Correlation of Intraluminal Thrombus Deposition, Biomechanics, and Hemodynamics with Surface Growth and Rupture in Abdominal Aortic Aneurysm—Application in a Clinical Paradigm

Background

The natural history of abdominal aortic aneurysm (AAA) can be investigated through longitudinal evaluation of localized aneurysm characteristics exploiting clinical images. The major challenge is to identify corresponding regions between follow-ups. We have recently developed an algorithm (VascForm) based on nonrigid registration that can obtain surface correspondence and quantify surface growth distribution.

Methods

A ruptured AAA with an initial computed tomography scan 2 years ago was studied. Following 3-dimensional reconstruction of outer wall and luminal surfaces, the wall/thrombus thickness was obtained. Wall stress distribution was computed with finite element analysis, and computational fluid dynamics simulation was performed. VascForm was applied and allowed for the ruptured wall site to be traced back to the initial wall surface and be correlated with local initial intraluminal thrombus thickness, wall stress, and hemodynamic parameters. It also allowed for the quantification of wall surface growth based on surface element growth.

Results

Rupture occurred at the posterolateral side. Initial wall surface growth was in most regions 40%. However, a large section of the posterior wall presented 110% growth. Initial thrombus deposition was more prevalent anteriorly, and a posterior thrombus-free isle was present. Peak wall stress (initial and follow-up) occurred at AAA neck. Nonrigid registration revealed that rupture originated from the vicinity of the initial thrombus-free isle. Furthermore, rupture occurred at the wall region with the largest growth (110%). No clear correlation between hemodynamics and rupture site could be identified.

Conclusions

High local surface growth correlates with rupture site and could therefore potentially become a marker of rupture risk. The ongoing application of this methodology to a large cohort of AAA patients will focus on identifying characteristic features of AAA regions that present high surface growth in follow-up evaluations, to assist in improved rupture risk estimation.

Introduction

Abdominal aortic aneurysm (AAA) is typically an asymptomatic vascular disease with a significant mortality rate in the developed world. These aneurysms are focal vascular dilatations that form in the distal region of the aorta and can progress to the point of rupture. Maximum diameter is the traditional criterion for rupture risk assessment: for an AAA with a maximum diameter ≥55 mm, the risk of rupture is considered in general to outweigh the risks associated with surgery. However, the analysis of autopsy data showed that nearly 13% of AAAs with a maximum diameter 5 cm or smaller were ruptured and 60% of the AAAs greater than 5 cm in diameter never ruptured.¹ It is thus reasonable that “maximum diameter criterion”, as a single parameter fitting all patients, is being questioned.2, 3, 4, 5, 6, 7 Several additional risk factors have been proposed that can potentially improve rupture risk assessment on a patient-specific base, exploiting developments in noninvasive imaging, software technologies, and numerical techniques, but they have not yet reached clinical practice.5, 6, 8, 9, 10, 11, 12

Along the lines of improving our understanding of this pathology and also improve rupture risk assessment, many important results can be obtained from longitudinal studies of AAAs.¹³ However, it is quite rare to have longitudinal data of an aneurysm that eventually ruptures. Most patients diagnosed with an AAA are consistently followed up until they meet the criteria for intervention. Most of the time, when an AAA ruptures, its prior existence was unknown. In the present study, a patient with a ruptured aneurysm was admitted to the emergency room. He had performed a computed tomography (CT) scan 2 years ago but failed to follow-up with doctors' recommendations for consistent re-evaluation of his AAA.

The existence of a previous CT scan, even 2-year old, provided the opportunity to shed light on aneurysm morphologic or biomechanic characteristics that could potentially be associated or even have triggered its rupture. A long-debated subject in aneurysm research is the role of peak wall stress (PWS) and intraluminal thrombus (ILT) in aneurysm growth and subsequent rupture. Although multifactorial in nature, AAA growth is from a mechanical point of view a result of reduced local wall strength relative to local wall stress. But which is really the role of thrombus? Does a thick ILT weaken the wall and is therefore a risk marker, or does it provide a cushioning effect reducing the wall stresses having thus a protective mechanism? A recent study has shed light toward answering these questions; Martufi et al.² investigated the effect of wall stress, thrombus, and local diameter on growth rate of 90 AAAs and found that the local growth rate was dependent on the local baseline diameter, the local ILT thickness and, for wall segments not covered by ILT, also on the local wall stress level.² On the other hand, for wall segments that were covered by a thick ILT layer, wall stress did not affect the growth rate. Driven by these findings, the present study focuses on the initial ILT thickness and PWS value at the location of rupture. Furthermore, as hemodynamics has a major influence on ILT deposition, computational fluid dynamics simulations were performed to characterize wall shear stress (WSS), oscillatory shear index (OSI), and relative residence time (RRT) at the site where rupture occurred.

While several localized characteristics can be quantified, the major challenge is to identify corresponding regions between follow-ups, which stem from the complexity of the aneurysm enlargement process. Especially, when the local growth is large and a small localized feature (such as rupture) needs to be traced back on the initial surface, a detailed correspondence is necessary. Such a case is the presented ruptured AAA. Toward this, we exploit our recently developed methodology based on nonrigid registration techniques to quantify surface growth distribution (a proposed future alternative for maximum diameter growth) and trace the rupture site back to the initial wall surface to correlate it with the local initial ILT thickness and wall stress.¹⁰ The aim is to correlate surface growth characteristics with rupture risk characteristics to identify AAAs at high rupture risk. Although, the results of a single study cannot be generalized, they do provide guidance for the design of future studies, a ground for discussion, and an opportunity to present the potential of the most recent advances in the field.10, 14