Characterization of fracture behavior of human atherosclerotic fibrous caps using a miniature single edge notched tensile test

Abstract

One well-established cause of ischemic stroke is atherosclerotic plaque rupture in the carotid artery. Rupture occurs when a tear in the fibrous cap exposes highly thrombogenic material in the lipid core. Though some fibrous cap material properties have been measured, such as ultimate tensile strength and stress-strain responses, there has been very little, if any, data published regarding the fracture behavior of atherosclerotic fibrous caps. This study aims to characterize the qualitative and quantitative fracture behavior of human atherosclerotic plaque tissue obtained from carotid endarterectomy samples using two different metrics. Uniaxial tensile experiments along with miniature single edge notched tensile (MSENT) experiments were performed on strips of isolated fibrous cap. Crack tip opening displacement (CTOD) and stress in the un-cracked segment (UCS) were measured at failure in fibrous cap MSENT specimens subjected to uniaxial tensile loading. Both CTOD and the degree of crack blunting, measured as the radius of curvature of the crack tip, increased as tearing propagated through the tissue. Higher initial stress in the UCS is significantly correlated with higher collagen content and lower macrophage content in the fibrous cap (ρ=0.77, P=0.009; ρ=-0.64, P=0.047; respectively). Trends in the data show that higher CTOD is inversely related to collagen content, though the sample size in this study is insufficient to statistically substantiate this relationship. To the authors' knowledge, this is the pioneering study examining the fracture behavior of fibrous caps and the first use of the CTOD metric in vascular tissue.

Statement of significance: A tear in the fibrous cap of atherosclerotic plaque can lead to ischemic stroke or myocardial infarction. While there is some information in the literature regarding quantitative measures of fibrous cap failure, there is little information regarding the behavior of the tissue during failure. This study examines the failure behavior of fibrous caps both qualitatively, by examining how and where the tissue fails, and quantitatively, by measuring (a) crack tip opening displacement (CTOD) in vascular tissue for the first time and (b) uniaxial stress in the un-cracked segment (UCS). This study shows that both metrics should be evaluated when assessing plaque vulnerability.

Keywords: Atherosclerosis; Crack blunting; Crack tip opening displacement (CTOD); Fracture mechanics; Plaque rupture.

Fig. 1

Typical examples of specimens obtained from carotid endarterectomy. An intact specimen immediately following harvest is shown on the left, with the common carotid artery at the bottom. The specimens are sliced into a series of 5 mm rings, as shown in the center. A transverse view of one slice is shown on the right, with the fibrous cap, atheroma, and underlying media labeled.

Fig. 2

A depiction of the CTOD measurement (a, b) and the geometry measurements required for the stress in the UCS calculations (c, d). CTOD is calculated by measuring the distance between the intersections of the sides of a 90 degree vertex centered at the crack tip. The axisymmetric case is shown in (a) and the non-axisymmetric case is shown in (b). The isolated fibrous cap is mounted in a pair of micro clamps in (c, d) and the current cross-sectional area is measured for the stress in the UCS calculations. The current cross-sectional area of the UCS is determined by measuring the current width (white line, c) and the current thickness (white line, d) of the UCS at the crack tip. The longitudinal (L), circumferential (C), and radial (R) directions are as shown.

Fig. 3

The stress-strain response of several fibrous caps. The dots represent experimental data points The solid curves represent predictions based on material model data fitting performed in Abaqus.

Fig. 4

The low-strain tangent modulus (LSTM), high-strain tangent modulus (HSTM) and the stretch ratio at shift are determined from the stress-strain curves. Data from the low-strain region and high-strain region of each stress-strain curve are fit by linear regression to determine the low-strain tangent modulus and high-strain tangent modulus, respectively. The stretch ratio at shift is determined by calculating the intersection of the HSTM line with the X-axis.

Fig. 5

The CTOD increases with crack extension (Δa) for each specimen. The dashed line shows a trendline through all CTOD data points.

Fig. 6

Stress in the UCS versus crack extension for each specimen. Samples III and IV only had one cycle of tearing and therefore were not evaluated at multiple crack extensions.

Fig. 7

Representative images from PSR staining (top left), total macrophage immunohistochemistry (IHC) (top right), SMC IHC (bottom left) and the quantified results (bottom right). Insets represent images used to quantify total tissue area. Collagen is significantly more abundant than SMCs or macrophages in the fibrous cap (*denotes p < 0.01). Error bars represent standard error.

Fig. 8

The initial stress in the UCS is affected by collagen and macrophage content. There are statistically significant relationships between both collagen content (top left) and macrophage content (top right) with the stress in the uncracked segment for the initial tearing cycle. While the trends are similar when the stress in the UCS is averaged over all cycles, the trends are no longer significant with either collagen content (bottom left) or macrophage content (bottom right). No significant relationship is observed between SMCs and stress in the UCS (data not shown). ρ denotes rho, and P represents the P-value associated with a significant relationship between the two variables. Error bars represent standard error.

Fig. 9

Initial CTOD does not depend on fibrous cap constituents but is related to radius of curvature. There is an inverse trend between total collagen content and initial CTOD and a direct trend between total macrophage content and initial CTOD, but neither relationship is statistically significant. There is no clear trend between SMC content and initial CTOD. There is a statistically significant relationship between initial CTOD and initial radius of curvature at the crack tip.

Fig. 10

Blunting of the crack tip progresses as the tissue tears. The first row shows the initial, intact specimen (a), the specimen immediately after the notch is made (b), and the specimen during the deformation before tearing initiated (c-f). The bottom row shows the progression of blunting as tearing progresses in the tissue (a-f).