The mechanical interaction between the surgical tools and the target soft tissue is mainly dictated by the fracture toughness of the tissue in several medical procedures, such as catheter insertion, robotic-guided needle placement, suturing, cutting or tearing, and biopsy. Despite the numerous experimental works on the fracture toughness of hard biomaterials, such as bone and dentin, only a very limited number of studies have focused on soft tissues, where the results do not show any consistency mainly due to the negligence of the puncturing/cutting tool geometry. In order to address this issue, we performed needle insertion experiments on 3 bovine livers with 4 custom-made needles having different diameters. A unique value for fracture toughness (J=164±6 J/m(2)) was obtained for the bovine liver by fitting a line to the toughness values estimated from the set of insertion experiments. In order to validate the experimental results, a finite element model of the bovine liver was developed and its hyper-viscoelastic material properties were estimated through an inverse solution based on static indentation and ramp-and-hold experiments. Then, needle insertion into the model was simulated utilizing an energy-based fracture mechanics approach. The insertion forces estimated from the FE simulations show an excellent agreement with those acquired from the physical experiments for all needle geometries.
Abstract-Nitinol stent oversizing is frequently performed in peripheral arteries to ensure a desirable lumen gain. However, the clinical effect of mis-sizing remains controversial. The goal of this study was to provide a better understanding of the structural and hemodynamic effects of Nitinol stent oversizing. Five patient-specific numerical models of noncalcified popliteal arteries were developed to simulate the deployment of Nitinol stents with oversizing ratios ranging from 1.1 to 1.8. In addition to arterial biomechanics, computational fluid dynamics methods were adopted to simulate the physiological blood flow inside the stented arteries. Results showed that stent oversizing led to a limited increase in the acute lumen gain, albeit at the cost of a significant increase in arterial wall stresses. Furthermore, localized areas affected by low Wall Shear Stress increased with higher oversizing ratios. Stents were also negatively impacted by the procedure as their fatigue safety factors gradually decreased with oversizing. These adverse effects to both the artery walls and stents may create circumstances for restenosis. Although the ideal oversizing ratio is stentspecific, this study showed that Nitinol stent oversizing has a very small impact on the immediate lumen gain, which contradicts the clinical motivations of the procedure.
Endovascular therapy in patients suffering from peripheral arterial disease shows high rates of restenosis. The poor clinical outcomes are commonly explained by the demanding mechanical environment due to leg movements, but the mechanisms responsible for restenosis remain unknown. In this study, we hypothesized that restenosis following revascularization is associated with hemodynamical markers derived from blood flow during leg flexion. Therefore, we performed personalized computational fluid dynamics (CFD) analyses of 20 patients, who underwent routine endovascular femoro-popliteal interventions. The CFD analyses were conducted using 3D models of the arterial geometry in straight and flexed positions, which were reconstructed from 2D angiographic images. Based on restenosis rates reported at 6-month follow-up, logistic regression analyses were performed to predict restenosis from hemodynamical parameters. Results showed that severe arterial deformations, such as kinking, induced by leg flexion in stented arteries led to adverse hemodynamic conditions that may trigger restenosis. A logistic regression analysis based solely on hemodynamical markers had an accuracy of 75%, which showed that flow parameters are sufficient to predict restenosis (p = 0.031). However, better predictions were achieved by adding the treatment method in the model. This suggests that a more accurate image acquisition technique is required to capture the localized modifications of blood flow following intervention, especially around the stented artery. This approach, based on the immediate postoperative configuration of the artery, has the potential to identify patients at increased risk of restenosis. Based on this information, clinicians could take preventive measures and more closely follow these patients to avoid complications.Electronic supplementary materialThe online version of this article (10.1007/s10237-019-01183-9) contains supplementary material, which is available to authorized users.
The choice of the treatment method affects the postinterventional axial deformations of the FP segment but does not influence the curvature behavior. 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, affect the long-term success of the procedure.
Purpose: To quantify the in-vivo deformations of the popliteal arterial segment (PAS) during leg flexion in subjects with clinically relevant (Fontaine Stage IIb) peripheral arterial disease (PAD).Methods: Five patients with varying calcification levels of the PAS undergoing endovascular revascularization underwent 3D rotational angiography. Image acquisition was performed with the leg straight and with a flexion of 70°/20° in the knee/hip joint. The arterial centerline and the corresponding branches in both positions were segmented to create 3D reconstructions of the arterial trees. Axial deformation, twisting, and curvatures were quantified. Furthermore, the relationships between the calcification levels and the deformations were investigated. Results:We observed an average shortening of the PAS of 5.9% ± 2.5% and twist rate of 3.8 Conclusions:The PAS of patients with symptomatic PAD is exposed to significant deformations during flexion of the knee joint. The curvature is directly affected by the extent of arterial calcification. In contrast, the changes in arterial length and twisting angles are comparable to those of healthy arteries. These measurements have the potential to be used for realistic arterial modeling to improve stent designs.This study also showed the ability of rotational angiography to quantify the three-dimensional deformations of the PAS in patients with various levels of calcification, and that this technique can be used to collect data on larger groups of patients.
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