Age-related differences between thinning of horizontal and vertical trabeculae in human lumbar bone as assessed by a new computerized method

Textural and geometric measures derived from digital tomosynthesis discriminate women with and without vertebral fracture

Vertebral fractures are a common and debilitating consequence of osteoporosis. Bone mineral density (BMD), measured by dual energy x-ray absorptiometry (DXA), is the clinical standard for assessing overall bone quantity but falls short in accurately predicting vertebral fracture. Fracture risk prediction may be improved by incorporating metrics of microstructural organization from an appropriate imaging modality. Digital tomosynthesis (DTS)-derived textural and microstructural parameters have been previously correlated to vertebral bone strength in vitro, but the in vivo utility has not been explored. Therefore, the current study sought to establish the extent to which DTS-derived measurements of vertebral microstructure and size discriminate patients with and without vertebral fracture. In a cohort of 93 postmenopausal women with or without history of vertebral fracture, DTS-derived microstructural parameters and vertebral width were calculated for T12 and L1 vertebrae, as well as lumbar spine BMD and trabecular bone score (TBS) from DXA images. Fracture patients had lower BMD and TBS, while DTS-derived degree of anisotropy and vertebral width were higher, compared to nonfracture (p < 0.02 to p < 0.003) patients. The addition of DTS-derived parameters (fractal dimension, lacunarity, degree of anisotropy and vertebral width) improved discriminative capability for models of fracture status (AUC = 0.79) compared to BMD alone (AUC = 0.67). For twelve additional participants who were imaged twice, in vivo repeatability errors for DTS parameters were low (0.2 % - 7.3 %). The current results support the complementary use of DTS imaging for assessing bone quality and improving the accuracy of fracture risk assessment beyond that achievable by DXA alone.

Trabecular bone structural units and their cement lines change with age, bone volume fraction, structure, and strength in female human vertebrae

A lifetime of successive bone remodeling events leads to trabeculae which are composed of a patchwork of bone structural units (BSUs) called hemi-osteons or trabecular packets. Traditionally, only intact surface BSUs have been studied, which are those that have been created most recently. Accordingly, the complex changes in the size and distribution of BSU throughout the trabeculae have been overlooked. In this study, the BSUs within the trabeculae of the second lumbar vertebrae were manually traced, using ImageJ software, in osteopontin immunostained sections of eight young women (aged 19-38 yr) and eight older women (aged 69-96 yr). A series of BSU profile properties including area, width, length, and perimeter were quantified, along with properties of each trabecular profile such as the number of BSU and cement line length. The relationships between these properties and age, as well as selected trabecular microstructural properties assessed with microcomputed tomography, and bone strength assessed on the neighboring third lumbar vertebrae, were investigated. The median BSU profile length and perimeter decreased with age, while the median BSU profile area and width was unchanged. Moreover, age was associated with an increase in the number of BSU profiles and cement line length per trabecular profile area. However, changes in BSU profile geometry, the number of BSU profiles, and the cement line length per trabecular profile were strongly correlated with trabecular bone volume fraction, structure model index, and bone strength. Further research is needed to understand how these changes in BSU properties affect the mechanical and failure properties of trabecular bone.

Dark-field radiography for the detection of bone microstructure changes in osteoporotic human lumbar spine specimens

BACKGROUND

Dark-field radiography imaging exploits the wave character of x-rays to measure small-angle scattering on material interfaces, providing structural information with low radiation exposure. We explored the potential of dark-field imaging of bone microstructure to improve the assessment of bone strength in osteoporosis.

METHODS

We prospectively examined 14 osteoporotic/osteopenic and 21 non-osteoporotic/osteopenic human cadaveric vertebrae (L2-L4) with a clinical dark-field radiography system, micro-computed tomography (CT), and spectral CT. Dark-field images were obtained in both vertical and horizontal sample positions. Bone microstructural parameters (trabecular number, Tb.N; trabecular thickness, Tb.Th; bone volume fraction, BV/TV; degree of anisotropy, DA) were measured using standard ex vivo micro-CT, while hydroxyapatite density was measured using spectral CT. Correlations were assessed using Spearman rank correlation coefficients.

RESULTS

The measured dark-field signal was lower in osteoporotic/osteopenic vertebrae (vertical position, 0.23 ± 0.05 versus 0.29 ± 0.04, p < 0.001; horizontal position, 0.28 ± 0.06 versus 0.34 ± 0.04, p = 0.003). The dark-field signal from the vertical position correlated significantly with Tb.N (ρ = 0.46, p = 0.005), BV/TV (ρ = 0.45, p = 0.007), DA (ρ = -0.43, p = 0.010), and hydroxyapatite density (ρ = 0.53, p = 0.010). The calculated ratio of vertical/horizontal dark-field signal correlated significantly with Tb.N (ρ = 0.43, p = 0.011), BV/TV (ρ = 0.36, p = 0.032), DA (ρ = -0.51, p = 0.002), and hydroxyapatite density (ρ = 0.42, p = 0.049).

CONCLUSION

Dark-field radiography is a feasible modality for drawing conclusions on bone microarchitecture in human cadaveric vertebral bone.

RELEVANCE STATEMENT

Gaining knowledge of the microarchitecture of bone contributes crucially to predicting bone strength in osteoporosis. This novel radiographic approach based on dark-field x-rays provides insights into bone microstructure at a lower radiation exposure than that of CT modalities.

KEY POINTS

Dark-field radiography can give information on bone microstructure with low radiation exposure. The dark-field signal correlated positively with bone microstructure parameters. Dark-field signal correlated negatively with the degree of anisotropy. Dark-field radiography helps to determine the directionality of trabecular loss.

Towards the diagnosis of osteoporosis - contributions from coincidental diagnostic imaging findings in chiropractors' practice

BACKGROUND

Osteoporosis is significantly associated with fractures and burdens the health of especially older people. Osteoporotic fractures cause pain, disability, and increased mortality. Early diagnosis of osteoporosis allows earlier initiation of treatment, thereby reducing the risk of osteoporotic fractures. Chiropractors encounter potential osteoporotic patients daily, and perform radiological evaluation of these and other patients, including evaluation of X-rays done for other purposes than osteoporosis. Therefore, chiropractors may identify vertebral fractures, vertebral deformity or osteopenia not otherwise suspected or recorded.

METHODS

This study examines procedures available to the chiropractor to describe conventional X-rays with the focus of osteoporosis related findings. We review the indications for radiological examination in chiropractic practice, and in the realm of osteoporosis we describe radiological methods available for examination of conventional radiographs, and the necessity of inter-disciplinary communication.

RESULTS

National guidelines are available regarding referral for X-rays in chiropractic practice. Standardized protocols ensure image acquisition of the highest quality in the chiropractors' radiological department. Conventional X-ray examination is not indicated on clinical suspicion of osteoporosis alone, as bone mineral density testing is the diagnostic test. Radiological assessment of all available X-rays of patients above the age of 50 years should include evaluation of the bone quality, and hip and vertebral fracture assessment. The Singh index, Genant Semi-Quantitative tool (GSQ), and Algorithm-Based Qualitative method (ABQ) should be used consistently during interpretation. Referral for additional imaging and evaluation should be prompt and systematic when needed.

