A braided stent becomes flattened inside a curved catheter tube: A micro-CT imaging study

Abstract

BACKGROUND:

The braided stent is a widely accepted endovascular treatment device consisting of woven metal wires. One of the unsolved issues for the braided stent is the stent flattening phenomena when deployed into highly curved arteries. Although a recent computational study highlighted that the mechanical state of the stent inside the catheter before the deployment plays an essential role in causing stent flattening, there is no experimental observation for the stent inside the curved catheter.

OBJECTIVE:

We investigated braided stent shapes in curved catheter tubes with various curvatures by micro-computed tomography (CT).

METHODS:

A braided stent was deployed into catheter tubes and set in rectangular cases with constant curvature. The three-dimensional shape of the stent was imaged by micro-CT, and its cross-sectional flatness was quantitatively assessed.

RESULTS:

Stent flattening occurred in cases of high curvatures of the outer side of the tube curvature, and the degree of flatness increased with increasing tube curvature. This demonstrates that stent flattening can be caused inside the highly curved catheter before deployment.

CONCLUSIONS:

This preliminary and first observational report provides new insight into the mechanism of stent flattening and emphasizes the importance of the geometrical and mechanical state of the stent inside the catheter.

Keywords

Braided stent stent flattening micro-computed tomography (CT)endovascular treatment cerebral aneurysm

1. Introduction

The braided stent is a widely accepted endovascular device that has recently attracted interest for cerebral aneurysm treatment as a self-expanding intracranial stent. The braided stent is a simple cylindrical structure that consists of woven metal wires (Fig. 1(a)) and is deployed in parent arteries of cerebral aneurysms via a catheter assisted by guidewire. The stent is mainly used for assisting endovascular coiling to prevent coil protrusion from aneurysms [1,2] and as a flow-diverter to isolate the aneurysm from parent arteries [3,4].

Fig. 1.

Braided stent that consists of 32 Co–Cr wires and two Pr wires (fully expanded (a) and bent by hand (b)).

A critical issue of the braided stent are the technical difficulties of its deployment compared to other kinds of stents [5–7], which may be due to the relatively weak radial force for expansion [5]. These difficulties may lead to the inappropriate stent expansion reported in existing studies [7,8]. Stent flattening is a fatal problem as observed in inappropriate stent expansion patterns during deployment into highly tortuous arteries, in which the cross-section of the braided stent becomes flattened in the midsection of the arterial curve [8,9]. These experimental observations suggest that the bending deformation of the stent in the curved arteries is an essential factor to cause stent flattening. However, the braided stent, which is fully expanded, rarely becomes flattened even under a large bending deformation (Fig. 1(b)). This fact highlights the necessities to deeply understand the mechanical characteristics of the braided stent to interpret the stent flattening mechanism.

Much attention has been paid to the technical characteristics of the braided stent, especially in the last two decades when the braided stent became widely accepted in clinical practice. Several theoretical approaches modeled the braided stent as an assembly of helical coils and estimated the radial force for stent expansion [10–13]. Computational approaches are also promising for estimating apparent stiffness of the braided stent, such as axial and flexural rigidities, with considering material nonlinearities and viscoelastic properties of stent wires [14–21]. In particular, a recent computational study [22] illustrated that the mechanical states of the braided stent inside the catheter before deployment and its transition during deployment play an important role in causing stent flattening. Based on these findings, we hypothesize that the geometric constraints and radial compression by the catheter may be essential to the stent flattening initiated inside the curved catheter before deployment. However, to the best of our knowledge, there are no reports that observe the stent shape inside the curved catheter. Understanding the three-dimensional (3D) stent shapes in a curved catheter may provide valuable references for further consideration of the stent flattening mechanism.

The present study investigates braided stent shapes in a curved catheter tube with various curvatures by micro-computed tomography (CT). For this aim, we developed an experimental setup to image the 3D braided stent and quantitatively evaluated the flatness of the stent cross-sections. Section 2 outlines the experimental setup of the 3D stent shape imaging and evaluates its cross-sectional flatness. The results and their implications are briefly illustrated in Section 3 and Section 4 notes the conclusions.

