Trabecular shear stress in human vertebral cancellous bone: intra- and inter-individual variations

Biomechanical evaluation of Percutaneous endoscopic posterior lumbar interbody fusion and minimally invasive transforaminal lumbar interbody fusion: a biomechanical analysis

In order to analyze and evaluate the stability of lumbar spine and the risk of cage subsidence after different minimally invasive fusion operations, two finite element models Percutaneous endoscopic posterior lumbar interbody fusion (PE-PLIF) and minimally invasive transforaminal lumbar interbody Fusion (MIS-TLIF) were established. The results showed that compared with MIS-TLIF, PE-PLIF had better segmental stability, lower pedicle screw rod system stress, and lower risk of cage subsidence. The results suggest that the cage with appropriate height should be selected to ensure the segmental stability and avoid the risk of the subsidence caused by the cage with large height.

A Review on Three-Dimensional Printed Silicate-Based Bioactive Glass/Biodegradable Medical Synthetic Polymer Composite Scaffolds

In recent years, tissue engineering scaffolds have turned into the preferred option for the clinical treatment of pathological and traumatic bone defects. In this field, silicate-based bioactive glasses (SBGs) and biodegradable medical synthetic polymers (BMSPs) have attracted a great deal of attention owing to their shared exceptional advantages, like excellent biocompatibility, good biodegradability, and outstanding osteogenesis. Three-dimensional (3D) printed SBG/BMSP scaffolds can not only replicate the mechanical properties and microstructure of natural bone but also degrade in situ after service and end up being replaced by regenerated bone tissue in vivo. This review first consolidates the research efforts in 3D printed SBG/BMSP scaffolds, and then focuses on their composite mechanism. This review may help to provide a fresh perspective for SBG/BMSP composite system in bone regeneration.

Effect of thermodisinfection on mechanic parameters of cancellous bone

Revision surgery of joint replacements is increasing and raises the demand for allograft bone since restoration of bone stock is crucial for longevity of implants. Proceedings of bone grafts influence the biological and mechanic properties differently. This study examines the effect of thermodisinfection on mechanic properties of cancellous bone. Bone cylinders from both femoral heads with length 45 mm were taken from twenty-three 6-8 months-old piglets, thermodisinfected at 82.5 °C according to bone bank guidelines and control remained native. The specimens were stored at -20 °C immediately and were put into 21 °C Ringer's solution for 3 h before testing. Shear and pressure modulus were tested since three point bending force was examined until destruction. Statistical analysis was done with non-parametric Wilcoxon, t test and SPSS since p < 0.05 was significant. Shear modulus was significantly reduced by thermodisinfection to 1.02 ± 0.31 GPa from 1.28 ± 0.68 GPa for unprocessed cancellous bone (p = 0.029) since thermodisinfection reduced pressure modulus not significantly from 6.30 ± 4.72 GPa for native specimens to 4.97 ± 2.23 GPa and maximum bending force was 270.03 ± 116.68 N for native and 228.80 ± 70.49 N for thermodisinfected cancellous bone. Shear and pressure modulus were reduced by thermodisinfection around 20 % and maximum bending force was impaired by about 15 % compared with native cancellous bone since only the reduction of shear modulus reached significance. The results suggest that thermodisinfection similarly affects different mechanic properties of cancellous bone and the reduction of mechanic properties should not relevantly impair clinical use of thermodisinfected cancellous bone.

Trabecular bone structure in lumbosacral transitional vertebrae: distribution and densities across sagittal vertebral body segments

BACKGROUND CONTEXT

Lumbosacral transitional vertebrae (LSTV) are associated with altered articular morphology at the L5-S1 junction. Studies related to lumbo-sacral trabecular architecture in LSTV are few. Altered lumbosacral load bearing at these anomalous junctions possibly results in changes in the number, density, and trajectory of the trabecular bone in transitional lumbosacral vertebral bodies.

PURPOSE

To investigate the pattern, distribution, and density of trabecular bone in the terminal lumbar vertebrae and the first sacral segments in LSTV-affected spines. Measurements were compared with those obtained from normal lumbosacral specimens.

