Effect of intraspecimen spatial variation in tissue mineral density on the apparent stiffness of trabecular bone.

This study investigated the effects of intraspecimen variations in tissue mineral density(TMD) on the apparent-level stiffness of human trabecular bone. High-resolution finite element (FE) models were created for each of 12 human trabecular bone specimens,using both microcomputed tomography (lCT) and "gold-standard" synchrotron radiation lCT (SRlCT) data. Our results confirm that incorporating TMD spatial variation reduces the calculated apparent stiffness compared to homogeneous TMD models. This effect exists for both lCT- and SRlCT-based FE models, but is exaggerated in lCT based models. This study provides a direct comparison of lCT to SRlCT data and is thereby able to conclude that the influence of including TMD heterogeneity is overestimated in lCT-based models.

Computational Studies on the Effects of Applied Apical Torsion for Cardiac Assist on Regional Wall Mechanics.

OBJECTIVE: Here we report the results of parametric computational simulations evaluating the biomechanical effects of applied apical torsion (AAT) on a patient-specific bi-ventricular failing heart model. METHODS: We examined the resulting effects on cardiac biomechanics with varying device coverage areas and applied rotation angles to determine the practical working limits of AAT on a dilated cardiomyopathy heart model. RESULTS: The largest maximum principal stresses and strains observed in the heart failure model were 80.21 kPa (at the basal node of the left ventricular epicardium) and 0.56 (at the node of the device base of the left ventricular free wall). Results show that increasing levels of AAT beyond 45 degrees produce supra-physiologic levels of stress and strain in the myocardium. CONCLUSION: Maximum principal stresses greater than 100 kPa were observed at multiple nodes along the epicardium and endocardium of the ventricular base and in the endocardium at the device base. Maximum principal strains greater than 0.60 were observed at multiple nodes along the epicardium and endocardium of the ventricular base. SIGNIFICANCE: This suggests that while AAT has the potential to provide meaningful returns to hemodynamic function in failing hearts, the large deformations produced by this approach with the upper bounds of applied rotation angle realistically excludes supra-physiological rotations as a means for cardiac support. However, lower AAT angles - closer to that of the native left-ventricular torsion - coupled with another means of external cardiac compression may prove to be a viable method of cardiac assist.

Computational Parametric Studies Investigating the Global Hemodynamic Effects of Applied Apical Torsion for Cardiac Assist.

Healthy hearts have an inherent twisting motion that is caused by large changes in muscle fiber orientation across the myocardial wall and is believed to help lower wall stress and increase cardiac output. It was demonstrated that applied apical torsion (AAT) of the heart could potentially treat congestive heart failure (CHF) by improving hemodynamic function. We report the results of parametric computational experiments where the effects of using a torsional ventricular assist device (tVAD) to treat CHF were examined using a patient-specific bi-ventricular computational model. We examined the effects on global hemodynamics as the device coverage area (CA) and applied rotation angle (ARA) were varied to determine ideal tVAD design parameters. When compared to a baseline, pretreatment CHF model, increases in ARA resulted in moderate to substantial increases in ejection fraction (EF), peak systolic pressures (PSP) and stroke work (SW) with concomitant decreases in end-systolic volumes (ESV). Increases in device CA resulted in increased hemodynamic function. The simulation representing the most aggressive level of cardiac assist yielded significant increases in left ventricular EF and SW, 49 and 72% respectively. Results with this more realistic computational model reinforce previous studies that have demonstrated the potential of AAT for cardiac assist.