2. Materials and methods

2.1. Experimental setup

An experimental setup using micro-CT imaging was developed to image the 3D braided stent shape in curved catheter tubes. The braided stent consisted of 32 Co–Cr wires, and two Pt wires with a diameter of 40 μm (Fig. 1), in which the stent diameter was 5 mm (fully expanded state) for a vascular reconstruction device (VRD) (bare braided material, not the products, provided by Allm Inc., Tokyo, Japan). The stent was deployed into the catheter tube (CNC-3/5H, Access Technologies Inc., IL, USA) with a diameter of 1 mm (Fig. 2(a)). The catheter tube with the stent was mounted in a groove in a fixing jig constructed by a 3D printer OBJECT30 Pro (Stratasys Ltd., MN, USA) with Vero Clear material (RGD810) (Fig. 2(b)). The shape of the groove is rectangular with a width of 1.0 mm and depth of 1.0 mm. Along the length of the groove, the groove is straight but partially curved with constant length (6.28 mm) and curvatures (0, 0.1, 0.16, 0.25, and 0.33 mm⁻¹) in the middle. The range of the curvature is consistent with those of the actual internal carotid arteries [23]. We defined the centerline coordinate S with S = 0 at the midpoint of the curved part along the centerline of the groove.

Fig. 2.

Experimental setup: (a) The braided stent was packed in the catheter tube; (b) fixing jig; (c) in-house micro-CT scanner; (d) representative raw image file output from the micro-CT scan; and (e) edited image file with eliminating artifacts.

Fig. 3.

(a) Braided stent shape in the curved catheter tube with a curvature of 0.25 mm⁻¹; (b) stent cross-sections at S = −4, −2, 0, 2, and 4 mm; and (c) cutaway view of the braided stent (cut at S = 0 mm in (a)).

2.2. Micro-CT imaging

The braided stent with a curved catheter tube was imaged using an in-house cone-beam micro-CT scanner (Fig. 2(c)). In this scanner, the X-ray source XWT-160TC (X-RAY WorX GmbH, Germany) and flat panel C7942CA-22 (Hamamatsu Photonics, Inc., Japan) were aligned on the horizontal table in a radiation shielding room. The image target was placed vertically on a rotation stage (ALZ-115-E1P, Chuo Precision Industrial Co., Ltd., Japan) between the X-ray source and flat panel. In the projection images, the pixel size was 8.6 μm × 8.6 μm and the number of pixels was 1120 × 1172. The CT scanning parameters were as follows: the electric voltage was 90 kV, the electric power was 7 W, the total rotation was 360° with a 0.3° rotation step and the exposure time in each angle was 2 seconds. The CT images were output from all projection images by the method of [24] (Fig. 2(d)) and the 3D stent shape was extracted from the CT images by Amira 5.5 (Thermo Fisher Scientific, Inc., MA, USA) considering the ring and metal artifacts and finally exported as a 3D tiff image file format (Fig. 2(e)).

2.3. Evaluations

To quantify stent flattening, we defined the flatness ratio 𝜑 of the stent cross-section. From the 3D images of the stents, two-dimensional (2D) cross-sectional images were extracted along the centerline of the rectangular grooves (e.g., Fig. 3(b)). The stent shapes on the cross-sections were approximated as an ellipsoid and evaluated by a gyration tensor $\mathbf{G}\in \mathbb{R}^{2\times 2}$ . Its component G _ij is defined as $\begin{eqnarray}\displaystyle G_{ij}=\frac{1}{N}\mathop{\sum }_{k=1}^{N}{x_{i}}^{k}{x_{j}}^{k}, & & \displaystyle\end{eqnarray}$ (1) where x is the position vector of the stent wire pixels in the cross-sectional images and N is number of those pixels. The length of longer and shorter axes of the ellipsoid, a and b, can be calculated from the eigenvalues and corresponding eigenvectors of G. Using a and b, the flatness ratio 𝜑 (0 ≤ 𝜑≤ 1) of the stent cross-section is given by $\begin{eqnarray}\displaystyle {\varphi}=\left|\frac{a-b}{a+b}\right|. & & \displaystyle\end{eqnarray}$ (2) 𝜑 will be 0 when the stent shape is a circle and will be close to 1 when flattened.

Fig. 4.

The flatness ratio of the stent cross-sections at the catheter curvatures is 0 (black), 0.1 (blue), 0.16 (green), 0.25 (yellow), and 0.33 mm⁻¹ (pink) (left) and images of each stent cross-sections at the midsection (right). The region between the two dashed lines corresponds to the curved parts of the catheter tubes.

3. Results and discussion

3.1. Stent cross-section flatness

Figure 3 shows a 3D shape of the braided stent in the catheter tube with the curvature of 0.25 mm⁻¹ reconstructed from micro-CT images. Representative stent cross-sections along the centerline at S = 0, ±2, and ±4 mm are shown in Fig. 3(b). The stent cross-sections were flattened in the midsection of the curved part and the degree of flattening became gradually modest from the midsection to both ends (Fig. 3(b)). At the flattened part (S = 0 mm), the stent cross-sections at the outer side of the tube curvature became concave and the wires tended to concentrate on the outer side (Fig. 3(b) S = 0 mm and (c)).

