Model reduction methodology for computational simulations of endovascular repair

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.

Patient-Specific Numerical Simulations of Endovascular Procedures in Complex Aortic Pathologies: Review and Clinical Perspectives

The endovascular technique is used in the first line treatment in many complex aortic pathologies. Its clinical outcome is mostly determined by the appropriate selection of a stent-graft for a specific patient and the operator's experience. New tools are still needed to assist practitioners with decision making before and during procedures. For this purpose, numerical simulation enables the digital reproduction of an endovascular intervention with various degrees of accuracy. In this review, we introduce the basic principles and discuss the current literature regarding the use of numerical simulation for endovascular management of complex aortic diseases. Further, we give the future direction of everyday clinical applications, showing that numerical simulation is about to revolutionize how we plan and carry out endovascular interventions.

Patient-specific computational modeling of endovascular aneurysm repair: State of the art and future directions

Endovascular aortic repair (EVAR) has become the preferred intervention option for aortic aneurysms and dissections. This is because EVAR is much less invasive than the alternative open surgery repair. While in-hospital mortality rates are smaller for EVAR than open repair (1%-2% vs. 3%-5%), the early benefits of EVAR are lost after 3 years due to larger rates of complications in the EVAR group. Clinicians follow instructions for use (IFU) when possible, but are left with personal experience on how to best proceed and what choices to make with respect to stent-graft (SG) model choice, sizing, procedural options, and their implications on long-term outcomes. Computational modeling of SG deployment in EVAR and tissue remodeling after intervention offers an alternative way of testing SG designs in silico, in a personalized way before intervention, to ultimately select the strategies leading to better outcomes. Further, computational modeling can be used in the optimal design of SGs in cases of complex geometries. In this review, we address some of the difficulties and successes associated with computational modeling of EVAR procedures. There is still work to be done in all areas of EVAR in silico modeling, including model validation, before models can be applied in the clinic, but much progress has already been made. Critical to clinical implementation are current efforts focusing on developing fast algorithms that can achieve (near) real-time solutions, as well as ways of dealing with inherent uncertainties related to patient aortic wall degradation on an individualized basis. We are optimistic that EVAR modeling in the clinic will soon become a reality to help clinicians optimize EVAR interventions and ultimately reduce EVAR-associated complications.

Numerical modeling and simulations of type B aortic dissection treated by stent-grafts with different oversizing ratios

Retrograde type A dissection after thoracic endovascular aortic repair has been a major drawback of endovascular treatment. This study investigated the biomechanical mechanism of stent-graft-induced new lesions after implantation and analyzed the relationship between radial force and spring-back force of the stent-graft when it was implanted virtually under different oversizing ratios. Based on the computed tomography angiography images, a three-dimensional geometric model of a patient-specific aortic dissection was established. The stent was designed in CAD software and the stent-graft implantation procedure under different oversizing ratios was simulated in the finite element analysis software. Implantation simulations were performed six times for each stent-graft model under 0%, 3%, 6%, 9%, 12%, and 15% oversizing ratios and the peak stress of the aorta was compared among groups. It was observed that the peak stress of the aorta was located where the proximal bare stent interacted with aortic wall and its value was increased by 62.2% from 0% to 15% oversizing ratio. The conclusions are reached that the long-term higher stress in the aortic wall may lead to the emergence of new lesions in these areas, and the radial force plays a key role in the formation of a new entry in the real aorta model.

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

Second-generation stent-grafts (SGs) have addressed many of the mechanical problems reported for first-generation endoprostheses, such as graft tear and stent rupture; however, suture wear and detachment due to pulsatile fatigue remains an issue. Numerical studies on the mechanical behavior of these endoprostheses usually model the attachment between stents and graft as a continuous ''tie'' constraint, which does not provide information on the mechanical loads acting on individual sutures. This paper presents a suitable approach for Finite Element (FE) simulations of SGs which allows for a qualitative evaluation of the loads acting on sutures. Attachment between stents and graft is modeled as rigid beams at discrete locations of the endoprostheses, and the reaction forces on the beams are analyzed. This modeling strategy is employed for four different SG models (two Z-stented commercial models and two circular-stented models) subjected to a severe 180° U-bend, followed by intraluminal pressurization. Results show that, for all models, the majority of sutures is experiencing fluctuating forces within a cardiac cycle (between 80 and 120 mmHg), which points to pulsatile fatigue as potential failure mode. In addition, the highest loads are concentrated in kinks and, for Z-stented models, at the apexes of stents. Moreover, suture loads for circular-stented models are lower than for Z-stented models, indicating better resistance to suture detachment. All these observations are in line with experimental results published in the literature, and, therefore, the procedure herein proposed may serve as a valuable tool in the development of new SG models with better suture resistance to pulsatile wear and fatigue.

Patient-specific in silico endovascular repair of abdominal aortic aneurysms: application and validation

Non-negligible postinterventional complication rates after endovascular aneurysm repair (EVAR) leave room for further improvements. Since the potential success of EVAR depends on various patient-specific factors, such as the complexity of the vessel geometry and the physiological state of the vessel, in silico models can be a valuable tool in the preinterventional planning phase. A suitable in silico EVAR methodology applied to patient-specific cases can be used to predict stent-graft (SG)-related complications, such as SG migration, endoleaks or tissue remodeling-induced aortic neck dilatation and to improve the selection and sizing process of SGs. In this contribution, we apply an in silico EVAR methodology that predicts the final state of the deployed SG after intervention to three clinical cases. A novel qualitative and quantitative validation methodology, that is based on a comparison between in silico results and postinterventional CT data, is presented. The validation methodology compares average stent diameters pseudo-continuously along the total length of the deployed SG. The validation of the in silico results shows very good agreement proving the potential of using in silico approaches in the preinterventional planning of EVAR. We consider models of bifurcated, marketed SGs as well as sophisticated models of patient-specific vessels that include intraluminal thrombus, calcifications and an anisotropic model for the vessel wall. We exemplarily show the additional benefit and applicability of in silico EVAR approaches to clinical cases by evaluating mechanical quantities with the potential to assess the quality of SG fixation and sealing such as contact tractions between SG and vessel as well as SG-induced tissue overstresses.