CONCLUSIONS

This article presents an overview of evidence-based radiological procedures for the purpose of promoting early diagnosis of osteoporosis. We present recommendations to the clinicians where we propose an opportunistic evaluation of X-rays, done for any reason, which include systematic evaluation of bone quality, presence of hip and vertebral fractures, and vertebral deformation of all patients above the age of 50 years. Detailed referral to healthcare professionals for further diagnostic evaluation is performed when needed. Consistent, high-quality radiological procedures in chiropractic practices could feasibly contribute to the timely diagnosis of osteoporosis, ultimately minimizing the impact of osteoporosis-related complications on patients' health.

Methionine as a regulator of bone remodeling with fasting

Caloric restriction improves metabolic health but is often complicated by bone loss. We studied bone parameters in humans during a 10-day fast and identified candidate metabolic regulators of bone turnover. Pro-collagen 1 intact N-terminal pro-peptide (P1NP), a bone formation marker, decreased within 3 days of fasting. Whereas dual-energy x-ray absorptiometry measures of bone mineral density were unchanged after 10 days of fasting, high-resolution peripheral quantitative CT demonstrated remodeling of bone microarchitecture. Pathway analysis of longitudinal metabolomics data identified one-carbon metabolism as fasting dependent. In cultured osteoblasts, we tested the functional significance of one-carbon metabolites modulated by fasting, finding that methionine - which surged after 3 days of fasting - affected markers of osteoblast cell state in a concentration-dependent manner, in some instances exhibiting a U-shaped response with both low and high concentrations driving putative antibone responses. Administration of methionine to mice for 5 days recapitulated some fasting effects on bone, including a reduction in serum P1NP. In conclusion, a 10-day fast in humans led to remodeling of bone microarchitecture, potentially mediated by a surge in circulating methionine. These data support an emerging model that points to a window of optimal methionine exposure for bone health.

Age- and sex-related changes in vertebral trabecular bone architecture in Neolithic and Mediaeval populations from Poland

This paper investigates trabecular bone ontogenetic changes in two different Polish populations, one prehistoric and the other historical. The studied populations are from the Brześć Kujawski region in Kujawy (north-central Poland), one from the Neolithic Period (4500-4000 BC) and one from the Middle Ages (twelfth-sixteenth centuries AD), in total 62 vertebral specimens (32 males, 30 females). Eight morphometric parameters acquired from microCT scan images were analysed. Two-way ANOVA after Box-Cox transformation and multifactorial regression model were calculated. A significant decrease in percentage bone volume fraction (BV/TV; [%]) with age at death was observed in the studied sample; Tb.N (trabecular number) was also significantly decreased with age; trabecular separation (Tb.Sp) increased with advancing age; connectivity density (Conn.D) was negatively correlated with biological age and higher in the Neolithic population. These data are found to be compatible with data from the current biomedical literature, while no loss of horizontal trabeculae was recorded as would be expected based on modern osteoporosis.

Dark-field X-ray imaging for the assessment of osteoporosis in human lumbar spine specimens

Background: Dark-field imaging is a novel imaging modality that allows for the assessment of material interfaces by exploiting the wave character of x-ray. While it has been extensively studied in chest imaging, only little is known about the modality for imaging other tissues. Therefore, the purpose of this study was to evaluate whether a clinical X-ray dark-field scanner prototype allows for the assessment of osteoporosis. Materials and methods: In this prospective study we examined human cadaveric lumbar spine specimens (vertebral segments L2 to L4). We used a clinical prototype for dark-field radiography that yields both attenuation and dark-field images. All specimens were scanned in lateral orientation in vertical and horizontal position. All specimens were additionally imaged with CT as reference. Bone mineral density (BMD) values were derived from asynchronously calibrated quantitative CT measurements. Correlations between attenuation signal, dark-field signal and BMD were assessed using Spearman's rank correlation coefficients. The capability of the dark-field signal for the detection of osteoporosis/osteopenia was evaluated with receiver operating characteristics (ROC) curve analysis. Results: A total of 58 vertebrae from 20 human cadaveric spine specimens (mean age, 73 years ±13 [standard deviation]; 11 women) were studied. The dark-field signal was positively correlated with the BMD, both in vertical (r = 0.56, p < .001) and horizontal position (r = 0.43, p < .001). Also, the dark-field signal ratio was positively correlated with BMD (r = 0.30, p = .02). No correlation was found between the signal ratio of attenuation signal and BMD (r = 0.14, p = .29). For the differentiation between specimens with and without osteoporosis/osteopenia, the area under the ROC curve (AUC) was 0.80 for the dark-field signal in vertical position. Conclusion: Dark-field imaging allows for the differentiation between spine specimens with and without osteoporosis/osteopenia and may therefore be a potential biomarker for bone stability.

2D size of trabecular bone structure units (BSU) correlate more strongly with 3D architectural parameters than age in human vertebrae

Bone tissue is continuously remodeled. In trabecular bone, each remodeling transaction forms a microscopic bone structural unit (BSU), also known as a hemiosteon or a trabecular packet, which is bonded to existing tissue by osteopontin-rich cement lines. The size and shape of the BSUs are determined by the size and shape of the resorption cavity, and whether the cavity is potentially over- or under-filled by the subsequent bone formation. The present study focuses on the recently formed trabecular BSUs, and how their 2D size and shape changes with age and trabecular microstructure. The study was performed using osteopontin-immunostained frontal sections of L2 vertebrae from 8 young (aged 18.5-37.6 years) and 8 old (aged 69.1-96.4 years) control females, which underwent microcomputed tomography (μCT) imaging prior to sectioning. The contour of 4230 BSU profiles (181-385 per vertebra) within 1024 trabecular profiles were outlined, and their 2D width, length, area, and shape were assessed. Of these BSUs, 22 (0.5%) were generated by modeling-based bone formation (i.e. without prior resorption), while 99.5% were generated by remodeling-based bone formation (i.e. with prior resorption). The distributions of BSU profile width, length, and area were significantly smaller in the old versus young females (p < 0.005), and the median profile width, length, and area were negative correlated with age (p < 0.018). Importantly, these BSU profile size parameters were more strongly correlated with trabecular bone volume (BV/TV, p < 0.002) and structure model index (SMI, p < 0.008) assessed by μCT, than age. Moreover, the 2D BSU size parameters were positively correlated to the area of the individual trabecular profiles (p < 0.0001), which were significantly smaller in the old versus young females (p < 0.024). The BSU shape parameters (aspect ratio, circularity, and solidity) were not correlated with age, BV/TV, or SMI. Collectively, the study supports the notion that not only the BSU profile width, but also its length and area, are more influenced by the age-related bone loss and shift from plates to rods (SMI), than age itself. This implies that BSU profile size is mainly driven by changes in the trabecular microstructure, which affect the size of the resorption cavity that the BSU refills.