STUDY DESIGN

Observational and descriptive human cadaveric study of vertebral trabecular architecture.

METHODS

Blocks of tissues were obtained from normal (n=20) and LSTV cadaveric specimens (n=16) by sectioning vertically through the fifth lumbar and the first sacral vertebra on either side of the midsagittal plane. Photographs of the cut surfaces were computationally enlarged and mapped for vertical and transverse trabecular numbers and surface areas using the software Image J. All parameters including the trabecular density were computed for anterior, middle, and posterior segments of each of the vertebral elements.

RESULTS

The anterior and the posterior segments showed greater number of trabeculae across all LSTV subtypes in both the terminal lumbar and first sacral vertebrae in comparison with the middle segment. L5 exhibited greater number of vertical trabeculae, whereas the first sacral segments demonstrated greater number and densities of transverse trabeculae. Transition-associated vertebrae showed overall reduced number of the lumbar trabeculae but relatively compact sacral posterior segments with greater number of horizontal trabeculae.

CONCLUSIONS

Findings suggest that some of these variations have overall reduced number of trabeculae across lumbo-sacral vertebrae in LSTV. Screw placements and subsequent pullouts in LSTV may be reviewed in light of different trabecular patterns as reported in this study.

Progress and challenges in biomaterials used for bone tissue engineering: bioactive glasses and elastomeric composites

Driven by the increasing economic burden associated with bone injury and disease, biomaterial development for bone repair represents the most active research area in the field of tissue engineering. This article provides an update on recent advances in the development of bioactive biomaterials for bone regeneration. Special attention is paid to the recent developments of sintered Na-containing bioactive glasses, borate-based bioactive glasses, those doped with trace elements (such as Cu, Zn, and Sr), and novel elastomeric composites. Although bioactive glasses are not new to bone tissue engineering, their tunable mechanical properties, biodegradation rates, and ability to support bone and vascular tissue regeneration, as well as osteoblast differentiation from stem and progenitor cells, are superior to other bioceramics. Recent progresses on the development of borate bioactive glasses and trace element-doped bioactive glasses expand the repertoire of bioactive glasses. Although boride and other trace elements have beneficial effects on bone remodeling and/or associated angiogenesis, the risk of toxicity at high levels must be highly regarded in the design of new composition of bioactive biomaterials so that the release of these elements must be satisfactorily lower than their biologically safe levels. Elastomeric composites are superior to the more commonly used thermoplastic-matrix composites, owing to the well-defined elastic properties of elastomers which are ideal for the replacement of collagen, a key elastic protein within the bone tissue. Artificial bone matrix made from elastomeric composites can, therefore, offer both sound mechanical integrity and flexibility in the dynamic environment of injured bone.

Photoelastic analisys in the lower region of vertebral body L4

Objective

To analyze the shear forces on the vertebral body L4 when submitted to a compression force by means of transmission photoelasticity.

Methods

Twelve photoelastic models were divided into three groups, with four models per group, according to the positioning of the sagittal section vertebrae L4-L5 (sections A, B and C). The simulation was performed using a 15N compression force, and the fringe orders were evaluated in the vertebral body L4 by the Tardy compensation method.

Results

Photoelastic analysis showed, in general, a homogeneous distribution in the vertebral bodies. The shear forces were higher in section C than B, and higher in B than A.

Conclusion

The posterior area of L4, mainly in section C, showed higher shear concentrations, corresponding to a more susceptible area for bone fracture and spondylolisthesis. Economic and Decision Analyses - Development of an Economic or Decision Model. Level I

Variability of trabecular microstructure is age-, gender-, race- and anatomic site-dependent and affects stiffness and stress distribution properties of human vertebral cancellous bone

