Experimental Validation of Planar Strain Theory for Predicting Bone Cross-sectional

Longitudinal and Shear Strains

Kari Jenson (Dr. Russell Main, advisor)

Abstract:  Understanding the diversity of in vivo skeletal loading regimes found in vertebrate long bones during locomotion has been significantly enhanced by the application of planar strain theory (PST), which predicts the distribution of longitudinal strains normal to the cross section of a static element and the location of the neutral axis of bending in the element. Although this technique is used frequently in in vivo skeletal biomechanics studies, the accuracy of this technique when applied to skeletal biomechanics and the possible effect of gauge distribution around the diaphysis have not been evaluated experimentally to our knowledge.  We evaluated the use of planar strain theory by attaching four strain gauges around the cross sections of adult Emu tibiotarsi, loading the instrumented bones in ex vivo four point bending, applying PST to different combinations of three gauges, and ultimately comparing experimentally measured to predicted (using PST) strain values at the four gauge locations. For all bending directions and gauge locations, the correlation between experimentally measured and predicted strain values was linearly related (R² ≥ 0.94) and a slope of ‘1’ fell within each 95% confidence interval, suggesting that planar strain theory does do a good job of predicting longitudinal strains around a bone cross section for a bone in pure bending. We also evaluated the use of PST-derived longitudinal strain ratios for extrapolating shear strains to other positions on the bone cross-section. To this end, guineafowl tibiotarsi were instrumented with rosette strain gauges, and in vivo longitudinal and shear strains were measured at two locations around the midshaft cortices at a specific point in the stride during treadmill running.  Scalar ‘multipliers’ relating the longitudinal strains between medial and posterior gauge sites were then used to extrapolate medial-site shear strain to posterior-site shear strain. Predicted posterior-site shear strains were then compared to experimentally measured values.  Measured and predicted shear strains at the posterior gauge site did not correlate linearly (slope: 0.23, R² = 0.10), suggesting that planar strain theory cannot reliably predict shear strains. 

Gut Microbial Metabolites Enhance T Cell Immunity and Homeostasis

Leon Friesen (Dr. Chang Kim, advisor)

Abstract: The intestinal tract provides essential functions of nutrient and fluid absorption and waste excretion, while maintaining a barrier between the intestinal microbiota and the rest of the body.  The intestinal immune system monitors and regulates this barrier, responding to pathogens to prevent infection while maintaining tolerance to harmless bacteria.  Dietary components affect the overall diversity and functions of the gut microbiome.  One example of this is soluble dietary fibers, which are metabolized by certain bacterial species through anaerobic fermentation into bioactive molecules called short-chain fatty acids (SCFAs).  SCFAs include the molecules acetate, propionate, and butyrate; these molecules can interact with SCFA receptors (e.g. GPR41 and GPR43) on the surface of cells or inhibit intracellular histone deacetylases (HDACs) causing changes in gene transcription.  A key cell population of the intestinal immune system is CD4⁺ T helper cells, which recognize specific antigens from antigen-presenting cells (APCs) and produce cytokines and chemokines to stimulate an appropriate immune response.  T helper cells are subdivided into Th1, Th2, Th17, and Treg populations, which all differentiate from naïve T helper cells based on the cytokine milieu and activation signals present in the tissue.  Recently published data has demonstrated that SCFAs enhances Th1 and Th17 differentiation, but they also enhance the expression of the anti-inflammatory cytokine IL-10 from these cells, limiting their inflammatory nature.  To determine if additional anti-inflammatory cytokines are similarly regulated by SCFAs, in vitro cultures of T helper cells were tested for RNA expression of IL-27 or IL-35.  RNA expression of these cytokines from ex vivo isolated CD4⁺ cells from mice fed acetate water or fiber diets containing low, medium, or high amounts of soluble fiber was similarly performed.  To summarize, IL-27 and/or IL-35 may be induced by SCFA in Th2 cells, indicating another link between dietary fiber and regulation of intestinal immune responses.

***Bring your lunch to seminar – BMEGSA will provide snacks and drinks***

*Seminar available via WebEx meeting in SL220A at IUPUI. Contact Sandy at smmay@purdue.edu for instructions to join the WebEx meeting.