Size and proximity of micro-scale hard-inclusions increase the risk of rupture in fibroatheroma-like laboratory models.

ABSTRACT

Increased mechanical stresses of the fibroatheroma cap tissue is a crucial risk factor on the pathogenesis of asymptomatic coronary artery disease events. Moreover, both numerical and analytical studies have shown that microcalcifications (µCalcs) located in the fibrous cap can multiply the cap tissue stress by a factor of 2-7. This stress amplification depends on the ratio of the gap between particles (h) and their diameter (D) when they are aligned along the tensile axis. However, the synergistic effect of cap stiffness and uCalcs on the ultimate stress and rupture risk of the atheroma cap has not been fully investigated. In this context, we studied the impact of micro-beads (µBeads) of varying diameters and concentration on the rupture of silicone-based laboratory models mimicking human fibroatheroma caps of different stiffness (shear moduli µsoft = 40 kPa, µstiff = 400 kPa) and thickness (650 µm and 100 µm). A total of 145 samples were tested under uniaxial tension up to failure and the true stress and strain response of each model was derived by means of Digital Image Correlation (DIC). Before testing, samples were scanned using high-resolution Micro-CT, to perform morphometry analyses of the embedded micro-beads and determine the number of closely spaced particles (h/D<0.5). The micro-beads structural and spatial features were then compared to the case of 29 non-ruptured human atheroma fibrous caps presenting µCalcs. Samples with and without µBeads exhibited a distinct hyperelastic behavior typical of arterial tissues. Regardless of the sample stiffness, large µBeads (>80 µm) significantly reduced the ultimate tensile stress (UTS) of the thick cap models with the effect being more pronounced as the particle diameter increases. Stiff models experienced early rupture in the presence of µBeads with 40 µm diameter. Smaller µBeads of 6 µm and 20 µm didn't affect the ultimate strength of the thick cap models. However, when 6 µm µBeads where introduced in thinner cap models, we observed more than 20% drop in UTS. Increasing the µBeads concentration was also positively correlated with lower stresses at rupture as more clusters formed resulting in lower values of h/D. Morphometry analyses of cap models and human atheroma show that the 6 µm µBeads groups present very similar size distributions to µCalcs and that human µCalcs occupy an average volume ratio of 0.79 ± 0.85%. Our results clearly capture the influence of µBeads on the rupture threshold of a vascular tissue mimicking material. This effect appears to be dependent on the µBeads-to-cap thickness size ratio as well as their proximity. These findings support previous numerical and analytical studies suggesting that µCalcs located within the fibroatheroma cap may be responsible for significantly increasing the risk of cap rupture that precedes myocardial infarction and sudden death.