No abstract

Haemodynamic forces have a synergistic effect on endothelial cell (EC) morphology, proliferation, differentiation and biochemical expression profiles. Alterations to haemodynamic force levels have been observed at curved regions and bifurcations of arteries but also around struts of stented arteries, and are also known to be associated with various vascular pathologies. Therefore, curvature in combination with stenting might create a proatherosclerotic environment compared with stenting in a straight vessel, but this has never been investigated. The goal of this study was to compare EC morphology, proliferation and differentiation within in vitro models of curved stented peripheral vessel models with those of straight and unstented vessels. These models were generated using both static conditions and also subjected to 24 h of stimulation in a peripheral artery bioreactor. Medicalgrade silicone tubes were seeded with human umbilical vein endothelial cells to produce pseudovessels that were then stented and subjected to 24 h of physiological levels of pulsatile pressure, radial distention and shear stress. Changes in cell number, orientation and nitric oxide (NO) production were assessed in straight, curved, non-stented and stented pseudovessels. We report that curved pseudovessels lead to higher EC numbers with random orientation and lower NO production per cell compared with straight pseudovessels after 24 h of biomechanical stimulation. Both stented curved and stented straight pseudovessels had lower NO production per cell than corresponding unstented pseudovessels. However, in contrast to straight stented pseudovessels, curved stented pseudovessels had fewer viable cells. The results of this study show, for the first time, that the response of the vascular endothelium is dependent on both curvature and stenting combined, and highlight the necessity for future investigations of the effects of curvature in combination with stenting to fully understand effects on the endothelial layer.

Physiological loads that act on the femoropopliteal artery, in combination with stenting, can lead to uncharacteristic deformations of the stented vessel. The overall goal of this study was to investigate the effect of stent length and stent location on the deformation characteristics of the superficial femoral artery (SFA) using an anatomically accurate, three-dimensional finite element model of the leg. For a range of different stent lengths and locations, the deformation characteristics (length change, curvature change, and axial twist) that result from physiological loading of the SFA along with the mechanical behavior of the vessel tissue are investigated. Results showed that stenting portions of the SFA leads to a change in global deformation characteristics of the vessel. Increased stress and strain values and altered deformation characteristics were observed in the various stented cases of this study, which are compared to previous results of an unstented vessel. The study concludes that shortening, twist and curvature characteristics of the stented vessel are dependent on stent length and stent location within the vessel.

The deformation characteristics of the femoropopliteal segment change in the presence of a stent, with the change to the deformation behavior dependent on stent type, stent length, location, flexibility, and intrinsic centerline curvature.

Computational simulations using finite element analysis are a tool commonly used to analyse stent designs, deployment geometries and interactions between stent struts and arterial tissue. Such studies require large computational models and efforts are often made to simplify models in order to reduce computational time while maintaining reasonable accuracy. The objective of the study is focused on computational modelling and specifically aims to investigate how different methods of modelling stent-artery interactions can affect the results, computational time taken and computational size of the model. Various different models, each with increasing levels of complexity, are used to simulate this analysis, representing the many assumptions and simplifications used in other similar studies in order to determine what level of simplification will still allow for an accurate representation of stent radial force and resulting stress concentrations on the inner lining of the vessel during self-expanding stent deployment. The main conclusions of the study are that methods used in stent crimping impact on the resulting predicted radial force of the stent; that accurate representation of stent-artery interactions can only be made when modelling the full length of the stent due to the incorporation of end effects; and that modelling self-contact of the stent struts greatly impacts on the resulting stress concentrations within the stent, but that the effect of this on the unloading behaviour and resulting radial force of the stent is negligible.

The femoropopliteal (FP) artery is a branch of the femoral artery, the main artery in the upper leg, providing blood to all muscles and superficial tissues in the thigh. It is the largest of the femoral artery branches, composed of the superficial femoral artery (SFA) in the proximal region and popliteal artery (PA) in the distal region which runs below the knee. It is characterised by its tortuous geometry, associating a high atherosclerotic plaque burden with it. However, due to the dynamic forces of the SFA and PA, peripheral stents are reported to have the highest failure rates, predominantly due to bending [1]. Worst case bending can be seen in regions of the SFA/PA behind and just above the knee [2] and this is detrimental to stent patency.

There are a range of physiological movements such as knee and hip flexion that result in the deformation and loading of the femoropopliteal artery, characterised as axial compression, radial compression, bending and torsion [1]. The SFA is a prevalent location for peripheral arterial disease often requiring percutaneous transluminal angioplasty followed by stenting to restore vessel patency and uncompromised blood flow. Whilst stent placement provides the required scaffolding of the artery, opening up blockages and preventing elastic recoil of the vessel, it alters axial rigidity, changing the way in which the vessel deforms. With extreme stent rigidity and long stent lengths, severe bending can occur in the unstented vessel portions, proximal and distal to the stent. This has been observed in angiographic images of stented vessels that show unstented artery portions bending in an exaggerated manner adjacent to stented regions [2, 3]. It is postulated that due to knee flexion, deformation characteristics of the vessel are changed with extreme curvatures induced, initiating large stresses in the vessel tissue which may contribute to vascular injury, intimal hyperplasia and restenosis [2]. The goal of this work was to determine the effect of stent placement on deformation characteristics of the SFA after knee flexion using an anatomically accurate, three dimensional finite element model of the leg. Deformation characteristics (length change, curvature change and axial twist) that result from knee flexion movements of the stented vessel are quantified and linked with stress and strain levels within the artery for various cases of stent length and location within the femoropopliteal artery.