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Aiyangar, A. K., Vivanco, J., Au, A. G., Anderson, P. A., Smith, E. L., & Ploeg, H. L. (2014). Dependence of Anisotropy of Human Lumbar Vertebral Trabecular Bone on Quantitative Computed Tomography-Based Apparent Density. J. Biomech. Eng.-Trans. ASME, 136(9), 10 pp.
Abstract: Most studies investigating human lumbar vertebral trabecular bone (HVTB) mechanical property-density relationships have presented results for the superior-inferior (SI), or “ on-axis” direction. Equivalent, directly measured data from mechanical testing in the transverse (TR) direction are sparse and quantitative computed tomography (QCT) density-dependent variations in the anisotropy ratio of HVTB have not been adequately studied. The current study aimed to investigate the dependence of HVTB mechanical anisotropy ratio on QCT density by quantifying the empirical relationships between QCT-based apparent density of HVTB and its apparent compressive mechanical propertieselastic modulus (E-app), yield strength (sigma(y)), and yield strain (epsilon(y))-in the SI and TR directions for future clinical QCT-based continuum finite element modeling of HVTB. A total of 51 cylindrical cores (33 axial and 18 transverse) were extracted from four L1 human lumbar cadaveric vertebrae. Intact vertebrae were scanned in a clinical resolution computed tomography (CT) scanner prior to specimen extraction to obtain QCT density, rho(CT). Additionally, physically measured apparent density, computed as ash weight over wet, bulk volume, rho(app), showed significant correlation with rho(CT) [rho(CT) = 1.0568 x rho(app), r = 0.86]. Specimens were compression tested at room temperature using the Zetos bone loading and bioreactor system. Apparent elastic modulus (E-app) and yield strength (sigma(y)) were linearly related to the rho(CT) in the axial direction [E-SI = 1493.8 x (rho(CT)), r = 0.77, p < 0.01; sigma(Y,SI) = 6.9 x (rho(CT)) = 0.13, r = 0.76, p < 0.01] while a power-law relation provided the best fit in the transverse direction [E-TR 3349.1 x (rho(CT))(1.94), r = 0.89, p < 0.01; sigma(Y,TR) 18.81 x (rho(CT)) 1.83, r = 0.83, p < 0.01]. No significant correlation was found between epsilon(y) and rho(CT) in either direction. E-app and sigma(y) in the axial direction were larger compared to the transverse direction by a factor of 3.2 and 2.3, respectively, on average. Furthermore, the degree of anisotropy decreased with increasing density. Comparatively, epsilon(y) exhibited only a mild, but statistically significant anisotropy: transverse strains were larger than those in the axial direction by 30%, on average. Ability to map apparent mechanical properties in the transverse direction, in addition to the axial direction, from CT-based densitometric measures allows incorporation of transverse properties in finite element models based on clinical CT data, partially offsetting the inability of continuum models to accurately represent trabecular architectural variations.
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Meyer, L. A., Johnson, M. G., Cullen, D. M., Vivanco, J. F., Blank, R. D., Ploeg, H. L., et al. (2016). Combined exposure to big endothelin-1 and mechanical loading in bovine sternal cores promotes osteogenesis. Bone, 85, 115–122.
Abstract: Increased bone formation resulting from mechanical loading is well documented; however, the interactions of the mechanotransduction pathways are less well understood. Endothelin-1, a ubiquitous autocrine/paracrine signaling molecule promotes osteogenesis in metastatic disease. In the present study, it was hypothesized that exposure to big endothelin-1 (big ET1) and/or mechanical loading would promote osteogenesis in ex vivo trabecular bone cores. In a 2 x 2 factorial trial of daily mechanical loading (-2000 μepsilon,120 cycles daily, “jump” waveform) and big ET1 (25 ng/mL), 48 bovine sternal trabecular bone cores were maintained in bioreactor chambers for 23 days. The bone cores' response to the treatment stimuli was assessed with percent change in core apparent elastic modulus (Delta E-app), static and dynamic histomorphometry, and prostaglandin E2 (PGE2) secretion. Two-way ANOVA with a post hoc Fisher's LSD test found no significant treatment effects on Delta E-app (p = 0.25 and 0.51 for load and big ET1, respectively). The Delta E-app in the “no load + big ET1” (CE, 13 +/- 12.2%, p = 0.56), “load + no big ET1” (LC, 17 +/- 3.9%, p = 0.14) and “load + big ETI” (LE, 19 +/- 4.2%, p = 0.13) treatment groups were not statistically different than the control group (CC, 3.3% +/- 8.6%). Mineralizing surface (MS/BS), mineral apposition (MAR) and bone formation rates (BFR/BS) were significantly greater in LE than CC (p = 0.037, 0.0040 and 0.019, respectively). While the histological bone formation markers in LC trended to be greater than CC (p = 0.055, 0.11 and 0.074, respectively) there was no difference between CE and CC (p = 0.61, 0.50 and 0.72, respectively). Cores in LE and LC had more than 50% greater MS/BS (p = 0.037, p = 0.055 respectively) and MAR (p = 0.0040, p = 0.11 respectively) than CC. The BFR/BS was more than two times greater in LE (p = 0.019) and LC (p = 0.074) than CC. The PGE2 levels were elevated at 8 days post-osteotomy in all groups and the treatment groups remained elevated compared to the CC group on days 15,19 and 23. The data suggest that combined exposure to big ET1 and mechanical loading results in increased osteogenesis as measured in biomechanical, histomorphometric and biochemical responses. (C) 2016 Elsevier Inc. All rights reserved.
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Vivanco, J. F., Burgers, T. A., Garcia-Rodriguez, S., Crookshank, M., Kunz, M., MacIntyre, N. J., et al. (2014). Estimating the density of femoral head trabecular bone from hip fracture patients using computed tomography scan data. Proc. Inst. Mech. Eng. Part H-J. Eng. Med., 228(6), 616–626.
Abstract: The purpose of this study was to compare computed tomography density (rho(CT)) obtained using typical clinical computed tomography scan parameters to ash density (rho(ash)), for the prediction of densities of femoral head trabecular bone from hip fracture patients. An experimental study was conducted to investigate the relationships between rho(ash) and rho(CT) and between each of these densities and rho(bulk) and rho(dry). Seven human femoral heads from hip fracture patients were computed tomography-scanned ex vivo, and 76 cylindrical trabecular bone specimens were collected. Computed tomography density was computed from computed tomography images by using a calibration Hounsfield units-based equation, whereas rho(bulk), rho(dry) and rho(ash) were determined experimentally. A large variation was found in the mean Hounsfield units of the bone cores (HUcore) with a constant bias from rho(CT) to rho(ash) of 42.5 mg/cm(3). Computed tomography and ash densities were linearly correlated (R-2 = 0.55, p < 0.001). It was demonstrated that rho(ash) provided a good estimate of rho(bulk) (R-2 = 0.78, p < 0.001) and is a strong predictor of rho(dry) (R-2 = 0.99, p < 0.001). In addition, the rho(CT) was linearly related to rho(bulk) (R-2 = 0.43, p < 0.001) and rho(dry) (R-2 = 0.56, p < 0.001). In conclusion, mineral density was an appropriate predictor of rho(bulk) and rho(dry), and rho(CT) was not a surrogate for rho(ash). There were linear relationships between rho(CT) and physical densities; however, following the experimental protocols of this study to determine rho(CT), considerable scatter was present in the rho(CT) relationships.
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