Intraluminal thrombus is associated with early rupture of abdominal aortic aneurysm

Background: The implications of intraluminal thrombus (ILT) in abdominal aortic aneurysm (AAA) are currently unclear. Previous studies have demonstrated that ILT provides a biomechanical advantage by decreasing wall stress, whereas other studies have associated ILT with aortic wall weakening. It is further unclear why some aneurysms rupture at much smaller diameters than others. In this study, we sought to explore the association between ILT and risk of AAA rupture, particularly in small aneurysms. Methods: Patients were retrospectively identified and categorized by maximum aneurysm diameter and rupture status: small (<60 mm) or large (>= 60 mm) and ruptured (rAAA) or nonruptured (non-rAAA). Three-dimensional AAA anatomy was digitally reconstructed from computed tomography angiograms for each patient. Finite element analysis was then performed to calculate peak wall stress (PWS) and mean wall stress (MWS) using the patient's systolic blood pressure. AAA geometric properties, including normalized ILT thickness (mean ILT thickness/maximum diameter) and % volume (100 x ILT volume/total AAA volume), were also quantified. Results: Patients with small rAAAs had PWS of 123 +/- 51 kPa, which was significantly lower than that of patients with large rAAAs (242 +/- 130 kPa; P = .04), small non-rAAAs (204 +/- 60 kPa; P < .01), and large non-rAAAs (270 +/- 106 kPa; P < .01). Patients with small rAAAs also had lower MWS(44 +/- 14 kPa vs 82 +/- 20 kPa; P < .02) compared with patients with large non-rAAAs. ILT % volume and normalized ILT thickness were greater in small rAAAs (68% +/- 11%; 0.16 +/- 0.04 mm) compared with small non-rAAAs (53% +/- 16% [P = .02]; 0.11 +/- 0.04 mm [P < .01]) and large non-rAAAs (57% +/- 12% [P = .02]; 0.12 +/- 0.03 mm [P < .01]). Increased ILT % volume was associated with both decreased MWS and decreased PWS. Conclusions: This study found that although increased ILT is associated with lower MWS and PWS, it is also associated with aneurysm rupture at smaller diameters and lower stress. Therefore, the protective biomechanical advantage that ILT provides by lowering wall stress seems to be outweighed by weakening of the AAA wall, particularly in patients with small rAAAs. This study suggests that high ILT burden may be a surrogate marker of decreased aortic wall strength and a characteristic of high-risk small aneurysms.