The Effect of Implant Length and Diameter on Stress Distribution around Single Implant Placement in 3D Posterior Mandibular FE Model Directly Constructed Form In Vivo CT

A three-dimensional (3D) finite element (FE) model of the mandibular bone was created from 3D X-ray CT scan images of a live human subject. Simulating the clinical situation of implant therapy at the mandibular first molar, virtual extraction of the tooth was performed at the 3D FE mandibular model, and 12 different implant diameters and lengths were virtually inserted in order to carry out a mechanical analysis. (1) High stress concentration was found at the surfaces of the buccal and lingual peri-implant bone adjacent to the sides of the neck in all the implants. (2) The greatest stress value was approximately 6.0 MPa with implant diameter of 3.8 mm, approx. 4.5 MPa with implant diameter of 4.3 mm, and approx. 3.2 MPa with implant diameter of 6.0 mm. (3) The stress on the peri-implant bone was found to decrease with increasing length and mainly in diameter of the implant.

CT Hounsfield unit and histogram analysis for differentiation of recent versus remote vertebral compression fractures

OBJECTIVES

The purpose of this study was to analyze the intraosseous tissue changes in recent vertebral compression fractures (VCFs) and to differentiate recent from remote VCFs using CT Hounsfield unit histogram analysis (HUHA).

METHODS

65 patients with T11 to L3 VCFs were included. HUHA of 2 vertebral bodies (VBs)- a fractured VB and the closest lower-level unaffected VB-was done. The mean Hounsfield unit (HU) value and HU proportions of 5 ranges (HU ≤ 0, 0 < HU ≤ 50, 50 < HU ≤ 100, 100 < HU ≤ 150, and HU > 150) were obtained. Then, ΔHU value and ΔHU proportion were calculated by subtracting the values from the two vertebrae. Finally, the obtained values were compared between the recent and remote VCF groups and subjected to ROC curve analysis.

RESULTS

In recent VCF group, the ΔHU proportion (HU ≤ 0) corresponding to normal fatty marrow was lower (-0.17 vs 0.01) and the ΔHU proportion (HU > 150) representing trabecular bone was higher (0.23 vs 0.04) than in remote VCF group (p < 0.001). In the differentiation of recent from remote VCF, the ΔHU value and ΔHU proportion (HU > 150) showed high area under the curve (AUC, 0.939 and 0.912, respectively).

CONCLUSION

CT HUHA demonstrated both trabecular bone and bone marrow changes in recent VCFs, and showed high diagnostic performance in differentiating between recent and remote VCFs.

ADVANCES IN KNOWLEDGE

With its vendor neutral applicability, CT HUHA can be used for the differentiation of recent and remote VCFs.

Characterization of fracture in topology-optimized bioinspired networks

Designing strong and robust bioinspired structures requires an understanding of how function arises from the architecture and geometry of materials found in nature. We draw from trabecular bone, a lightweight bone tissue that exhibits a complex, anisotropic microarchitecture, to generate networked structures using multiobjective topology optimization. Starting from an identical volume, we generate multiple different models by varying the objective weights for compliance, surface area, and stability. We examine the relative effects of these objectives on how resultant models respond to simulated mechanical loading and element failure. We adapt a network-based method developed initially in the context of modeling trabecular bone to describe the topology-optimized structures with a graph-theoretical framework, and we use community detection to characterize locations of fracture. This complementary combination of computational methods can provide valuable insights into the strength of bioinspired structures and mechanisms of fracture.

Aging-associated microstructural deterioration of vertebra in zebrafish

Zebrafish, a small teleost fish, is currently emerging as an animal model of local and systemic aging. In this study, we assessed age-related degenerative changes in the vertebral bone of zebrafish (3–12 month-post-fertilisation [mpf]) using micro-CT scanning. The bone volume (BV) of the trabecular bone in the male and female fish peaked at 6 mpf and reduced with age. In contrast to BV, bone mineral density and tissue volume did not change after 6 mpf, implying that the total mineral volume in the trabecular area remains unchanged, retaining the strength of vertebra. In addition, we performed micro-structural analysis of the trabecular thickness, trabecular number, and star volume of the tissue space and trabeculae, and found that the size of the trabecular bone reduced with age. Furthermore, aged zebrafish (45 mpf) exhibited ectopic ossification inside or outside of their vertebrae.

In summary, we analysed bone structural parameters in adult zebrafish vertebra, which are also used in humans, and demonstrated that aged zebrafish have deteriorated microarchitecture (trabecular thickness and number, tissue space star volume and trabecular star volume) with reduction of trabecular bones, similar to that observed during aging in humans. Zebrafish can be utilised as an animal model to understand the pathology of human bone aging, and the discovery of new therapeutic agents against age-related osteoporosis.

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We analysed bone structural parameters in adult zebrafish vertebrae.

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Microstructural changes in aged-zebrafish are similar to those in humans.

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Aged zebrafish exhibited ectopic ossification inside or outside of their vertebrae.

Ultrastructure Organization of Human Trabeculae Assessed by 3D sSAXS and Relation to Bone Microarchitecture

Although the organization of bone ultrastructure, i.e. the orientation and arrangement of the mineralized collagen fibrils, has been in the focus of research for many years for cortical bone, and many models on the osteonal arrangement have been proposed, limited attention has been paid to trabecular bone ultrastructure. This is surprising because trabeculae play a crucial role for the mechanical strength of several bone sites, including the vertebrae and the femoral head. On this account, we first validated a recently developed method (3D sSAXS or 3D scanning small-angle X-ray scattering) for investigating bone ultrastructure in a quantitative and spatially resolved way, using conventional linearly polarized light microscopy as a gold standard. While both methods are used to analyze thin tissue sections, in contrast to polarized light microscopy, 3D sSAXS has the important advantage that it provides 3D information on the orientation and arrangement of bone ultrastructure. In this first study of its kind, we used 3D sSAXS to investigate the ultrastructural organization of 22 vertebral trabeculae of different alignment, types and sizes, obtained from 4 subjects of different ages. Maps of ultrastructure orientation and arrangement of the trabeculae were retrieved by stacking information from consecutive 20-μm-thick bone sections. The organization of the ultrastructure was analyzed in relation to trabecular microarchitecture obtained from computed tomography and to relevant parameters such as distance to trabecular surface, local curvature or local bone mineralization. We found that (i) ultrastructure organization is similar for all investigated trabeculae independent of their particular characteristics, (ii) bone ultrastructure exhibiting a high degree of orientation was arranged in domains, (iii) highly oriented ultrastructural areas were located closer to the bone surface, (iv) the ultrastructure of the human trabecular bone specimens followed the microarchitecture, being oriented mostly parallel to bone surface, and (v) local surface curvature seems to have an effect on the ultrastructure organization. Further studies that investigate bone ultrastructure orientation and arrangement are needed in order to understand its organization and consequently its relation to bone biology and mechanics.

