Estimation of rupture sites through fluid-structure interaction simulations using the immersed boundary method

Rupture sites in thin-cap fibroatheromas are important, as thrombosis and embolism following plaque rupture are influenced by blood flow, and vice versa. Although well-developed commercial codes are available, simulating the rupture phenomenon using body-fitted grid systems is difficult due to the formation of discontinuities within the cap. To overcome the limitation, we developed an in-house code using the immersed boundary method. A semi-circular deformable stenosis with moderate severity, composed of a soft lipid core and a stiff fibrous cap, was placed in a straight, rigid channel. Assuming an incompressible Newtonian fluid, the pulsatile blood flow was solved on staggered grids using a semi-implicit fractional-step method. Lagrangian points within the stenosis were connected via Hookean springs, and rupture was assumed to occur when the local von Mises stress exceeded 300 kPa. With decreasing cap stiffness, the first rupture site over one pulsatile cycle shifted from the distal to the proximal shoulder of the cap. Given the elevated shear rate and shear stress at the proximal shoulder, proximal rupture may promote thrombus formation and heighten the long-term risk of embolism. Consequently, softening of the cap can negatively affect clinical outcomes after rupture.