Cancellous bone microstructure is an important determinant of the mechanical integrity of vertebrae. The numerous microstructural parameters that have been studied extensively are generally represented as a single value obtained as an average over a sample. The range of the intra-sample variability of cancellous microstructure and its effect on the mechanical properties of bone are less well-understood. The objectives of this study were to investigate the extent to which human cancellous bone microstructure within a vertebra i) is related to bone modulus and stress distribution properties and ii) changes along with age, gender and locations thoracic 12 (T12) vs lumbar 1 (L1). Vertebrae were collected from 15 male (66±15 years) and 25 female (54±16 years) cadavers. Three dimensional finite element models were constructed using microcomputed tomography images of cylindrical specimens. Linear finite element models were used to estimate apparent modulus and stress in the cylinders during uniaxial compression. The intra-specimen mean, standard deviation (SD) and coefficient of variation (CV) of microstructural variables were calculated. Mixed model statistical analysis of the results demonstrated that increases in the intra-specimen variability of the microstructure contribute to increases in the variability of trabecular stresses and decreases in bone stiffness. These effects were independent from the contribution from intra-specimen average of the microstructure. Further, the effects of microstructural variability on bone stiffness and stress variability were not accounted for by connectivity and anisotropy. Microstructural variability properties (SD, CV) generally increased with age, were greater in females than in males and in T12 than in L1. Significant interactions were found between age, gender, vertebra and race. These interactions suggest that microstructural variability properties varied with age differently between genders, races and vertebral levels. The current results collectively demonstrate that microstructural variability has a significant effect on mechanical properties and tissue stress of human vertebral cancellous bone. Considering microstructural variability could improve the understanding of bone fragility and improve assessment of vertebral fracture risk.

Effects of preexisting microdamage, collagen cross-links, degree of mineralization, age, and architecture on compressive mechanical properties of elderly human vertebral trabecular bone

Previous studies have shown that the mechanical properties of trabecular bone are determined by bone volume fraction (BV/TV) and microarchitecture. The purpose of this study was to explore other possible determinants of the mechanical properties of vertebral trabecular bone, namely collagen cross-link content, microdamage, and mineralization. Trabecular bone cores were collected from human L2 vertebrae (n = 49) from recently deceased donors 54-95 years of age (21 men and 27 women). Two trabecular cores were obtained from each vertebra, one for preexisting microdamage and mineralization measurements, and one for BV/TV and quasi-static compression tests. Collagen cross-link content (PYD, DPD, and PEN) was measured on surrounding trabecular bone. Advancing age was associated with impaired mechanical properties, and with increased microdamage, even after adjustment by BV/TV. BV/TV was the strongest determinant of elastic modulus and ultimate strength (r²  = 0.44 and 0.55, respectively). Microdamage, mineralization parameters, and collagen cross-link content were not associated with mechanical properties. These data indicate that the compressive strength of human vertebral trabecular bone is primarily determined by the amount of trabecular bone, and notably unaffected by normal variation in other factors, such as cross-link profile, microdamage and mineralization.

Apparent damage accumulation in cancellous bone using neural networks

In this paper, a neural network model is developed to simulate the accumulation of apparent fatigue damage of 3D trabecular bone architecture at a given bone site during cyclic loading. The method is based on five steps: (i) performing suitable numerical experiments to simulate fatigue accumulation of a 3D micro-CT trabecular bone samples taken from proximal femur for different combinations of loading conditions; (ii) averaging the sample outputs in terms of apparent damage at whole specimen level based on local tissue damage; (iii) preparation of a proper set of corresponding input-output data to train the network to identify apparent damage evolution; (iv) training the neural network based on the results of step (iii); (v) application of the neural network as a tool to estimate rapidly the apparent damage evolution at a given bone site. The proposed NN model can be incorporated into finite element codes to perform fatigue damage simulation at continuum level including some morphological factors and some bone material properties. The proposed neural network based multiscale approach is the first model, to the author's knowledge, that incorporates both finite element analysis and neural network computation to rapidly simulate multilevel fatigue of bone. This is beneficial to develop enhanced finite element models to investigate the role of damage accumulation on bone damage repair during remodelling.

