Characterization of the Deformation Behavior and Mechanical Response of the Femoro-popliteal Arterial Tract after Stent Placement

Gökgöl, Can (2016). Characterization of the Deformation Behavior and Mechanical Response of the Femoro-popliteal Arterial Tract after Stent Placement. (Dissertation, University of Bern, Faculty of Medicine)

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The femoro-popliteal (FP) segment is the most commonly diseased artery of the peripheral circulation. Obstructions of these lower-limb arteries are frequent and even with the new generation Nitinol stents (drug-eluting or otherwise), long-term restenosis rates following endovascular procedures range from 15 to 40% and are much higher compared with the long-term outcomes after coronary artery interventions.
The major difference between peripheral and coronary arteries concerns their mechanical environments, with the FP arterial segments being subjected to repeated external deformations during leg flexion. It has been widely hypothesized that the high distortion of the tissues due to the un-physiological deformations of the arteries following stent implantation is the main cause for restenosis. However, there is very limited information on the FP artery deformations of patients with Peripheral Arterial Disease (PAD). Furthermore, the effects of endovascular therapy on the deformation behavior of the PAD-afflicted FP arteries are currently unknown. As such, further research on the deformations of the FP arteries is warranted to not only improve existing stent designs, but to also determine the correct interventional procedure.
The main objectives of this thesis were to characterize the deformation behavior and mechanical response of the FP arterial tract through clinical and numerical investigations. The former was achieved by, first, investigating the pre-angioplasty deformations of the FP arteries during leg flexion in a pilot study of five patients with PAD and utilizing 3D rotational angiography. The methodology was then adapted to perform a clinical study of 35 patients with PAD, in which X-ray angiography was used to image the FP arteries in straight and flexed positions prior to endovascular therapy and following either Percutaneous Transluminal Angioplasty (PTA) or primary Nitinol stent implantation. The 3D models of the FP arteries were reconstructed from the 2D X-ray angiograms and the deformations of axial deformation and
curvature were quantified. Both studies showed that the PAD-afflicted FP arterial segment undergoes significant shortening and an increase in curvature with leg flexion. Comparisons between the pre- and post-treatment deformations, as well as between the different treatment methods, suggested that the choice of the treatment method significantly affects the post-interventional axial deformations of the FP arteries (post-balloon: 7.6% ± 4.9%; post-stent: 3.2% ± 2.9%; P: 0.004). As such, while PTA results in a more flexible artery, stents restrict the arteries’ shortening capabilities. Depending on the anatomical position of the stents, this axial stiffening of the arteries may lead to chronic kinking, which may cause occlusions and, consequently, impact the long-term success of the procedure. As current stent designs were found to conform to the curvature behavior of the FP arterial tract, improvements should be focused on reproducing the native axial stiffness of the artery to reduce the risk of restenosis for patients that will have to undergo stent implantation.
The complexities caused by leg flexion are further exacerbated by controversial clinical practices, such as Nitinol stent oversizing. The procedure is frequently performed in peripheral arteries to ensure a desirable acute lumen gain and strong wall apposition, and to prevent stent migration. However, the increased radial force exerted onto the arterial walls by the oversized stents could lead to significant arterial damage and, in turn, restenosis. The contradictory findings between animal and clinical studies, in conjunction with the majority of the numerical studies focusing on balloon-expandable stents, suggests that the efficacy of the procedure remains as an issue to be answered.
The mechanical behavior of the FP artery under Nitinol stent oversizing was investigated by creating a validated finite element (FE) framework, which included numerical models of healthy FP arteries with patient-specific geometries and idealized arteries with clinically relevant levels of PAD. Based on the artery model, either only stent implantation or the complete endovascular therapy (PTA + stent implantation) was simulated. Four different stent-to-artery ratios ranging from 1.0 to 1.8 were used in the simulations. For the healthy arteries, additional analyses, in the form of computational fluid dynamics (CFD) analyses and fatigue behavior of the stents, were performed to observe the hemodynamic behavior of the arteries with respect to increased oversizing ratios. For the calcified arteries, three different plaque types were modeled to report the influence of the plaque behaviors on the outcomes of endovascular therapy and stent oversizing. Regardless of the presence of a plaque tissue, results showed that Nitinol stent oversizing was found to produce a marginal lumen gain in
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contrast to a significant increase in arterial stresses. For the lightly and moderately calcified arteries, oversizing was found to be non-critical; whereas for healthy and heavily calcified arteries, the procedure should be avoided due to a risk of tissue failure. These adverse effects to both the artery walls and stents may create circumstances for restenosis. Although the ideal oversizing ratio is stent-specific, the studies showed that Nitinol stent oversizing has a very small impact on the immediate lumen gain, which contradicts the clinical motivations of the procedure.
In order to predict the possibility of restenosis through mechanical markers that are associated with the effects of leg movement following stent implantation, clinical investigations should be complimented with patient-specific numerical analyses. Combining intra-arterial imaging methodologies with in-vivo arterial deformations, which can be translated to FE simulations as boundary conditions, and building upon the numerical framework that is introduced in the 2nd part of this thesis, it’s possible to generate accurate patient-specific models. These models, evaluated in conjunction with clinical follow-ups, are expected to provide a deeper understanding of the mechanical background of restenosis in peripheral arteries.

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