Relative strain is a novel predictor of aneurysmal degeneration of the thoracic aorta: An ex vivo mechanical study

OBJECTIVE: Current guidelines for prophylactic replacement of the thoracic aorta, primarily based on size alone, may not be adequate in identifying patients at risk for either progression of disease or aortic catastrophe. We undertook the current study to determine whether the mechanical properties of the aorta might be able to predict aneurysmal dilatation of the aorta using a clinical database and benchtop mechanical testing of human aortic tissue. METHODS: Using over 400 samples from 31 patients, mechanical properties were studied in (a) normal aorta and then (b) between normal and diseased aorta using linear mixed-effects models. A machine learning technique was used to predict aortic growth rate over time using mechanical properties and baseline clinical characteristics. RESULTS: Healthy aortic tissue under in vivo loading conditions, after accounting for aortic segment location, had lower longitudinal elastic modulus compared with circumferential elastic modulus: −166.8 kPa (95% confidence interval [CI]: −210.8 to −122.7, P < .001). Fracture toughness was also lower in the longitudinal vs circumferential direction: −201.2 J/m(3) (95% CI: −272.9 to −129.5, P < .001). Finally, relative strain was lower in the longitudinal direction compared with the circumferential direction: −0.01 (95% CI: −0.02 to −0.004, P = .002). Patients with diseased aorta, after accounting for segment location and sample direction, had decreased toughness compared with normal aorta, −431.7 J/m(3) (95% CI: −628.6 to −234.8, P < .001), and increased relative strain, 0.09 (95% CI: 0.04 to 0.14, P = .003). CONCLUSIONS: Increasing relative strain was identified as a novel independent predictor of aneurysmal degeneration. Noninvasive measurement of relative strain may aid in the identification and monitoring of patients at risk for aneurysmal degeneration. (JVS–Vascular Science 2021;2:1-12.) CLINICAL RELEVANCE: Aortic aneurysm surveillance and prophylactic surgical recommendations are based on computed tomographic angiogram aortic dimensions and growth rate measurements. However, aortic catastrophes may occur at small sizes, confounding current risk stratification models. Herein, we report that increasing aortic relative strain, that is, greater distensibility, is associated with growth over time, thus potentially identifying patients at risk for dissection/rupture.