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Biomechanical rupture risk prediction with AAA growth.

    Open AccessPublished:November 17, 2022DOI:https://doi.org/10.1016/j.jvssci.2022.11.002
        Abdominal aortic aneurysms (AAA) are repaired when they meet diameter criteria, or when they become symptomatic or rupture. The use of aortic diameter as the primary criterion in the decision to repair fails address the considerable numbers of AAA which rupture below this operative threshold (
        • Reed W.W.
        • Hallett Jr., J.W.
        • Damiano M.A.
        • Ballard D.J.
        Learning from the last ultrasound. A population-based study of patients with abdominal aortic aneurysm.
        ); particularly in women (
        • Wilson K.A.
        • Lee A.J.
        • Lee A.J.
        • Hoskins P.R.
        • Fowkes F.G.R.
        • Ruckley C.V.
        • et al.
        The relationship between aortic wall distensibility and rupture of infrarenal abdominal aortic aneurysm.
        ). Improved prediction of AAA behavior is required to prevent significant morbidity and mortality.
        AAA are thought to rupture when aortic wall mechanical stress exceeds wall strength (
        • Gasser T.C.
        • Miller C.
        • Polzer S.
        • Roy J.
        A quarter of a century biomechanical rupture risk assessment of abdominal aortic aneurysms. Achievements, clinical relevance, and ongoing developments.
        ,
        • Singh T.P.
        • Moxon J.V.
        • Gasser T.C.
        • Golledge J.
        Systematic Review and Meta-Analysis of Peak Wall Stress and Peak Wall Rupture Index in Ruptured and Asymptomatic Intact Abdominal Aortic Aneurysms.
        ). Finite element analysis (FEA) has been used to model the physical characteristics of the AAA wall from CT reconstructions; allowing determination of peak wall stress (PWS) and peak wall rupture index (PWRI). PWS is a measure of maximal tensile stress; whereas, PWRI is a ratio of maximal wall stress to strength. PWRI serves as a predictor of AAA rupture risk; however, to date it has yet to consistently predict the outcome for specific AAA geometries (
        • Fillinger M.F.
        • Marra S.P.
        • Raghavan M.L.
        • Kennedy F.E.
        Prediction of rupture risk in abdominal aortic aneurysm during observation: wall stress versus diameter.
        ,
        • Fillinger M.F.
        • Raghavan M.L.
        • Marra S.P.
        • Cronenwett J.L.
        • Kennedy F.E.
        In vivo analysis of mechanical wall stress and abdominal aortic aneurysm rupture risk.
        ). This lack of predictability likely reflects the multifactorial involvement of hemodynamics, intraluminal thrombus (ILT), and inflammatory effects of cytokines and proteases (
        • Boyd A.J.
        Intraluminal thrombus: Innocent bystander or factor in abdominal aortic aneurysm pathogenesis?.
        ); and may also reflect constant aortic remodeling associated with growth (
        • Gasser T.C.
        • Miller C.
        • Polzer S.
        • Roy J.
        A quarter of a century biomechanical rupture risk assessment of abdominal aortic aneurysms. Achievements, clinical relevance, and ongoing developments.
        ).
        The current manuscript examined the biomechanical and morphological changes associated with AAA growth using a FEA linear transformation-based comparison. The authors hypothesized that AAA growth would accompanied by significant changes in biomechanical and geometrical characteristics. It was also hypothesized that PWRI, PWS and ILT would change location with AAA growth. AAA diameter and volume, neck configuration, α and β angulation, vessel tortuosity, ILT volume, PWS, and PWRI were determined at 2 time points.
        There was a significant increase in AAA and ILT volume, maximal ILT thickness, neck angulation and iliac tortuosity with AAA growth. The change in PWRI was most correlated with an increase in AAA volume; whereas the change in PWS was best correlated with neck angulation. These findings suggest that AAA wall stresses vary with growth and that AAA volume may be a better predictor of AAA rupture risk than diameter.
        The observation that maximum ILT thickness, in comparison to positions of maximum PWS and PWRI, was the most pronounced with AAA growth suggests that ILT volume may play an important role in predicting AAA behavior. Using a computational fluid dynamics approach, Lasheras and coworkers (
        • Salsac A.V.
        • Sparks S.R.
        • Chomaz J.M.
        • Lasheras J.C.
        Evolution of the wall shear stresses during the progressive enlargement of symmetric abdominal aortic aneurysms.
        ,
        • Salsac A.V.
        • Sparks S.R.
        • Lasheras J.C.
        Hemodynamic changes occurring during the progressive enlargement of abdominal aortic aneurysms.
        ) have previously shown that alterations in aortic length and tortuosity with AAA growth have been associated with the development of turbulent flow vortexes, with changes in wall shear stress (WSS). Low WSS is thought to play a role in initiating and propagating ILT deposition in AAA which may play a role in promoting aortic wall degeneration and rupture (
        • Boyd A.J.
        Intraluminal thrombus: Innocent bystander or factor in abdominal aortic aneurysm pathogenesis?.
        ). Future studies aimed at improving understanding of AAA rupture risk will require both hemodynamic and finite element analyses in a large cohort with patient-specific physiologic and anatomic data.

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