Modeling the Mechanical Consequences of Age-Related Trabecular Bone Loss by XFEM Simulation

The elderly are more likely to suffer from fracture because of age-related trabecular bone loss. Different bone loss locations and patterns have different effects on bone mechanical properties. Extended finite element method (XFEM) can simulate fracture process and was suited to investigate the effects of bone loss on trabecular bone. Age-related bone loss is indicated by trabecular thinning and loss and may occur at low-strain locations or other random sites. Accordingly, several ideal normal and aged trabecular bone models were created based on different bone loss locations and patterns; then, fracture processes from crack initiation to complete failure of these models were observed by XFEM; finally, the effects of different locations and patterns on trabecular bone were compared. Results indicated that bone loss occurring at low-strain locations was more detrimental to trabecular bone than that occurring at other random sites; meanwhile, the decrease in bone strength caused by trabecular loss was higher than that caused by trabecular thinning, and the effects of vertical trabecular loss on mechanical properties were more severe than horizontal trabecular loss. This study provided a numerical method to simulate trabecular bone fracture and distinguished different effects of the possible occurrence of bone loss locations and patterns on trabecular bone.

Modelling of bone fracture and strength at different length scales: a review

In this paper, we review analytical and computational models of bone fracture and strength. Bone fracture is a complex phenomenon due to the composite, inhomogeneous and hierarchical structure of bone. First, we briefly summarize the hierarchical structure of bone, spanning from the nanoscale, sub-microscale, microscale, mesoscale to the macroscale, and discuss experimental observations on failure mechanisms in bone at these scales. Then, we highlight representative analytical and computational models of bone fracture and strength at different length scales and discuss the main findings in the context of experiments. We conclude by summarizing the challenges in modelling of bone fracture and strength and list open topics for scientific exploration. Modelling of bone, accounting for different scales, provides new and needed insights into the fracture and strength of bone, which, in turn, can lead to improved diagnostic tools and treatments of bone diseases such as osteoporosis.

Mapping trabecular disconnection "hotspots" in aged human spine and hip

Trabecular bone disconnection is an independent factor in age-related skeletal failure where real termini (ReTm; rare in youth) may cause weakness disproportionate to tissue loss, yet their structural contribution at vulnerable locations remains uncertain. ReTm (previously recorded at the iliac crest) were mapped in "normal" aged vertebral bodies (T11-L5 autopsy; 20 females, 10 males) and corresponding proximal femora (autopsy; 10 females). Results were compared with biomechanically failed femora from orthopaedic subjects aged >58 yr (osteoporosis OP, 10 females; osteoarthritis OA, 10 females). A novel direct 2D/3D histological method was applied to large, thick (300 μm) slices superficially silver-stained to separate ReTm (unstained) from apparent termini (planar artefacts, brown). Light microscope field co-ordinates enabled ReTm mapping and statistical testing relative to i) sex, ii) tissue sector and iii) slicing plane. In men ReTm populations were small and random while in women they were large and sector-specific. In vertebrae they clustered anterior/superior being rare posterior/inferior; in the femoral head they concentrated distal/superior and also near the fovea, being fewer distal/inferior. A distribution polarity was evident with 100% more ReTm observed transversely (i.e., on tensile-related cross struts) than longitudinally (i.e., on compression-related vertical struts). Their numbers rose in OP (BV/TV<14%, microCT) and in OA (BV/TV>14%), remaining polarised and sector-specific in OP only. Comparative experimentation by marrow elution of an OP animal model demonstrated "floating segments" as a possible outcome. Conclusions were supported statistically that trabecular disconnection "hotspots" at vulnerable locations are sex- and sector-specific, mainly transaxial, and subject to disease modulation.

Poromicromechanics reveals that physiological bone strains induce osteocyte-stimulating lacunar pressure

Mechanical loads which are macroscopically acting onto bony organs, are known to influence the activities of biological cells located in the pore spaces of bone, in particular so the signaling and production processes mediated by osteocytes. The exact mechanisms by which osteocytes are actually able to “feel” the mechanical loading and changes thereof, has been the subject of numerous studies, and, while several hypotheses have been brought forth over time, this topic has remained a matter of debate. Relaxation times reported in a recent experimental study of Gardinier et al. (Bone 46(4):1075–1081, 2010) strongly suggest that the lacunar pores are likely to experience, during typical physiological load cycles, not only fluid transport, but also undrained conditions. The latter entail the buildup of lacunar pore pressures, which we here quantify by means of a thorough multiscale modeling approach. In particular, the proposed model is based on classical poroelasticity theory, and able to account for multiple pore spaces. First, the model reveals distinct nonlinear dependencies of the resulting lacunar (and vascular) pore pressures on the underlying bone composition, highlighting the importance of a rigorous multiscale approach for appropriate computation of the aforementioned pore pressures. Then, the derived equations are evaluated for macroscopic (uniaxial as well as hydrostatic) mechanical loading of physiological magnitude. The resulting model-predicted pore pressures agree very well with the pressures that have been revealed, by means of in vitro studies, to be of adequate magnitude for modulating the responses of biological cells, including osteocytes. This underlines that osteocytes may respond to many types of loading stimuli at the same time, in particular so to fluid flow and hydrostatic pressure.

Age-related changes in vertebral and iliac crest 3D bone microstructure--differences and similarities

Age-related changes of vertebra and iliac crest 3D microstructure were investigated, and we showed that they were in general similar. The 95th percentile of vertebral trabecular thickness distribution increased with age for women. Surprisingly, vertebral and iliac crest bone microstructure was only weakly correlated (r = 0.38 to 0.75), despite the overall similar age-related changes.

INTRODUCTION

The purposes of the study were to determine the age-related changes in iliac and vertebral bone microstructure for women and men over a large age range and to investigate the relationship between the bone microstructure at these skeletal sites.

METHODS

Matched sets of transiliac crest bone biopsies and lumbar vertebral body (L2) specimens from 41 women (19-96 years) and 39 men (23-95 years) were micro-computed tomography (μCT) scanned, and the 3D microstructure was quantified.

RESULTS

For both women and men, bone volume per total volume (BV/TV), connectivity density (CD), and trabecular number (Tb.N) decreased significantly, while structure model index (SMI) and trabecular separation (Tb.Sp) increased significantly with age at either skeletal site. Vertebral trabecular thickness (Tb.Th) was independent of age for both women and men, while iliac Tb.Th decreased significantly with age for men, but not for women. In general, the vertebral and iliac age-related changes were similar. The 95th percentile of the Tb.Th distribution increased significantly with age for women but was independent of age for men at the vertebral body, while it was independent of age for either sex at the iliac crest. The Tb.Th probability density functions at the two skeletal sites became significantly more similar with age for women, but not for men. The microstructural parameters at the iliac crest and the vertebral bodies were only moderately correlated from r = 0.38 for SMI in women to r = 0.75 for Tb.Sp in men.

