Is there a role for bone quality in fragility fractures?

The Influence of Chronic Kidney Disease on the Structural and Mechanical Properties of Canine Bone

Background

Chronic kidney disease (CKD) is common in companion animals. Secondary hyperparathyroidism is an inevitable consequence of the disease and may have deleterious effect on the bone; however, the information regarding CKD‐associated bone abnormalities in companion animals is scarce.

Hypothesis/Objectives

Dogs with CKD have decreased bone quality compared to dogs without CKD.

Animals

Nine dogs diagnosed with naturally occurring CKD for at least 6 months and 9 age‐matched controls.

Methods

Dogs with CKD were enrolled and compared to 9 age‐, weight‐, and sex‐matched control dogs with no evidence of CKD. Samples were assessed using light microscopy, mechanical testing, and microcomputed tomography. Variables evaluated included microstructural features such as number, size, and density of Haversian canals, resorption cavities and osteocytic lacunae, bone mineral density, porosity and Young's modulus.

Results

Median lacunae size was significantly smaller in the CKD group compared to the control group (P = 0.001). Resorption cavity density was higher in the CKD compared to the control group (10 [8–14] vs. 7 [4–9]/mm², respectively, P = 0.001). Overall porosity was significantly (2.3‐fold) higher in the CKD compared to the control group. There was no difference in Young's moduli between groups.

Conclusions and Clinical Importance

Naturally occurring CKD affects bone quality in dogs, but these changes are relatively mild and likely not to be manifested clinically. The duration of the disease in dogs evaluated here is short compared to cats and human patients, likely accounting for the more subtle changes in dogs compared to other species.

Bone Material Properties and Skeletal Fragility

Deformations of vertebrae and sudden fractures of long bones caused by essentially normal loading are a characteristic problem in osteoporosis. If the loading is normal, then the explanation for and prediction of unexpected bone failure lies in understanding the mechanical properties of the whole bone-which come from its internal and external geometry, the mechanical properties of the hard tissue, and from how well the tissue repairs damage. Modern QCT and MRI imaging systems can measure the geometry of the mineralized tissue quite well in vivo-leaving the mechanical properties of the hard tissue and the ability of bone to repair damage as important unknown factors in predicting fractures. This review explains which material properties must be measured to understand why some bones fail unexpectedly despite our current ability to determine bone geometry and bone mineral content in vivo. Examples of how to measure the important mechanical properties are presented along with some analysis of potential drawbacks of each method. Particular attention is given to methods useful to characterize the loss of bone toughness caused by mechanical fatigue, drug side effects, and damage to the bone matrix.

A new approach to iliac bone histomorphometry: implications for biomechanics and cell biology

I devised a method for obtaining information on cancellous bone structure from iliac bone histomorphometry that led to the demonstration that architecture is an important component of bone strength and bone fragility. Furthermore, this method contributed to the recognition of the importance of changes in osteoclast and osteocyte apoptosis in response to estrogen deficiency and replacement.

Retrospective 3D registration of trabecular bone MR images for longitudinal studies

PURPOSE

To evaluate an automatic 3D registration algorithm for serial high-resolution images of trabecular bone (TB) in studies designed to evaluate the response of the trabecular architecture to intervention or disease progression.

MATERIALS AND METHODS

An efficient algorithm for registering high-resolution 3D images of TB is presented. The procedure identifies the six parameters of rigid displacement between two scans performed at different timepoints. By assuming a relatively small through-plane rotation, considerable time is saved by combining the results of a collection of regional 2D registrations throughout the TB region of interest (ROI). The algorithm was applied to 26 pairs of MR images acquired 6 months apart. Reproducibility of local TB structural parameters (plate, rod, and junction density) computed in manually selected regions were compared between baseline and registered follow-up images.

RESULTS

All 26 registrations were completed successfully in less than 30 seconds per image pair. The resampled follow-up images agreed with baseline to around one pixel throughout the volume at 137 x 137 x 410 microm(3) image resolution. Structural parameters in each region correlated well from baseline to follow-up with intraclass correlation coefficients ranging between 85%-97% for TB plate density. Interregional variations in the parameters were large as compared with intraregion reproducibility.

CONCLUSION

The proposed algorithm was successful in automatically registering baseline and follow-up TB images in a translational study, and may be useful in regional analyses in longitudinal MR studies of TB architecture.

Bone morphometry strongly predicts cortical bone stiffness and strength, but not toughness, in inbred mouse models of high and low bone mass

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

The compositional and physicochemical homogeneity of male femoral cortex increases after the sixth decade

The temporal and spatial fluctuations in the dynamics of secondary osteonal remodeling impart heterogeneity to the compositional quality of bone. Bone mineral density (BMD) fails to reflect this heterogeneity as being a single score, and thus it cannot resolve the overlap between healthy individuals and those who experience fractures. Such information on tissue heterogeneity is lacking in the literature. In the current study, specimens were prepared from mid-diaphyseal portions of human femora (N=16, age range 52-85 years old) and grouped based on the anatomical location (anterior, lateral, medial and posterior quadrants). Raman microscopy was used to obtain multiple measurements from each specimen which allowed the construction of histograms of mineralization, crystallinity and carbonation. The coefficient of variation (COV) and skewness were extracted from histograms as measures of heterogeneity. Results demonstrated that average mineralization of the medial quadrant and the data pooled over quadrants significantly increased with age. The mean carbonation increased within the observed age range for the pooled data. The variations of values about the mean became tighter for mineralization, crystallinity and type-B carbonation with age, indicating an overall reduction in compositional heterogeneity of aging femoral cortex. Skewness values indicated that the distributions of histograms were not Gaussian. We conclude that age-related changes in mean tissue composition are confounded with changes in the variation of tissue make-up about the mean. Future studies will establish as to whether compositional heterogeneity correlates with the mechanical strength of bone.

Bone Strength: The Whole Is Greater Than the Sum of Its Parts

OBJECTIVE

To summarize the current knowledge regarding the various determinants of bone strength.

