Micromechanical heterogeneity in abdominal aortic aneurysms is associated with rupture risk

Abdominal aortic aneurysms (AAA) are common, especially in men, and associated with a high mortality when ruptured. Clinical guidance for surgical repair is based on the maximum aortic diameter (>5.5 cm in men or 5 cm in women) but this is a poor indicator of clinical risk. Here, we examined micromechanical and biochemical properties of tissue excised from 21 patients undergoing repair for degenerative AAA using nanoindentation and biochemical assays (collagen elastin and glycosaminoglycans (GAGs)), along with 6 control aortic samples. AAA tissue properties were compared with peak wall stress (PWS), peak wall rupture risk (PWRR) localised rupture risk index (RRI) and wall stress (WS) determined from patient-specific finite element (FE) models, and with abdominal aortic calcification (AAC) scoring obtained from CT scans. The AAA samples had a lower median elastic modulus (72.4 kPa) and a higher IQR (86.8 kPa) relative to the controls (median 91.2 kPa, IQR 53.8 kPa). A heteroscedastic relationship was found in the AAA samples; patients with the highest median stiffness exhibited the largest IQR. Relative to controls, collagen was higher in the AAAs, whilst GAG and elastin were lower. Microcalcification was higher in the inner and middle layers of the vessel wall, matching the trend observed with stiffness. Correlative analysis showed that E was related to RRI but a complex, interplay of tissue properties contributed to overall PWRR. AAC was found to be inversely correlated with PWRR. Random forest modelling demonstrated that RRI is most influenced by E measured in the belly of the aneurysm, GAG, and collagen. In conclusion, micromechanical properties and calcification may be useful for patient-specific rupture risk prediction. STATEMENT OF SIGNIFICANCE: Abdominal aortic aneurysms (AAA) are more prevalent with age, and rupture is associated with a high mortality rate. Maximum aortic diameter, the main clinical criteria for surgical repair is a poor indicator of rupture risk (RR). We used micromechanical and biochemical characterisation, and computational modelling to understand RR in degenerative AAAs. The tissue elastic modulus was found to be an indicator of RR as was the in vivo abdominal aortic calcification (AAC) score with the latter having an inverse relationship with RR. Collagen and glycosaminoglycans levels were also key to RR. We demonstrate that RR is better indicated by AAC and tissue elastic properties than conventional clinical markers such as diameter alone. These findings can be exploited for patient-specific RR determination.