Evaluation of a New Approach for Modeling Full Ring Stent Bundles with the Inclusion of Manufacturing Strains

Abstract

Ring stent bundles have been used in several biomedical stent-graft devices for decades, yet in the published literature, the numerical models of these structures always present significant simplifications. In this paper, a finite element (FE) ring stent bundle has been developed and evaluated with a combination of beam and surface elements. With this approach, the shape, the global stiffness and the strains of the structure can all be well predicted at a low computational cost while the approach is suitable for application to non-symmetrical, patient-specific implant simulations. The model has been validated against analytical and experimental data showing that the manufacturing strains can be predicted to a 0.1% accuracy and the structural stiffness with 0-7% precision. The model has also been compared with a more computationally expensive FE model of higher fidelity, revealing a discrepancy of 0-5% of the strain value. Finally, it has been shown that the exclusion of the manufacturing process from the simulation, a technique used in the literature, quadruples the analysis error. This is the first model that can capture the mechanical state of a full ring stent bundle, suitable for complex implant geometry simulations, with such accuracy.

Keywords: Anaconda; Aneurysm; Finite element analysis; Ring bundle; Stent.

Figure 1

The Anaconda™ stent graft placed inside an abdominal aortic aneurysm (AAA). The 1st proximal ring bundle is shown in detail illustrating the multiple turns of Nitinol wire sutured onto the fabric.

Figure 2

During manufacturing, a wire is turned n times to create a bundle (a); yet herein, n wires that occupy the same space are turned once (b).

Figure 3

Section of the BF model. n superimposed wire turns are enclosed inside a shell that represents the bundle (a). For the identification of the bundle diameter, an approximation is used. More specifically, the cross-section of any realistic bundle configuration (b) is reconfigured according to the circle packing theory (c) and the minimum circle, R, that can enclose the wire turns is calculated. The BF model’s cross-section (d) has a bundle diameter, R and hosts all the turns overlapped at its center. All the wire turns have radius, r, as in the original configuration.

Figure 4

The experimental set-up of the ‘saddle pull test’. In the schematic, pairs A, C and B, D represent the peaks and valleys of the ring bundle respectively.

Figure 5

After ring formation, the ring was compacted with the help of a cylindrical sheath (a) down to its delivery size (b). Subsequently, the sheath was inflated (c), allowing the final deployment of the ring inside the vessel, which pulsated between the diastolic and systolic pressure (d). The internal cylindrical surface represents the inner tube present in the physical delivery system.

Figure 6

The CQ model. A quarter of the ring stent is modeled with continuum elements. Each wire turn is considered separately.

Figure 7

Comparative results of all four ring configurations tested in the ‘saddle pull test’. The gray area corresponds to the standard deviation of the experiments while the coloured regions represent the regions of interest (operational range of motion) for each ring bundle. The model with (a) and without (b) the manufacturing strains is assessed.

Figure 8

Each turn of the CQ model experiences tension and compression (a). Similarly, all overlapping turns of the BF model have some integration points under tension (b) and some under compression (c).