Stiffness of the endplate boundary layer and endplate surface topography are associated with brittleness of human whole vertebral bodies

Stress magnitude and variability as estimated from large scale finite element (FE) analyses have been associated with compressive strength of human vertebral cancellous cores but these relationships have not been explored for whole vertebral bodies. In this study, the objectives were to investigate the relationship of FE-calculated stress distribution parameters with experimentally determined strength, stiffness, and displacement based ductility measures in human whole vertebral bodies, investigate the effect of endplate loading conditions on vertebral stiffness, strength, and ductility and test the hypothesis that endplate topography affects vertebral ductility and stress distributions. Eighteen vertebral bodies (T6-L3 levels; 4 female and 5 male cadavers, aged 40-98 years) were scanned using a flat-panel CT system and followed with axial compression testing with Wood's metal as filler material to maintain flat boundaries between load plates and specimens. FE models were constructed using reconstructed CT images and filler material was added digitally. Two different FE models with different filler material modulus simulating Wood's metal and intervertebral disc (W-layer and D-layer models) were used. Element material modulus to cancellous bone was based on image gray value. Average, standard deviation, and coefficient of variation of von Mises stress in vertebral bone for W-layer and D-layer models and also the ratios of FE parameters from the two models (W/D) were calculated. Inferior and superior endplate surface topographical distribution parameters were calculated. Experimental stiffness, maximum load and work to fracture had the highest correlation with FE-calculated stiffness while experimental ductility measures had highest correlations with FE-calculated average von Mises stress and W-layer to D-layer stiffness ratio. Endplate topography of the vertebra was also associated with its structural ductility and the distribution parameter that best explained this association was kurtosis of inferior endplate topography. Our results indicate that endplate topography variations may provide insight into the mechanisms responsible for vertebral fractures.

Cancellous bone properties and matrix content of TGF-beta2 and IGF-I in human tibia: a pilot study

Transforming and insulin-like growth factors are important in regulating bone mass. Thus, one would anticipate correlations between matrix concentrations of growth factors and functional properties of bone. We therefore investigated the relationships of (1) TGF-beta2 and (2) IGF-I matrix concentrations with the trabecular microstructure, stress distribution, and mechanical properties of tibial cancellous bone from six male human cadavers. Trabecular stress amplification (VMExp/sigma(app)) and variability (VMCOV) were calculated using microcomputed tomography (muCT)-based finite element simulations. Bone volume fraction (BV/TV), surface/volume ratio (BS/BV), trabecular thickness (Tb.Th), number (Tb.N) and separation (Tb.Sp), connectivity (Eu.N), and anisotropy (DA) were measured using 3-D morphometry. Bone stiffness and strength were measured by mechanical testing. Matrix concentrations of TGF-beta2 and IGF-I were measured by ELISA. We found higher matrix concentrations of TGF-beta2 were associated with higher Tb.Sp and VMExp/sigma(app) for pooled data and within subjects. Similarly, a higher matrix concentration of IGF-I was associated with lower stiffness, strength, BV/TV and Tb.Th and with higher BS/BV, Tb.Sp, VMExp/sigma(app) and VMCOV for pooled data and within subjects. IGF-I and Tb.N were negatively associated within subjects. It appears variations of the stress distribution in cancellous bone correlate with the variation of the concentrations of TGF-beta2 and IGF-I in bone matrix: increased local matrix concentrations of growth factors are associated with poor biomechanical and architectural properties of tibial cancellous bone.

Differences in early osteogenesis and bone micro-architecture in anterior lumbar interbody fusion with rhBMP-2, equine bone protein extract, and autograft

PURPOSE

To investigate the microstructural differences and responsible mechanisms in early bone formation in anterior lumbar interbody fusion (ALIF) in the spine using rhBMP-2 (INFUSE), equine bone protein extract (COLLOSS E) or autograft.

METHODS

Twelve Danish female landrace pigs underwent a 3-level ALIF procedure at L3-6. PEEK interbody cages packed with rhBMP-2, COLLOSS E, or autograft were inserted. The animals were divided into two groups of six, and observed for four and eight weeks postoperatively. MicroCT was performed for evaluation of microstructure of the bone within the cage. A mathematical finite element model was developed to investigate the aqueous behavior within the cages when exposed to external compressive forces.

