Lumen Irregularity Dominates the Relationship Between Mechanical Stress Condition, Fibrous-Cap Thickness, and Lumen Curvature in Carotid Atherosclerotic Plaque

High mechanical stress condition over the fibrous cap (FC) has been widely accepted as a contributor to plaque rupture. The relationships between the stress, lumen curvature, and FC thickness have not been explored in detail. In this study, we investigate lumen irregularity-dependent relationships between mechanical stress conditions, local FC thickness $(LTFC)$⁠, and lumen curvature $(LClumen)$⁠. Magnetic resonance imaging slices of carotid plaque from 100 patients with delineated atherosclerotic components were used. Two-dimensional structure-only finite element simulations were performed for the mechanical analysis, and maximum principal stress (stress-$P1$⁠) at all integral nodes along the lumen was obtained. $LTFC$ and $LClumen$ were computed using the segmented contour. The lumen irregularity $(L-δir)$ was defined as the difference between the largest and the smallest lumen curvature. The results indicated that the relationship between stress-$P1$⁠, $LTFC$⁠, and $LClumen$ is largely dependent on $L-δir$⁠. When $L-δir≥1.31$ (irregular lumen), stress-$P1$ strongly correlated with lumen curvature and had a weak/no correlation with local FC thickness, and in 73.4% of magnetic resonance (MR) slices, the critical stress (maximum of stress-$P1$ over the diseased region) was found at the site where the lumen curvature was large. When $L-δir≤0.28$ (relatively round lumen), stress-$P1$ showed a strong correlation with local FC thickness but weak/no correlation with lumen curvature, and in 71.7% of MR slices, the critical stress was located at the site of minimum FC thickness. Using lumen irregularity as a method of identifying vulnerable plaque sites by referring to the lumen shape is a novel and simple method, which can be used for mechanics-based plaque vulnerability assessment.