic modulus and ultimate stress. The structural properties of whole tibias were measured in torsion to determine the torsional stiffness and failure torque. The elastic modulus and maximum compressive stress of the trabecular bone samples were measured by compression tests of trabecular cores. Intermittent treatment with pamidronate did not change the pattern of mechanical properties that occurs naturally around the femur or the torsional stiffness and failure torque of the tibias. By contrast, pamidronate did significantly increase the modulus of elasticity (by 19%) and maximum compressive stress (by 33%) of vertically aligned cylindrical trabecular specimens taken from the vertebrae of the beagles.     

Effects of osteoporosis medications on bone quality

In clinical practice, the quantitative evaluation of bone tissue relies on dual-energy X-ray absorptiometry (DXA) measurements of bone mineral density (BMD) values, which are closely associated with the risk of osteoporotic fracture. However, only a small fraction of the antifracture effect of bone resorption inhibitors is ascribable to BMD gains (4% with raloxifene and 16-28% with alendronate and risedronate). Bone quality encompasses a number of bone tissue properties that govern mechanical resistance, such as bone geometry, cortical properties, trabecular microarchitecture, bone tissue mineralization, quality of collagen and bone apatite crystal, and presence of microcracks. All these properties are dependent on bone turnover and its variations. In populations, the decreases in bone resorption markers achieved with resorption inhibitors may predict in part the decrease in fracture risk. At the spine, however, this correlation exists down to a 40% fall in bone resorption markers; larger drops did not provide further protection against fractures in patients taking risedronate in one evaluation of this relationship. Osteoporosis medications can exert favorable effects on bone size and cortical thickness. Such effects have been documented with teriparatide (PTH 1-34), which is the unique purely anabolic treatment for osteoporosis available to date. More surprising are the favorable effects on bone size seen with some of the bone resorption inhibitors such as neridronate in adults with osteogenesis imperfecta. Similarly, estrogens and alendronate can increase femoral neck size in postmenopausal women. Preservation of the trabecular microarchitecture was demonstrated first with risedronate and subsequently with alendronate. In placebo-controlled studies, a deterioration in trabecular microarchitecture occurred within 1 to 3 years in the placebo groups but not in the bisphosphonate groups. Teriparatide, in contrast, improves trabecular microarchitecture, in particular by increasing connectivity and improving the plate-rod distribution. The minerals within trabecular or cortical bone can be evaluated using microradiography or synchrotron micro-computed tomography. Marked or prolonged secondary mineralization may result in poor bone quality. Increased bone mineralization is among the key effects of bone resorption inhibitors, most notably bisphosphonates. Prolonged use of the most potent bisphosphonates may lead to unwanted effects related to excessive mineralization. Microcracks may play a physiological role; however, a large number of microcracks may be deleterious via an effect on osteocytes. Excessive mineralization may promote the development of multiple microcracks. Studies of bone crystal and collagen properties with several bone resorption inhibitors, including risedronate and raloxifene, showed no harmful effects. An increasing number (several hundreds) of mandibular osteonecrosis associated with bisphosphonate therapy has been reported. The typical patient was receiving injectable bisphosphonate therapy for bone cancer and had undergone dental work shortly before bisphosphonate administration. The mechanism of this adverse effect is poorly understood.     

Fractographic Examination of Racing Greyhound Central (Navicular) Tarsal Bone Failure Surfaces Using Scanning Electron Microscopy

The greyhound is a fatigue fracture model of a short distance running athlete. Greyhounds have a high incidence of central (navicular) tarsal bone (CTB) fractures, which are not associated with overt trauma. We wished to determine whether these fractures occur because of accumulation of fatigue microdamage. We hypothesized that bone from racing dogs would show site-specific microdamage accumulation, causing predisposition to structural failure. We performed a fractographic examination of failure surfaces from fractured bones using scanning electron microscopy and assessed microcracking observed at the failure surface using a visual analog scale. Branching arrays of microcracks were seen in failure surfaces of CTB and adjacent tarsal bones, suggestive of compressive fatigue failure. Branching arrays of microcracks were particularly prevalent in remodeled trabecular bone that had become compact. CTB fractures showed increased microdamage when compared with other in vivo fractures (adjacent tarsal bone and long bone fractures), and ex vivo tarsal fractures induced by monotonic loading (P < 0.02). It was concluded that greyhound racing and training often results in CTB structural failure, because of accumulation and coalescence of branching arrays of fatigue microcracks, the formation of which appears to be predisposed to adapted bone. Received: 12 November 1999 / Accepted: 10 March 2000     