CONCLUSION

Age-related changes in vertebral and iliac bone microstructure were in general similar. The iliac and vertebral Tb.Th distributions became more similar with age for women. Despite the overall similar age-related changes in trabecular bone microstructure, the vertebral and iliac bone microstructural measures were only weakly correlated (r = 0.38 to 0.75).

Bone three-dimensional microstructural features of the common osteoporotic fracture sites

Osteoporosis is a common metabolic skeletal disorder characterized by decreased bone mass and deteriorated bone structure, leading to increased susceptibility to fractures. With aging population, osteoporotic fractures are of global health and socioeconomic importance. The three-dimensional microstructural information of the common osteoporosis-related fracture sites, including vertebra, femoral neck and distal radius, is a key for fully understanding osteoporosis pathogenesis and predicting the fracture risk. Low vertebral bone mineral density (BMD) is correlated with increased fracture of the spine. Vertebral BMD decreases from cervical to lumbar spine, with the lowest BMD at the third lumbar vertebra. Trabecular bone mass of the vertebrae is much lower than that of the peripheral bone. Cancellous bone of the vertebral body has a complex heterogeneous three-dimensional microstructure, with lower bone volume in the central and anterior superior regions. Trabecular bone quality is a key element to maintain the vertebral strength. The increased fragility of osteoporotic femoral neck is attributed to low cancellous bone volume and high compact porosity. Compared with age-matched controls, increased cortical porosity is observed at the femoral neck in osteoporotic fracture patients. Distal radius demonstrates spatial inhomogeneous characteristic in cortical microstructure. The medial region of the distal radius displays the highest cortical porosity compared with the lateral, anterior and posterior regions. Bone strength of the distal radius is mainly determined by cortical porosity, which deteriorates with advancing age.

Endplate deflection is a defining feature of vertebral fracture and is associated with properties of the underlying trabecular bone

Endplate deflection frequently occurs with vertebral failure, but the relationship between the two remains poorly defined. This study examined associations between endplate deflection under compressive loading and characteristics of the neighboring subchondral bone and intervertebral disc (IVD). Ten L1 vertebrae with adjacent IVDs were dissected, compressed axially in a stepwise manner to failure, and imaged with micro-computed tomography before each loading step. From the images, deflection was measured across the surface of each endplate at each step. Trabecular microstructure and endplate volume fraction were evaluated in 5 mm regions just under the superior endplate. IVDs were assessed using computed tomography and histology. A marked increase in superior endplate deflection coincided with a drop in the load-displacement curve. Endplate deflection was higher in regions with less robust bone microstructure (p < 0.009), though these associations tended to weaken as loading progressed. Immediately following the ultimate point, endplate deflection was higher in regions underlying the nucleus pulposus versus annulus fibrosus (p = 0.035), irrespective of disc grade (p = 0.346). These results indicate that a sudden increase in endplate deflection signals that the mechanical competence of the vertebra has been compromised. The mechanisms of endplate failure likely relate to anatomical features of the endplate, neighboring trabecular bone, and IVD.

Biaxial normal strength behavior in the axial-transverse plane for human trabecular bone--effects of bone volume fraction, microarchitecture, and anisotropy

The biaxial failure behavior of the human trabecular bone, which has potential relevance both for fall and gait loading conditions, is not well understood, particularly for low-density bone, which can display considerable mechanical anisotropy. Addressing this issue, we investigated the biaxial normal strength behavior and the underlying failure mechanisms for human trabecular bone displaying a wide range of bone volume fraction (0.06-0.34) and elastic anisotropy. Micro-computed tomography (CT)-based nonlinear finite element analysis was used to simulate biaxial failure in 15 specimens (5 mm cubes), spanning the complete biaxial normal stress failure space in the axial-transverse plane. The specimens, treated as approximately transversely isotropic, were loaded in the principal material orientation. We found that the biaxial stress yield surface was well characterized by the superposition of two ellipses--one each for yield failure in the longitudinal and transverse loading directions--and the size, shape, and orientation of which depended on bone volume fraction and elastic anisotropy. However, when normalized by the uniaxial tensile and compressive strengths in the longitudinal and transverse directions, all of which depended on bone volume fraction, microarchitecture, and mechanical anisotropy, the resulting normalized biaxial strength behavior was well described by a single pair of (longitudinal and transverse) ellipses, with little interspecimen variation. Taken together, these results indicate that the role of bone volume fraction, microarchitecture, and mechanical anisotropy is mostly accounted for in determining the uniaxial strength behavior and the effect of these parameters on the axial-transverse biaxial normal strength behavior per se is minor.

Dependence of mechanical properties of trabecular bone on plate-rod microstructure determined by individual trabecula segmentation (ITS)

Individual trabecula segmentation (ITS) technique can decompose the trabecular bone network into individual trabecular plates and rods and is capable of quantifying the plate/rod-related microstructural characteristics of trabecular bone. This novel technique has been shown to be able to provide in-depth insights into micromechanics and failure mechanisms of human trabecular bone, as well as to distinguish the fracture status independent of area bone mineral density in clinical applications. However, the plate/rod microstructural parameters from ITS have never been correlated to experimentally determined mechanical properties of human trabecular bone. In this study, on-axis cylindrical trabecular bone samples from human proximal tibia (n=22), vertebral body (n=10), and proximal femur (n=21) were harvested, prepared, scanned using micro computed-tomography (µCT), analyzed with ITS and mechanically tested. Regression analyses showed that the plate bone volume fraction (pBV/TV) and axial bone volume fraction (aBV/TV) calculated by ITS analysis correlated the best with elastic modulus (R(2)=0.96-0.97) and yield strength (R(2)=0.95-0.96). Trabecular plate-related microstructural parameters correlated highly with elastic modulus and yield strength, while most rod-related parameters were found inversely and only moderately correlated with the mechanical properties. In addition, ITS analysis also identified that trabecular bone at human femoral neck had the highest trabecular plate-related parameters while the other sites were similar with each other in terms of plate-rod microstructure.

Age-related changes of vertical and horizontal lumbar vertebral trabecular 3D bone microstructure is different in women and men

The study presents a 3D method for subdividing a trabecular network into horizontal and vertical oriented bone. This method was used to investigate the age related changes of the bone volume fraction and thickness of horizontal and vertical trabeculae in human lumbar vertebral bone estimated with unbiased 3D methods in women and men over a large age-range. The study comprised second lumbar vertebral body bone samples from 40 women (aged 21.7-96.4years, median 56.6years) and 39 men (aged 22.6-94.6years, median 55.6years). The bone samples were μCT scanned and the 3D microstructure was quantified. A voxel based algorithm inspecting the local neighborhood is presented and used to segment the trabecular network into horizontal and vertical oriented bone. For both women and men BV/TV decreased significantly with age, Tb.Th* was independent of age, while SMI increased significantly with age. Vertical (BV.vert/TV) and horizontal (BV.horz/TV) bone volume fraction decreased significantly with age for both sexes. BV.vert/TV decreased significantly faster with age for women than for men. Vertical (Tb.Th*.vert) and horizontal (Tb.Th*.horz) trabecular thickness were independent of age, while Tb.Th*.horz/Tb.Th*.vert decreased significantly with age for both sexes. Additionally, the 95th percentile of the trabecular thickness distribution increased significantly with age for vertical trabeculae in women, whereas it was independent of age in men. In conclusion, we have shown that vertical and horizontal oriented bone density decreases with age in both women and men, and that vertical oriented bone is lost more quickly in women than in men. Furthermore, vertical and horizontal trabecular thickness were independent of age, whereas the horizontal to vertical trabecular thickness ratio decreased significantly with age indicating a relatively more pronounced thinning of horizontal trabeculae. Finally, the age-related loss of trabecular elements appeared to result in a compensatory hypertrophy of vertical trabeculae in women, but not in men.

