Cortical bone continuum damage mechanics constitutive model with stress triaxiality criterion to predict fracture initiation and pattern.

A primary objective of finite element human body models (HBMs) is to predict response and injury risk in impact scenarios, including cortical bone fracture initiation, fracture pattern, and the potential to simulate post-fracture injury to underlying soft tissues. Current HBMs have been challenged to predict the onset of failure and bone fracture patterns owing to the use of simplified failure criteria. In the present study, a continuum damage mechanics (CDM) model, incorporating observed mechanical response (orthotropy, asymmetry, damage), was coupled to a novel phenomenological effective strain fracture criterion based on stress triaxiality and investigated to predict cortical bone response under different modes of loading. Three loading cases were assessed: a coupon level notched shear test, whole bone femur three-point bending, and whole bone femur axial torsion. The proposed material model and fracture criterion were able to predict both the fracture initiation and location, and the fracture pattern for whole bone and specimen level tests, within the variability of the reported experiments. There was a dependence of fracture threshold on finite element mesh size, where higher mesh density produced similar but more refined fracture patterns compared to coarser meshes. Importantly, the model was functional, accurate, and numerically stable even for relatively coarse mesh sizes used in contemporary HBMs. The proposed model and novel fracture criterion enable prediction of fracture initiation and resulting fracture pattern in cortical bone such that post-fracture response can be investigated in HBMs.

Importance of asymmetry and anisotropy in predicting cortical bone response and fracture using human body model femur in three-point bending and axial rotation.

Modeling of cortical bone response and failure is critical for the prediction of Crash Induced Injuries (CII) using advanced finite element (FE) Human Body Models (HBM). Although cortical bone is anisotropic and asymmetric in tension and compression, current HBM often utilize simple isotropic, symmetric, elastic-plastic constitutive models. In this study, a 50th percentile male femur FE model was used to quantify the effect of asymmetry and anisotropy in three-point bending and axial torsion. A complete set of cortical bone mechanical properties was identified from a literature review, and the femur model was used to investigate the importance of material asymmetry and anisotropy on the failure load/moment, failure displacement/rotation and fracture pattern. All models were able to predict failure load in bending, since this was dominated by the cortical bone material tensile response. However, only the orthotropic model was able to predict the torsional response and failure moment. Only the orthotropic model predicted the fracture initiation location and fracture pattern in bending, and the fracture initiation location in torsion; however, the anticipated spiral fracture pattern was not predicted by the models for torsional loading. The results demonstrated that asymmetry did not significantly improve the prediction capability, and that orthotropic material model with the identified material properties was able to predict the kinetics and kinematics for both three-point bending and axial torsion. This will help to provide an improved method for modeling hard tissue response and failure in full HBM.

Multipotential human adipose-derived stromal stem cells exhibit a perivascular phenotype in vitro and in vivo.

Mesenchymal stem-like cells identified in different tissues reside in a perivascular niche. In the present study, we investigated the putative niche of adipose-derived stromal/stem cells (ASCs) using markers, associated with mesenchymal and perivascular cells, including STRO-1, CD146, and 3G5. Immunofluorescence staining of human adipose tissue sections, revealed that STRO-1 and 3G5 co-localized with CD146 to the perivascular regions of blood vessels. FACS was used to determine the capacity of the CD146, 3G5, and STRO-1 specific monoclonal antibodies to isolate clonogenic ASCs from disassociated human adipose tissue. Clonogenic fibroblastic colonies (CFU-F) were found to be enriched in those cell fractions selected with either STRO-1, CD146, or 3G5. Flow cytometric analysis revealed that cultured ASCs exhibited similar phenotypic profiles in relation to their expression of cell surface markers associated with stromal cells (CD44, CD90, CD105, CD106, CD146, CD166, STRO-1, alkaline phosphatase), endothelial cells (CD31, CD105, CD106, CD146, CD166), haematopoietic cells (CD14, CD31, CD45), and perivascular cells (3G5, STRO-1, CD146). The immunoselected ASCs populations maintained their characteristic multipotential properties as shown by their capacity to form Alizarin Red positive mineralized deposits, Oil Red O positive lipid droplets, and Alcian Blue positive proteoglycan-rich matrix in vitro. Furthermore, ASCs cultures established from either STRO-1, 3G5, or CD146 selected cell populations, were all capable of forming ectopic bone when transplanted subcutaneously into NOD/SCID mice. The findings presented here, describe a multipotential stem cell population within adult human adipose tissue, which appear to be intimately associated with perivascular cells surrounding the blood vessels.