Numerical study on the effect of stent shape on suture forces in stent-grafts

Fatigue strength and life prediction of lower limb venous stents under three-stage loading conditions

After the implantation of lower limb artery stents, the complex loading conditions imposed on the limb can lead to fatigue failure, which may induce inflammation and restenosis. To investigate the effect of multi-axial loading conditions on the fatigue performance of stents, five stents, namely APsolute Pro (APbott Vascular, USA), Complete SE (Medtronic, USA), Protégé EverFlex (PE3, USA), Pulsar-35 (Biotronik, Germany), and E-luminexx-B (Bard, USA), were analyzed based on the finite element method (FEM). Besides, their fatigue strength was determined under three levels of loading conditions, including tension-bending-torsion and compression-bending-torsion. Based on that, the fatigue life of these stents was predicted. The results showed that based on the nominal stress method, tension-bending-torsion loading had a more significant impact on the fatigue life of stents than compression-bending-torsion loading. Besides, two different types of initial cracks were analyzed by the fracture mechanics method. The results suggested that both the initial crack and the external load were the main causes of stent fatigue fractures. Compared with the loading nature, the influence of the initial crack on stent fatigue life was more significant. Under the same loading condition, the APsolute Pro stent had the longest fatigue life, while the E-luminexx-B stent had the shortest. Moreover, the mechanism of stent fatigue failure was revealed by exploring the fatigue performance and life prediction of stents under complex loading conditions. These findings have important implications for improving the structural design of stents and their clinical selection.

Computational Comparison of the Mechanical Behavior of Aortic Stent-Grafts Derived from Auxetic Unit Cells

PURPOSE

Inappropriate stent-graft (SG) flexibility has been frequently associated with endovascular aortic repair (EVAR) complications such as endoleaks, kinks, and SG migration, especially in tortuous arteries. Stents derived from auxetic unit cells have shown some potential to address these issues as they offer an optimum trade-off between radial stiffness and bending flexibility.

METHODS

In this study, we utilized an established finite element (FE)-based approach to replicate the mechanical response of a SG iliac limb derived from auxetic unit cells in a virtual tortuous iliac aneurysm using a combination of a 180° U-bend and intraluminal pressurization. This study aimed to compare the mechanical performance (flexibility and durability) of SG limbs derived from auxetic unit cells and two commercial SG limbs (Z-stented SG and circular-stented SG models) in a virtual tortuous iliac aneurysm. Maximal graft strain and maximum stress in stents were employed as criteria to estimate the durability of SGs, whereas the maximal luminal reduction rate and the bending stiffness were used to assess the flexibility of the SGs.

RESULTS

SG limbs derived from auxetic unit cells demonstrated low luminal reduction (range 4-12%) with no kink, in contrast to Z-stented SG, which had a kink in its central area alongside a high luminal reduction (44%).

CONCLUSIONS

SG limbs derived from auxetic unit cells show great promise for EVAR applications even at high angulations such as 180°, with acceptable levels of durability and flexibility.

Coronary stent-graft use to salvage a juxta-anastomotic arterial rupture complicating a case of radio-cephalic fistuloplasty

Background

Stenosis is a common complication of haemodialysis arteriovenous accesses. Endovascular approaches with percutaneous transluminal fistuloplasty have largely replaced open surgical approaches as first line treatment. Vessel rupture is an uncommon complication of fistuloplasty and most reports describe venous rupture. Stent-graft deployment can salvage this, however, its use requires careful assessment of the distal vasculature. Arterial rupture with fistuloplasty has rarely been described in the literature. This is a novel case describing the use of a BeGraft coronary stent-graft to manage juxta-anastomotic arterial rupture and pseudoaneurysm complicating fistuloplasty.

Case presentation

A 77 year old female with end stage renal failure secondary to systemic amyloid light chain type amyloidosis was referred for a suspected radio-cephalic arteriovenous fistula stenosis after difficulty cannulating with poor flow during dialysis and clinical reduction in the fistula thrill. Both Doppler ultrasound and intravenous fistulography confirmed a venous stenosis 2 cm distal to the anastomosis. The stenosis was treated by fistuloplasty, however, this was complicated by a rupture of the juxta-anastomotic arterial segment intraoperatively. Intermittent balloon tamponade was used to minimise extravasation although a pseudoaneurysm formed within the damaged arterial segment. The patient’s distal neurovascular status was assessed using the Barbeau test and we sonographically confirmed adequate retrograde arterial flow via a complete palmar arch directing blood from the ulnar artery. After discussion with the renal transplant team, a 4 mm BeGraft coronary stent-graft was deployed to control haemorrhage and bypass the pseudoaneurysm until adequate haemostasis and fistula flow was achieved. Follow-up 3 months post-procedure reported the patient continued with haemodialysis using the stented fistula with no further complications.

Conclusions

To our knowledge, this is the first case report describing the application of BeGraft coronary stent-grafts to salvage fistuloplasty of a radio-cephalic arteriovenous fistula stenosis complicated by juxta-anastomotic arterial rupture and pseudoaneurysm formation. We demonstrate the safety and short-term efficacy of this technology.

Validation and Verification of High-Fidelity Simulations of Thoracic Stent-Graft Implantation

Thoracic Endovascular Aortic Repair (TEVAR) is the preferred treatment option for thoracic aortic pathologies and consists of inserting a self-expandable stent-graft into the pathological region to restore the lumen. Computational models play a significant role in procedural planning and must be reliable. For this reason, in this work, high-fidelity Finite Element (FE) simulations are developed to model thoracic stent-grafts. Experimental crimp/release tests are performed to calibrate stent-grafts material parameters. Stent pre-stress is included in the stent-graft model. A new methodology for replicating device insertion and deployment with explicit FE simulations is proposed. To validate this simulation, the stent-graft is experimentally released into a 3D rigid aortic phantom with physiological anatomy and inspected in a computed tomography (CT) scan at different time points during deployment with an ad-hoc set-up. A verification analysis of the adopted modeling features compared to the literature is performed. With the proposed methodology the error with respect to the CT is on average 0.92 ± 0.64%, while it is higher when literature models are adopted (on average 4.77 ± 1.83%). The presented FE tool is versatile and customizable for different commercial devices and applicable to patient-specific analyses.