Analysis of mechanical properties of the stent's outflow segment during thoracic endovascular aortic repair

Yu Shen; Zheng Chen; Qingsheng Lu

doi:10.4103/2589-9686.377612

ORIGINAL STUDY

Year : 2023 | Volume : 6 | Issue : 1 | Page : 6-10

Analysis of mechanical properties of the stent's outflow segment during thoracic endovascular aortic repair

Yu Shen¹, Zheng Chen², Qingsheng Lu²
¹ Department of General Surgery, Eastern Theater General Hospital, Nanjing; Department of Vascular Surgery, Changhai Hospital, Shanghai, China
² Department of Vascular Surgery, Changhai Hospital, Shanghai, China

Date of Submission	03-Nov-2022
Date of Decision	15-Jan-2023
Date of Acceptance	28-Jan-2023
Date of Web Publication	26-May-2023

Correspondence Address:

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2589-9686.377612

Abstract

AIMS AND OBJECTIVES: To discuss the mechanical properties of stents and optimize the stents' selection scheme of overlapping segment through in vitro mechanical simulation of stent grafts.
MATERIALS AND METHODS: The radial force tester was used to measure the radial force of different aortic covered stents, bare stents and the combination of the two. Linear relationship between the radial force at the distal end of the stent and oversize rate was plotted. The stent-induced aortic wall shear stress was evaluated intuitively by mechanical simulation.
RESULTS: The radial force curve of the distal end of the thoracic aortic covered stent reflected the superelastic characteristics of aortic stent, and the characteristics was more obvious in stents composed of nickel-titanium than stainless; When different size of Hercules stents were combined with the same sized restrictive bare stent, the oversize rate of overlapping segment was the same, and the radial force was similar; At the initial compression diameter, the use of restrictive bare stent can improve the chronic outward force, but at high compression level, the chronic outward force of the overlapping segment was higher than that of the individual aortic covered stent.
CONCLUSIONS: Based on in vitro mechanical simulation, the stent-vessel wall interaction after implantation can be more intuitively understood. The application of restrictive bare stents can change the radial force of the overlapping segment of the stents by reducing the oversize rate of the distal end of the stent-graft, but it can be harmful at a higher compression level.

Keywords: Thoracic endovascular aortic repair, stent's outflow segment, oversize rate

How to cite this article:
Shen Y, Chen Z, Lu Q. Analysis of mechanical properties of the stent's outflow segment during thoracic endovascular aortic repair. Vasc Invest Ther 2023;6:6-10

How to cite this URL:
Shen Y, Chen Z, Lu Q. Analysis of mechanical properties of the stent's outflow segment during thoracic endovascular aortic repair. Vasc Invest Ther [serial online] 2023 [cited 2023 Jun 4];6:6-10. Available from: https://www.vitonline.org/text.asp?2023/6/1/6/377612

Introduction

Recent years, mounting number of researches have reported stent-induced new entry tear (SINE) in the treatment of thoracic aortic dissection.^[1],[2] Currently, the main focus is on oversize rate of the stent's outflow segment.^[1],[3],[4] As a result, the researchers suggest that the use of restrictive bare-stent can effectively reduce the occurrence of the complication, improve aortic remodeling, and lower the endovascular re-intervention rate.^[5],[6] However, there is still the possibility of SINE and overlapping segment rupture.^[7] This may be relevant to the mismatch of stent selection in the overlapping segment but lacking favorable data and evidence. To our knowledge, there is no research analyzing the mechanical properties of the overlapping segment. Therefore, this study intends to compare the mechanical properties of the stent's outflow segment through in vitro mechanical test during thoracic endovascular aortic repair (TEVAR) and provide a better stent selection strategy for TEVAR.

Materials and Methods

Samples

MicroPort Hercules (30–26–160 mm, 32–28–160 mm, 34–30–160 mm, 36–32–160 mm), Cook Zenith (36–36 mm), Gore TAG (34–34–100 mm), Medtronic Valiant (32–32–155 mm) [Table 1] and restrictive bare stent (RBS) - Optimed Sinus-XL (20–20–80 mm, 22–22–80 mm, 24–24–80 mm) aortic stents were selected and evaluated.

