A novel framework for quantifying the subject-specific three-dimensional residual stress field in the aortic wall.

BACKGROUND: Quantification of subject-specific residual stress field remains a challenge that prohibits accurate stress analysis and refined understanding of the biomechanical behavior of the aortic wall. METHOD: This study presents a framework combining experiments, constitutive modeling, and computer simulation to quantify the subject-specific three-dimensional residual stress field of the aortic wall. The material properties and residual deformations were acquired from the same porcine aortic sample, so that the subject-specific residual stress field was quantified analytically. Consequently, a novel stress-driven tissue growth model was developed and incorporated in a finite element aortic model to recover the subject-specific residual stress with the help of analytical solution. We then evaluated the framework's efficacy by simulating the residual stress distribution in the aortic dissection (AD). RESULT: Subject-specific residual stress field of the aortic sample was quantified analytically. No appreciable discrepancy was observed between the numerically simulated and analytically derived residual stress distributions, indicating the effectiveness of the tissue growth model. Errors arising from the numerically simulated circumferential opening angle and axial bending angle were within 5% relative to experimental results, highlighting that the framework was accurate in terms of subject-specific residual stress estimation. Finally, numerical simulations recovered the buckling behavior of the intimal flap of the dissected aorta and revealed the expansion of the false lumen and compression of the true lumen as the tear propagates circumferentially. CONCLUSION: The proposed framework is effective in quantifying the three-dimensional subject-specific residual stress field and it is potentially applicable in more sophisticated scenarios involving residual stress.

Biomechanical Effect of Different Graft Heights on Adjacent Segment and Graft Segment Following C4/C5 Anterior Cervical Discectomy and Fusion: A Finite Element Analysis.

BACKGROUND The finite element analysis (FEA) was used to explore the effect of different graft heights on adjacent segment and graft segment stress after C4/5 anterior cervical discectomy and fusion (ACDF). MATERIAL AND METHODS A detailed, geometrically accurate 3-dimensional cervical spine model was successfully built from computed tomography (CT) scanning of a healthy adult male. We changed the graft height in C4-C5 to be 90%, 150%, 175%, and 200% of the preoperative disc height and simulated the postoperative scenarios with different bone graft height, respectively. A stress analysis was conducted on the adjacent segment and graft segment. RESULTS The maximum von Mises stress on C3-C4 showed that when the graft height was 200%, the values were 0.99 MPa, 0.85 MPa, 0.91 MPa, and 0.89 MPa in different loading conditions. For C5-C6, the maximum von Mises stress was 0.77 MPa, 0.83 MPa, 0.91 MPa, and 0.81 MPa, observed when the graft height was 175%, except in extension condition. With regard to graft segment (C4-C5), the biggest von Mises stress was 1.25 MPa, 1.77 MPa, 1.75 MPa, and 1.81 MPa observed at 200% graft height. For these 3 segments, the smallest von Mises stress was found at 150% graft height under the 4 loading conditions. CONCLUSIONS The graft height makes an important difference on the stress on the adjacent segment and the graft segment after anterior cervical discectomy and fusion. A 150% graft height was considered the proper graft height in C4/C5 ACDF, with the lowest stress on the adjacent segment and the graft segment.

Finite element simulation of three dimensional residual stress in the aortic wall using an anisotropic tissue growth model.

Residual stress is believed to play a significant role in the in vivo stress state of the arterial wall, but quantifying residual stress in vivo is challenging. Based on the well-known assumptions that residual stress is a result of heterogeneous arterial growth and that it homogenizes the transmural distribution of arterial wall stress, we propose a new anisotropic tissue growth model for the aorta to recover the three-dimensional residual stress field in a bi-layer human aortic wall. Finite element simulations showed that the predicted residual stress magnitude with this method are within the documented range for human aorta. Particularly, the homeostatic inter-layer stress difference is identified as a key parameter to quantify the opening angle. To the authors' knowledge, this is the first finite element study employing anisotropic growth of aortic tissue in a bi-layer model to generate three-dimensional residual stress field, and the resultant opening angle can match with the experiments. A parametric study found that inter-layer stress homogeneity, arterial blood pressure, axial pre-stretch, and material stiffness strongly affect the residual stress field.

Three-dimensional flows in a hyperelastic vessel under external pressure.

We study the collapsible behaviour of a vessel conveying viscous flows subject to external pressure, a scenario that could occur in many physiological applications. The vessel is modelled as a three-dimensional cylindrical tube of nonlinear hyperelastic material. To solve the fully coupled fluid-structure interaction, we have developed a novel approach based on the Arbitrary Lagrangian-Eulerian (ALE) method and the frontal solver. The method of rotating spines is used to enable an automatic mesh adaptation. The numerical code is verified extensively with published results and those obtained using the commercial packages in simpler cases, e.g. ANSYS for the structure with the prescribed flow, and FLUENT for the fluid flow with prescribed structure deformation. We examine three different hyperelastic material models for the tube for the first time in this context and show that at the small strain, all three material models give similar results. However, for the large strain, results differ depending on the material model used. We further study the behaviour of the tube under a mode-3 buckling and reveal its complex flow patterns under various external pressures. To understand these flow patterns, we show how energy dissipation is associated with the boundary layers created at the narrowest collapsed section of the tube, and how the transverse flow forms a virtual sink to feed a strong axial jet. We found that the energy dissipation associated with the recirculation does not coincide with the flow separation zone itself, but overlaps with the streamlines that divide the three recirculation zones. Finally, we examine the bifurcation diagrams for both mode-3 and mode-2 collapses and reveal that multiple solutions exist for a range of the Reynolds number. Our work is a step towards modelling more realistic physiological flows in collapsible arteries and veins.