A computational assessment of the independent contribution of changes in canine trabecular bone volume fraction and microarchitecture to increased bone strength with suppression of bone turnover

This study addressed the effects of changes in trabecular microarchitecture induced by suppressed bone turnover-including changes to the remodeling space-on the trabecular bone strength-volume fraction characteristics independent of changes in tissue material properties. Twenty female beagle dogs, aged 1-2 years, were treated daily with either oral saline (n=10 control) or high doses of oral risedronate (0.5mg/kg/day, n=10 suppressed) for a period of 1 year, the latter designed (and confirmed) to substantially suppress bone turnover. High-resolution micro-CT-based finite element models (18-mum voxel size) of canine trabecular bone cores (n=2 per vertebral body) extracted from the T-10 vertebrae were analyzed in both compressive and torsional loading cases. The same tissue-level material properties were used in all models, thus providing measures of tissue-normalized strength due only to changes in the microarchitecture. Suppressed bone turnover resulted in more plate-like architecture with a thicker and more dense trabecular structure, but the relationship between the microarchitectural parameters and volume fraction was unaltered (p>0.05). Though the suppressed group had a greater tissue-normalized strength as compared to the control group (p<0.001) for both compressive and torsional loading, the relationship between tissue-normalized strength and volume fraction was not significantly altered for compression (p>0.13) or torsion (p>0.09). In this high-density, non-osteoporotic animal model, the increases in tissue-normalized strength seen with suppression of bone turnover were entirely commensurate with increases in bone volume fraction and thus, no evidence of microarchitecture-related or "stress-riser" effects which may disproportionately affect strength were found.     

Comparison of trabecular bone behavior in core and whole bone samples using high-resolution modeling of a vertebral body

Computational analysis of trabecular bone normally involves the modeling of (experimental tests of) cored samples. However, the lack of constraint on the sides of the extracted trabecular bone samples limits the information that can be inferred regarding true in situ behavior. Here, the element-by-element voxel-based finite element method was applied via, a custom-written software suite (FEEBE), to a 72 μm resolution model of an ovine vertebra. The difference between the apparent modulus of eight concentric core cylinders when modeled as part of the whole bone (containing 84 × 10⁶ degrees of freedom) and independent of the whole bone was investigated. The results showed that cored trabecular bone apparent modulus depended significantly on the core diameter when modeled as an extracted core (r ² = 0.975) and as part of a whole bone (r ² = 0.986). The cause of this result was separated into the side-artifact effect and bone volume fraction (BV/TV) effect. For the independently modeled cores, the apparent modulus of an inner core region of interest varied with increasing thickness of the outer annulus. This was attributed to the side-artifact effect, given that the BV/TV of the core region was constant. Within the whole trabecular structure, the side artifact was eliminated as the entire bone structure was modeled. However, a BV/TV effect influenced the apparent modulus depending on the size of the core selected for determining apparent modulus. Changing the size of the core varied the overall BV/TV of the core, and this significantly (r ² = 0.999) influences the apparent modulus. Therefore, determining a ‘true’ apparent modulus for trabecular bone was not achievable. The independently modeled cores consistently under-predict the in vivo apparent modulus. It is recommended that if a ‘true’ apparent modulus is required, the BV/TV at which it is required needs to be first determined. Apparent modeling of entire bones at microscale resolution allowed regions of low and high tissue strains to be identified, consistent with patterns of trabecular bone remodeling and resorption reported in literature. The basivertebral vein cavity underwent the highest strains within the entire vertebral body, suggesting that failure might initiate here, despite containing visibly thicker struts and plate trabeculae. Although computationally expensive, analysis of the entire vertebral body provided a full picture of in situ trabecular bone deformation.     

A morphological model of vertebral trabecular bone

In their micro-structures, typical natural cellular materials such as vertebral trabecular bone have a network of doubly tapered struts, thickening near the strut joints. However, past analytical models for vertebral trabecular bone do not take account of the effect of strut taper on the mechanical properties.This paper presents an analytical cell model comprised of doubly tapered struts to predict the global mechanical properties of vertebral trabecular bone. The predicted results for male, female, and both sexes fit the experimental data well. By considering several strut taper geometries, it is shown that the horizontal Young's modulus and the horizontal uniaxial collapse stress are, in some cases, approximately 1.8- and 2.2-fold higher, respectively, than those of the uniform strut model. This finding illustrates the importance of increased trabecular thickening near the strut joints (i) for improving the accuracy of calculating the mechanical properties and (ii) for the effective treatment of aged bone using drug therapy. It also highlights the need to combine trabecular architecture measurements with information about the morphology near the strut joints.     