METHODS

Relevant English-language articles acquired from Medline from 1966 up to January 2005 were reviewed. Searches included the keywords bone AND 1 of the following: strength, remodeling, microcrack, structur*, mineralization, collagen, organic, crystallinity, osteocyte, porosity, diameter, anisotropy, stress risers, or connectivity. Abstracts from applicable conference proceedings were also reviewed for pertinent information.

RESULTS

Bone strength is determined from both its material and its structural properties. Material properties such as its degree of mineralization, crystallinity, collagen characteristics, and osteocyte viability have substantial impacts on bone strength. Structural properties such as the diameter and thickness of the cortices, the porosity of the cortical shell, the connectivity and anisotropy of the trabecular network, the thickness of trabeculae, and the presence of trabecular stress risers and microcracks impact bone strength in diverse manners. Remodeling activity either directly or indirectly impacts all of these processes.

CONCLUSIONS

Bone strength is dependent on numerous, interrelated factors. Remodeling activity has a direct impact on almost all of the components of bone strength and requires further investigation as to its impact on these factors in isolation and in unison.

Osteonal crack barriers in ovine compact bone

Bone is an anisotropic structure which can be compared to a composite material. Discontinuities within its microstructure may provide stress concentration sites for crack initiation, but act as a barrier to its propagation. This study looks specifically at the relationship between crack length and propagation in compact bone. Beam-shaped bone samples from sheep radii were prepared and stained with fluorochrome dyes and tested in cyclic fatigue under four-point bending in an INSTRON 1341 servo-hydraulic fatigue-testing machine. Samples were tested at a frequency of 30 Hz and stress range of 100 MPa under load control. Specimens were sectioned transversely using a diamond saw, slides prepared and examined using epifluorescence microscopy. Cracks in transverse sections were classified in terms of their location relative to cement lines surrounding secondary osteons. Mean crack length, crack numerical density and crack surface density were examined. Short microcracks (100 microm or less) were stopped at the cement lines surrounding osteons, microcracks of intermediate length (100-300 microm) were deflected as they hit the cement line, and microcracks that were able to penetrate through cement lines were longer (> 400 microm). These data show that bone microstructure allows the initiation of microcracks but acts as a barrier to crack propagation.

Susceptibility of aging human bone to mixed-mode fracture increases bone fragility

Since bone-mass-based measurements have demonstrated limited success in predicting age-related increase in fracture incidence, recent emphasis has been placed on quantification of bone quality. Specifically, material parameters such as strength, stiffness, and toughness have been quantified to characterize bone quality through mechanical testing. This study is the first one to report multiaxial failure characteristics of bone (quality) by conducting fatigue tests on human cortical bone specimens under physiologically relevant loading involving simultaneous application of axial and torsional loading to produce previously reported in vivo shear/normal stress ratios. Our results show that, compared to uniaxial tests, multiaxial fatigue tests show up to a 20-fold reduction in the fatigue life of human cortical bone. More significantly, the susceptibility of mixed-mode failure increases bone fragility in aging human bone. Furthermore, since the magnitude of mixed-mode loading varies with physiological activities, results of this study also suggest that a reduction in the activities involving significant mixed-mode loading may lower the overall fracture incidence among older individuals.

Damage in trabecular bone at small strains

Evidence that damage decreases bone quality, increases fracture susceptibility, and serves as a remodeling stimulus motivates further study of what loading magnitudes induce damage in trabecular bone. In particular, whether damage occurs at the smaller strains characteristic of habitual, as opposed to traumatic, loading is not known. The overall goal of this study was to characterize damage accumulation in trabecular bone at small strains (0.20 - 0.45% strain). A continuum damage mechanics approach was taken whereby damage was quantified by changes in modulus and residual strain. Human vertebral specimens (n = 7) were tested in compression using a multi-cycle load - unload protocol in which the maximum applied strain for each cycle, epsilonmax, was increased incrementally from epsilonmax = 0.20% on the first loading cycle to epsilonmax = 0.45% on the last cycle. Modulus and residual strain were measured for each cycle. Both changes in modulus and residual strains commenced at small strains, beginning as early as 0.24 and 0.20% strain, respectively. Strong correlations between changes in modulus and residual strains were observed (r = 0.51 - 0.98). Fully nonlinear, high-resolution finite element analyses indicated that even at small apparent strains, tissue-level strains were sufficiently high to cause local yielding. These results demonstrate that damage in trabecular bone occurs at apparent strains less than half the apparent compressive yield strain reported previously for human vertebral trabecular bone. Further, these findings imply that, as a consequence of the highly porous trabecular structure, tissue yielding can initiate at very low apparent strains and that this local failure has detectable and negative consequences on the apparent mechanical properties of trabecular bone.

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.

Sensitivity of multiple damage parameters to compressive overload in cortical bone

Damage accumulation plays a key role in weakening bones prior to complete fracture and in stimulating bone remodeling. The goal of this study was to characterize the degradation in the mechanical properties of cortical bone following a compressive overload. Longitudinally oriented, low-aspect ratio specimens (n=24) of bovine cortical bone were mechanically tested using an overload-hold-reload protocol. No modulus reductions greater than 5% were observed following overload magnitudes less than 0.73% strain. For each specimen, changes in strength and Poisson's ratio were greater (p=0.02) than that in modulus by 10.8- and 26.6-fold, respectively, indicating that, for the specimen configuration used in this study, longitudinal elastic modulus is one of the least sensitive properties to a compressive overload. Residual strains were also proportionately greater by 6.4-fold (p=0.01) in the transverse than axial direction. These results suggest that efforts to relate microcrack density and morphology to changes in compressive mechanical properties of cortical bone may benefit from considering alternative parameters to modulus reductions.

Microdamage in bone: surface analysis and radiological detection

Microdamage accumulation leads to reduced bone strength and fracture. Intact, damaged and Rose Bengal stained cortical bone specimens were studied using SEM and EDXA imaging. SEM coupled with EDXA studies showed selective labelling of surface damage due to binding of dye at free lattice sites. A series of novel iodinated X-ray contrast agent were synthesised. These agents demonstrated excellent stability, water solubility and lack of atropisomerism. Preliminary imaging studies, using cone-beam mu-CT, demonstrated their ability to provide visible contrast in the solid state on bone surfaces.

