[Show abstract][Hide abstract] ABSTRACT: The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals is of limited value. However, there is growing evidence that information on trabecular microarchitecture can improve the assessment of fracture risk. One current strategy is to exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular bone structures obtained from non-invasive imaging modalities for the prediction of apparent mechanical properties. However, there is a lack of FE damage models, based on solid experimental facts, which are needed to validate such approaches and to provide criteria marking elastic-plastic deformation transitions as well as microdamage initiation and accumulation. In this communication, we present a strategy that could elegantly lead to future damage models for FEA: direct measurements of local strains involved in microdamage initiation and plastic deformation in single trabeculae. We use digital image correlation to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values. Our results show that the whitening zones, i.e. damage formation, in the presented loading case of a three-point bending test correlate best with areas of elevated tensile strains oriented parallel to the long axis of the samples. The average local strains along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagating over a large range of tensile strains.
Journal of the mechanical behavior of biomedical materials. 05/2011; 4(4):523-34.
[Show abstract][Hide abstract] ABSTRACT: High doses of sodium fluoride in bones lead to severe softening, by weakening interfacial properties between the inorg. minerals and the org. components, while leaving mineralization unchanged. This leads to redn. of microdamage and assocd. stress-whitening pointing to a change in failure mode. Accordingly, elastic modulus, failure stress, and indentation-distance increase are decreased, whereas failure strain is increased. [on SciFinder (R)]
[Show abstract][Hide abstract] ABSTRACT: The bone diagnostic instrument (BDI) is being developed with the long-term goal of providing a way for researchers and clinicians to measure bone material properties of human bone in vivo. Such measurements could contribute to the overall assessment of bone fragility in the future. Here, we describe an improved BDI, the Osteoprobe IItrade mark. In the Osteoprobe IItrade mark, the probe assembly, which is designed to penetrate soft tissue, consists of a reference probe (a 22 gauge hypodermic needle) and a test probe (a small diameter, sharpened rod) which slides through the inside of the reference probe. The probe assembly is inserted through the skin to rest on the bone. The distance that the test probe is indented into the bone can be measured relative to the position of the reference probe. At this stage of development, the indentation distance increase (IDI) with repeated cycling to a fixed force appears to best distinguish bone that is more easily fractured from bone that is less easily fractured. Specifically, in three model systems, in which previous mechanical testing and/or tests reported here found degraded mechanical properties such as toughness and postyield strain, the BDI found increased IDI. However, it must be emphasized that, at this time, neither the IDI nor any other mechanical measurement by any technique has been shown clinically to correlate with fracture risk. Further, we do not yet understand the mechanism responsible for determining IDI beyond noting that it is a measure of the continuing damage that results from repeated loading. As such, it is more a measure of plasticity than elasticity in the bone.
Review of Scientific Instruments 07/2008; 79(6):064303. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Practical protocols are presented to reproducibly prepare micrometer-sized Au(111) substrates. Au(111) terraces of micrometer dimensions and atomic smoothness were crystallized by flame-annealing vacuum-deposited gold films on glass and on millimetric amorphous gold shots. Gold films and shots that were slowly cooled in a moderately applied stream of nitrogen gas exhibited large and stable crystal surfaces with Au(111) morphologies. Similarly, flame-annealed gold samples cooled with another protocol--in much rougher streams of nitrogen gas--produced morphologically unstable and highly mobile Au(111) layers. Within the first hour after preparation, however, rapid microscale restructuring in the layers produced complex morphologies of hexagonal channel networks and islands that were predominantly triangular. These channeled gold layers fused slowly in the following hours, with velocities of 0.01-0.2 A/s, as quantified by digital image correlation (DIC). Atomically smooth, stable, and predominantly triangular Au(111) terraces on the scale of micrometers were observed approximately 24 h after the sample preparations.
[Show abstract][Hide abstract] ABSTRACT: Mechanical testing of trabecular bone is mainly motivated by the huge impact of osteoporosis in post-menopausal women and the aged in society in terms of social and health care costs. Trabecular bone loss and impairment of its mechanical properties reduce bone strength and increase fracture risk, especially in vertebrae. It is generally accepted that in addition to bone mineral density, microarchitecture and material properties of bone also play important roles for bone strength and fracture risk. In order to overcome the limitations of standard mechanical tests delivering merely integral information about complicated samples, experiments were designed for step-wise mechanical testing with concurrent imaging of trabecular and cortical bone. In this communication we present an approach for real-time imaging of trabecular bone during compression using high-speed photography and investigate the hypothesis whether the whitening of deformed trabeculae is due to microdamage. Experiments on human trabecular bone samples from a healthy male donor revealed that failure of such samples is highly localized in fracture bands. Moreover, strongly deformed trabeculae were seen to whiten, an effect similar to stress whitening in polymers. Scanning Electron Microscopy of the same regions of interest revealed that whitened trabeculae were strongly damaged by microscopic cracks and mostly failed in delamination. Higher resolution images uncovered mineralized collagen fibrils spanning the cracks. The whitening partially faded after unloading of the samples, presumably due to partial crack closure. Overall, high-speed photography enables microdamage detection in real-time during a mechanical test and provides a correlation to recorded stress strain curves.
[Show abstract][Hide abstract] ABSTRACT: Research in bone diseases like osteoporosis is motivated by its immense social impact and health care costs. While there are numerous studies about the influence of bone mineral density and microarchitectural properties on the mechanical properties of trabecular bone, little is known about the influence of bone matrix material properties. In this communication, we present novel ways for combining mechanical testing of single trabeculae with imaging on both the micro- and nanoscale to further investigate these material properties. Our results indicate microdamage in an ellipsoid zone on the tension side of the trabeculae tested in three-point bending. We estimated the highest tensile strains in this region to be about 3.5%. Quantitatively global whitening versus distance curves correlate well with retrieved force-distance data. Scanning electron microscopy investigations of the microdamaged and optically whitened zones suggest that damage formation happens primarily in the bone and not on the surface. In addition to whitening/microdamage assessment on the microscale, we used atomic force microscopy together with a custom made three-point bending device and a region based digital image correlation tool to obtain quantitative local surface displacements on the bending side of single trabeculae. We found that bone deformation is heterogeneous with different surface domains shearing off each other. The two novel methods presented make it possible to investigate the dynamics of plasticity and failure of bone both on the micro- and on the nanoscale.