Compressive fatigue behavior of human vertebral trabecular bone

Damage accumulation under compressive fatigue loading is believed to contribute significantly to non-traumatic, age-related vertebral fractures in the human spine. Only few studies have explored trabecular bone fatigue behavior under compressive loading and none examined the influence of trabecular architecture on fatigue life. In this study, trabecular bone samples of human lumbar and thoracic vertebrae (4 donors from age 29 to 86, n=29) were scanned with a microCT system prior to compressive fatigue testing to determine morphology-mechanical relationships for this relevant loading mode. Inspired from previous fabric-based relationships for elastic properties and quasi-static strength of trabecular bone, a simple power relationship between volume fraction, fabric eigenvalue, applied stress and the number of cycles to failure is proposed. The experimental results demonstrate a high correlation for this relationship (R2=0.95) and detect a significant contribution of the degree of anisotropy towards prediction of fatigue life. Step-wise regression for total and residual strains at failure suggested a weak, but significant correlation with volume fraction. From the obtained results, we conclude that the applied stress normalized by volume fraction and axial fabric eigenvalue can estimate fatigue life of human vertebral trabecular bone in axial compressive loading.     

Changes in trabecular bone turnover and bone marrow cell development in tail-suspended mice

Skeletal unloading induces trabecular bone loss in loaded bones. The tail-suspended mouse model simulates conditions associated with lack of mechanical stress such as space flight for the loaded bones. In such a model, the tail supports the body weight. The forelimbs are normally loaded and the movement of its hindlimbs is free without weight bearing. Histomorphometric analyses of the murine tibiae of the elevated hindlimbs show that trabecular bone volume rapidly diminishes within one week and stabilizes at that level in the subsequent week of tail suspension. Two-week reloading after one-week unloading completely restores trabecular bone volume, but this does not happen after two-week unloading. Unloading for one or two weeks significantly reduces bone formation rate and increases both the osteoclast surface and number compared with age-matched ground control mice. Subsequent reloading restores reduced bone formation and suppresses increased bone resorption. In bone marrow cell cultures, the numbers of alkaline phosphatase (ALP)-positive colony-forming units-fibroblastic (CFU-f) and mineralized nodules are significantly reduced, but the numbers of adherent marrow cells and total CFU-f are unaltered after tail suspension. On the other hand, subsequent reloading increases the number of adherent marrow cells. Unloading for one week significantly increases the number of tartrate-resistant acid phosphatase (TRAP)- positive multinucleated cells compared with the control level. Our data demonstrate that tail suspension in mice reduces trabecular bone formation, enhances bone resorption, and is closely associated with the formation of mineralized nodules and TRAP-positive multinucleated cells in bone marrow cultures obtained from tibiae. Two-week reloading restores bone volume reduced after one-week unloading, but does not after two-week unloading. The tail-suspended model provides a unique opportunity to evaluate the physiological and cellular mechanisms of the skeletal response to unloading and reloading.     

Damage accumulation in vertebral trabecular bone depends on loading mode and direction