Stent flattening were quantitatively assessed with five patterns of curvatures using the flatness ratio defined in Section 2.3. The flatness ratio of the stent along the centerline of the groove is illustrated in Fig. 4 (left). Although the flatness ratio was constantly zero in the straight case (curvature = 0 mm⁻¹), it gradually increased with increasing curvatures and approached 0.8 for the highest curvature. From the snapshots (Fig. 4 (right)), the stent cross-section apparently became flattened when the curvature was 0.25 mm⁻¹ or greater. Figure 5 summarizes the maximum flatness ratio of the braided stent in the curved catheter tubes with various curvatures. The maximum flatness ratio monotonically increased with increasing curvature and reached 0.8.

Fig. 5.

Maximum flatness ratio of the braided stent cross-sections in the curved catheter with various curvatures.

3.2. Implications for stent flattening mechanisms

The observations of this study illustrate stent flattening in the catheter tube with high curvatures. This result demonstrates that the stent flattening can be caused not only during the deployment process, but also before the deployment process when the stent is inside the catheter. This observation supports our hypothesis that radial compression and geometric constraints by the catheter are essential factors that can cause stent flattening. Because the catheter wall is significantly stiffer than the braided stent, the braided stent is compressed in radial directions and its expansion is geometrically constrained in the catheter. Although the association between stent flattening inside the catheter and that inside the arteries is not clear, further investigation of the braided stent mechanics under these mechanical conditions may be valuable to deepening the understanding of the stent flattening mechanism.

In general, bending buckling of cylindrical tube is caused at the inner side of tube curvature, c.f. [25]. However, the stent cross-sections became concave at the outer side of the tube curvature. Because the stent wires do not strictly constrain each other and the wires could slide with increasing tube curvatures, this finding suggests that the braided stent has different mechanical characteristics compared to typical continuum material tube.

3.3. Limitations

This study aims to investigate a 3D braided stent shape under radial compression and geometric constraints, and the preliminary observations conducted have differences in the experimental protocol from clinical situation. Thus, these differences give rise to mainly three limitations to consider the braided stent flattening in clinical practice. First, the guidewire, which is commonly set in the catheter to assist stent deployment, was not considered because the guidewire causes a severe metal artifact in the CT images making it difficult to segment the stent outlines from the images. The guidewire in the catheter acts as the geometric constraints on the braided stent, and thus this constraint may affect the stent behaviors inside the catheter in a clinical situation. Second, the diameter of the catheter tube used in this study was larger than the catheter used in cerebrovascular treatment, owing to difficulties of setting the braided stent in the catheter tube with a small diameter. Although stent flattening was observed in the curved tubes in this study, the catheter diameter might be an influential factor causing braided stent flattening. Third, we considered the stent flattening in 2D curved catheter with no torsion, as idealized condition, whereas the actual arteries contain 3D complexity (e.g., torsion). Further consideration of the actual arterial shape might provide more precise knowledge of the flattening mechanism.

4. Conclusions

The present study investigated braided stent shapes in the curved catheter tubes with various curvatures. An experimental setup to image the 3D braided stent shapes in the curved tubes was developed with using a micro-CT scanner and the flatness of the stent cross-sections with different catheter curvatures were quantitatively evaluated. Stent flatness was observed in the curved catheter tubes and the degree of flatness increased with increasing catheter curvatures. This finding highlights that the stent flattening can occur inside the curved catheter at the curvature where the flattening does not occur in the case of the expanded stents. This fact suggests that the mechanical characteristics of braided stent in clinically-reasonable situations (inside the catheter) should be considered, whereas the mechanical characteristics of stents has been investigated usually at fully expanded state. In addition, the stent cross-section became flattened and the concave shape at the outer side of the tube curvature. This implies that the braided stent mechanics cannot be directly interpreted by typical continuum material tube mechanics.

This study demonstrates that stent flattening can occur inside the catheter even before deployment into the curved arteries. This preliminary but first observation report provides new insight into the mechanisms of stent flattening and would be helpful for designing the stent shape and deployment protocol.

Footnotes

Acknowledgements

We thank Makoto Nishigishi for providing the braided stent used in this study. We thank Hiroyuki Takao, Soichiro Fujimura and Shigeru Yoneyama for clinical input. We also thank Toshihiro Sera, Kaori Nagaike, and Katsuki Kashiyama for micro-CT setups and Naoki Takeishi, Shunichi Ishida, and Shun Takenaka for fruitful discussion. We thank Ashleigh Cooper, PhD, from Edanz Group () for editing the draft of this manuscript.

Conflict of interest

None to report.

Funding

This work was supported by research grants from JSPS Grants-in-Aid for Scientific Research (18H18367 to Otani) and MEXT as “Priority Issue on PostK computer (Supercomputer Fugaku)” (Integrated Computational Life Science to Support Personalized and Preventive Medicine) (hp190187 to Wada).

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