RESULTS

At 4 weeks postoperative bone surface volume fraction (BS/TV) using rhBMP-2 was higher than with use of COLLOSS E and autograft, while trabecular thickness (Tb.Th.) was lower using rhBMP-2 at this time-point. At eight weeks BS/TV and trabecular number (Tb.N.) were still higher using rhBMP-2 than autograft and COLLOSS E. Connectivity density was significantly higher using rhBMP-2 than using autograft or COLLOSS E at both time-points. Between four- and eight-week time-points BV/TV and Tb.Th. rose while Tb.N. declined using rhBMP-2. The degree of anisotropy and the calculated amount of trabeculae with main direction along the spinal axis, were higher at four weeks using COLLOSS E. rhBMP-2 had the highest amount of trabeculae directed along the spinal axis at eight weeks. A change in main direction between four and eight weeks was observed for both autograft and rhBMP-2. The numerical results from the finite element model verify that significantly different flow pattern emerges as the boundary conditions are altered. At four weeks there was an evident correlation between trabecular orientation and flow pattern using rhBMP-2.

CONCLUSION

This study reveals large differences in microstructure in the early osteogenesis and explains important mechanisms of early bone formation using rhBMP-2, COLLOSS E or autograft treatment. These differences might explain some of the unfortunate events reported such as edema, swelling, and excessive bone formation using different bone graft substitutes in spinal fusion procedures.

Human cancellous bone from T12-L1 vertebrae has unique microstructural and trabecular shear stress properties

Increase of trabecular stress variability with loss of bone mass has been implicated as a mechanism for increased cancellous bone fragility with age and disease. In the current study, a previous observation that trabecular shear stress estimates vary along the human spine such that the cancellous tissue from the thoracic 12 (T12)-lumbar 1 (L1) junction experiences the highest trabecular stresses for a given load was tested as a formal hypothesis using multiple human spines. Thoracic 4, T5, T7, T9, T10, T12, L1, L2, L4 and L5 vertebrae from 10 human cadaver spines were examined. One specimen in the central anterior region was cored in the supero-inferior (SI) direction and another in the postero-lateral region was cored in the transverse (TR) direction from each vertebra. Micro-CT-based large-scale finite element models were constructed for each specimen and compression in the long axis of the cylindrical specimens was simulated. Cancellous bone modulus and the mean, the standard deviation, variability and amplification of trabecular von Mises stresses were computed. Bone volume fraction, trabecular number, trabecular thickness, trabecular separation, connectivity density and degree of anisotropy were calculated using 3D stereology. The results were analyzed using a mixed model in which spine level was modeled using a quadratic polynomial. The maximum of trabecular shear stress amplification and minimum of bone volume fraction were found in the cancellous tissue from the T12-L1 location when results from the samples of the same vertebra were averaged. When groups were separated, microstructure and trabecular stresses varied with spine level, extrema being at the T12-L1 levels, for the TR specimens only. SI/TR ratio of measured parameters also had quadratic relationships with spine level, the extrema being located at T12-L1 levels for most parameters. For microstructural parameters, these ratios approached to a value of one at the T12-L1 level, suggesting that T12-L1 vertebrae have more uniform cancellous tissue properties than other levels. The mean intercept length in the secondary principal direction of trabecular orientation could account for the variation of all mechanical parameters with spine level. Our results support that cancellous tissue from T12-L1 levels is unique and may explain, in part, the higher incidence of vertebral fractures at these levels.

Fatigue damage in cancellous bone: an experimental approach from continuum to micro scale

Repeated loadings may cause fatigue fractures in bony structures. Even if these failure types are known, data for trabecular bone exposed to cyclic loading are still insufficient as the majority of fatigue analyses on bone concentrate on cortical structures. Despite its highly anisotropic and inhomogeneous structure, trabecular bone is treated with continuum approaches in fatigue analyses. The underlying deformation and damage mechanism within trabecular specimens are not yet sufficiently investigated. In the present study different types of trabecular bone were loaded in monotonic and cyclic compression. In addition to the measurement of integral specimen deformations, optical deformation analysis was employed in order to obtain strain distributions at different scale levels, from the specimens' surface to the trabeculae level. These measurements allowed for the possibility of linking the macroscopic and microscopic mechanical behaviour of cancellous bone. Deformations were found to be highly inhomogeneous across the specimen. Furthermore strains were found to already localise at very low load levels and after few load cycles. Microcracks in individual trabeculae were induced in the very early stage of cyclic testing. The results provide evidence of the capability of the method to supply essential data on the failure behaviour of individual trabeculae in future studies.

