Interaction of expanding abdominal aortic aneurysm with surrounding tissue: Retrospective CT image studies.

Objectives: Abdominal aortic aneurysms (AAA) that rupture have a high mortality rate. Rupture occurs when local mechanical stress exceeds the local mechanical strength of an AAA, so stress profiles such as those from finite element analysis (FEA) are useful. The role and effect of surrounding tissues, like the vertebral column, which have not been extensively studied, are examined in this paper.

Methods: Longitudinal CT scans from ten patients with AAAs were studied to see the effect of surrounding tissues on AAAs. Segmentation was performed to distinguish the AAA from other tissues and we studied how these surrounding tissues affected the shape and curvature of the AAA. Previously established methods by Veldenz et al. were used to split the AAA into 8 sections and examine the specific effects of surrounding tissues on these sections [1]. Three-dimensional models were created to better examine these effects over time. Registration was done in order to compare AAAs longitudinally.

Results: The vertebral column and osteophytes were observed to have been affecting the shape and the curvature of the AAA. Interaction with the spine caused focal flattening in certain areas of the AAA. In 16 of the 41 CT scans, the right posterior dorsal section (section 5), had the highest radius of curvature, which was by far the section that had the maximum radius for a specified CT scan. Evolution of the growing AAA showed increased flattening in this section when comparing the last CT scan to the first scan.

Conclusion: Surrounding tissues have a clear influence on the geometry of an AAA, which may in turn affect the stress profile of AAA. Incorporating these structures in FEA and G&R models will provide a better estimate of stress.

Clinical Relevance: Currently, size is the only variable considered when deciding whether to undergo elective surgery to repair AAA since it is an easy enough measure for clinicians to utilize. However, this may not be the best indicator of rupture risk because small aneurysms also contribute to a high mortality rate. AAA's wall stress is a superior indicator and may be better predicted with the inclusion of these surrounding tissues, which then could be used by clinicians in their decision-making process on whether to operate on an AAA.