Measurement of pulsatile haemodynamic forces in a model of a bifurcated stent graft for abdominal aortic aneurysm repair

Abstract

The longitudinal haemodynamic force (LF) acting on a bifurcated stent graft for abdominal aortic aneurysm repair has been estimated previously using a simple one-dimensional analytical model based on the momentum equation which assumes steady flow of an inviscid fluid. Using an instrumented stent-graft model an experimental technique was developed to measure the LF under pulsatile flow conditions. The physical stent-graft model, with main trunk diameter of 30mm and limb diameters of 12 mm, was fabricated from aluminium. Strain gauges were bonded on to the main trunk to determine the longitudinal strain which is related to the LF. After calibration, the model was placed in a pulsatile flow system with 40 per cent aqueous glycerol solution as the circulating fluid. The LF was determined using a Wheatstone bridge signal-conditioning circuit. The signals were averaged over 590 cardiac cycles and saved to a personal computer for subsequent processing. The LF was strongly dependent on the pressure but less so on the flowrate. The measured forces were higher than those predicted by the simplified mathematical model by about 6-18 per cent during the cardiac cycle. The excess measured forces are due to the viscous drag and the effect of pulsatile flow. The peak measured LF in this model of 30 mm diameter may exceed the fixation force of some current clinical endovascular stent grafts.

An Improved Methodology for Investigating the Parameters Influencing Migration Resistance of Abdominal Aortic Stent-Grafts

“…It has previously been shown that the drag force acting on an implanted stent-graft is largely dependent on the pressure acting on the graft. 44 In this study, the systolic pressure was increased by 16.6%, so the drag was also increased by a similar percentage; this caused ,13% reduction in the displacement force.…”

“…8,9 It has been hypothesized that excessive oversizing can lead to proximal aneu- rysm neck dilatation after EVAR and can greatly increase the risk of migration. 44 Because of this, the current practice is to oversize stent-grafts by 10% to 20% in vivo. 34 The oversizing of the devices in the current study was 16.66% in the aneurysm neck and in the iliac legs, which is more representative of the situation of a stent-graft implanted in the human aorta.…”

Experimental Measurement and Mathematical Modeling of Pulsatile Forces on a Symmetric, Bifurcated Endoluminal Stent Graft Model

“…Only Šutalo and colleagues and Zhou and colleagues compared theory with force measurements, but they made no comparisons with CFD-predicted forces. 13,14 Stent graft designers must allow for fatigue factors when using the peak forces in their design calculations because the forces will still be applying for 10 years. Engineers should not compromise structural strength, which includes the safety margin for fatigue and peak performance, to reduce the size for delivery.…”

“…The other two studies that have compared theory with force measurements also used rigid stent graft models. 13,14 We assumed a parabolic input velocity profile with the justification that Morris and colleagues found that the input velocity profile did not affect the magnitude of the drag forces. 21 We assumed that the graft was oriented horizontally for our comparison, which is approximately equivalent to the case when a patient is lying down.…”

Computational Fluid Dynamics Modeling of Redundant Stent-Graft Configurations in Endovascular Aneurysm Repair

“…However, one cannot apply causation based on the similarity of observed trends. There are very few experimental in vitro studies available to which we can compare CFD results, and the studies that are available do not use parameters and conditions comparable to those that CFD studies attempt to simulate [23,37]. Even within the literature on CFD of stent-graft blood flow, the geometries (dimensions) as well as the boundary conditions and blood physical properties vary widely, as presented in Tables 1 and 3.…”