Revised microcalcification hypothesis for fibrous cap rupture in human coronary arteries

Abstract

Using 2.1-µm high-resolution microcomputed tomography, we have examined the spatial distribution, clustering, and shape of nearly 35,000 microcalcifications (µCalcs) ≥ 5 µm in the fibrous caps of 22 nonruptured human atherosclerotic plaques. The vast majority of these µCalcs were <15 µm and invisible at the previously used 6.7-µm resolution. A greatly simplified 3D finite element analysis has made it possible to quickly analyze which of these thousands of minute inclusions are potentially dangerous. We show that the enhancement of the local tissue stress caused by particle clustering increases rapidly for gap between particle pairs (h)/particle diameter (D) < 0.4 if particles are oriented along the tensile axis of the cap. Of the thousands of µCalcs observed, there were 193 particle pairs with h/D ≤ 2 (tissue stress factor > 2), but only 3 of these pairs had h/D ≤ 0.4, where the local tissue stress could increase a factor > 5. Using nondecalcified histology, we also show that nearly all caps have µCalcs between 0.5 and 5 µm and that the µCalcs ≥ 5 µm observed in high-resolution microcomputed tomography are agglomerations of smaller calcified matrix vesicles. µCalcs < 5 µm are predicted to be not harmful, because the tiny voids associated with these very small particles will not explosively grow under tensile forces because of their large surface energy. These observations strongly support the hypothesis that nearly all fibrous caps have µCalcs, but only a small subset has the potential for rupture.

Keywords: clustered microcalcifications; finite element analysis of fibrous caps; microcomputed tomography imaging of microcalcifications; vulnerable plaque.

Fig. 1.

HR-µCT images of human coronary atheroma with µCalcs embedded in the fibrous cap proper. A shows images scanned at 6.7-µm resolution. B was scanned at 2.1-µm resolution. Multiple µCalcs in the cap are visible in B Inset that were previously undetected in A. A Inset and B Inset show the difference between what appears to be a single µCalc at 6.7 µm and µCalc clusters when viewed at 2.1-µm resolution. (Scale bar: 100 µm.)

Fig. 2.

(A) 3D FEA results of stress concentration factor calculated for the area between two particles located along the tensile axis in a fibrous cap. Stress concentration factor rises exponentially when the distance between the two spherical µCalcs decreases. Results are compared with previous 2D FEA reported in the work by Maldonado et al. (10). B and C show FEA results for particles with initial h/D = 0.4 oriented along and tensile the transverse axis, respectively.

Fig. 3.

Ratio h/D for 193 pairs of µCalcs embedded in fibrous caps where h/D < 2 and the corresponding stress concentration factor when embedded in a fibrous cap along the tensile axis. Lines indicate mean ± SD.

Fig. 4.

TEM and histology-based FEA. (A) TEM image of aggregated calcifying matrix vesicles forming µCalcs in a mouse atheroma. (B) Image of a µCalc embedded in a human fibrous cap, obtained from nondecalcified histology, and stained with von Kossa. (C and D) Stress distribution at the interface of the µCalcs in A and B, respectively, assuming that they are embedded in fibrous caps under tension. Numbers show calculated stress concentration factors at the poles.