Effects of Wall Calcifications in Patient-Specific Wall Stress Analyses of Abdominal Aortic Aneurysms

Abstract

It is generally acknowledged that rupture of an abdominal aortic aneurysm (AAA) occurs when the stress acting on the wall over the cardiac cycle exceeds the strength of the wall. Peak wall stress computations appear to give a more accurate rupture risk assessment than AAA diameter, which is currently used for a diagnosis. Despite the numerous studies utilizing patient-specific wall stress modeling of AAAs, none investigated the effect of wall calcifications on wall stress. The objective of this study was to evaluate the influence of calcifications on patient-specific finite element stress computations. In addition, we assessed whether the effect of calcifications could be predicted directly from the CT-scans by relating the effect to the amount of calcification present in the AAA wall. For 6 AAAs, the location and extent of calcification was identified from CT-scans. A finite element model was created for each AAA and the areas of calcification were defined node-wise in the mesh of the model. Comparisons are made between maximum principal stress distributions, computed without calcifications and with calcifications with varying material properties. Peak stresses are determined from the stress results and related to a calcification index (CI), a quantification of the amount of calcification in the AAA wall. At calcification sites, local stresses increased, leading to a peak stress increase of 22% in the most severe case. Our results displayed a weak correlation between the CI and the increase in peak stress. Additionally, the results showed a marked influence of the calcification elastic modulus on computed stresses. Inclusion of calcifications in finite element analysis of AAAs resulted in a marked alteration of the stress distributions and should therefore be included in rupture risk assessment. The results also suggest that the location and shape of the calcified regions--not only the relative amount--are considerations that influence the effect on AAA wall stress. The dependency of the effect of the wall stress on the calcification elastic modulus points out the importance of determination of the material properties of calcified AAA wall.

Biomechanical factors in Finite Element Analysis of abdominal aortic aneurysms

“…Although, there is a lack of agreement on how calcifications actually affect it. Since the discussed constituent is stiffer than the rest of the wall, it presents possible stress concentration which is consistent with higher rupture probability [32,33]. On the other hand, calcifications cause local wall thickening [11] and demonstrate significant loadbearing effects, which results in lowering the wall stress [34].…”

“…It is important to employ ECG-gaited CT in order to obtain AAA geometry in diastolic phase of cardiac cycle. Some studies presented results of FEM analysis after applying the full systolic pressure to AAA model [32,52]. The used systolic pressure is either obtained from the patient or assumed as a value from the range 110-130 mm Hg.…”

Towards patient-specific risk assessment of abdominal aortic aneurysm

“…For stiffer calcifications, however, the different modeling approaches and the different geometries lead to significantly different results. A recent study confirmed that calcifications can significantly influence the local wall stress [43]. This indicates the importance of further study into methods for accurately determining and modeling the vessel wall material properties.…”

“…The material properties reported in literature for calcifications and the material properties actually used for simulations show great variation. Previous studies have focused on simplified modeling of the calcification shapes within a realistic aneurysm shape [20,42]. In our study, an accurate representation of the calcification geometry and a simplified model for the AAA were used.…”

Improved assessment and treatment of abdominal aortic aneurysms: the use of 3D reconstructions as a surgical guidance tool in endovascular repair

“…The aortic wall was assumed to have a uniform thickness of 1.9mm [10,20,25], and was modeled as a non-linear hyperelastic material [10]. These boundary conditions have been implemented in many previous studies [10,19,21,22,24,[26][27][28]. …”