Nic Debusschere

The influence of vascular anatomy on carotid artery stenting: A parametric study for damage assessment

Carotid artery stenting is emerging as an alternative technique to surgery for the treatment of symptomatic severe carotid stenosis. Clinical and experimental evidence demonstrates that both plaque morphology and biomechanical changes due to the device implantation can be possible causes of an unsuccessful treatment. In order to gain further insights of the endovascular intervention, a virtual environment based on structural finite element simulations was built to emulate the stenting procedure on generalized atherosclerotic carotid geometries which included a damage model to quantify the injury of the vessel. Five possible lesion scenarios were simulated by changing both material properties and vascular geometrical features to cover both presumed vulnerable and stable plaques. The results were analyzed with respect to lumen gain and wall stresses which are potentially related to the failure of the procedure according to previous studies. Our findings show that an elliptic lumen shape and a thinner fibrous cap with an underlying lipid pool result in higher stenosis reduction, while large calcifications and fibrotic tissue are more prone to recoil. The shielding effect of a thicker fibrous cap helps to reduce local compressive stresses in the soft plaque. The presence of a soft plaque reduces the damage in the healthy vascular structures. Contrarily, the presence of hard plaque promotes less damage volume in the fibrous cap and reduces stress peaks in this region, but they seem to increase stresses in the media-intima layer. Finally the reliability of the achieved results was put into clinical perspective.

A finite element strategy to investigate the free expansion behaviour of a biodegradable polymeric stent

Bioresorbable stents represent a promising technological development within the field of cardiovascular angioplasty because of their ability to avoid long-term side effects of conventional stents such as in-stent restenosis, late stent thrombosis and fatigue induced strut fracture. Finite element simulations have proven to present a useful research tool for the design and mechanical analysis of stents. However, biodegradable stents pose new challenges because of their transitional mechanical behaviour. For polymeric biodegradable stents, viscoplastic effects have to be accounted for. This paper presents a method to analyse the mechanical behaviour of polymeric bioresorbable stents using an implicit finite-element solver. As an example, we investigate the mechanical behaviour of a commercially available bioresorbable stent. We examine how, due to the visco-elastic properties of the stent material, the balloon deployment rate influences the mechanical integrity of the stent.

Assessment of shear stress related parameters in the carotid bifurcation using mouse-specific FSI simulations

The ApoE(-)(/)(-) mouse is a common small animal model to study atherosclerosis, an inflammatory disease of the large and medium sized arteries such as the carotid artery. It is generally accepted that the wall shear stress, induced by the blood flow, plays a key role in the onset of this disease. Wall shear stress, however, is difficult to derive from direct in vivo measurements, particularly in mice. In this study, we integrated in vivo imaging (micro-Computed Tomography-µCT and ultrasound) and fluid-structure interaction (FSI) modeling for the mouse-specific assessment of carotid hemodynamics and wall shear stress. Results were provided for 8 carotid bifurcations of 4 ApoE(-)(/)(-) mice. We demonstrated that accounting for the carotid elasticity leads to more realistic flow waveforms over the complete domain of the model due to volume buffering capacity in systole. The 8 simulated cases showed fairly consistent spatial distribution maps of time-averaged wall shear stress (TAWSS) and relative residence time (RRT). Zones with reduced TAWSS and elevated RRT, potential indicators of atherosclerosis-prone regions, were located mainly at the outer sinus of the external carotid artery. In contrast to human carotid hemodynamics, no flow recirculation could be observed in the carotid bifurcation region.

A Computational Framework to Model Degradation of Biocorrodible Metal Stents Using an Implicit Finite Element Solver

Bioresorbable stents represent an emerging technological development within the field of cardiovascular angioplasty. Their temporary presence avoids long-term side effects of non-degradable stents such as in-stent restenosis, late stent thrombosis and fatigue induced strut fracture. Several numerical modelling strategies have been proposed to evaluate the transitional mechanical characteristics of biodegradable stents using a continuum damage framework. However, these methods rely on an explicit finite-element integration scheme which, in combination with the quasi-static nature of many simulations involving stents and the small element size needed to model corrosion mechanisms, results in a high computational cost. To reduce the simulation times and to expand the general applicability of these degradation models, this paper investigates an implicit finite element solution method to model degradation of biodegradable stents.

