Cases

Cardiovascular disease is the leading cause of morbidity and premature death of modern- era medicine. An abdominal aortic aneurysm (AAA) is a permanent and irreversible localized dilatation of the aorta greater than 50% of its normal diameter. Nowadays, surgical intervention is often performed when the AAA’s maximum diameter exceeds 5 cm. However, research has shown that an AAA with a diameter smaller than 5 cm can also rupture, while an AAA with a diameter greater than 5 cm might not. The researchers in this case were seeking a way of assessing the risk of an AAA rupturing based on finite element analyses (FEA) determining wall stress in the AAA. They found Mimics to be the perfect solution to prepare their CT data for FE analysis. The software also aided them in determining the diameter, size, volume, overall geometry, asymmetry and curvature of the AAAs they examined. These are important parameters for the surgical intervention and are hard to determine from 2D scans.

An AAA will rupture when the stress acting on the blood vessel is greater than the vessel’s wall strength. Therefore, the researchers defined a finite element analysis rupture index (FEARI) which calculates the rupture potential of a blood vessel as wall stress exercised on the blood vessel divided by the vessel’s wall strength. FEARI can have a value between 0 and 1, meaning a low or high rupture potential respectively. For this study, the researchers collected CT data from 10 patients who were either awaiting or had received AAA repair at the time of the scan.

In order to determine the AAA’s wall stress, Doyle and McGloughlin relied on Mimics to construct 3D models based on the patients’ CT scans. Construction time for virtual models ranged from two minutes in cases where minor details were ignored to one hour where all details were included. They were thrilled by the extremely short preparation time. The 3D reconstructions were then meshed for further FE analysis. Mimics’ automatic remesher allowed for effortless reduction of data complexity needed for FEA, while guaranteeing the necessary accuracy.

The remeshed data were then imported in ABAQUS for FE analysis. Analysis revealed that regions of elevated and peak wall stress occurred at inflection points on the AAA surface, which were previously reported to act as stress raisers. Asymmetric, irregularly shaped and tortuous AAAs have many regions of inflection. This means they are more complex, have a higher risk of rupturing and should be monitored more closely. In fact, peak wall stress turned out to be independent of the AAA’s maximum diameter. As a result, this parameter alone doesn’t offer sufficient information to assess the likelihood of the AAA rupturing. Geometry, on the other hand, has a greater influence on wall stress. A sudden change in geometry causes large peak stress. Therefore smaller AAAs with complex geometry may be more likely to rupture than greater ones with simpler shapes.

In order to calculate the FEARI for each individual patient, the researchers used experimental data from previous research to determine the ultimate tensile strength (UTS) value for wall strength of the region where peak stress occurred. Although the rupture index needs additional elaboration, it is already clear that it can be an interesting tool in endovascular aneurysm repair preparation, if combined with current techniques.

Mimics was indispensable in helping the researchers visualize and measure the diseased AAAs and to prepare each patient’s data for stress analysis. Mimics’ swift reconstruction methods and user-friendly measurement tools allowed them to assess the AAA’s morphology in detail. The researchers were thrilled by Mimics’ effortless ability to calculate important EVAR parameters such as length, degree of curvature, angles and exact distances over surfaces. Having the data at hand aids the surgeon in examining his patient, and choosing the correct stent-graft. It also eliminates many unforeseen problems during surgical repair, reducing operating time and increasing its efficiency. Mimics is also the ideal partner for post-operative patient follow-up. Mimics’ centerline tool, even makes it possible to design stent-grafts customized to the patient’s aortic geometry.