Four key bench-top tests, including trackability, conformability, wall-apposition, and bending stiffness, were performed to understand the mechanical characteristics in 3 different types of stents applicable for treatment of intracranial atherosclerotic stenosis: Balloon-expandable D+Storm, Pro-Kinetic Energy, and self-expandable Wingspan stents.
Trackability was assessed by measuring the tracking forces of each stent with its delivery systems. Conformability and wall apposition were quantified and analyzed using curved vessel models. A 3-point bending test was employed to evaluate bending stiffness.
D+Storm showed the lowest tracking forces while the conformability of the Wingspan stent was superior to that of the tested stents. Pro-Kinetic Energy and D+Storm had better wall apposition in curved vessels than the Wingspan stent. Bending stiffness of the Wingspan stent was notably lower, whereas no significant differences were found between D+Storm and Energy. Pro-Kinetic Energy and D+Storm not only indicated lower gap ratios between the struts and the vessel wall but also maintained good wall apposition even in the curved model.
These bench-top measurements may provide clinicians with useful information in regard to selecting suitable stents for treatment of intracranial atherosclerotic stenosis.
Stroke is one of the leading causes of death worldwide, and ischemic stroke is made up of 87 percent of stroke cases [
In the early stage of endovascular treatment using intracranial stenting, balloon-expandable stents (BES) are the only available option for intracranial atherosclerotic stenosis, which is intended to be designed for treating cardiovascular disease [
In this study, we conducted an
Three intracranial stenosis stents, Wingspan (15 mm length; Boston Scientific), PRO-Kinetic Energy (15 mm length; Biotronik), and D+Storm (16 mm length; CGBio), were investigated for benchtop comparative tests in the study. All stent models were uncovered metallic stents with a diameter of 3.5 mm. Stent material, strut dimensions, stent type, and structure analysis of the tested stent samples are summarized in
The trackability describes the ability of the stent and its delivery system to be delivered softly to the target lesion through a tortuous vessel anatomy. Trackability of the stent and its system was evaluated using interventional device testing equipment (IDTE-3000; MSI, Flagstaff, AZ, USA) as shown in
Conformability played a major role in determining the degree of geometric changes in intracranial arteries when stents were deployed. To investigate the ability of stents in conforming to their original vessels, a benchtop test of conformability was performed. Conformability was assessed by quantifying changes in angle values of a curved vessel model after stent deployment when compared to the angle value in the natural shape as seen in
Wall-apposition addresses a stent’s ability to maintain strut apposition to the vessel wall when deployed in curved vessels. A silicone curved vessel model (Circle of Willis Model; United Biologics, Santa Ana, CA, USA) with a 3.5 mm inner diameter was used in this study, and the stents were deployed in a curved segment having a radius with a curvature of approximately 3 mm. During the test, the vessel model was filled with water containing lubricant, and the solution was kept flowing through a flow pump system (FlowTek 125 System; United Biologics). Stent deployment was undertaken 3 times in each stent model including Wingspan, PRO-Kinetic Energy, and D+Storm stents. Images were photographed to analyze the apposition of each stent model to the curved vessel wall, and ImageJ software (NIH) was used to measure the gap between the vessel wall and the deployed stents. The measurements were calibrated by using a 3.5 mm inner diameter of the vessel model as a reference. The gap ratio was measured by the gap distance divided by the diameter of the vessel.
Bending stiffness is a measure of the stent platform’s resistance to bending deformation which relates to its flexibility. To characterize the bending stiffness, a 3-point bending test was applied in the study. The bending force of the 3-point bending test was evaluated by using a universal tensile-compression machine with its apparatus. Forces were measured with 5.0 N load cells at a speed of 10 mm/min. The measurement of bending force was performed in the expanded state as demonstrated in
Mechanical properties of stents are crucial in evaluating the clinical performance of in-stent treatment for intracranial atherosclerotic stenosis. Currently, there are 2 types of intracranial stenosis stents available. One is a self-expanding stent made of nitinol materials, and the other is a cobalt-chromium-based balloon-expanding stent. Clinical studies have shown pros and cons for patients treated with balloon-expanding and self-expanding stents for ICAS [
In our study, D+Storm showed the lowest tracking forces, while the conformability of the Wingspan stent was superior to that of the tested stents. Pro-Kinetic Energy and D+Storm had better wall apposition in curved vessels than the Wingspan stent. Bending stiffness of the Wingspan stent was notably lower, whereas no significant differences were found between D+Storm and Energy. Pro-Kinetic Energy and D+Storm not only indicated lower gap ratios between the struts and the vessel wall but also maintained good wall apposition even in the curved model.
The evaluation of trackability is an essential aspect in determining the proper intracranial stenosis stent, as they must go through tortuous vascular anatomy in the implantation process. In the present study, the trackability of self-expanding Wingspan and 2 different BES systems (D+Storm, Pro-kinetic Energy) were assessed by measuring track force while advancing along the pathway of the vessel model. The trackability relies on varied parameters such as the crimped cross profile of the stent, stent platform design, strut dimensions, and stiffness of the delivery system. The investigations of trackability demonstrate that D+Storm stent-catheter systems have the lowest track force among the examined stents, indicating they can provide easier delivery to the target lesion compared to Pro-Kinetic Energy and Wingspan stent systems. In contrast, the Wingspan stent exhibits a higher tracking force than both the Pro-Kinetic Energy and D+Storm stent systems.