Detection of occult vertebral fractures by quantitative assessment of bone marrow attenuation values at MDCT

OBJECTIVES

To determine a cut-off value of Hounsfield attenuation units (HU) at multidetector computed tomography (MDCT) for valid and reliable detection of bone marrow oedema (BME) related to occult vertebral fractures.

METHODS

36 patients underwent both MDCT and Magnetic Resonance Imaging (MRI) for evaluation of vertebral fractures of the thoracolumbar spine and were included in this retrospective study. Two readers independently assessed HU values at MDCT in a total of 196 vertebrae. Reliability was assessed by intraclass correlation coefficient and Bland-Altman analysis. For each patient we determined the vertebra with the lowest HU value and calculated the HU-difference to each other vertebral body. HU-differences were subjected to receiver operating characteristic (ROC) curve analysis to determine the diagnostic accuracy for detection of BME as determined by MRI, which served as the reference standard. Results of HU-measurements were compared with standard visual evaluation of MDCT.

RESULTS

HU measurements demonstrated a high interrater reliability (ICC=0.984). ROC curve analysis (AUC=0.978) exhibited an ideal cut-off value of 29.6 HU for detection of BME associated with vertebral fractures with an accuracy of 97.4% as compared to 93.4% accuracy of visual evaluation. Particularly, HU-measurements increased the sensitivity for detection of vertebral fractures from 78.0% to 92.7% due to the detection of 7 of 9 occult fractures that were missed by visual evaluation alone.

CONCLUSIONS

Assessing bone marrow density by HU measurements using the cut-off of 29.6 HU is a valid and reliable tool for detection of BME related to occult vertebral fractures in MDCT. The introduced technique may allow more accurate treatment decisions and may make further diagnostic work-up with MRI unnecessary.

Effect of specimen-specific anisotropic material properties in quantitative computed tomography-based finite element analysis of the vertebra

Intra- and inter-specimen variations in trabecular anisotropy are often ignored in quantitative computed tomography (QCT)-based finite element (FE) models of the vertebra. The material properties are typically estimated solely from local variations in bone mineral density (BMD), and a fixed representation of elastic anisotropy ("generic anisotropy") is assumed. This study evaluated the effect of incorporating specimen-specific, trabecular anisotropy on QCT-based FE predictions of vertebral stiffness and deformation patterns. Orthotropic material properties estimated from microcomputed tomography data ("specimen-specific anisotropy"), were assigned to a large, columnar region of the L1 centrum (n = 12), and generic-anisotropic material properties were assigned to the remainder of the vertebral body. Results were compared to FE analyses in which generic-anisotropic properties were used throughout. FE analyses were also performed on only the columnar regions. For the columnar regions, the axial stiffnesses obtained from the two categories of material properties were uncorrelated with each other (p = 0.604), and the distributions of minimum principal strain were distinctly different (p ≤ 0.022). In contrast, for the whole vertebral bodies in both axial and flexural loading, the stiffnesses obtained using the two categories of material properties were highly correlated (R2 > 0.82, p < 0.001) with, and were no different (p > 0.359) from, each other. Only moderate variations in strain distributions were observed between the two categories of material properties. The contrasting results for the columns versus vertebrae indicate a large contribution of the peripheral regions of the vertebral body to the mechanical behavior of this bone. In companion analyses on the effect of the degree of anisotropy (DA), the axial stiffnesses of the trabecular column (p < 0.001) and vertebra (p = 0.007) increased with increasing DA. These findings demonstrate the need for accurate modeling of the peripheral regions of the vertebral body in analyses of the mechanical behavior of the vertebra.

Stochastic multi-scale prediction on the apparent elastic moduli of trabecular bone considering uncertainties of biological apatite (BAp) crystallite orientation and image-based modelling

An assessment of the mechanical properties of trabecular bone is important in determining the fracture risk of human bones. Many uncertainty factors contribute to the dispersion of the estimated mechanical properties of trabecular bone. This study was undertaken in order to propose a computational scheme that will be able to predict the effective apparent elastic moduli of trabecular bone considering the uncertainties that are primarily caused by image-based modelling and trabecular stiffness orientation. The effect of image-based modelling which focused on the connectivity was also investigated. A stochastic multi-scale method using a first-order perturbation-based and asymptotic homogenisation theory was applied to formulate the stochastically apparent elastic properties of trabecular bone. The effective apparent elastic modulus was predicted with the introduction of a coefficient factor to represent the variation of bone characteristics due to inter-individual differences. The mean value of the predicted effective apparent Young's modulus in principal axis was found at approximately 460 MPa for respective 15.24% of bone volume fraction, and this is in good agreement with other experimental results. The proposed method may provide a reference for the reliable evaluation of the prediction of the apparent elastic properties of trabecular bone.

Age-related changes in trabecular and cortical bone microstructure

The elderly population has substantially increased worldwide. Aging is a complex process, and the effects of aging are myriad and insidious, leading to progressive deterioration of various organs, including the skeleton. Age-related bone loss and resultant osteoporosis in the elderly population increase the risk for fractures and morbidity. Osteoporosis is one of the most common conditions associated with aging, and age is an independent risk factor for osteoporotic fractures. With the development of noninvasive imaging techniques such as computed tomography (CT), micro-CT, and high resolution peripheral quantitative CT (HR-pQCT), imaging of the bone architecture provides important information about age-related changes in bone microstructure and estimates of bone strength. In the past two decades, studies of human specimens using imaging techniques have revealed decreased bone strength in older adults compared with younger adults. The present paper addresses recently studied age-related changes in trabecular and cortical bone microstructure based primarily on HR-pQCT and micro-CT. We specifically focus on the three-dimensional microstructure of the vertebrae, femoral neck, and distal radius, which are common osteoporotic fracture sites.