Osteoporotic vertebral fractures constitute a major clinical problem in ageing societies. A third of all vertebral fractures is caused by falls, 15% by lifting heavy loads or traffic accidents and over 50% are not relatable to a traumatic event. In the latter case vertebrae show sinter processes which indicate the accumulation of damage and permanent deformation. Accumulated damage may not be visible on radiographs but increases the risk of fracture and could lead to vertebral collapse. Clear understanding of the accumulation of damage and residual strains and their dependence on loading mode and direction is important for understanding vertebral fractures. Altogether, 251 cylindrical samples (8×18-25mm) were obtained from 50 male and 54 female fresh frozen human vertebrae (T1-L3) of 65 (21-94) years. Vertebrae were randomly assigned to three groups cranial-caudal, anterior-posterior and latero-lateral. Specimens were mechanically loaded in compression, tension or torsion in five load steps at a strain rate of 0.2%/s. Three conditioning cycles were driven per load step. Stress-strain curves were reconstructed from the force-displacement or from the moment-twist angle curves. Damage accumulated from 0 to 86% in compression, from 0 to 76% in tension and from 0 to 86% in torsion through the five load steps. Residual strains accumulated from 0 to -0.008mm/mm in compression, 0 to 0.006mm/mm in tension and 0 to 0.026rad/rad in torsion. Significantly less damage (p<0.05) but not residual strains accumulated in transverse directions. This study provides detailed experimental insights into the damage behaviour of vertebral trabecular bone under various loads occurring in vivo. Damage but not residual strain evolution seems to be anisotropic. Both seem to evolve differently under different loading modes. The results could be of importance in understanding vertebral fractures.     

Development of residual strains in human vertebral trabecular bone after prolonged static and cyclic loading at low load levels

Development of irreversible residual strains in trabecular bone may be a mechanism by which age-related non-traumatic vertebral fractures occur. To investigate this concept, static and cyclic loading tests were conducted at low loading levels for cylindrical cores of cadaveric vertebral trabecular bone. Stresses were applied equivalent to elastic strains of either 750 or 1,500 microstrain. Creep strains were measured during the tests, which lasted for 125,000 seconds (about 35 h), and for an additional 125,000 seconds after complete unloading. Emphasis was placed on the residual strains that developed, defined as the strain remaining at the end of the unloading phase. The results indicated that appreciable residual strains did develop, and were similar for static and cyclic loading. Irrespective of the applied load levels and loading modes, the residual strains that remained after the unloading phase were similar in magnitude to the originally applied elastic strain. Extrapolation of the observed residual strains to full recovery indicated that the time that would be required for full recovery was over 20 times longer than the duration of the applied loads. These results indicate that human vertebral trabecular bone does not creep in a linear viscoelastic fashion at low stress levels, and that creep mechanisms dominate the residual strains regardless of the loading mode. Taken together, these findings support the concept that non-traumatic vertebral fractures may be related to long-term creep effects because the trabecular bone does not have sufficient time to recover mechanically from creep deformations accumulated by prolonged static or cyclic loading.     

The effect of damage on the viscoelastic behavior of human vertebral trabecular bone

The present study examines the viscoelastic behavior of cancellous bone at low strains and the effects of damage on this viscoelastic behavior. It provides experimental evidence of interaction between stress relaxation behavior and the effect of accumulated damage. The results suggest that damage is at least orthotropic in trabecular bone specimens under uniaxial loading. Simple linear models of viscoelasticity described the time-dependent stress-strain behavior at low strains before and after specimen damage, although better fits of these models were obtained prior to damage. Modeling the observed changes in relaxation times with damage accumulation appears necessary to successfully predict the post-damage viscoelastic response.     

The elastic moduli of human subchondral, trabecular, and cortical bone tissue and the size-dependency of cortical bone modulus

The elastic moduli of human subchondral, trabecular, and cortical bone tissue from a proximal tibia were experimentally determined using three-point bending tests on a microstructural level. The mean modulus of subchondral specimens was 1.15 GPa, and those of trabecular and cortical specimens was 4.59 GPa and 5.44 GPa respectively. Significant differences were found in the modulus values between bone tissues, which may have mainly resulted from the differences in the microstructures of each bone tissue rather than in the mineral density. Furthermore, the size-dependency of the modulus was examined using eight different sizes of cortical specimens (heights h = 100-1000 microns). While the modulus values for relatively large specimens (h greater than 500 microns) remained fairly constant (approximately 15 GPa), the values decreased as the specimens became smaller. A significant correlation was found between the modulus and specimen size. The surface area to volume ratio proved to be a key variable to explain the size-dependency.     

