Conference Agenda

Introduction

The human aorta undergoes lifelong morphological changes driven by mechanical and biological factors. Aging and hypertension, in particular, increase aortic tortuosity, disrupting flow and wall stresses and raising the risk of cardiovascular complications. A key driver of these changes is the progressive loss of axial pre-stretch, a crucial factor for maintaining biomechanical stability. While previous studies have linked reduced axial pre-stretch to aortic elongation and altered mechanical behavior, its precise role in growth and remodeling (G&R) and its potential to induce pathological deformations, such as aortic buckling, remain poorly understood. This study aims to elucidate the influence of axial pre-stretch on aortic morphology evolution using a patient-specific finite element simulation.

Method

A computational model of the human ascending aorta was developed based on the constrained mixture theory, accounting for the distinct mechanical contributions of elastin, collagen, and smooth muscle cells. The model captures the tissue’s capacity to restore homeostatic stress via biological adaptation mechanisms such as mass growth and structural remodeling. It was implemented in Abaqus through a UMAT subroutine to simulate the G&R of the ascending aorta.

Using age-related physiological axial loading conditions, we considered two cases to evaluate effects of axial pre-stretch on the G&R of the ascending aorta in hypertensive patients. In the first case, representing a 70-year-old patient, the model was subjected only to 80 mmHg without axial pre-stretch. In the second case, corresponding to a 40-year-old patient, the same pressure was applied with 20% axial pre-stretch. After computing the pre-stress fields, both models were subjected to a 40% increase in pressure to simulate hypertensive conditions.

Results

The simulations revealed distinct remodeling patterns between the two scenarios. In both cases, the model predicted increases in aortic diameter and wall thickness, consistent with clinical observations of hypertension-related remodeling. However, the presence or absence of axial pre-stretch had a significant impact on aortic morphology and mechanical stability.

In the model without axial pre-stretch, the ascending aorta exhibited localized kinking along the inner curvature under elevated pressure, indicative of pathological deformation and reduced structural integrity. In contrast, the model with 20% axial pre-stretch preserved a smoother geometry, even following a 40% increase in blood pressure.

An analysis of stress evolution during G&R revealed a gradual reduction in axial stress over time in both models. This effect was more pronounced in the non-pre-stretched aorta, where the loss of axial tension led to localized pathological deformations. These findings are consistent with prior studies suggesting that buckling pressure decreases as axial pre-stretch diminishes. Our results confirm that insufficient axial tension compromises the mechanical stability of the aortic wall, increasing the risk of tortuous geometries.

Overall, the results underscore the critical biomechanical role of axial pre-stretch in maintaining aortic structure under hypertensive loading. The loss of axial tension appears to drive maladaptive remodeling and may serve as a key factor in the onset of tortuosity and buckling in hypertensive patients.

Background: Abdominal aortic aneurysm (AAA) is a degenerative vascular disease characterized by significant changes in the geometry and microstructure of the aortic wall. AAA particularly affects elderly men, with a prevalence ranging from 1.3% to 8.9% in men over 60. Rupture of the aneurysm wall is associated with a high mortality rate of up to 80 %, so that adequate identification of patients at risk is crucial for their survival. Clinically, rupture risk is assessed using the maximum diameter criterion, which guides surgical decision-making. While statistically validated, this criterion lacks precision as it fails to account for patient-specific variations in aortic wall strength and local stress concentrations. The need for more patient-specific biomarkers is therefore widely acknowledged. It is hypothesized that Wall Motion Indices (WMI), derived from 4D ultrasound (4D-US)-based local strain measurements, can provide a more accurate assessment of patient-specific rupture risk by capturing aortic wall kinematics.

Methods: Preoperative data from 9 AAA patients were collected, including 4D-US strain imaging, CT angiography, maximum diameter, and blood pressure. Patients underwent open surgical repair, during which tissue samples were harvested for mechanical testing. WMI, describing the distribution of the aortic wall’s strain field, were computed from 4D-US strain data. The analysis focused specifically on wall regions corresponding to the locations from which tissue samples were obtained during surgery. These indices were correlated with a normalized experimental rupture potential (NERP), calculated as the ratio between in vivo wall tension (derived from Finite Element Analysis based on CT-A and mean arterial pressure) and the experimental failure tension (yield and maximum tension) determined via uniaxial tensile tests on the harvested tissue.

Results: Several WMI demonstrated significant correlations with NERP (r>0.75, p<0.05), particularly indices describing heterogeneous strain distributions within the aneurysmal wall. These WMI also correlated significantly with the experimental failure tension determined from uniaxial testing of the tissue samples (r<-0.67, p<0.05). Conversely, no significant correlation was found between maximum diameter and NERP, highlighting the limitation of size alone in predicting the individual mechanical failure risk.

Conclusion: This study indicates that 4D-US-derived WMI can provide patient-specific information correlated to rupture potential of AAA independent of the maximum diameter. The strong correlation between heterogeneous strain patterns and tension-based rupture potential highlights the potential of WMI to refine clinical decision-making. As a non-invasive modality, 4D-US derived WMI could potentially improve patient selection for elective repair, identifying individuals truly at high risk irrespective of diameter alone, thus personalizing AAA management. Further validation in larger patient cohorts is necessary to confirm these findings.

We propose a novel model for a vascularized poroelastic composite representing the myocardium which incorporates both mechanical deformations and electrical conductivity. Our structure comprises a poroelastic extracellular matrix with embedded vasculature and elastic inclusions (representing the myocytes) and we consider the electrical conductance between these two solid compartments. There is a distinct length scale separation between the scale where we can visibly see the connected fluid compartment (vessels) separated from the poroelastic matrix and the elastic myocyte and the overall size of the heart muscle.

We therefore apply the asymptotic homogenisation technique to derive the new model. The effective governing equations that we obtain describe the behaviour of the myocardium in terms of the zero-th order stresses, current densities, relative fluid-solid velocities, pressures, electric potentials and elastic displacements. It effectively accounts for the fluid filling in the pores of the poroelastic matrix, flow in the vessels, the transport of fluid between the vessels and the matrix, and the elastic deformation and electrical conductance between the poroelastic matrix and the myocyte.

This work paves the way towards a myocardium model that incorporates multiscale deformations and electrical conductivity whilst also considering the effects of the vascularisation and indeed the impact on mechanotransduction.