Age-related differences in microstructure, density and biomechanics of vertebral cancellous bone of Chinese males

The conventional lumbar separation was performed by removing soft tissue, subsidiary structures and leaving only the vertebral body. The vertebral body was cut into two halves along the median sagittal plane, keeping the upper and lower end plates of each half, which were subsequently used for biomechanical, morphological and density experiments. From the age of 20-29 to 30-39 years, both the horizontal trabecular thickness (Tb.Th) and vertical Tb.Th decreased; the horizontal and vertical Tb.Sp increased; the plate-like trabecular Tb.Th decreased; the apparent density and volume ratio decreased; and the elastic modulus and the ultimate stress decreased; with all changes being statistically significant (p < 0.01). Similar trends were obtained from ages 40-49 to 50-59, although the changes were not significant (p > 0.05), except for the reduction in ultimate stress (p < 0.05). With aging, the collagen cross-linking capacity declined; the thicknesses of the collagen fibrils were variable, ranging from almost the same to loose, sparse or disordered thickness; and the finer collagen fibrils between the thick filaments were disorganized. In males aged from 20 to 59 years old, the horizontal and vertical Tb.Th and the plate-like Tb.Th of the vertebral body decreased, while the horizontal and vertical Tb.Sp increased. Additionally, the density, elastic modulus and the ultimate stress of the cancellous bone decreased with age. Thus, the associated changes of bone microstructure, density and biomechanics with age may lead to an increased risk of osteoporosis and fracture.

Vertebral fragility and structural redundancy

The mechanisms of age-related vertebral fragility remain unclear, but may be related to the degree of "structural redundancy" of the vertebra; ie, its ability to safely redistribute stress internally after local trabecular failure from an isolated mechanical overload. To better understand this issue, we performed biomechanical testing and nonlinear micro-CT-based finite element analysis on 12 elderly human thoracic ninth vertebral bodies (age 76.9 ± 10.8 years). After experimentally overloading the vertebrae to measure strength, we used nonlinear finite element analysis to estimate the amount of failed tissue and understand the failure mechanisms. We found that the amount of failed tissue per unit bone mass decreased with decreasing bone volume fraction (r(2)  = 0.66, p < 0.01). Thus, for the weak vertebrae with low bone volume fraction, overall failure of the vertebra occurred after failure of just a tiny proportion of the bone tissue (<5%). This small proportion of failed tissue had two sources: the existence of fewer vertically oriented load paths to which load could be redistributed from failed trabeculae; and the vulnerability of the trabeculae in these few load paths to undergo bending-type failure mechanisms, which further weaken the bone. Taken together, these characteristics suggest that diminished structural redundancy may be an important aspect of age-related vertebral fragility: vertebrae with low bone volume fraction are highly susceptible to collapse because so few trabeculae are available for load redistribution if the external loads cause any trabeculae to fail.

New advances in imaging osteoporosis and its complications

Tremendous advances have been made over the past several decades in assessing osteoporosis and its complications. High resolution imaging combined with sophisticated computational techniques now provide a detailed analysis of bone structure and a much more accurate prediction of bone strength. These techniques have shown how different mechanisms of age-related bone weakening exist in males and females. Limitations peculiar to these more advanced imaging techniques currently hinder their adoption into mainstream clinical practice. As such, the ultimate quest remains a readily available, safe, high resolution technique capable of fully predicting bone strength, capable of showing how bone strength is faltering and precisely monitoring treatment effect. Whether this technique will be based on acquisition of spine/hip data or data obtained at peripheral sites reflective of changes happening in the spine and hip regions is still not clear. In the meantime, mainstream imaging will continue to improve the detection of osteoporosis related insufficiency fracture in the clinical setting. We, as clinicians, should aim to increase awareness of this fracture type both as a frequent and varied source of pain in patients with osteoporosis and as the ultimate marker of severely impaired bone strength.

Trabecular architecture and vertebral fragility in osteoporosis

Osteoporosis heightens vertebral fragility owing to the biomechanical effects of diminished bone structure and composition. These biomechanical effects are only partially explained by loss in bone mass, so additional factors that are independent of bone mass are also thought to play an important role in vertebral fragility. Recent advances in imaging equipment, imaging-processing methods, and computational capacity allow researchers to quantify trabecular architecture in the vertebra at the level of the individual trabecular elements and to derive biomechanics-based measures of architecture that are independent of bone mass and density. These advances have shed light on the role of architecture in vertebral fragility. In addition to the adverse biomechanical consequences associated with trabecular thinning and loss of connectivity, a reduction in the number of vertical trabecular plates appears to be particularly harmful to vertebral strength. In the clinic, detailed architecture analysis is primarily applied to peripheral sites such as the distal radius and tibia. Analysis of trabecular architecture at these peripheral sites has shown mixed results for discriminating between patients with and without a vertebral fracture independent of bone mass, but has the potential to provide unique insight into the effects of therapeutic treatments. Overall, it does appear that trabecular architecture has an independent role on vertebral strength. Additional research is required to determine how and where architecture should be measured in vivo and whether assessment of trabecular architecture in a clinical setting improves prospective fracture risk assessment for the vertebra.

Statistics of the envelope of ultrasonic backscatter from human trabecular bone

The paper describes the investigations intended to compare the results of experimental measurements of backscattering properties of the trabecular bone with the results of computer simulations. Ultrasonic RF echoes were collected using two bone scanners operating at 0.58 and 1.3 MHz. The simulations of the backscattered RF echoes were performed using the scattering model of the trabecular bone that consisted of cylindrical and spherical elements uniformly distributed in water-like medium. For each measured or simulated RF backscatter the statistical properties of the signal envelope were determined. Experimental results suggest deviations of the backscattering properties from the Rayleigh distribution. The results of simulation suggest that deviation from Rayleigh distribution depends on the variation of trabeculae diameters and the number of thin trabeculae. Experimentally determined deviations corresponded well to the deviations calculated from simulated echoes assuming trabeculae thickness variation equaled to the earlier published histomorphometric study results.

Analysis of bone mineral density distribution at trabecular bones in thoracic and lumbar vertebrae using X-ray CT images

The number of participants in thoracic or abdominal examinations using multi-detector-row CT (MDCT) has been increasing recently. If the degree of progress of osteoporosis can be estimated using these images, it may be useful as it will allow predictions of vertebral fractures without an additional radiation exposure. The aims of this study were to investigate segmental variations in bone mineral density (BMD) distributions of thoracic and lumbar vertebral bodies and to show specific differences according to age and gender. A large database including 1,031 Japanese subjects for whom MDCT was used to examine various organs and tissues was utilized in this study for trabecular BMD at thoracic and lumbar vertebrae. In relationship to vertebral level, L3 had the lowest trabecular BMD. BMD tended to gradually increase from L3 to T1 in all age categories. Also, there was a moderate correlation between vertebrae whose distance from each other was great whereas there was a high correlation between adjacent vertebrae. It may be appropriate to use an arbitrary vertebra as a first approximation for assessing vertebrae that are in the area of predilection for the fracture; however, to better understand their behavior, it may be necessary to measure BMD directly in this region. This study showed trabecular BMD distribution at healthy thoracic and lumbar vertebrae in Japanese subjects and specific differences in age and gender. Improved knowledge about vertebral BMD may help with the diagnosis of primary osteoporosis using MDCT.

