Energy Loss of Aortic Wall in Aneurysms Predicts Tissue Delamination

Mingyi Tang (1,3), Edwin Wong (1,2,3), Jennifer Chung (4),

Maral Ouzounian (4), Craig Simmons (1,2,3)

1. Department of Mechanical and Industrial Engineering, University of Toronto

2. Institute of Biomaterials and Biomedical Engineering, University of Toronto

3. Translational Biology and Engineering Program, Ted Rogers Centre for Heart Research

4. Division of Cardiac Surgery, University of Toronto

The biomechanics of aortic aneurysms are poorly understood, making it difficult to predict which patients should be operated on because their aneurysms are at risk of dissection. Dissections are life-threatening events that occur when blood enters the medial layer of the aortic vessel wall causing it to delaminate. Currently, risk is assessed by measuring the aortic aneurysm (AA) diameter from clinical computed tomography. However, this metric leads to missing many patients who then present with aortic dissection. One proposed clinical biomarker is energy loss (ΔUL): the percentage of mechanical energy/unit volume dissipated between loading and unloading of a material. For AAs, ΔUL has been found to positively correlate with pathological changes to the microstructure of the AA tissues. Other potential biomechanical markers include the tissue’s Young’s modulus (E) and the modulus anisotropy index (AI). There have yet to be any studies evaluating correlations between a failure criterion such as the delamination strength of the tissue (Sd) and a biomechanical parameter. Our aim is to determine if biomechanical parameters such as ΔUL, E and AI can be used to predict the forces needed to dissect AAs.

Ascending AAs were collected during surgery (n = 43) while non-diseased aorta controls were collected from transplant donors (n = 8). Square samples (14 x 14 mm2) were subjected to 25% equibiaxial strain to calculate ΔUL and E in both directions of loading as well as AI for both parameters. 6 mm x 30 mm strips were delaminated via uniaxially peeling to calculate Sd. ΔUL was significantly and negatively correlated to Sd (r2 > 0.42, p < 0.0001). However, E and AI for both ΔUL and E were not significantly correlated to Sd (r2 < 0.08, p > 0.05). AA diameters were significantly but poorly correlated to Sd (r2 = 0.18, p < 0.05). This suggest that ΔUL maybe a stronger predictive parameter to assess dissection risk compared to E, AI and AA diameter. When stratified based on valve morphology, it was determined that AAs associated with bicuspid valves had higher Sd (35.8 ± 1.97 N/mm vs. 27.0 ± 2.4 N/mm, p < 0.01) and lower ΔUL (6.6% ± 1.5 vs. 9.7% ± 2.8, p < 0.0001) compared to tricuspid valve patients.

Our study suggests that ΔUL is a biomechanical marker that is a stronger predictor of AA dissection risk compared to the clinical standard of using aneurysm diameter. For future investigations, we aim to trial clinical imaging techniques to measure ΔUL in vivo.