PSI - Issue 49

S. McLennan et al. / Procedia Structural Integrity 49 (2023) 51–58

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Mongrain et al. / Structural Integrity Procedia 00 (2023) 000–000

1. Introduction 1.1 Endovascular Aortic Aneurysm Repair

Since the 1990s, endovascular aortic aneurysm repair (EVAR) has become a common alternative to open surgery for treatment of abdominal aortic aneurysms (AAAs). The advantages of this approach being a faster recovery time and lower peri-operative mortality [19]. Over the past two decades, EVAR technology has evolved substantially to enhance procedural success Wanken et al. (2020). Current planning of stent-graft procedures are based on pre-operative computed tomography (CT) imaging that does not take into account the presenceofcalcificationRafi-Tari et al. (2013). The procedure has been noted to fail in certain cases where arterial calcification is present Manunga et al. (2013). 1.2 Aortic Calcification and Numerical Simulations Most AAAs contain a noticeable amount of calcification Maier et al. (2010) while atherosclerotic calcification in major vessel beds is associated with an increased risk of death including in patients undergoing endovascular aortic aneurysm repair (EVAR), Boss et al. (2015) and TerBush et al. (2019). While it has been established that calcifications change aortic tissue mechanical properties and alter the distribution of wall stress Hermanetal.(2018), there is debate over whether this change leads to an increase, Gallitoto et al. (2017) and Komen et al. (2011) or decrease Giannini et al. (2019) in the risk of rupture. Meanwhile, computational studies investigating the impact of calcification on interventional procedures are limited due to lack of information on the mechanical properties of calcification Calahane et al. (2018). To date, the impact of calcification has been poorly treated in computational wall stress analyses and neglecting calcifications therein is common practice in literature, Haller et al. (2020) and Maier et al. (2010). Despite the gap in the literature, there does exist at least one study where investigation of calcification impact on general vascular behaviour has been carried out. Speelman et al. (2007) performed finite element analysis (FEA) on patient-specific AAA geometries (N=6) to establish the effect of calcification on aortic wall stress. Interestingly, they found that if calcifications are located in areas predicted to have high stress, the effect of the calcifications on wall stress is pronounced, even if its stiffness is only slightly higher than that of the AAA wall. Finite element analysis (FEA) provides a means for calculating aortic wall stress on a patient-specific basis. The simulations utilize clinical data that has already been acquired as part of the routine AAA evaluations such as patient CT scans and systolic blood pressure (SBP) readings Halleretal.(2020). Certain studies have investigated the aortic peak wall stress (PWS) in AAA patients with the aim of establishing if PWS can be used as a predictor of rupture Fernandez et al. (2009) and Gallitto et al. (2017), however, these studies are not specific to PWS during EVAR. The goal of this numerical simulation study is to develop a model for assessing the feasibility of EVAR intervention for patients with moderate to severe aortic calcification presence. In addition, we use the simulation technique to investigate the stress response of different regions of the aorto-iliac structure during EVAR and whether the patient specific calcification presence impacts this stress response. The aorta wall was modelled as a simplified hyperelastic, homogenous, isotropic, incompressible material model through the second-order Yeoh hyperelastic material strain energy function with the properties taken from Raghavan et al. (2000). Calcium was modelled as a linear elastic, homogenous, incompressible, and isotropic material. The ILT was modelled as a single-layered incompressible tissue using the one-parameter Ogden-like strain energy function. The various mechanical parameters are presented in Mohammadi et al. (2018) and McLennan (2021).

Fig. 1 FEA model: (a) abdomen (b) bone (c) ILT and aortic calcification and (d) combined model