Trabecular shear stress amplification and variability in human vertebral cancellous bone: relationship with age, gender, spine level and trabecular architecture

Trabecular shear stress magnitude and variability have been implicated in damage formation and reduced bone strength associated with bone loss for human vertebral bone. This study addresses the issue of whether these parameters change with age, gender or anatomical location, and if so whether this is independent of bone mass. Additionally, 3D-stereology-based architectural parameters were examined in order to establish the relationship between stress distribution parameters and trabecular architecture. Eighty cancellous bone specimens were cored from the anterior region of thoracic 12 and donor-matched lumbar 1 vertebrae from a randomly selected population of 40 cadavers. The specimens were scanned at 21-microm voxel size using microcomputed tomography (microCT) and reconstructed at 50microm. Bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), bone surface-to-volume ratio (BS/BV), degree of anisotropy (MIL1/MIL3), and connectivity density (-#Euler/Vol) were calculated directly from micro-CT images. Large-scale finite element models were constructed and superoinferior compressive loading was simulated. Apparent cancellous modulus (EFEM) was calculated. The average trabecular von Mises stress generated per uniaxial apparent stress (sigma (-)VM / sigmaapp) and coefficient of variation of trabecular von Mises stresses (COV) were calculated as measures of the magnitude and variability of shear stresses in the trabeculae. Mixed-models and regression were used for analysis. sigma(-)VM / sigmaapp and COV were not different between genders and vertebrae. Both sigma(-)VM / sigmaapp and COV increased with age accompanied by a decrease in BV/TV. Strong relationship of sigma(-)VM / sigmaapp with BV/TV was found whereas COV was strongly related to EFEM/(BV/TV). The results from T12 and L1 were not different and highly correlated with each other. The relationship of sigma(-)VM / sigmaapp with COV was observed to be different between males and females. This difference could not be explained by architectural parameters considered in this study. Our results support the relevance of trabecular shear stress amplification and variability in age-related vertebral bone fragility. The relationships found are expected to help understand the micro-mechanisms by which cancellous bone mass and mechanical properties are modulated through a collection of local stress parameters.

The Effect of Regional Variations of the Trabecular Bone Properties on the Compressive Strength of Human Vertebral Bodies

Cancellous centrum is a major component of the vertebral body and significantly contributes to its structural strength and fracture risk. We hypothesized that the variability of cancellous bone properties in the centrum is associated with vertebral strength. Microcomputed tomography (micro-CT)-based gray level density (GLD), bone volume fraction (BV/TV), and finite element modulus (E) were examined for different regions of the trabecular centrum and correlated with vertebral body strength determined experimentally. Two sets of images in the cancellous centrum were digitally prepared from micro-CT images of eight human vertebral bodies (T10-L5). One set included a cubic volume (1 per vertebral centrum, n = 8) in which the largest amount of cancellous material from the centrum was included but all the shell materials were excluded. The other set included cylindrical volumes (6 per vertebral centrum, n = 48) from the anterior (4 regions: front, center, left, and right of the midline of vertebra) and the posterior (2 regions: left and right) regions of the centrum. Significant positive correlations of vertebral strength with GLD (r (2) = 0.57, p = 0.03) and E (r (2) = 0.63, p = 0.02) of the whole centrum and with GLD (r (2) = 0.65, p = 0.02), BV/TV (r (2) = 0.72, p = 0.01) and E (r (2) = 0.85, p = 0.001) of the central region of the vertebral centrum were found. Vertebral strength decreased with increasing coefficient of variation of GLD, BV/TV, and E calculated from subregions of the vertebral centrum. The values of GLD, BV/TV, and E in centrum were significantly smaller for the anterior region than for the posterior region. Overall, these findings supported the significant role of regional variability of centrum properties in determining the whole vertebral strength.