Variations in mineralization affect the stress and strain distributions in cortical and trabecular bone

The mechanical properties of bone depend largely on its degree and distribution of mineralization. The present study analyzes the effect of an inhomogeneous distribution of mineralization on the stress and strain distributions in the human mandibular condyle during static clenching. A condyle was scanned with a micro-CT scanner to create a finite element model. For every voxel the degree of mineralization (DMB) was determined from the micro-CT scan. The Young's moduli of the elements were calculated from the DMB using constant, linear, and cubic relations, respectively. Stresses, strains, and displacements in cortical and trabecular bone, as well as the condylar deformation (extension along the antero-posterion axis) and compliance were compared. Over 90% of the bone mineral was located in the cortical bone. The DMB showed large variations in both cortical bone (mean: 884, SD: 111 mg/cm(3)) and trabecular bone (mean: 738, SD: 101 mg/cm(3)). Variations of the stresses and the strains were small in cortical bone, but large in trabecular bone. In the cortical bone an inhomogeneous mineral distribution increased the stresses and the strains. In the trabecular bone, however, it decreased the stresses and increased the strains. Furthermore, the condylar compliance remained relatively constant, but the condylar deformation doubled. It was concluded that neglect of the inhomogeneity of the mineral distribution results in a large underestimation of the stresses and strains of possibly more than 50%. The stiffness of trabecular bone strongly influences the condylar deformation. Vice versa, the condylar deformation largely determines the magnitude of the strains in the trabecular bone.     

The differential response of cortical and trabecular bone to aluminum administration in the rat

Osteomalacia has been noted following in vivo aluminum (Al) loading in the rat by some investigators but not by others. To determine whether the response of bone to Al differs as a function of the skeletal site examined, quantitative histology of cortical and trabecular bone was done in the tibiae from control (C, n = 10), Al-treated (AL, n = 9), nephrectomized control (NX-C, n = 7), and nephrectomized Al-treated (NX-AL, n = 8) rats given 2 mg/day of Al for 4 weeks. Bone Al content was determined by histochemical methods. In cortical bone, osteoid seam width, osteoid volume, and percent osteoid area were similar for all groups. In contrast, for trabecular bone, both forming surface (means +/- SD) (5.2 +/- 3.4 vs 1.8 +/- 1.1%, P less than 0.05) and osteoid volume (1.7 +/- 0.7 vs 1.0 +/- 0.4%, P less than 0.05) increased from control values in AL, although osteoid seam width did not differ. In NX-AL, trabecular forming surface (20.2 +/- 6.7 vs 6.2 +/- 2.4%, P less than 0.01), osteoid area (13.2 +/- 5.7 vs 3.5 +/- 0.8%, P less than 0.01), and osteoid width (18.7 +/- 5.7 vs 9.7 +/- 2.3 micron, P less than 0.01) all were greater than in NX-C. Deposits of Al were undetectable in C and NX-C, were minimal in cortical bone in AL and NX-AL, but were present at 40.5 +/- 11.5 and 71.1 +/ 6.5% of trabecular surfaces in AL and NX-AL, respectively. Osteoid area and osteoid surface each correlated with trabecular bone Al. Thus, (a) osteoid accumulates in trabecular, but not in cortical, bone after 4 weeks of Al loading; (b) the extent of osteoid accumulation correlates with the bone Al content; and (c) the histologic response to Al in cortical and trabecular bone is related to local differences in the uptake of Al into bone.     

A comparison of the fatigue behavior of human trabecular and cortical bone tissue

The fatigue properties of trabecular bone tissue (single trabeculae) and similarly sized cortical bone specimens from human tibia were experimentally determined on a microstructural level using four-point bending cyclic tests, and they were compared based on modulus, mineral density, and microstructural characteristics. The results showed that trabecular specimens had significantly lower moduli and lower fatigue strength than cortical specimens, despite their higher mineral density values. Fracture surface and microdamage analyses illustrated different fracture and damage patterns between trabecular and cortical bone tissue, depending upon their microstructural characteristics. Based on the results from mechanical tests and qualitative observations, a possible mechanical role of the cement lines in trabecular tissue microfracture was suggested.     