Medieval trabecular bone architecture: the influence of age, sex, and lifestyle

Osteoporosis has become a growing health concern in developed countries and an extensive area of research in skeletal biology. Despite numerous paleopathological studies of bone mass, few studies have measured bone quality in past populations. In order to examine age- and sex-related changes in one aspect of bone quality in the past, a study was made of trabecular bone architecture in a British medieval skeletal sample. X-ray images of 5-mm-thick coronal lumbar vertebral bone sections were taken from a total of 54 adult individuals divided into three age categories (18-29, 30-49, and 50+ years), and examined using image analysis to evaluate parameters related to trabecular bone structure and connectivity. Significant age-related changes in trabecular bone structure (trabecular bone volume (BV/TV), trabecular number (Tb.N), trabecular separation (Tb.Sp), and anisotropic ratio (Tb.An)) were observed to occur primarily by middle age with significant differences between the youngest and two older age groups. Neither sex showed continuing change in trabecular structure between the middle and old age groups. Age-related changes in bone connectivity (number of nodes (N.Nd) and node-to-node strut length (Nd.Nd)) similarly indicated a change in bone connectivity only between the youngest and two older age groups. However, females showed no statistical differences among the age groups in bone connectivity. These patterns of trabecular bone loss and fragility contrast with those generally found in modern populations that typically report continuing loss of bone structure and connectivity between middle and old age, and suggest greater loss in females. The patterns of bone loss in the archaeological samples must be interpreted cautiously. We speculate that while nutritional factors may have initiated some bone loss in both sexes, physical activity could have conserved bone architecture in old age in both sexes, and reproductive factors such as high parity and extended periods of lactation could have played a key role in female bone maintenance in this historic population. The study of qualitative elements (such as trabecular architecture) is vital if we are to understand bone maintenance and fragility in the past.

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.

Mechanisms of uniformity of yield strains for trabecular bone

Variations in yield strains for trabecular bone within a specific anatomic site are only a small fraction of the substantial variations that exist for elastic modulus and strength, and yet the source of this uniformity is not known. Our goal was to investigate the underlying mechanisms by using high-resolution, materially nonlinear finite element models of 12 human femoral neck trabecular bone specimens. The finite element models, used to obtain apparent yield strains in both tension and compression, assumed that the tissue-level yield strains were the same across all specimens. Comparison of the model predictions with the experimental data therefore enabled us to isolate the combined roles of volume fraction and architecture from the role of tissue material properties. Results indicated that, for both tensile and compressive loading, natural variations in volume fraction and architecture produced a negligible coefficient of variation (less than 3%) in apparent yield strains. Analysis of tissue-level strains showed that while bending of individual trabeculae played only a minor role in the apparent elastic behavior, the combined effects of this bending and tissue-level strength asymmetry produced apparent-level failure strains in compression that were 14% lower than those at the tissue level. By contrast, tissue and apparent-level yield strains were equivalent for tensile loading. We conclude that the uniformity of apparent yield strains is primarily the result of the highly oriented architecture that minimizes bending. Most of the variation that does occur is the result of the non-uniformity of the tissue-level yield strains.

Identification of hip fracture patients from radiographs using Fourier analysis of the trabecular structure: a cross-sectional study

BACKGROUND: This study presents an analysis of trabecular bone structure in standard radiographs using Fourier transforms and principal components analysis (PCA) to identify contributions to hip fracture risk. METHODS: Radiographs were obtained from 26 hip fracture patients and 24 controls. They were digitised and five regions of interest (ROI) were identified from the femoral head and neck for analysis. The power spectrum was obtained from the Fourier transform of each region and three profiles were produced; a circular profile and profiles parallel and perpendicular to the preferred orientation of the trabeculae. PCA was used to generate a score from each profile, which we hypothesised could be used to discriminate between the fracture and control groups. The fractal dimension was also calculated for comparison. The area under the receiver operating characteristic curve (Az) discriminating the hip fracture cases from controls was calculated for each analysis. RESULTS: Texture analysis of standard radiographs using the fast Fourier transform yielded variables that were significantly associated with fracture and not significantly correlated with age, body mass index or femoral neck bone mineral density. The anisotropy of the trabecular structure was important; both the perpendicular and circular profiles were significantly better than the parallel-profile (P < 0.05). No significant differences resulted from using the various ROI within the proximal femur. For the best three groupings of profile (circular, parallel or perpendicular), method (PCA or fractal) and ROI (Az = 0.84 - 0.93), there were no significant correlations with femoral neck bone mineral density, age, or body mass index. PCA analysis was found to perform better than fractal analysis (P = 0.019). CONCLUSIONS: Both PCA and fractal analysis of the FFT data could discriminate successfully between the fracture and control groups, although PCA was significantly stronger than fractal dimension. This method appears to provide a powerful tool for the assessment of bone structure in vivo with advantages over standard fractal methods.

The elusive concept of bone quality

This paper outlines information from recent publications that aid our understanding of bone quality in relation to osteoporosis. In practical terms, bone quality designates the properties of bone that contribute to strength but are not assessed by bone densitometry. While osteoporosis is still defined in terms of bone density, the limitations of this approach, long questioned, have become indisputable. In parallel, the results of treatment trials of antiresorptive agents demonstrate that bone density is a flawed surrogate for bone fragility and a weak indicator of antifracture efficacy. The case for emphasizing bone turnover in assessing fracture risk, has become increasingly strong, and a redefinition of osteoporosis on this basis may well occur. New technologies for studying bone microstructure and matrix composition, merging with sophisticated biomechanical assessments, are advancing our ideas regarding bone "damageability" and its effects over time.