A finite element model to study the effect of tissue anisotropy onex vivoarterial shear wave elastography measurements

Shear wave elastography (SWE) is an ultrasound (US) diagnostic method for measuring the stiffness of soft tissues based on generated shear waves (SWs). SWE has been applied to bulk tissues, but in arteries it is still under investigation. Previously performed studies in arteries or arterial phantoms demonstrated the potential of SWE to measure arterial wall stiffness-a relevant marker in prediction of cardiovascular diseases. This study is focused on numerical modelling of SWs in ex vivo equine aortic tissue, yet based on experimental SWE measurements with the tissue dynamically loaded while rotating the US probe to investigate the sensitivity of SWE to the anisotropic structure. A good match with experimental shear wave group speed results was obtained. SWs were sensitive to the orthotropy and nonlinearity of the material. The model also allowed to study the nature of the SWs by performing 2D FFT-based and analytical phase analyses. A good match between numerical group velocities derived using the time-of-flight algorithm and derived from the dispersion curves was found in the cross-sectional and axial arterial views. The complexity of solving analytical equations for nonlinear orthotropic stressed plates was discussed.

Tissue prolapse and stresses in stented coronary arteries

Among the many factors influencing the effectiveness of cardiovascular stents, tissue prolapse indicates the potential of a stent to cause restenosis. The deflection of the arterial wall between the struts of the stent and the tissue is known as a prolapse or draping. The prolapse is associated with injury and damage to the vessel wall due to the high stresses generated around the stent when it expands. The current study investigates the impact of stenosis severity and plaque morphology on prolapse in stented coronary arteries. A finite element method is applied for the stent, plaque, and artery set to quantify the tissue prolapse and the corresponding stresses in stenosed coronary arteries. The variable size of atherosclerotic plaques is considered. A plaque is modelled as a multi-layered medium with different thicknesses attached to the single layer of an arterial wall. The results reveal that the tissue prolapse is influenced by the degree of stenosis severity and the thickness of the plaque layers. Stresses are observed to be significantly different between the plaque layers and the arterial wall tissue. Higher stresses are concentrated in fibrosis layer of the plaque (the harder core), while lower stresses are observed in necrotic core (the softer core) and the arterial wall layer. Moreover, the morphology of the plaque regulates the magnitude and distribution of the stress. The fibrous cap between the necrotic core and the endothelium constitutes the most influential layer to alter the stresses. In addition, the thickness of the necrotic core and the stenosis severity affect the stresses. This study reveals that the morphology of atherosclerotic plaques needs to be considered a key parameter in designing coronary stents.

Finite Element Modeling of Biodegradable Stents

Biodegradable stents, which temporarily support a stenotic blood vessel and afterwards fully disappear, have recently gained a lot of interest. They avoid long-term complications associated with conventional stents such as late stent thrombosis and in-stent restenosis. Moreover, degradable stents allow for a restoration of vasomotion and vessel growth which makes them particularly suitable for pediatric applications [1]. Finite element simulations have proven to be an efficient and cost-effective tool to investigate and optimize the mechanical performance of minimal invasive devices such as stents [2]. Biodegradable stents have however created new challenges in their design and optimization via finite element analysis because of their complex time-varying material behavior. To correctly simulate the mechanical behavior of biodegradable stents, a model should be developed that incorporates the effect of degradation upon all material characteristics. By combining existing constitutive material models based on continuum damage theory we were able to create such a virtual environment in which the transitional mechanical behavior of biodegradable stents can be investigated.Copyright

Modeling of Coated Biodegradable Stents

A bioresorbable stent supports the stenosed blood vessel during the healing period after coronary angioplasty and then gradually disappears. Unlike permanent stents, the biodegradable stent forms no obstacle for future interventions. Moreover, the degradable stent material presents an ideal vehicle for local drug delivery. Long term side effects inherent to drug eluting stents such as in-stent restenosis and late stent thrombosis might be avoided [1]. To date, several bioresorbable stents are being developed or are currently being tested in clinical trials. Two classes of biomaterials are being used in biodegradable stent technology: biodegradable polymers and bioerodible metal alloys. Polymers can be tailored to have a well-defined degradational behaviour but have relatively poor mechanical properties. Biocorrodible metals such as magnesium alloys have good mechanical characteristics but display a more complex an less predictive degradational behaviour. A biocorrodible metallic stent coated with a biodegradable polymer might be able to combine the benefits of both metallic and polymeric biodegradable stents. Finite element modelling can play an important role in the study of nevel stent designs. To correctly simulate the behaviour of degradable stents a material model must be developed that incorporates the effect of degradation on all material characteristics. In case of a coated biocorrodible magnesium stent this includes corrosion modelling, the effect of the coating and the influence of mechanical loading on the corrosion rate.Copyright