High-frequency oscillatory motions enhance the simulated mechanical properties of non-weight bearing trabecular bone

Extremely low-level oscillatory accelerations, applied without constraint, can increase bone formation. Here, we tested the hypothesis that high-frequency oscillations, applied in the absence of functional weight bearing, can be sensed by trabecular bone to produce a structure that is more efficient in sustaining applied loads. The left leg of anesthetized adult female mice (n=18) was subjected to high-frequency oscillations at 45 Hz, 0.6g for 20 min/day, 5 days/week for 3 weeks, while the contralateral leg served as an internal control. To remove the potential interference of the habitual strain environment with the imposed physical signal, the hindlimbs of these mice were chronically unloaded. In vivo microCT scans of the proximal metaphyseal region of the tibia were transformed into finite element meshes to evaluate trabecular and cortical mechanical properties. Simulated longitudinal compression tests showed that the short applications of high-frequency oscillations were sensed primarily by trabecular bone. At the end of the experimental period, apparent trabecular stiffness of the oscillated bones was 38% (p<0.001) greater than that of non-weight bearing controls. Simulated uniaxial loads applied to trabecular bone induced 21%, 52%, and 131% greater (p<0.05) median, peak compressive, and peak tensile longitudinal stresses in control than in stimulated bones. Non-weight bearing control bones were also characterized by greater transverse normal and shear stresses (77% and 54%, respectively, p<0.001) as well as 35% greater (p=0.03) longitudinal shear stresses. Compared to normal age-matched controls (n=18), oscillations were able to attenuate, but not fully prevent, the decline in trabecular mechanical properties associated with the removal of weight bearing. These data indicate not only that bone cells can sense low-level, high-frequency oscillatory accelerations, but also that they can orchestrate a structural response that produces a stiffer trabecular structure that may be less prone to fracture.

Effect of Microcomputed Tomography Voxel Size on the Finite Element Model Accuracy for Human Cancellous Bone

The level of structural detail that can be acquired and incorporated in a finite element (FE) analysis might greatly influence the results of microcomputed tomography (μCT)-based FE simulations, especially when relatively large bones, such as whole vertebrae, are of concern. We evaluated the effect of scanning and reconstruction voxel size on the μCT-based FE analyses of human cancellous tissue samples for fixed- and free-end boundary conditions using different combinations of scan/reconstruction voxel size. We found that the bone volume fraction (BV/TV) did not differ considerably between images scanned at 21 and 50 μm and reconstructed at 21, 50, or 110 μm (−0.5% to 7.8% change from the 21/21 μm case). For the images scanned and reconstructed at 110 μm, however, there was a large increase in BV/TV compared to the 21/21 μm case (58.7%). Fixed-end boundary conditions resulted in 1.8% [coefficient of variation (COV)] to 14.6% (E) difference from the free-end case. Dependence of model output parameters on scanning and reconstruction voxel size was similar between free- and fixed-end simulations. Up to 26%, 30%, 17.8%, and 32.3% difference in modulus (E), and average (VMExp), standard deviation (VMSD) and coefficient of variation (COV) of von Mises stresses, respectively, was observed between the 21/21 μm case and other scan/reconstruction combinations within the same (free or fixed) simulation group. Observed differences were largely attributable to scanning resolution, although reconstruction resolution also contributed significantly at the largest voxel sizes. All 21/21 μm results (taken as the gold standard) could be predicted from the 21/50 radj2=0.91-0.99;p<0.001, 21/110 radj2=0.58-0.99;p<0.02 and 50/50 results radj2=0.61-0.97;p<0.02. While BV/TV, VMSD, and VMExp/σz from the 21/21 could be predicted by those from the 50/110 radj2=0.63-0.93;p<0.02 and 110/110 radj2=0.41-0.77;p<0.05 simulations as well, prediction of E, VMExp, and COV became marginally significant 0.04<p<0.13 at 50/110 and nonsignificant at 110/110 0.21<p<0.70. In conclusion, calculation of cancellous bone modulus, mean trabecular stress, and other parameters are subject to large errors at 110/110 μm voxel size. However, enough microstructural details for studying bone volume fraction, trabecular shear stress scatter, and trabecular shear stress amplification VMExp/σz can be resolved using a 21/110 μm, 50/110 μm, and 110/110 μm voxels for both free- and fixed-end constraints.