Influence of vertical trabeculae on the compressive strength of the human vertebra

Vertebral strength, a key etiologic factor of osteoporotic fracture, may be affected by the relative amount of vertically oriented trabeculae. To better understand this issue, we performed experimental compression testing, high-resolution micro-computed tomography (µCT), and micro-finite-element analysis on 16 elderly human thoracic ninth (T(9)) whole vertebral bodies (ages 77.5 ± 10.1 years). Individual trabeculae segmentation of the µCT images was used to classify the trabeculae by their orientation. We found that the bone volume fraction (BV/TV) of just the vertical trabeculae accounted for substantially more of the observed variation in measured vertebral strength than did the bone volume fraction of all trabeculae (r(2) = 0.83 versus 0.59, p < .005). The bone volume fraction of the oblique or horizontal trabeculae was not associated with vertebral strength. Finite-element analysis indicated that removal of the cortical shell did not appreciably alter these trends; it also revealed that the major load paths occur through parallel columns of vertically oriented bone. Taken together, these findings suggest that variation in vertebral strength across individuals is due primarily to variations in the bone volume fraction of vertical trabeculae. The vertical tissue fraction, a new bone quality parameter that we introduced to reflect these findings, was both a significant predictor of vertebral strength alone (r(2) = 0.81) and after accounting for variations in total bone volume fraction in multiple regression (total R(2) = 0.93). We conclude that the vertical tissue fraction is a potentially powerful microarchitectural determinant of vertebral strength.

Role of trabecular microarchitecture and its heterogeneity parameters in the mechanical behavior of ex vivo human L3 vertebrae

Low bone mineral density (BMD) is a strong risk factor for vertebral fracture risk in osteoporosis. However, many fractures occur in people with moderately decreased or normal BMD. Our aim was to assess the contributions of trabecular microarchitecture and its heterogeneity to the mechanical behavior of human lumbar vertebrae. Twenty-one human L(3) vertebrae were analyzed for BMD by dual-energy X-ray absorptiometry (DXA) and microarchitecture by high-resolution peripheral quantitative computed tomography (HR-pQCT) and then tested in axial compression. Microarchitecture heterogeneity was assessed using two vertically oriented virtual biopsies--one anterior (Ant) and one posterior (Post)--each divided into three zones (superior, middle, and inferior) and using the whole vertebral trabecular volume for the intraindividual distribution of trabecular separation (Tb.Sp*SD). Heterogeneity parameters were defined as (1) ratios of anterior to posterior microarchitectural parameters and (2) the coefficient of variation of microarchitectural parameters from the superior, middle, and inferior zones. BMD alone explained up to 44% of the variability in vertebral mechanical behavior, bone volume fraction (BV/TV) up to 53%, and trabecular architecture up to 66%. Importantly, bone mass (BMD or BV/TV) in combination with microarchitecture and its heterogeneity improved the prediction of vertebral mechanical behavior, together explaining up to 86% of the variability in vertebral failure load. In conclusion, our data indicate that regional variation of microarchitecture assessment expressed by heterogeneity parameters may enhance prediction of vertebral fracture risk.

Vertebral fracture

Vertebral fractures are usually the first to occur in osteoporosis, provide indisputable evidence of reduced bone strength, and are frequently a harbinger of further vertebral and nonvertebral fracture. Radiologists are best placed to draw attention to the presence of vertebral fractures, most of which are clinically silent. Magnetic resonance imaging supplemented if necessary by computed tomography is usually sufficient to enable distinction between osteoporotic and non-osteoporotic vertebral fracture, without a need for percutaneous biopsy.

Type and orientation of yielded trabeculae during overloading of trabecular bone along orthogonal directions

Trabecular architecture plays a major role in bone mechanics. Osteoporosis leads to a transition from a plate-like to a more rod-like trabecular morphology, which may contribute to fracture risk beyond that predicted by changes in density. In this study, microstructural finite element analysis results were analyzed using individual trabeculae segmentation (ITS) to identify the type and orientation of trabeculae where tissue yielded during compressive overloads in two orthogonal directions. For both apparent loading conditions, most of the yielded tissue was found in longitudinally oriented plates. However, the primary loading mode of yielded trabeculae was axial compression with superposed bending for on-axis loading in contrast to bending for transverse loading. For either loading direction, most plate-like trabeculae yielded in the same loading mode, regardless of their orientation. In contrast, rods oriented parallel to the loading axis yielded in compression, while rods oblique or perpendicular to the loading axis yielded in combined bending and tension. The predominance of tissue yielding in plates during both on-axis and transverse overloading explains why on-axis overloading is detrimental to the off-axis mechanical properties. At the same time, a large fraction of the tissue in rod-like trabeculae parallel to the loading direction yielded in both on-axis and transverse loading. Hence, rods may be more likely to be damaged and potentially resorbed by damage mediated remodeling.

Ultrasonic wave propagation in stereo-lithographical bone replicas

Predictions of a modified anisotropic Biot-Allard theory are compared with measurements of pulses centered on 100 kHz and 1 MHz transmitted through water-saturated stereo-lithographical bone replicas. The replicas are 13 times larger than the original bone samples. Despite the expected effects of scattering, which is neglected in the theory, at 100 kHz the predicted and measured transmitted waveforms are similar. However, the magnitude of the leading negative edge of the waveform is overpredicted, and the trailing parts of the waveforms are not predicted well. At 1 MHz, although there are differences in amplitudes, the theory predicts that the transmitted waveform is almost a scaled version of that incident in conformity with the data.

Effects of trabecular type and orientation on microdamage susceptibility in trabecular bone

Trabecular architecture becomes more rod-like and anisotropic in osteoporotic and aging trabecular bone. In order to address the effects of trabecular type and orientation on trabecular bone damage mechanics, microstructural finite element modeling was used to identify the yielded tissue in ten bovine tibial trabecular bone samples compressed to 1.2% on-axis apparent strain. The yielded tissue was mapped onto individual trabeculae identified by an Individual Trabeculae Segmentation (ITS) technique, and the distribution of the predicted yielding among trabecular types and orientations was compared to the experimentally measured microdamage. Although most of the predicted yielded tissue was found in longitudinal plates (73+/-11%), the measured microcrack density was positively correlated with the proportion of the yielded tissue in longitudinal rods (R(2)=0.52, p=0.02), but not in rods of other directions or plates. The overall fraction of rods and the fractions of rods along the longitudinal and transverse axes were also correlated with the measured microcrack density. In contrast, diffuse damage area did not correlate with any of these quantities. These results agree with the findings that both in vitro and in vivo microcrack densities are correlated with Structure Model Index (SMI), and are also consistent with decreased energy to failure in more rod-like trabecular bone. Together the results suggest that bending or buckling deformations of rod-like trabeculae may make trabecular structures more susceptible to microdamage formation. Moreover, while simple strain-based tissue yield criteria may account for macroscopic yielding, they may not be suitable for identifying damage.