Table 1: Aortic covered stents

Click here to view

Measurement method of radial force

RF was measured with an radial force (RF) measurement machine (Rx-650, Machine Solutions Inc., AZ) [Figure 1]. The test environment was maintained at 37°C with nonliquid circulation. The testing standard followed the recommendations in the ISO standard (BS EN ISO 25539-1: 2009 Cardiovascular Implants - Endovascular devices - Part 1: Endovascular prostheses D.5.3.15 RF). The outflow segment of the stent in a fully expanded state was positioned in the Rx machine. The stent was crimped by a cylinder to its minimum size, after that the force on the cylinder was reversed by the re-expansion force of the stent until it recovered to a fully expanded state. In the course of the procedure, their RF profiles were continuously measured.^[8] The experiment went through three crimping re-expanding cycles.

Figure 1: Rx-650 system. (a) Test environment, (b) The stent is placed in the machine jaw of Rx-650

Click here to view

Measurement index

Chronic outward force (COF), referring to RF in the course of stent expansion; resistive radiation force (RRF), referring to RF during stent crimping.^[9]

Results

The COF profile of the aortic stents is shown in [Figure 2]. The results of Hercules stents demonstrate that at the start of crimping, the aortic stent had a sharp increase in COF, followed by a relatively stable growth rate beyond a certain point (oversize rate 14.749%–17.035%) [Figure 2]a. Compared to Cook Zenith 36–36 mm, Gore TAG 34–34 mm, and Medtronic Valiant 32–32 mm, the trend in the relationship between the COF and the oversize rate of the stent was similar, the inflexion of Cook stent's COF profile appears earlier and is less obvious [Figure 2]b.

Figure 2: COF profile of aortic stents. (a) COF profile of the stent's outflow segment of Hercules in difference sizes. (b) COF profile of the stent's outflow segment of Cook Zenith 36–36 mm, Gore TAG 34–34 mm and Medtronic Valiant 32–32 mm. COF: Chronic outward force

Click here to view

The RF profile is similar between bare-stents of the same size and between aortic stents of different sizes under the same compression degree. However, when the aortic stents are used alone, taking Hercules of different sizes as an instance, there are apparent differences in their RF profiles under the same compression degree [Figure 2]a. It shows that the bare stent performs a major function in the RF profile of the overlapping segment at the same compression diameter [Figure 3].

Figure 3: RF profile of Hercules combined Optimed. (a) RF profile of Hercules in different sizes combined Optimed 20 mm. (b) RF profile of Hercules in different sizes combined Optimed 22 mm. (c) RF profile of Hercules in different sizes combined Optimed 24 mm. RF: Radial force. COF: Chronic outward force, RRF: Resistive radiation force

Click here to view

At the initial compression degree, aortic stents combined with a bare stent can improve the COF intensity of the overlapping segment to some extent. However, the COF intensity of the overlapping segment is higher than that of a single aortic stent at a high compression degree. Taking Valiant 32–32 plus Optimed 20 mm as an example, the COF of the overlapping segment was less than that of the aortic stent alone until they were compressed to 18.18 mm in diameter [Figure 4]e.