Bone's mechanostat: a 2003 update

The still-evolving mechanostat hypothesis for bones inserts tissue-level realities into the former knowledge gap between bone's organ-level and cell-level realities. It concerns load-bearing bones in postnatal free-living bony vertebrates, physiologic bone loading, and how bones adapt their strength to the mechanical loads on them. Voluntary mechanical usage determines most of the postnatal strength of healthy bones in ways that minimize nontraumatic fractures and create a bone-strength safety factor. The mechanostat hypothesis predicts 32 things that occur, including the gross anatomical bone abnormalities in osteogenesis imperfecta; it distinguishes postnatal situations from baseline conditions at birth; it distinguishes bones that carry typical voluntary loads from bones that have other chief functions; and it distinguishes traumatic from nontraumatic fractures. It provides functional definitions of mechanical bone competence, bone quality, osteopenias, and osteoporoses. It includes permissive hormonal and other effects on bones, a marrow mediator mechanism, some limitations of clinical densitometry, a cause of bone "mass" plateaus during treatment, an "adaptational lag" in some children, and some vibration effects on bones. The mechanostat hypothesis may have analogs in nonosseous skeletal organs as well.

Comparison of the elastic and yield properties of human femoral trabecular and cortical bone tissue

The ability to determine trabecular bone tissue elastic and failure properties has biological and clinical importance. To date, trabecular tissue yield strains remain unknown due to experimental difficulties, and elastic moduli studies have reported controversial results. We hypothesized that the elastic and tensile and compressive yield properties of trabecular tissue are similar to those of cortical tissue. Effective tissue modulus and yield strains were calibrated for cadaveric human femoral neck specimens taken from 11 donors, using a combination of apparent-level mechanical testing and specimen-specific, high-resolution, nonlinear finite element modeling. The trabecular tissue properties were then compared to measured elastic modulus and tensile yield strain of human femoral diaphyseal cortical bone specimens obtained from a similar cohort of 34 donors. Cortical tissue properties were obtained by statistically eliminating the effects of vascular porosity. Results indicated that mean elastic modulus was 10% lower (p<0.05) for the trabecular tissue (18.0+/-2.8 GPa) than for the cortical tissue (19.9+/-1.8 GPa), and the 0.2% offset tensile yield strain was 15% lower for the trabecular tissue (0.62+/-0.04% vs. 0.73+/-0.05%, p<0.001). The tensile-compressive yield strength asymmetry for the trabecular tissue, 0.62 on average, was similar to values reported in the literature for cortical bone. We conclude that while the elastic modulus and yield strains for trabecular tissue are just slightly lower than those of cortical tissue, because of the cumulative effect of these differences, tissue strength is about 25% greater for cortical bone.

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.

Reconsidering the effects of antiresorptive therapies in reducing osteoporotic fracture

Concepts of what constitutes osteoporosis have evolved from the single criterion of low bone mass to a more inclusive consideration of bone strength, based on both quantity and quality. The evidence driving this shift is drawn from many sources. For example, recent studies of bone geometry have shown what engineers have always known: material properties and structural strength are inseparable. Genetic factors also argue against a one-dimensional (ID) view of osteoporosis. Large-scale family studies present a strong case for genetic influences on bone mass and predisposition to fracture. The contribution of aging to fracture risk has long been known, but we are only now beginning to understand what happens to bone remodeling and microstructure in an aging skeleton. The recognition that osteoporosis is far more complex than previously thought suggests that factors in addition to bone mineral density (BMD) may be useful for evaluating bone fragility and therapeutic effectiveness. Although assessment of BMD is noninvasive and widely available, the degree of increase in BMD alone fails to account for the broader effectiveness of antiresorptive agents in reducing the risk of fractures related to osteoporosis. Indeed, the very multiplicity of factors that determine fracture risk implies that response to therapy may be equally complex. Studies of response to antiresorptive agents and the cellular processes they induce are at best preliminary at this time. Although new technologies have been applied to studying bone microarchitecture, their invasive nature limits wide use. New methods are needed to provide insight into the causes and effects of bone fragility. The definition of osteoporosis, meanwhile, must still be considered a work in progress.

Bone brittleness varies with genetic background in A/J and C57BL/6J inbred mice

The contribution of genetic and environmental factors to variations in bone quality are understood poorly. We tested whether bone brittleness varies with genetic background using the A/J and C57BL/6J inbred mouse strains. Whole bone four-point bending tests revealed a 70% decrease in postyield deflection of A/J femurs compared with C57BL/6J, indicating that A/J femurs failed in a significantly more brittle manner. Cyclic loading studies indicated that A/J femurs accumulated damage differently than C57BL/6J femurs, consistent with their increased brittleness. Differences in matrix composition also were observed between the two mouse strains. A/J femurs had a 4.5% increase in ash content and an 11.8% decrease in collagen content. Interestingly, a reciprocal relationship was observed between femoral geometry and material stiffness; this relationship may have contributed to the brittle phenotype of A/J femurs. A/J femurs are more slender than those of C57BL/6J femurs; however, their 47% smaller moment of inertia appeared to be compensated by an increased tissue stiffness at the expense of altered tissue damageability. Importantly, these differences in whole bone mechanical properties between A/J and C57BL/6J femurs could not have been predicted from bone mass or density measures alone. The results indicated that bone brittleness is a genetically influenced trait and that it is associated with genetically determined differences in whole bone architecture, bone matrix composition, and mechanisms of cyclical damage accumulation.

From Wolff's law to the Utah paradigm: insights about bone physiology and its clinical applications