Simulation of vertebral trabecular bone loss using voxel finite element analysis

Trabecular bone loss in human vertebral bone is characterised by thinning and eventual perforation of the horizontal trabeculae. Concurrently, vertical trabeculae are completely lost with no histological evidence of significant thinning. Such bone loss results in deterioration in apparent modulus and strength of the trabecular core. In this study, a voxel-based finite element program was used to model bone loss in three specimens of human vertebral trabecular bone. Three sets of analyses were completed. In Set 1, strain adaptive resorption was modelled, whereby elements which were subject to the lowest mechanical stimulus (principal strain) were removed. In Set 2, both strain adaptive and microdamage mechanisms of bone resorption were included. Perforation of vertical trabeculae occurred due to microdamage resorption of elements with strains that exceeded a damage threshold. This resulted in collapse of the trabecular network under compression loading for two of the specimens tested. In Set 3, the damage threshold strain was gradually increased as bone loss progressed, resulting in reduced levels of microdamage resorption. This mechanism resulted in trabecular architectures in which vertical trabeculae had been perforated and which exhibited similar apparent modulus properties compared to experimental values reported in the literature. Our results indicate that strain adaptive remodelling alone does not explain the deterioration in mechanical properties that have been observed experimentally. Our results also support the hypothesis that horizontal trabeculae are lost principally by strain adaptive resorption, while vertical trabeculae may be lost due to perforation from microdamage resorption followed by rapid strain adaptive resorption of the remaining unloaded trabeculae.     

Precision of nanoindentation protocols for measurement of viscoelasticity in cortical and trabecular bone

Nanoindentation has recently gained attention as a characterization technique for mechanical properties of biological tissues, such as bone, on the sub-micron level. However, optimal methods to characterize viscoelastic properties of bones are yet to be established. This study aimed to compare the time-dependent viscoelastic properties of bone tissue obtained with different nanoindentation methods. Bovine cortical and trabecular bone samples (n=8) from the distal femur and proximal tibia were dehydrated, embedded and polished. The material properties determined using nanoindentation were hardness and reduced modulus, as well as time-dependent parameters based on creep, loading-rate, dissipated energy and semi-dynamic testing under load control. Each loading protocol was repeated 160 times and the reproducibility was assessed based on the coefficient of variation (CV). Additionally, three well-characterized polymers were tested and CV values were calculated for reference.The employed methods were able to characterize time-dependent viscoelastic properties of bone. However, their reproducibility varied highly (CV 9–40%). The creep constant increased with increasing dwell time. The reproducibility was best with a 30 s creep period (CV 18%). The dissipated energy was stable after three repeated load cycles, and the reproducibility improved with each cycle (CV 23%). The viscoelastic properties determined with semi-dynamic test increased with increase in frequency. These measurements were most reproducible at high frequencies (CV 9–10%). Our results indicate that several methods are feasible for the determination of viscoelastic properties of bone material. The high frequency semi-dynamic test showed the highest precision within the tested nanoindentation protocols.     

Computational simulation of simultaneous cortical and trabecular bone change in human proximal femur during bone remodeling

In this study, we developed a numerical framework that computationally determines simultaneous and interactive structural changes of cortical and trabecular bone types during bone remodeling, and we investigated the structural correlation between the two bone types in human proximal femur. We implemented a surface remodeling technique that performs bone remodeling in the exterior layer of the cortical bone while keeping its interior area unchanged. A micro-finite element (μFE) model was constructed that represents the entire cortical bone and full trabecular architecture in human proximal femur. This study simulated and compared the bone adaptation processes of two different structures: (1) femoral bone that has normal cortical bone shape and (2) perturbed femoral bone that has an artificial bone lump in the inferomedial cortex. Using the proposed numerical method in conjunction with design space optimization, we successfully obtained numerical results that resemble actual human proximal femur. The results revealed that actual cortical bone, as well as the trabecular bone, in human proximal femur has structurally optimal shapes, and it was also shown that a bone abnormality that has little contribution to bone structural integrity tends to disappear. This study also quantitatively determined the structural contribution of each bone: when the trabecular adaptation was complete, the trabecular bone supported 54% of the total load in the human proximal femur while the cortical bone carried 46%.     