The result of a higher track force in the Wingspan stent system was unexpected because self-expanding stent systems commonly are considered better at stent deliverability, due to not only their raw materials but also their thinner strut dimensions. This phenomenon might be attributed to a difference in the stent delivery system applied under bench-top experimental conditions when compared to the other 2 stents employing the balloon-expandable delivery system. The Pro-Kinetic Energy and D+Storm stent systems consist only of stents pre-mounted on its balloon catheter. In contrast, the Wingspan stent system has a more complex delivery system containing a pre-loaded stent, a microcatheter for stent delivery, and an additional Gateway angioplasty balloon combined together for pre-dilating the lesion. Therefore, the microcatheter with the balloon catheter system of the Wingspan stent may result in recording a higher track force when compared to the other 2 balloon catheter systems. However, further studies are needed to understand these tracking force behaviors, since clinical conditions and scenarios are much more complicated and hostile than those of bench-top experimental conditions.
Conformability is an important factor that affects the mechanical properties of intracranial stenosis stents. In particular, it may be closely associated with bending stiffness [
In regards to flexibility, bending stiffness was examined using 3 point-bending methods. The results of bending stiffness tests also showed a similar tendency as shown in the conformability test in which the Wingspan stent had the least amount of changes in bending stiffness, followed by D+Storm and Pro-Kinetic Energy. Here, it should be emphasized that different types of stent materials play a key role in determining their conformability as well as flexibility.
Wall apposition is the ability of the stent struts to sustain attachment to the vessel walls when deployed in curved and tortuous cerebral vessels. It is obvious that incomplete stent apposition leads to adverse clinical events such as late stent thrombosis, early/delayed stent migrations, and related vessel occlusions [
Although a mechanical comparison of the 3 different stents using bench-top measurements furthered our understanding of particular stents, the bench tests carried out in our study have several limitations. Namely, the conditions set up in our experiment do not fully reflect those in actual clinical performance. The results in clinical trials can change in vessel properties with diameters as well as tortuosity, since each stent model and type may have different mechanical strength and characteristics depending on stent diameters and length. Also, the surface friction of the tested vessel models is not identical to that of a human vessel, which may influence the apposition behavior of the tested stents. Lastly, the vessel tortuosity under bench-top conditions is less tortuous than in actual intracranial vascular arteries. Thus, further bench-top tests are needed to understand the effects of different types of stents in various experimental conditions. Despite these limitations, the bench-top results of this study can provide clinicians with useful insights on choosing the appropriate stent system for the treatment of intracranial atherosclerosis.
As demonstrated in the study results, Wingspan stents are better in conformability due to not only the lowest angle variation but also the lowest bending stiffness. However, the results of wall-apposition tests indicate that they perform the largest separation of the struts such as kinking and incomplete expansions from the curved-vessel walls, which might be explained by their inferior bending force compared to D+Storm and Pro-Kinetic Energy stents.
However, there is still a limited amount of bench-top studies on current intracranial stenosis stents to better understand their mechanical properties, particularly those that are related to clinical implications. Therefore, further investigation is required, which will facilitate in selecting the most appropriate stent system for certain target sites as well as lesion types.
This study was supported by the Korean Society of Interventional Neuroradiology (KSIN) research grant 2018.
This study was approved by the Institutional Review Board of Jeonbuk National University Hospital.
The authors have no conflicts to disclose.
Concept and design: HSK. Analysis and interpretation: JWK. Data collection: JSP and JWK. Writing the article: JSP and JWK. Final approval of the article: HSK
Vessel model and stent system for trackability tests: (
Schematic images of angle variations of the tested stents (
Images of test setup of the 3-point bending test for measuring bending stiffness.
Tracking force-distance curves of trackability measurements: (
Images of angle variation results for measurements of conformability (in the following order): D+Storm, Pro-Kinetic Energy, and Wingspan.
Images of stent-vessel wall apposition measurements (D+Storm, Pro-Kinetic Energy, and Wingspan stents, respectively).
Images of apposed stents to the curved vessel wall: D+Storm, Pro-Kinetic Energy, and Wingspan.
Graphs displaying bending force-displacement curves for measurement of bending stiffness: (
Stent material, strut dimensions, stent, type, and structure analysis of the tested stents (Pro-Kinetic Energy, D+Storm, and Wingspan)
Pro-Kinetic Energy | D+Storm | Wingspan | |
---|---|---|---|
Materials | Co-Cr alloy | Co-Cr alloy | Ni-Ti alloy |
Strut thickness (μm) | 60 (Di 2.25–3 mm) | 80 | 75 |
80 (Di 3.5–4 mm) | |||
Strut width (μm) | 60 (Di 2.25–3 mm) | 80 | 70 |
80 (Di 3.5–4 mm) | |||
Stent type | Balloon-expandable | Balloon-expandable | Self-expandable |
Cells | 12 crowns | 6&8 crowns | 9 crowns |
Connectors | 4 linkers | 2 linkers | 3 linkers |
Di, diameter.
Summary of the bench-top testing results of the tested stents: D+Storm, Pro-Kinetic Energy, and Wingspan
Stent | Max track force (N) | Angle variation (°) | Gap ratio between stent and wall (%) | Bending force (N) |
---|---|---|---|---|
D+Storm | 0.525±0.021 | 10.76±0.46 | 9.28±3.10 | 0.084±0.005 |
Pro-Kinetic Energy | 0.625±0.020 | 11.80±0.97 | 8.82±4.88 | 0.100±0.005 |
Wingspan | 1.186±0.321 | 8.78±0.60 | 12.66±3.23 | 0.047±0.003 |