Analyses of the distributions of stress and strain within individual bone trabeculae have not yet been reported. In this study, four trabeculae were imaged and finite elements models were generated in an attempt to quantify the variability of stress/strain in real trabeculae. In three of these trabeculae, cavities were identified with depths comparable to values reported for resorption lacunae ( approximately 50 microm)-although we cannot be certain, it is most probable that they are indeed resorption lacunae. A tensile load was applied to each trabeculum to simulate physiological loading and to ensure that bending was minimized. The force carried by each trabecula was calculated from this value using the average cross sectional area of each trabecula. The analyses predict that very high stresses (>100 MPa) existed within bone trabecular tissue. Stress and strain distributions were highly heterogeneous in all cases, more so in trabeculae with the presumptive resorption lacunae where at least 30% of the tissue had a strain greater than 4000 micoepsilon in all cases. Stresses were elevated at the pit of the lacunae, and peak stress concentrations were located in the longitudinal direction ahead of the lacunae. Given these high strains, we suggest that microdamage is inevitable around resorption lacunae in trabecular bone, and may cause the bone multicellular unit to proceed to resorb a packet of bone in the trabeculum rather than just resorb whatever localized area was initially targeted.
"Any movement of the root inside the lamina dura of the alveolar bone due to an orthodontic loading regime–or due to physiological masticatory loading for that matter–would generate high local stresses and strains in these spiculae. McNamara et al. (2006) have demonstrated that irregular bony surfaces, like for example resorption lacunae, give rise to local stress amplification. According to Frost's Mechanostat theory (1987), bone needs at least a lower threshold of ca. "
"such micro-scale features as intra-specimen variation in mineral density and tissue material properties, or fine details of resorption spaces or microcracks. Although inclusion of such details would likely influence absolute values of any strength estimates (Bourne and van der Meulen, 2004; Easley et al., 2010; Jaasma et al., 2001; McNamara et al., 2006; Vaughan et al., 2012; Verhulp et al., 2008a), it is not clear that their exclusion would appreciably influence the direct comparison between the two behaviors. On the one hand, it may be that such micro-scale effects might similarly influence the brittle and ductile failure mechanisms if failure of individual trabeculae is dominated more by overall load sharing across the trabecular network than by local material behavior; on the other hand, it is possible that fine geometric details of resorption spaces or microcracks might have a more deleterious ''stress riser'' effect in more brittle bone tissue. "
[Show abstract][Hide abstract] ABSTRACT: The role of tissue-level post-yield behavior on the apparent-level strength of trabecular bone is a potentially important aspect of bone quality. To gain insight into this issue, we compared the apparent-level strength of trabecular bone for the hypothetical cases of fully brittle versus fully ductile failure behavior of the trabecular tissue. Twenty human cadaver trabecular bone specimens (5mm cube; BV/TV=6-36%) were scanned with micro-CT to create 3D finite element models (22-micron element size). For each model, apparent-level strength was computed assuming either fully brittle (fracture with no tissue ductility) or fully ductile (yield with no tissue fracture) tissue-level behaviors. We found that the apparent-level ultimate strength for the brittle behavior was only about half the value of the apparent-level 0.2%-offset yield strength for the ductile behavior, and the ratio of these brittle to ductile strengths was almost constant (mean±SD=0.56±0.02; n=20; R(2)=0.99 between the two measures). As a result of this small variation, although the ratio of brittle to ductile strengths was positively correlated with the bone volume fraction (R(2)=0.44, p=0.01) and structure model index (SMI, R(2)=0.58, p<0.01), these effects were small. Mechanistically, the fully ductile behavior resulted in a much higher apparent-level strength because in this case about 16-fold more tissue was required to fail than for the fully brittle behavior; also, there was more tensile- than compressive-mode of failure at the tissue level for the fully brittle behavior. We conclude that, in theory, the apparent-level strength behavior of human trabecular bone can vary appreciably depending on whether the tissue fails in a fully ductile versus fully brittle manner, and this effect is largely constant despite appreciable variations in bone volume fraction and microarchitecture.
Journal of Biomechanics 03/2013; 46(7). DOI:10.1016/j.jbiomech.2013.02.011 · 2.75 Impact Factor
"To reduce computational expense, only regions corresponding to a prescribed overlap ± camera positioning repeatability are used to achieve the tiled mosaic. Vertical alignment of each crosssection is similarly subject to stage positioning inaccuracy and is addressed using a set of fiduciary markers (in this case perpendicular holes drilled into the embedding media near the specimen) (McNamara et al., 2006). "
[Show abstract][Hide abstract] ABSTRACT: Serial block face imaging is a microscopy technique in which the top of a specimen is cut or ground away and a mosaic of images is collected of the newly revealed cross-section. Images collected from each slice are then digitally stacked to achieve 3D images. The development of fully automated image acquisition devices has made serial block face imaging more attractive by greatly reducing labour requirements. The technique is particularly attractive for studies of biological activity within cancellous bone as it has the capability of achieving direct, automated measures of biological and morphological traits and their associations with one another. When used with fluorescence microscopy, serial block face imaging has the potential to achieve 3D images of tissue as well as fluorescent markers of biological activity. Epifluorescence-based serial block face imaging presents a number of unique challenges for visualizing bone specimens due to noise generated by sub-surface signal and local variations in tissue autofluorescence. Here we present techniques for processing serial block face images of trabecular bone using a combination of non-uniform illumination correction, precise tiling of the mosaic in each cross-section, cross-section alignment for vertical stacking, removal of sub-surface signal and segmentation. The resulting techniques allow examination of bone surface texture that will enable 3D quantitative measures of biological processes in cancellous bone biopsies.
Journal of Microscopy 10/2009; 236(1):52-9. DOI:10.1111/j.1365-2818.2009.03204.x · 2.33 Impact Factor
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