Efforts to understand our anatomy and physiology can involve four often overlapping phases. We study what occurs, then how, then ask why, and then seek clinical applications. In that regard, in 1960 views, bone's effector cells (osteoblasts and osteoclasts) worked chiefly to maintain homeostasis under the control of nonmechanical agents, and that physiology had little to do with anatomy, biomechanics, tissue-level things, muscle, and other clinical applications. But it seems later-discovered tissue-level mechanisms and functions (including biomechanical ones, plus muscle) are the true key players in bone physiology, and homeostasis ranks below the mechanical functions. Adding that information to earlier views led to the Utah paradigm of skeletal physiology that combines varied anatomical, clinical, pathological, and basic science evidence and ideas. While it explains in a general way how strong muscles make strong bones and chronically weak muscles make weak ones, and while many anatomists know about the physiology that fact depends on, poor interdisciplinary communication left people in many other specialties unaware of it and its applications. Those applications concern 1.) healing of fractures, osteotomies, and arthrodeses; 2.) criteria that distinguish mechanically competent from incompetent bones; 3.) design criteria that should let load-bearing implants endure; 4.) how to increase bone strength during growth, and how to maintain it afterwards on earth and in microgravity situations in space; 5.) how and why healthy women only lose bone next to marrow during menopause; 6.) why normal bone functions can cause osteopenias; 7.) why whole-bone strength and bone health are different matters; 8.) why falls can cause metaphyseal and diaphyseal fractures of the radius in children, but mainly metaphyseal fractures of that bone in aged adults; 9.) which methods could best evaluate whole-bone strength, "osteopenias" and "osteoporoses"; 10.) and why most "osteoporoses" should not have bone-genetic causes and some could have extraosseous genetic causes. Clinical specialties that currently require this information include orthopaedics, endocrinology, radiology, rheumatology, pediatrics, neurology, nutrition, dentistry, and physical, space and sports medicine. Basic science specialties include absorptiometry, anatomy, anthropology, biochemistry, biomechanics, biophysics, genetics, histology, pathology, pharmacology, and cell and molecular biology. This article reviews our present general understanding of this new bone physiology and some of its clinical applications and implications. It must leave to other times, places, and people the resolution of questions about that new physiology, and to understand the many devils that should lie in its details. (Thompson D'Arcy, 1917).

Ageing human bone: factors affecting its biomechanical properties and the role of collagen

The incidence of fractures increases with age. This is partly due to extraosseous factors and partly to the increased fragility of the bone material itself. Ageing adversely affects the "quality" of human bone material, its elastic and ultimate properties. The hypothesis here is that these effects are caused by factors such as architectural changes, compositional changes, physicochemical changes, changes at the micromechanical level, and the degree of prior in vivo microdamage. Examination of the extent of the secondary osteonal area, the porosity level, the calcium content, the mineral/wet weight fraction, the dry density, the condition of the collagen and its content in mature x-links, the elasticity of osteonal and interstitial lamellae at the microscopic level and the numerical- and surface-density of the in vivo fatigue microcracks has been undertaken. The findings show that some factors simply affect the stiffness and the strength of bone, while others soley affect its toughness. We discuss the implications of these findings in the context of the composite nature of the ageing bone material matrix.

Bone mineral density, osteopenia, and osteoporosis in the rhesus macaques of Cayo Santiago

This cross-sectional study investigates metabolic bone disease and the relationship between age and bone mineral density (BMD) in males and females of a large, well-documented skeletal population of free-ranging rhesus monkeys (Macaca mulatta), from the Caribbean Primate Research Center Museum collection from Cayo Santiago, Puerto Rico. The sample consists of 254 individuals aged 1.0-20+ years. The data consist of measurements of bone mineral content and bone mineral density, obtained from dual-energy X-ray absorptiometry (DEXA), of the last lumbar vertebra from each monkey. The pattern of BMD differs between male and female rhesus macaques. Females exhibit an initial increase in BMD with age, with peak bone density occurring around age 9.5 years, and remaining constant until 17.2 years, after which there is a steady decline in BMD. Males acquire bone mass at a faster rate, and attain a higher peak BMD at an earlier age than do females, at around 7 years of age, and BMD remains relatively constant between ages 7-18.5 years. After age 7 there is no apparent effect of age on BMD in the males of this sample; males older than 18.5 years were excluded due to the presence of vertebral osteophytosis, which interferes with DEXA. The combined frequency of osteopenia and osteoporosis in this population is 12.4%. BMD values of monkeys with vertebral wedge fractures are generally higher than those of virtually all of the nonfractured osteopenic/osteoporotic individuals, thus supporting the view that BMD as measured by DEXA is a useful but imperfect predictor of fracture risk, and that low BMD may not always precede fractures in vertebral bones. Other factors such as bone quality (i.e., trabecular connectivity) should also be considered. The skeletal integrity of a vertebra may be compromised by the loss of key trabeculae, resulting in structural failure, but the spine may still show a BMD value within normal limits, or within the range of osteopenia.

Does suppression of bone turnover impair mechanical properties by allowing microdamage accumulation?

One plausible purpose of bone turnover is to repair bone microdamage. We hypothesized that suppression of bone turnover impairs bone quality by allowing accumulation of microdamage. We investigated the effect of high-dose etidronate (EHDP) on bone's mechanical properties and microdamage accumulation. Skeletally mature beagles, 1-2 years old at the beginning of the study, were treated with daily injections of vehicle or EHDP at 0.5 mg/kg per day or 5.0 mg/kg per day for 1 year. X-rays were taken at baseline and monthly from 7 to 12 months. Bones were taken upon sacrifice and biomechanical tests, histomorphometry, and microdamage analyses were performed. Fractures of ribs and/or thoracic spinous processes were found in 10 of 11 dogs treated with the higher dose EHDP. Only one fracture of a thoracic spinous process was found in dogs treated with the lower dose EHDP, and no fractures were found in the vehicle controls. Biomechanical tests showed reduced mechanical strength in ribs and lumbar vertebrae, but not in the femoral diaphysis or thoracic spinous process in the higher dose EHDP group. Histomorphometric measurements showed a significant reduction of cancellous bone turnover in both EHDP-treated groups compared with controls. In dogs treated with the higher dose EHDP, activation frequency was reduced to zero in both cortical and cancellous bone. Osteoid volume increased significantly, especially in trabecular bone, resulting in reduced mineralized bone volume in the higher dose EHDP group. Microcrack numerical density (Cr.Dn) increased significantly only in the lumbar vertebral body in the higher dose EHDP group, but not in the rib or thoracic spinous process where fractures occurred. These findings show that suppression of bone turnover using high doses of EHDP is associated with fractures of the ribs and spinous processes in dogs. This is most likely the result of excessive amounts of unmineralized bone produced by the inhibition of mineralization at these high doses, rather than by the accumulation of microdamage.

