Introduction: Randomized controlled trials have now proven the endovascular stent-grafts safety and effectiveness many years after they were first implanted in humans in 1991[1]-[3]. With that said, the occasionally observed in-vivo damage and deterioration of fabrics is still poorly understood[4],[5]. As a newly recognized failure mode, the buckling of the stent-graft could result in major damage to the fabrics at the apexes of sharp-angled zig-zag stents seriously impairing its durability[6]. Furthermore, the presumed twisting of stent-grafts leading to failure has been observed in explanted grafts in the past but we do not have an experimental model to verify this[7],[8]. We hereby report a series of in vitro experiments to investigate the potential damage to stent-grafts from accelerated twisting whose fabrics are supported with zig-zag stents and ringed stents.
Materials and Methods: The zig-zag shaped stent-grafts were manufactured at the College of Textiles, Donghua University. Each stent had 5 apexes with strut angles at 35° and 45° respectively (Figure 1). The ringed Anaconda stent-grafts devices were used as controls.
Figure 1. Three different shaped stents in endovascular stent-graft.
(a) 45° zig-zag stent; (b) 35° zig-zag stent; (c) ringed stent
Figure 2. The twisting process of stent-grafts.
The twisting test machine was developed at Donghua University to mimic the pulsatile dynamics of the blood flow within human tortuous iliac arteries in Figure 2. The twisting angle was 60° at 1 Hz and the distilled water was used instead of blood. The tests were scheduled for durations of 24 h, 48 h, and 168 h and the corresponding number of pulsations and twists were listed in Table 1. All stent-grafts were analyzed for any changes in the fabrics from before and after the accelerating twisting test: i.e. surface morphology (microscopy and SEM) and fabric characteristics (fabric count, thickness, mass and crystallinity).
Table 1. The Number of pulsations and twists
Results and Discussion: After the twisting experiments, stent-grafts with zig-zag stents in 35° angles had the most severe distortion and damage to the fabrics. The zig-zag stents at 45° had less fabric damage. The ringed stent-graft had the least damage observed. Furthermore, the suture holes were consistently stretched, creating channels for possible fluid percolation through the wall among all devices.
The fabric characteristic did not show major differences following the twisting simulation test except for the crystallinity. Compared to the control fabrics, the decrease of the crystallinity of fabrics supported with 35° zig-zag stents was larger than those supported with the 45° zig-zag and ringed stents. The low crystallinity suggests that there is a decline of the mechanical property of fabrics.
Conclusion: Twisting in stent-grafts could cause device-related adverse events: sharp angles of zig-zag stents deserve more attention in order to decrease the damage to the fabrics. The ringed stents presented a better support to the fabrics than the zig-zag stents.
This research program was supported by the 111 Project (B07024), Shanghai Construction of College Experimental Technique Team Project (101-07-0053014) and the National Students Innovation Plans of China (15T10105).
References:
[1] Parodi JC, Palmaz JC, Barone HD. Transfemoral Intraluminal Graft Implantation for Abdominal Aortic Aneurysms. Ann Vasc Surg. 1991;5:491-9.
[2] EVAR trial participants. Endovascular aneurysm repair versus open repair in patients with abdominal aortic aneurysm (EVAR Trial 1): randomised control trial. Lancet 2005; 365: 2179-2186.
[3] Makaroun MS, Tuchek M, Massop D, et al. One year outcomes of the United States regulatory trial of the Endurant stent graft system. J Vasc Surg. 2011; 54: 601-608.
[4] Mehta M, Valdés FE, Nolte T, et al. One-year outcomes from an international study of the Ovation abdominal stent graft system for endovascular aneurysm repair. J Vasc Surg. 2014; 59: 65-73.
[5] Chakfe N, Dieval F, Riepe G, et al. Influence of the textile structure on the degradation of explanted aortic endoprostheses. Eur J Vasc Endovasc Surg. 2004; 27: 33-41.
[6] Yao T, Choules BD, Rust JP, King MW. The development of an in vitro test method for predicting the abrasion resistance of textile and metal components of endovascular stent grafts. J Biomed Mater Res Part B. 2014; 102: 488-499.
[7] Lin J, Wang L, Guidoin R, et al. Stent fabric fatigue of grafts supported by Z-stents versus ringed stents: An in vitro buckling test. J Biomat Appl. 2014; 28: 965-977.
[8] Nutley M, Guidoin R, Yin T, et al. Detailed analysis of a series of explanted Talent AAA stent-grafts: biofunctionality assessment. J long Term Eff Med Implants. 2011; 21:299-319.