Development of biomechanical models incorporating microstructures for vascular tissue rupture and implant design optimization

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Vascular tissue rupture is a common phenomenon associated with many catastrophic cardiovascular diseases including coronary plaque rupture, aortic dissection and aortic aneurysm rupture. Biomechanical criteria based on peak stress have been proposed to assess the severity of the diseases and the potential of rupture. However, these indexes cannot account for all observed ruptures. For example, for coronary plaque rupture, the conventional load analysis which predicts stress concentration at the plaque shoulders cannot account for about 30 % of the ruptures occurring in the mid portion of the plaque. In fact, peak stress predominates so much the literature that certain results have essentially become paradigms (rupture at plaque shoulders). The fact is that the biomechanics of vascular tissue rupture is incompletely understood. There is a need for a better model capable of explaining the various clinically observed ruptures. A core hypothesis is that toughness (resistance to tear) is the principal physical criterion governing vascular tissue rupture and that diseased vessels present microstructure alterations (plaque inclusions, medial degeneration, collagen degradation) that constitute flaws that can induce a crack propagation causing rupture. It is proposed to use the new micro-indenter just obtained with a NSERC RTI to directly measure the inherent mechanical properties of the plaque microstructures (fibrous cap, inclusions) and aortic tissue wall alterations (medial degeneration). In analogy with endurance S-N curve, a toughness exhaustion J-N curve is proposed. The shape of the experimental J-N curves can be fitted with functions of exponential type and allows for extrapolating the number of cycles required to reach a certain threshold of toughness at which the tissue would rupture. To the best of the applicant's knowledge, the concept of toughness exhaustion has not been applied to vascular tissue. Finally, the toughness concept will be used to identify the conditions associated with injuries sometimes caused by implants. Indeed, vascular implants interact with the vascular tissue and for unclear reasons can induce injuries. This would constitute the first design methodology incorporating the microstructure and the tissue mesoscale (cells agglomeration).