Activation of bone remodeling after fatigue: Differential response to linear microcracks and diffuse damage

Recent experiments point to two predominant forms of fatigue microdamage in bone: linear microcracks (tens to a few hundred microns in length) and “diffuse damage” (patches of diffuse stain uptake in fatigued bone comprised of clusters of sublamellar-sized cracks). The physiological relevance of diffuse damage in activating bone remodeling is not known. In this study microdamage amount and type were varied to assess whether linear or diffuse microdamage has similar effects on the activation of intracortical resorption. Activation of resorption was correlated to the number of linear microcracks (Cr.Dn) in the bone (R² = 0.60, p < 0.01). In contrast, there was no activation of resorption in response to diffuse microdamage alone. Furthermore, there was no significant change in osteocyte viability in response to diffuse microdamage, suggesting that osteocyte apoptosis, which is known to activate remodeling at typical linear microcracks in bone, does not result from sublamellar damage. These findings indicate that inability of diffuse microdamage to activate resorption may be due to lack of a focal injury response. Finally, we found that duration of loading does not affect the remodeling response. In conclusion, our data indicate that osteocytes activate resorption in response to linear microcracks but not diffuse microdamage, perhaps due to lack of a focal injury-induced apoptotic response.     

3D scanning SAXS: A novel method for the assessment of bone ultrastructure orientation

The arrangement and orientation of the ultrastructure plays an important role for the mechanical properties of inhomogeneous and anisotropic materials, such as polymers, wood, or bone. However, there is a lack of techniques to spatially resolve and quantify the material's ultrastructure orientation in a macroscopic context. In this study, a new method is presented, which allows deriving the ultrastructural 3D orientation in a quantitative and spatially resolved manner. The proposed 3D scanning small-angle X-ray scattering (3D sSAXS) method was demonstrated on a thin trabecular bone specimen of a human vertebra. A micro-focus X-ray beam from a synchrotron radiation source was used to raster scan the sample for different rotation angles. Furthermore, a mathematical framework was developed, validated and employed to describe the relation between the SAXS data for the different rotation angles and the local 3D orientation and degree of orientation (DO) of the bone ultrastructure. The resulting local 3D orientation was visualized by a 3D orientation map using vector fields. Finally, by applying the proposed 3D scanning SAXS method on consecutive bone sections, a 3D map of the local orientation of a complete trabecular element could be reconstructed for the first time. The obtained 3D orientation map provided information on the bone ultrastructure organization and revealed links between trabecular bone microarchitecture and local bone ultrastructure. More specifically, we observed that trabecular bone ultrastructure is organized in orientation domains of tens of micrometers in size. In addition, it was observed that domains with a high DO were more likely to be found near the surface of the trabecular structure, and domains with lower DO (or transition zones) were located in-between the domains with high DO. The method reproducibility was validated by comparing the results obtained when scanning the sample under different sample tilt angles. 3D orientation maps such as the ones created using 3D scanning SAXS will help to quantify and understand structure–function relationships between bone ultrastructure and bone mechanics. Beyond that, the proposed method can also be used in other research fields such as material sciences, with the aim to locally determine the 3D orientation of material components.     

Compressive behavior of human bone-cement composites

Current surgical practice in the implantation of cemented total joint arthroplasties generally creates a zone of variable thickness in which polymethylmethacrylate (PMMA) is intermixed with trabecular bone. The authors' objectives in these experiments were to characterize the compressive mechanical properties of this bone-cement composite material. They found that the mechanical properties of bone-cement composite specimens, fabricated under in vitro conditions that would promote nearly complete cement filling, are closer to the properties of trabecular bone than to those of cement. For both low-viscosity cement (LVC) and PMMA specimens, with the cement introduced by either hand-packing or pressurized injection at periods of 2 and 7 minutes, the compressive strengths ranged from 29 MPa to 50 MPa and the compressive moduli from 539 MPa to 1,210 MPa. Cement volume fractions achieved using different filling methods ranged from 76% to 87%. In contrast to previous studies of bone-cement composites using high-density bovine bone, neither mechanical properties nor filling parameters correlated significantly with bone porosity measured prior to filling. The authors expect that the mechanical properties of bone-cement regions created at surgery under less than these ideal in vitro filling conditions will only approach their values as an upper limit. Thus, bone-cement composites created in situ at surgery will also exhibit mechanical properties well below previously assumed values.