Figure 4: (a-f) The red curve represents the COF profile of a single aortic stent, the green curve represents the COF profile of the overlapping segment, the red dot represents the COF intensity of the overlapping segment is equal to that of the aortic stent alone at a certain compression diameter

Click here to view

Discussion

Since 2010, a number of studies on the occurrence of SINE after endovascular repair of aortic dissection have been published.^[1],[2] Most notably, Dong et al.^[10] highlighted that the compliance of the aortic true lumen, the stress property of the stent graft, and the re-expansion trend of the stent graft were the important causes of SINE. Besides, researchers discussed the relevant risk factors leading to the SINE after endovascular repair of aortic dissection and reported that a greater oversize rate of the stent's outflow segment was a significant predictor of major adverse events, because it could exert an excessively high COF to the aortic wall.^[3],[4] Currently, multiple strategies are used to reduce the risk of SINE in patients with aortic dissection. Among them, a certain amount of researchers recommend the application of RBS, which has been proved to reduce the incidence of SINE.^[5],[6],[11] Nevertheless, if the bare-stents do not match the main stents, there is also a possibility of stent migration, bare stent fracture and vessel wall injury. Therefore, there is no clear view on how to regulate the use of bare stents.

The RF created by stent-graft maintains and expands the true lumen patency at the segment where the stent is deployed. Two parameters describing a stent's strength characteristics have been defined by Duerig et al.^[12] RRF – the force that the stent exerts as it resists squeezing by constriction of the artery, and COF – the force that the stent exerts on the artery as it tries to expand to its norminal diameter when the artery is relaxed. Although the RRF is greater than the COF in the course of the entire stent compression process, the COF is usually considered to be the main factor that contributes to vascular wall damage due to the different timing of the two actions.

In the study, we found that lowering the oversize rate at the distal end of the stent could reduce COF. Corresponding to the above results, Jang et al.^[4] reported a 28% increase in SINE risk for every 10% change in oversize rate of the stent's outflow segment (hazard ratio 1.28, 95% confidence interval: 1.12–1.48, P < 0.001). The inflexion of Cook stent's COF profile appears earlier and is less obvious than other aortic-covered stents [Figure 2], this may be related to the use of stainless steel as the supporting metal. In comparison with Nitinol, stainless steel can offer more supporting force, so that the inner cavity of the aortic stents can be expanded more fully. In addition, we used bare stents of the same size combined with aortic stents of different sizes, the COF profile is similar under the same compression level. We believe that the distal diameter of the stent's outflow segment can be adjusted to make the stent better adapt to the distal anatomical environment of the true lumen, and the COF of the stent's outflow segment can be reduced. Furthermore, compare to aortic stent alone, we observed that the COF of stent's outflow segment in the initial compression section was smaller in the overlapping segment, and that was higher in high compression level. Therefore, we believe that the use of RBS can improve the RF at the stent's outflow segment by reducing the oversize rate, and when exceeding certain compression level, it will increase the COF and may damage the vessel wall. In clinical practice, it implies that if we don't have a bare stent of the right size, it is more dangerous to use a bare stent. According to the systematic analysis by Canaud et al.^[13] about 9.2% (10/108) of stent-related complications were achieved by PETTICOAT, including 5 stent migrations, 4 stent misalignments (lack of complete adherence of the stent to the aortic wall), and 1 focal rupture.

Based on the above results, we are inspired in stent design and clinical treatment: (1) The selection of bare stents with appropriate oversize rate is beneficial in reducing the risk of distal intimal injury. It can effectively fit to the vessel wall, optimize distal aortic remodeling, and reduce the risk of intimal injury; (2) In the absence of properly sized bare stents, it may be safer to use the main stent alone; (3) The above results also suggest that the RF at the stent's outflow segment can be optimized by changing the structure of the stent. In order to achieve the ideal outcomes, cooperation between clinicians and manufacturers is vitally necessary.

Limitations

There are limitations to this study. Whether there should be different considerations for acute and chronic aortic dissection, and how to balance the remodeling of the true lumen with the damage to the vessel wall caused by COF need further demonstrations in clinical practice. The test in this study was carried out by an RF measurement machine, which measures the circular diameter, while in clinical practice, the selection of distal stents is usually based on the long diameter of the true lumen at the level of the intended distal end of the stent graft, and the two are significantly different in morphology.

Conclusion

The use of an RBS may reduce the distal oversize rate and lower the incidence of distal intimal injury. However, our results suggest that an excessive compression of the overlapping segment may increase the risk of intimal injury on the contrary. Therefore, more complete tests and case observation studies are still needed for a better clinical therapeutic effect.

