S. De Bock

Our capricious vessels: The influence of stent design and vessel geometry on the mechanics of intracranial aneurysm stent deployment.

There is a growing interest in virtual tools to assist clinicians in evaluating different procedures and devices for endovascular treatment. In the present study we use finite element analysis to investigate the influence of stent design and vessel geometry for stent assisted coiling of intracranial aneurysms. Nine virtual stenting procedures were performed: three nitinol stent designs ((i) an open cell stent resembling the Neuroform, (ii) a generic stiff and (iii) a more flexible closed cell design), were deployed in three patient-specific cerebral aneurysmatic vessels. We investigated the percentage of strut area covering the aneurysm neck, the straightening induced on the cerebrovasculature by the stent placement (quantified by the reduction in tortuosity), and stent apposition to the wall (quantified as the percentage of struts within 0.2mm of the vessel). The results suggest that the open cell design better covers the aneurysm neck (11.0±1.1%) compared to both the stiff (7.8±1.6%) and flexible (8.7±1.6%) closed cell stents, and induces less straightening of the vessel (-5.1±1.6% vs. -42.9±9.8% and -26.9±11.9% ). The open cell design has, however, less struts apposing well to the vessel wall (56.0±6.4%) compared to the flexible (73.4±4.6%) and stiff (70.4±5.1%) closed cell design. With the presented study, we hope to contribute to and improve aneurysm treatment, using a novel patient specific environment as a possible pre-operative tool to evaluate mechanical stent behavior in different vascular geometries.

Virtual evaluation of stent graft deployment: a validated modeling and simulation study.

The presented study details the virtual deployment of a bifurcated stent graft (Medtronic Talent) in an Abdominal Aortic Aneurysm model, using the finite element method. The entire deployment procedure is modeled, with the stent graft being crimped and bent according to the vessel geometry, and subsequently released. The finite element results are validated in vitro with placement of the device in a silicone mock aneurysm, using high resolution CT scans to evaluate the result. The presented work confirms the capability of finite element computer simulations to predict the deformed configuration after endovascular aneurysm repair (EVAR). These simulations can be used to quantify mechanical parameters, such as neck dilations, radial forces and stresses in the device, that are difficult or impossible to obtain from medical imaging.

Supersonic Shear Wave Imaging to Assess Arterial Nonlinear Behavior and Anisotropy: Proof of Principle via Ex Vivo Testing of the Horse Aorta

Supersonic shear wave imaging (SSI) is a noninvasive, ultrasound-based technique to quantify the mechanical properties of bulk tissues by measuring the propagation speed of shear waves (SW) induced in the tissue with an ultrasound transducer. The technique has been successfully validated in liver and breast (tumor) diagnostics and is potentially useful for the assessment of the stiffness of arteries. However, SW propagation in arteries is subjected to different wave phenomena potentially affecting the measurement accuracy. Therefore, we assessed SSI in a less complex ex vivo setup, that is, a thick-walled and rectangular slab of an excised equine aorta. Dynamic uniaxial mechanical testing was performed during the SSI measurements, to dispose of a reference material assessment. An ultrasound probe was fixed in an angle position controller with respect to the tissue to investigate the effect of arterial anisotropy on SSI results. Results indicated that SSI was able to pick up stretch-induced stiffening of the aorta. SW velocities were significantly higher along the specimens circumferential direction than in the axial direction, consistent with the circumferential orientation of collagen fibers. Hence, we established a first step in studying SW propagation in anisotropic tissues to gain more insight into the feasibility of SSI-based measurements in arteries.

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.

Filling the void: A coalescent numerical and experimental technique to determine aortic stent graft mechanics

The presented study details a combined experimental and computational method to assess and compare the mechanical behavior of the main body of 4 different stent graft designs. The mechanical response to a flat plate compression and radial crimping of the devices is derived and related to geometrical and material features of different stent designs. The finite element modeling procedure is used to complement the experimental results and conduct a solution sensitivity study. Finite element evaluations of the mechanical behavior match well with experimental findings and are used as a quantitative basis to discuss design characteristics of the different devices.

What if you stretch the IFU? A mechanical insight into stent graft Instructions For Use in angulated proximal aneurysm necks

Endovascular treatment for patients with a proximal neck anatomy outside instructions for use is an ongoing topic of debate in endovascular aneurysm repair. This paper employs the finite element method to offer insight into possible adverse effects of deploying a stent graft into an angulated geometry. The effect of angulation, straight neck length and device oversize was investigated in a full factorial parametric analysis. Stent apposition, area reduction of the graft, asymmetry of contact forces and the ability to find a good seal were investigated. Most adverse effects are expected for combinations of high angulation and short straight landing zones. Higher oversize has a beneficiary effect, but not enough to compensate the adverse effects of (very) short and angulated angles. Our analysis shows that for an angle between the suprarenal aorta and proximal neck above 60°, proximal kinking of the device can occur. The method used offers a engineering view on the morphological limits of EVAR for a clinically used device.

A Simulation Tool for Virtual Stent Graft Deployment in Patient-Specific Abdominal Aortic Aneurysms

An Abdominal Aortic Aneurysm (AAA) is a pathological dilatation of the aorta, commonly involving the region between the renal arteries and the aortic bifurcation, with a high degree of mortality. Endovascular repair of AAA has proven to be a valid alternative for the conventional open repair, with clear benefits in terms of length of hospital stay, reduced operational mortality and less trauma [1]. However, stent grafts are still associated with various problems and failings, which may lead to AAA rupture or rise the need for an additional open surgery.Copyright

Abdominal Aortic Stent Graft Mechanics: Sizing Them Up

An abdominal aortic aneurysm (AAA) of the aorta is a local widening of the aorta in a region between the renal arteries and the aortic bifurcation. The disease impacts the structural integrity of the AAA wall, weakening the tissue and predisposing it to rupture. Preventive treatment of the disease is often performed minimally invasive with endovascular repair by stent graft deployment, as an alternative to open surgical repair. During endovascular aneurysm repair, a metallic stent, covered with a polymer membrane is first crimped and mounted onto the delivery system and inserted through the iliac artery. It is advanced to the AAA region, and expanded to cover the weakened, ballooning aorta. The technique has excellent clinical outcome, yet it is still associated with long term problems such as migration, a downward displacement of the device, and endoleakage, with blood reentering and pressurizing the aneurismal sack.Copyright

Virtual deployment of self-expanding stents in aneurysmatic intracranial vessels: do stent design and vessel geometry matter?

Cerebral aneurysms are weakened, balloon like dilations of the intracranial vessels. Rupture of these swelled vessels, with a prevalence of 1 to 5% not uncommon [1], is associated with a high mortality and morbidity [2]. Clipping (by craniotomy) or coiling (passing platinum wires into the aneurysm) is often performed to reduce the risk of rupture. Coiling of wide necked or fusiform aneurysms, which is not easily achieved without risk of coil displacement, is often assisted by placement of intracranial stents, serving as a scaffold for the platinum coils. Cerebral stents can be of either closed or open cell design. Both designs have shown to be successful at assisting coiling, yet each design has its own strengths and weaknesses [3].Copyright