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

mechanical damage of the tissue. We also investigated PWS at the following regions of the aorto-iliac structure: neck, aneurysm, bifurcation, iliacs, and the healthy-calcified wall boundary. The anatomical locations were classified in regions (Figure 1c). By doing so, we could investigate which regions experience the greatest stress during EVAR and which regions will require particular care whennavigatinginterventionaltools during the procedure. Moreover, we ran two simulations for each patient, one with calcification inclusion With-Ca and one without calcification inclusion No-Ca . The No-Ca simulations were created by changing the calcified wall material properties to that of the healthy wall. Only after running these two simulations for each patient, we investigated the impact of calcification inclusion on the aortic PWS. Calcification inclusion for each patient was quantified using percent relative calcification presence (RCP), calculated as follows: RCP =Vc/(Vc + Vh)X100 with V c the calcified tissue volume in mm 3 , V h the ‘healthy’ tissue volume in mm 3 . The assumed aortic wall thickness of 1.5 mm is based on prior research efforts Mohammadietal.(2018), however,Raghavan et al. (2000)showed that wall thickness varies regionally in AAAs from as low as 0.23 mm at a rupture site to 4.26 mm at a calcified site (median of 1.48 mm). Despite the wall thickness variation between the aortic and calcified wall, we idealised the calcification as being the same thickness as the aortic wall. This was done to create a smooth inner surface of the aorto-iliac structure in order to prevent jamming of the endovascular tools, mainly between the guidewires and calcification piece edges. To assess the impact of this idealisation on the aortic PWS, we ran a validation study using a simplified FEA model of a single piece of calcification embedded in a 30x30 mm patch of the aortic wall. We found that when using the real calcification geometry with a thickness of approximately 2 times the wall, the PWS stress only increased by 0.08%. 3 Results and Discussion 3.1 Impact of Calcification Presence at each Aorto-iliac Region during Endovascular Aortic Aneurysm Repair As shown in Table 1, none of the 12 With-Ca patient simulations exceeded the empirically calculated wall strength, suggesting interventional tool deployment during EVAR will not cause mechanical damage to the tissue. For each region of interest on the aorto-iliac structure, the mean PWS, across all 12 patients, always increased upon inclusion of calcification into the simulation (Fig. 3). The greatest mean PWS was observed at the iliac region of the With-Ca simulations (576.13±140.10 kPa) and the lowest in the neck (271.49±98.46 kPa).

Table 1 PWS in the aorto-iliac structure for each With-Ca patient simulation, with the corresponding wall strength, PWS to wall strength ratio, and the RCP. A value greater than or equal to 1 for the PWS to wall strength ratio is indicative of rupture occurring Patient PWS (kPa) Wall Strength (kPa) PWS/Wall Strength RCP (%) 1 459.20 549.08 0.84 1.34 2 733.29 814.70 0.90 2.26 3 795.00 844.99 0.94 3.00 4 469.03 656.99 0.71 4.55 5 698.88 762.06 0.92 5.72 6 643.19 743.96 0.86 5.87 7 412.56 690.40 0.60 6.61 8 601.13 748.32 0.80 7.27 9 675.00 771.82 0.87 7.48 10 614.42 660.06 0.93 9.70 11 282.34 347.56 0.81 10.40 12 682.51 926.75 0.74 12.41 Mean ± SD 588.88 ± 151.24 709.72 ± 150.23 0.83 ± 0.10 6.38 ± 3.35

When comparing the increase in PWS of the entire aorto-iliac structure between the No-Ca and With-Ca simulations, a positive correlation ( R = 0 . 794 , p < 0 . 001) was found between the RCP and the PWS increase (Figure2). This finding supports the notion that severe calcification presence can limit the feasibility of EVAR as higher PWS in our simulations suggests a higher likelihood of rupture due to tool deployment.