The effect of restriction of dietary calcium on trabecular and cortical bone mineral density in the rats

This study aimed to investigate effects of restricted calcium intake on cortical and trabecular bone density in white rats. Low Ca diet was fed for six weeks, and bone density and bone metabolism parameters were assessed in blood. This study was carried out on 12 male white rats aged 12 weeks (Sprague-Dawley; SD). These rats were bred for 1 week and randomly assigned to the standard calcium diet group (SCa group, n = 6) and the low calcium diet group (LCa group; n = 6). The SCa group was given a modified AIN-93M mineral mix (with 0.5% Ca), which was made by adding calcium to a standard AIN93 diet, and the LCa Group was fed a modified AIN-93 Mineral mix (with 0.1% Ca). Femoral BMD and BMC were measured by DEXA in each rat. After trabecular bone was separated from cortical bone, volumetric bone mineral density (vBMD) was measured using pQCT. Serum Ca and P levels were measured as parameters of bone metabolism, and S-ALP, S-TrACP and-Dpd levels were also measured. The results revealed no significant differences in weight, growth rate, feed consumption and feed efficiency between the two groups before and after calcium-restricted diet (p > .05). No significant differences were also observed in bone length and bone mass between the two groups (p > .05). Although bilateral femoral BMDs were not significantly different between the two groups, bilateral femoral BMCs significantly decreased in the LCa group, compared with the SCa group (p = .023, p = .047). Bilateral cortical MDs were not significantly different between the two groups, either. However, trabecular BMD significantly decreased in the LCa group, compared with the SCa group (p = .041). U-Dpd and S-TrACP levels significantly declined in the LCa group, compared to the SCa group (p = .039, p = .010). There were no significant differences in serum Ca and P levels between the two groups (p > .05). However, a significant decrease in urinary Ca level (p = .001) and a significant increase in urinary P (p = .001) were observed in the LCa group, compared to the Sca group. These findings described that six-week low calcium diet led to decreased trabecular bone density, reduced urinary excretion of Ca and increased urinary excretion of P. As a result, Ca hemeostasis can be maintained.     

Micromechanical analyses of vertebral trabecular bone based on individual trabeculae segmentation of plates and rods

Trabecular plates play an important role in determining elastic moduli of trabecular bone. However, the relative contribution of trabecular plates and rods to strength behavior is still not clear. In this study, individual trabeculae segmentation (ITS) and nonlinear finite element (FE) analyses were used to evaluate the roles of trabecular types and orientations in the failure initiation and progression in human vertebral trabecular bone. Fifteen human vertebral trabecular bone samples were imaged using micro computed tomography (μCT), and segmented using ITS into individual plates and rods by orientation (longitudinal, oblique, and transverse). Nonlinear FE analysis was conducted to perform a compression simulation for each sample up to 1% apparent strain. The apparent and relative trabecular number and tissue fraction of failed trabecular plates and rods were recorded during loading and data were stratified by trabecular orientation. More trabecular rods (both in number and tissue fraction) failed at the initiation of compression (0.1–0.2% apparent strain) while more plates failed around the apparent yield point (>0.7% apparent strain). A significant correlation between plate bone volume fraction (pBV/TV) and apparent yield strength was found (r²=0.85). From 0.3% to 1% apparent strain, significantly more longitudinal trabecular plate and transverse rod failed than other types of trabeculae. While failure initiates at rods and rods fail disproportionally to their number, plates contribute significantly to the apparent yield strength because of their larger number and tissue volume. The relative failed number and tissue fraction at apparent yield point indicate homogeneous local failure in plates and rods of different orientations.     

A 3D elastic micropolar model of vertebral trabecular bone from lattice homogenization of the bone microstructure

A 3D anisotropic micropolar continuum model of vertebral trabecular bone is presently developed accounting for the influence of microstructure-related scale effects on the macroscopic effective properties. Vertebral trabecular bone is modeled as a cellular material with an idealized periodic structure made of open 3D cells. The micromechanical approach relies on the discrete homogenization technique considering lattice microrotations as additional degrees of freedom at the microscale. The effective elastic properties of 3D lattices made of articulated beams taking into account axial, transverse shearing, flexural, and torsional deformations of the cell struts are derived as closed form expressions of the geometrical and mechanical microparameters. The scaling laws of the effective moduli versus density are determined in situations of low and high effective densities to assess the impact of the transverse shear deformation. The classical and micropolar effective moduli and the internal flexural and torsional lengths are identified versus the same microparameters. A finite element model of the local architecture of the trabeculae gives values of the effective moduli that are in satisfactory agreement with the homogenized moduli.     

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.