A new approach to determine the accuracy of morphology–elasticity relationships in continuum FE analyses of human proximal femur

AbstractContinuum finite element (FE) models of bones are commonly generated based on CT scans. Element material properties in such models are usually derived from bone density values using some empirical relationships. However, many different empirical relationships have been proposed. Most of these will provide isotropic material properties but relationships that can provide a full orthotropic elastic stiffness tensor have been proposed as well. Presently it is not clear which of these relationships best describes the material behavior of bone in continuum models, nor is it clear to what extent anisotropic models can improve upon isotropic models. The best way to determine the accuracy of such relationships for continuum analyses would be by quantifying the accuracy of the calculated stress/strain distribution, but this requires an accurate reference distribution that does not depend on such empirical relationships. In the present study, we propose a novel approach to generate such a reference stress distribution. With this approach, stress results obtained from a micro-FE model of a whole bone, that can represent the bone trabecular architecture in detail, are homogenized and the homogenized stresses are then used as a reference for stress results obtained from continuum models. The goal of the present study was to demonstrate this new approach and to provide examples of comparing continuum models with anisotropic versus isotropic material properties.Continuum models that implemented isotropic and orthotropic material definitions were generated for two proximal femurs for which micro-FE results were available as well, one representing a healthy and the other an osteoporotic femur. It was found that the continuum FE stress distributions calculated for the healthy femur compared well to the homogenized results of the micro-FE although slightly better for the orthotropic model (r=0.83) than for the isotropic model (r=0.79). For the osteoporotic bone also, the orthotropic model did better (r=0.83) than the isotropic model (r=0.77). We propose that this approach will enable a more relevant and accurate validation of different material models than experimental methods used so far

An efficient strategy to describe the propagation of variation through multi-stage metal forming processes

In this work an efficient strategy to describe the propagation of uncertainty throughout a production process is proposed and validated. A two-stage metal forming process is used to demonstrate the proposed strategy. A metamodel of the first stage is built using a singular value decomposition combined with multiquadric radial basis functions. The strategy is validated by comparing the propagated results from the first stage, of both the metamodel and a finite element model, to a second stage finite element model for a new set of validation points. Using the proposed strategy, it will be possible to efficiently optimize multi-stage processes under uncertainty.</p

The Content of Native American Cultural Stereotypes in Comparison to Other Racial Groups

abstract: Despite a large body of research on stereotypes, there have been relatively few empirical investigations of the content of stereotypes about Native Americans. The primary goal of this research was to systematically explore the content of cultural stereotypes about Native Americans and how stereotypes about Native Americans differ in comparison to stereotypes about Asian Americans and African Americans. Building on a classic paradigm (Katz and Braly, 1933), participants were asked to identify from a list of 145 adjectives those words associated with cultural stereotypes of Native Americans and words associated with stereotypes of Asian Americans (Study 1) or African Americans (Study 2). The adjectives associated with stereotypes about Native Americans were significantly less favorable than the adjectives associated with stereotypes about Asian Americans, but were significantly more favorable than the adjectives associated with stereotypes about African Americans. Stereotypes about Native Americans, Asian Americans and African Americans were also compared along the dimensions of the stereotype content model (SCM; Fiske, et al., 2002), which proposes that stereotypes about social groups are based on the core dimensions of perceived competence, warmth, status, and competitiveness. Native Americans were rated as less competent, less of a source of competition, and lower in social status than Asian Americans, and less competent and lower in social status than African Americans. No significant differences were found in perceived warmth across the studies. Combined, these findings contribute to a better understanding of stereotypes about Native Americans and how they may differ from stereotypes about other racial groups.Dissertation/ThesisM.S. Psychology 201

Mapping anisotropy improves QCT-based finite element estimation of hip strength in pooled stance and side-fall load configurations

Hip fractures are one of the most severe consequences of osteoporosis. Compared to the clinical standard of DXA-based aBMD at the femoral neck, QCT-based FEA delivers a better surrogate of femoral strength and gains acceptance for the calculation of hip fracture risk when a CT reconstruction is available. Isotropic, homogenised voxel-based, finite element (hvFE) models are widely used to estimate femoral strength in cross-sectional and longitudinal clinical studies. However, fabric anisotropy is a classical feature of the architecture of the proximal femur and the second determinant of the homogenised mechanical properties of trabecular bone. Due to the limited resolution, fabric anisotropy cannot be derived from clinical CT reconstructions. Alternatively, fabric anisotropy can be extracted from HR-pQCT images of cadaveric femora. In this study, fabric anisotropy from HR-pQCT images was mapped onto QCT-based hvFE models of 71 human proximal femora for which both HR-pQCT and QCT images were available. Stiffness and ultimate load computed from anisotropic hvFE models were compared with previous biomechanical tests in both stance and side-fall configurations. The influence of using the femur-specific versus a mean fabric distribution on the hvFE predictions was assessed. Femur-specific and mean fabric enhance the prediction of experimental ultimate force for the pooled, i.e. stance and side-fall, (isotropic: r2=0.81, femur-specific fabric: r2=0.88, mean fabric: r2=0.86,p<0.001) but not for the individual configurations. Fabric anisotropy significantly improves bone strength prediction for the pooled configurations, and mapped fabric provides a comparable prediction to true fabric. The mapping of fabric anisotropy is therefore expected to help generate more accurate QCT-based hvFE models of the proximal femur for personalised or multiple load configurations