Heterogeneity of bone lamellar-level elastic moduli

Advances in our ability to assess fracture risk, predict implant success, and evaluate new therapies for bone metabolic and remodeling disorders depend on our understanding of anatomically specific measures of local tissue mechanical properties near and surrounding bone cells. Using nanoindentation, we have quantified elastic modulus and hardness of human lamellar bone tissue as a function of tissue microstructures and anatomic location. Cortical and trabecular bone specimens were obtained from the femoral neck and diaphysis, distal radius, and fifth lumbar vertebra of ten male subjects (aged 40-85 years). Tissue was tested under moist conditions at room temperature to a maximum depth of 500 nm with a loading rate of 10 nm/sec. Diaphyseal tissue was found to have greater elastic modulus and hardness than metaphyseal tissues for all microstructures, whereas interstitial elastic modulus and hardness did not differ significantly between metaphyses. Trabecular bone varied across locations, with the femoral neck having greater lamellar-level elastic modulus and hardness than the distal radius, which had greater properties than the fifth lumbar vertebra. Osteonal, interstitial, and primary lamellar tissues of compact bone had greater elastic moduli and hardnesses than trabecular bone when comparing within an anatomic location. Only femoral neck interstitial tissue had a greater elastic modulus than its osteonal counterpart, which suggests that microstructural distinctions can vary with anatomical location and may reflect differences in the average tissue age of cortical bone or mineral and collagen organization.

Age, gender, and bone lamellae elastic moduli

To enhance preventative and therapeutic strategies for metabolic bone diseases and bone fragility disorders, we began to explore the physical properties of bone tissue at the cellular level. Proximal femurs were harvested from 27 cadavera (16 male and 11 female) for in vitro measurement of the mechanical properties. We measured the variations in lamellar-level elastic modulus and hardness in human bone as a function of age and gender to identify microstructural properties responsible for age and gender-related reductions in the mechanical integrity. The lateral femoral necks were examined, and age, gender, height, body mass, and body mass index were not found to correlate with lamellar-level elastic modulus or hardness. This result was consistent for osteonal, interstitial, and trabecular tissue. These data suggest that increased bone mass maintenance, known to occur in heavier individuals, is not accompanied by increases in the lamellar-level elastic modulus or hardness. The independence of elastic modulus and hardness from age and gender suggests that age and gender-related decreases in mechanical integrity do not involve alterations in elastic modulus or hardness of the extracellular matrix. Lamellar-level ultimate, fatigue, and fracture toughness properties should also be investigated. Other factors, such as tissue mass and organization, may also contribute to age and gender-related decreases in the mechanical integrity.

Vertebral trabecular bone microscopic tissue elastic modulus and hardness do not change in ovariectomized rats

Ovariectomized rats have been used extensively and have received substantial acceptance as animal models for postmenopausal osteoporosis. However, little is known about the microscopic tissue properties of rat vertebral bone, especially during osteoporosis caused by estrogen depletion. This study applied a new nanoindentation technique to quantify the microscopic mechanical properties of vertebral trabecular bone tissue in ovariectomized rats. Seventeen-week-old Sprague-Dawley rats underwent an ovariectomy. After death at 37 weeks, the fraction of the trabecular bone area of the lumbar vertebrae (L4) was measured with scanning electron microscopy and the elastic modulus and hardness were determined with the nanoindentation technique. The bone area fraction was reduced significantly after the ovariectomy. However, the elastic modulus and hardness did not change significantly at the microscopic level. The results indicate that estrogen-dependent osteoporosis in rats manifests in a loss of bone mass whereas the elastic and hardness properties of the surviving bone tissue remain relatively unchanged.

The role of collagen in the declining mechanical properties of aging human cortical bone

The importance of the mechanical role of collagen in bone is becoming increasingly more clear as evidence mounts on the detrimental effects of altered collagen on the mechanical properties of bone. We previously examined a set of mechanical properties (material stiffness, strength, and toughness) of human femoral bone (ages 35-92) and found that a gradual deterioration in these properties occurs with age. The present study examines the collagen of the same specimens and relates the collagen properties to the mechanical ones. In the collagen we measured the concentration of stable mature crosslinks, the shrinkage temperature, and the rate of contraction during isometric heating. The changes in the concentration of mature (pyridinium and deoxypyridinium) crosslinks showed no clear relationship to age nor did they correlate with the mechanical properties. The shrinkage temperature declined with age and correlated with a bone's toughness. The maximum rate of contraction was strongly correlated with three different measures of tissue toughness, but much less to stiffness and strength. Our results reinforce speculation regarding the toughening role of collagen in bone mechanics and suggest that the fragility of aging bone may be related to collagen changes.

Fluorescence-aided detection of microdamage in compact bone

En bloc staining with basic fuchsin is an established method for demonstrating microdamage in bone. Using transmitted light microscopy, variations in light intensity, depth of focus and magnification are necessary to distinguish fully-stained microcracks generated in vivo, from partially-stained or unstained artefactual cracks due to cutting and machining. This process is both difficult and time-consuming. In this study, 2 methods were used to examine fuchsin-stained microcracks in human rib sections, transmitted light and epifluorescence microscopy. No differences were found in crack number, density or length between the 2 methods indicating comparable accuracy. Using green epifluorescence, only microcracks containing fuchsin fluoresced orange against the darkfield background, enabling unstained, artefactual cracks to be screened out. Under UV epifluorescence, microcracks stained through the full 100 microm depth of the section fluoresced purple. Partially-stained artefactual cracks failed to fluoresce and were screened out. Epifluorescence is a simple, rapid and accurate screening method for differentiating fully-stained from artefactual microcracks in bone.

Pathophysiology of osteoporosis

As with many chronic diseases that express themselves late in life, osteoporosis is distinctly multifactorial, both in etiology and pathophysiology. Osteoporotic fractures occur because of a combination of injury and intrinsic bony fragility. Injury comes most often from a combination of falls, falling to the side, poor postural reflexes that fail to protect bony parts from impact, and reduced soft-tissue padding over bony prominences. The bony fragility itself is a composite of geometry, low mass density, severance of microarchitectural connections in trabecular structures, and altered bone material quality. The latter is primarily the result of accumulated fatigue damage, but reduced collagen cross-links and other intrinsic material defects may play a role as well. Reduced bone mass, in turn, is the result of varying combinations of gonadal hormone deficiency, inadequate intakes of calcium and vitamin D, decreased physical activity, comorbidity, and the effects of drugs used to treat various unrelated medical conditions. Finally, the often poor outcome from hip fracture in the elderly is partly due to associated protein-calorie malnutrition. An adequate preventive program for osteoporotic fracture must address as many of these factors as possible and be as multifaceted as the disease is multifactorial.