At the initial compression degree, aortic stents combined with a bare stent can improve the COF intensity of the overlapping segment to some extent. However, the COF intensity of the overlapping segment is higher than that of a single aortic stent at a high compression degree [Figure 4]a, [Figure 4]b, [Figure 4]c, [Figure 4]d, [Figure 4]e, [Figure 4]f. Taking Valiant 32–32 plus Optimed 20 mm as an example, the COF of the overlapping segment was less than that of the aortic stent alone until they were compressed to 18.18 mm in diameter [Figure 4]e.

Acknowledgment

The authors thank Zhuoyan Xu for her valuable suggestions in preparing the manuscript.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1.	Li Q, Wang LF, Ma WG, Xu SD, Zheng J, Xing XY, et al. Risk factors for distal stent graft-induced new entry following endovascular repair of type B aortic dissection. J Thorac Dis 2015;7:1907-16.
2.	Zhu C, Huang B, Zhao J, Ma Y, Yuan D, Yang Y, et al. Influence of distal entry tears in acute type B aortic dissection after thoracic endovascular aortic repair. J Vasc Surg 2017;66:375-85.
3.	Huang CY, Weng SH, Weng CF, Chen WY, Chen IM, Hsu CP, et al. Factors predictive of distal stent graft-induced new entry after hybrid arch elephant trunk repair with stainless steel-based device in aortic dissection. J Thorac Cardiovasc Surg 2013;146:623-30.
4.	Jang H, Kim MD, Kim GM, Won JY, Ko YG, Choi D, et al. Risk factors for stent graft-induced new entry after thoracic endovascular aortic repair for stanford type B aortic dissection. J Vasc Surg 2017;65:676-85.
5.	Kazimierczak A, Rynio P. Extended petticoat strategy in Type B aortic dissection. Eur J Vasc Endovasc Surg 2019;57:302.
6.	Feng J, Lu Q, Zhao Z, Bao J, Feng X, Qu L, et al. Restrictive bare stent for prevention of stent graft-induced distal redissection after thoracic endovascular aortic repair for type B aortic dissection. J Vasc Surg 2013;57:44S–52S.
7.	Duque Santos Á, Reyes Valdivia A, Gordillo Alguacil S, Ocaña Guaita J, Gandarias Zúñiga C. Symptomatic aortic bare-metal stent fracture after petticoat technique for complicated type b aortic dissection. Ann Vasc Surg 2019;59:311.e1-4.
8.	Egron S, Fujita B, Gullón L, Pott D, Schmitz-Rode T, Ensminger S, et al. Radial force: An underestimated parameter in oversizing transcatheter aortic valve replacement prostheses: In vitro analysis with five commercialized valves. ASAIO J 2018;64:536-43.
9.	Duda SH, Wiskirchen J, Tepe G, Bitzer M, Kaulich TW, Stoeckel D, et al. Physical properties of endovascular stents: An experimental comparison. J Vasc Interv Radiol 2000;11:645-54.
10.	Dong Z, Fu W, Wang Y, Wang C, Yan Z, Guo D, et al. Stent graft-induced new entry after endovascular repair for stanford type B aortic dissection. J Vasc Surg 2010;52:1450-7.
11.	Sultan I, Dufendach K, Kilic A, Bianco V, Trivedi D, Althouse AD, et al. Bare metal stent use in type B aortic dissection may offer positive remodeling for the distal aorta. Ann Thorac Surg 2018;106:1364-70.
12.	Duerig TW, Pelton AR, Stöckel D. The utility of superelasticity in medicine. Biomed Mater Eng 1996;6:255-66.
13.	Canaud L, Faure EM, Ozdemir BA, Alric P, Thompson M. Systematic review of outcomes of combined proximal stent-grafting with distal bare stenting for management of aortic dissection. Ann Cardiothorac Surg 2014;3:223-33.