PACS-ILT: A proposed morphobiochemical scoring system for rupture risk stratification of asymptomatic abdominal aortic aneurysms.

Objective: Intraluminal thrombus (ILT) is increasingly recognized as a dynamic factor in abdominal aortic aneurysm (AAA) progression. It can play a dual role-providing mechanical cushioning while simultaneously destabilizing the aortic wall through its biochemical activity. Fissuring and bleeding within the ILT remain underinvestigated but may serve as early indicators of wall failure, independent of aneurysm size. Our aim was to propose a novel biology-driven framework-the PACS-ILT classification (Proteolytic Activity and Clinical Stability of Intraluminal Thrombus)-to refine the assessment of rupture risk by integrating thrombus biology, imaging characteristics, and patient-specific factors. Methods: A targeted literature review of biomechanical and biochemical studies was conducted to inform the development of the PACS-ILT system. The weighting of individual parameters was guided by their reported impact on aortic wall vulnerability. Results: The analysis indicates that diameter-based risk assessment alone does not capture the local pathophysiological changes of the aortic wall. Two key controllers of rupture risk beyond size were identified: (1) biomechanical effects, in which ILT thickness and morphology significantly influence wall stress distribution; and (2) biochemical effects, in which proteolytic activity, including matrix metalloproteinases and imbalances in their inhibitors, contributes to mural degradation. Integrating these findings with established clinical risk factors, the PACS-ILT algorithm stratifies rupture risk on a 1 to 5 scale (P1-P5). Scores are assigned across four domains: Geometric (G), Morphologic (M), Biochemical (B), and Clinical (C). A key insight is the recognition of "proteolytic hyperactivity" within thin ILTs, which may explain early rupture in small AAAs. Conversely, some larger aneurysms may remain relatively stable due to the "thrombus paradox," where a thick, mature ILT serves as a mechanical shield. Conclusions: PACS-ILT provides a conceptual, hypothesis-generating framework that moves AAA risk assessment beyond size alone toward a more dynamic, biology-informed approach. Future validation-incorporating computed tomography-based morphology, fluid-structure interaction, and computational fluid dynamics simulations-is required to confirm its utility in identifying "small-but-unstable" aneurysms and preventing catastrophic rupture.