Calcaneus as a site for the assessment of bone mass

The calcaneus is a skeletal site frequently used for monitoring bone loss after spaceflight, because it is sensitive to microgravity-induced bone mineral loss and reflects the degree of demineralization in the vertebra and the femoral neck. In this article, methods for assessing the calcaneus are reviewed, and their potential applications and limitations as the monitoring site for bone loss in weightlessness are discussed. Currently, single or dual energy X-ray absorptiometry appears to be most sensitive for monitoring bone mineral loss in weightlessness. The results of recent studies suggest two- to threefold longer follow-up times required for ultrasound techniques. However, ultrasound devices can be designed to be portable, making them attractive for inflight use, and ultrasound techniques are expected to provide information related to bone quality. Additional investigations that assess new ultrasound techniques would be important to determine and utilize the full potential of this technology for monitoring bone loss in weightlessness.

Fracture and material degradation properties of cortical bone under accelerated stress

The fracture stress and material property degradation of bovine cortical bone specimens were investigated experimentally under accelerated cyclic tensile stress testing. The fracture stress of a typical specimen was found from a static tensile test, and the cyclic loading/unloading was calculated as a percentage of this fracture stress. The results of accelerated cyclic stress tests were compared to monotonically increased static tests to determine if loading/unloading has an effect on the damage mechanism in bone. It was found that fracture stress of the bone increases due to accelerated stress cycling whereas the modulus decreases in a logarithmic fashion with increasing cyclic stress.

Assessment of bone quantity and 'quality' by ultrasound attenuation and velocity in the heel

OBJECTIVE: To determine if ultrasound measurements in the heel are related to bone quality in addition to quantity. DESIGN: In situ and in vitro experiments on cadaver heels. BACKGROUND: It has been suggested, but not demonstrated, that clinical ultrasound - used to screen for osteoporosis in clinical trials - provides a measure of 'bone quality' as distinct from bone quantity. METHODS: Ultrasound transmission velocity (UTV) and the slope of the linear dependence of broadband ultrasound attenuation on frequency (BUA) were measured in situ in 32 heels of 16 cadavers and in vitro in cores of calcaneal trabecular bone. RESULTS: After adjusting for Young's modulus, in situ UTV explains 33% (P = 0.03) and in situ BUA explains none of the remaining variance in density (r(2) = 0.02, P = 0.60). After adjusting for density, in situ BUA explains 29% (P = 0.04) and in situ UTV explains none of the remaining variance in Young's modulus (r(2) = 0.01, P = 0.79). By comparison, in vitro BUA explains 58% (P = 0.001) of the remaining variance in Young's modulus, after adjusting for density. CONCLUSIONS: In situ BUA reflects 'bone quality' independently of bone quantity, whereas in situ UTV reflects bone quantity independently of 'bone quality'.

Decrease in canine proximal femoral ultimate strength and stiffness due to fatigue damage

Fractures of the proximal femur represent a significant health concern especially in the elderly. Fatigue damage and microfractures have been implicated in the etiology of hip fractures; however, the extent to which these factors are sufficient to bring about significant reductions in proximal femur strength and stiffness is unknown. This study examined the hypothesis that fatigue loading of the proximal femur results in highly correlated decreases in bone stiffness and strength through the accumulation of bone microdamage. One canine femur from each of 10 pairs was monotonically loaded to failure to determine the ultimate strength. The contralateral femur was then cyclically loaded at 50% of the ultimate load value for either 3600 cycles or until a 40% reduction in stiffness was achieved. This femur was then monotonically loaded to failure. For two additional femur pairs, the fatigued femur was histologically processed to reveal bone microdamage. In support of the hypothesis, the data demonstrated a linear relationship between strength loss and stiffness loss (Adj. R2 = 0.79, p < 0.0004) with significant decreases in residual whole bone strength (p < 0.004) following cyclic loading. In addition, damage (microcracks) in the cortical bone and broken trabeculae were observed in the neck and head region of the femur fatigued until its stiffness was reduced by 40% but not fractured subsequent to cyclic loading.

Bone quantity and quality in past populations

BACKGROUND

The study of osteoporosis in past populations offers valuable insight into the patterns and prevalence of the disease in both the past and in the present.

METHODS AND RESULTS

A review is made of different paleopathological studies better to understand bone loss in past populations and to examine the contribution such studies can make to our knowledge of osteoporosis in modern populations. The review includes studies of bone mass in past populations from different geographic regions. Nutritionally based hypotheses, traditionally used to explain bone loss in past populations, are reviewed and assessed against the current clinical and epidemiological findings. In general, the various studies have revealed different degrees of low bone mass in past populations; however, the pattern of bone loss and fragility seen in age-related and postmenopausal osteoporosis today is not evident in the past. Bone loss in earlier populations is often found in both sexes, whereas significant bone loss in females occurs often among the young-age categories. In addition, a prevalence of osteoporotic fracture is absent.

CONCLUSIONS

We suggest that, despite influences that may have reduced bone mass in past populations, a protection of bone quality may have occurred, reducing the likelihood of bone fragility and fracture typically seen in modern osteoporotics. It seems evident that, although reduced bone mass may be prevalent in past history, clinically recognized osteoporosis is not.

Pathophysiology of osteoporosis

As with many chronic diseases that express themselves late in life, osteoporosis is distinctly multifactorial both in etiology and in pathophysiology. Osteoporotic fractures occur because of a combination of injury and intrinsic bony fragility. The injury comes most often from a combination of falls, poor postural reflexes that fail to protect bony parts from impact, and reduced soft tissue padding over bony prominences. The bony fragility itself is a composite of geometry, low mass density, severance of microarchitectural connections in trabecular structures, and accumulated fatigue damage. Reduced bone mass, in turn, is caused by varying combinations of gonadal hormone deficiency, inadequate intakes of calcium and vitamin D, decreased physical activity, comorbidity, and the effects of drugs used to treat various unrelated medical conditions. Finally, the often poor outcome from hip fracture in the elderly is partly caused by associated protein-calorie malnutrition. An adequate preventive program for osteoporotic fracture must address as many of these factors as possible, ie, it must be as multifaceted as the disease is multifactorial.

Bone quality factor analysis: a new noninvasive technique for the measurement of bone density and bone strength

The sensitivity of bone mineral density (BMD) as a predictor of fracture risk is limited by the fact that this index does not take into account the geometrical and material characteristics of bone. In contrast, both BMD and bone architecture influence the quality factor (QF), the fraction of the inverse of the energy lost in one cycle of deformation. In this study we have compared the sensitivity of a QF analyzer and dual-energy X-ray absorptiometry (DXA) in detecting the changes induced by ovariectomy (OVX) on the QF, impact strength, and BMD of the femur of mature rats. QF and BMD were measured noninvasively before and 4 weeks after OVX or sham operation using a QF analyzer developed in our laboratory and a Hologic QDR 2000 bone densitometry, respectively. Impact strength was measured in excised femurs at the end of the study. The in vivo short-term precision (coefficient of variation) of the QF analyzer was 1.9%. BMD and QF measurements were highly correlated (r = 0.80,p <0.0001). At baseline, QF and BMD were similar in OVX and sham-operated rats. At 4 weeks, BMD was 14.7 + or - 0.9% lower than at baseline (p < 0.001) in OVX rats and 5.3 + or - 1.3% lower in sham-operated rats (p <0.05). QF decreased 36.0 + or - 2.8% (p <0.0001) in OVX and 10.6 + or - 3.6% in sham rats (p <0.01). As a result, at 4 weeks the difference between sham-operated and OVX rats was larger (p < 0.05) by QF than by BMD. Moreover, QF correlated better than BMD with impact strength and the difference in impact strength between sham and OVX mice was closer to that in QF than that in BMD. These data demonstrate that QF analysis is a precise technique that is more sensitive than DXA in detecting the changes in bone density and strength induced by OVX. QF analysis may represent a new, simple, and economic technique for predicting fracture risk.

Mechanical validation of a tomographic (pQCT) index for noninvasive estimation of rat femur bending strength

Cross-sectional moment of inertia (CSMI) and volumetric cortical bone mineral density (vCtBMD) were assessed by peripheral quantitative computed tomography (pQCT) at femur midshafts from 103 Wistar female rats receiving 0 (n = 12) or 15-1000 mu g/kg/day sc of dexamethasone (n = 46) from 5 to 9 weeks of age, or 0 or 80 mg/kg 3/wk of AI(OH)(3) IP (n = 23,22) from 4 to 10 months of age. A bone strength index (BSI), calculated as the product CSMI x vCtBMD, was found to closely correlate (r = 0.94, R(2) = 0.89, p < 0.001) with the actual, mechanically tested bending breaking force of all bones. Correlation and determination coefficients obtained were higher than those usually reported employing different long-bone strength predictive formulae. The curve approached the origin and was linear throughout the wide range of CSMI, vCtBMD and BSI achieved because of age- and treatment-induced differences, showing a very low standard error of the estimate. Instead, different curve slopes and/or intercepts were found in separate analysis between data from each of the experiments when breaking force was correlated with CSMI or vCtBMD alone, or with the DEXA-assessed BMD of the mechanically assayed bone portion. Results suggest that noninvasive assessment of the BSI by means of pQCT technology provides an original tool for a precise and accurate estimation of long-bone bending strength that can be advantageously applied in crosssectional as well as longitudinal, in vivo studies employing animal models.

Measurement of femoral geometry in type I and type II osteoporosis: differences in hip axis length consistent with heterogeneity in the pathogenesis of osteoporotic fractures

The epidemiologic patterns of vertebral and femoral fractures are sufficiently different to suggest that they represent distinct disorders (type I versus type II osteoporosis) although osteopenia is common in both. To determine whether differences in femoral geometry, one of the main determinants of bone quality, might contribute to the heterogeneity in osteoporotic fractures, we obtained dual energy X-ray absorptiometry scans on 210 women age 60 or older, including 105 type I fracture cases, 30 type II patients, and 75 controls. Hip axis length, measured on the scan printout, was significantly increased (p < 0.01) in hip fracture patients compared with women with postmenopausal osteoporosis, whereas femoral neck density (BMD) was equal in both groups. The best discrimination between both fracture types was obtained by a logistic regression model based on age and axis length. Adding BMD to the model did not improve the discriminative power (p = 0.67). These data provide further evidence that geometric characteristics may be implicated in hip fracture risk. Furthermore, these findings suggest that an increase in hip axis length may predispose osteopenic subjects to a femoral localization of fragility fractures, consistent with the postulated heterogeneity in the pathogenesis of osteoporotic fractures.

Aging and matrix microdamage accumulation in human compact bone

Bone matrix microdamage in bone matrix, evidenced as microcracks, occurs consequent to cyclic loading. Microdamage caused by in vivo loading has been described in human rib cortex; however, the existence and extent of microcracks in human long bone cortices are largely unknown. Using histomorphometric methods to examine the incidence and localization of microcracks in human femoral compact bone specimens, we found that the amount of microdamage present in femoral compact bone increases dramatically with increasing age. Least squares regression analysis showed that in males, microcrack density (Cr.De., #/mm2) increases exponentially with age (r2 = 0.70). In females, Cr.De. also increases as an exponential function of increasing age (r2 = 0.79), at a significantly higher rate than in male specimens (p < 0.001). The current studies indicate that with increasing age, bone microdamage accumulates more rapidly than intrinsic processes can effect its repair. A combination of cumulative loading history, focal changes in material properties and alteration in the ability of the tissue to perceive and/or react to microcracks may all play role in this accumulation of bone microdamage with aging. This accumulation of microdamage in bone will contribute to decreased strength and stiffness. In addition, and perhaps most significantly for understanding aging and increased bone fragility, matrix microdamage in composite materials like bone will result in a profoundly reduced resistance to fracture. The importance of this accumulation of matrix microdamage in human bone with increasing age in contributing to the increased fragility of the aging skeleton is discussed.