The Human Cardiovascular System - Computational Hemodynamics: Theory, Modelling and Applications - page 37

the mechanisms of calcium formation involved, histology shows that calcium is a
common but variable component in advanced atherosclerotic plaques.
Plaque rupture is dependent on biomechanical events acting on the fibrous cap
such as haemodynamic shear stresses (Gertz and Roberts 1990), turbulent pressure
fluctuations (Loree et al. 1991), cyclic variation of intraluminal pressure and maxi-
mum principal stress by the pulsatile blood pressure (Loree et al. 1992; Richardson
et al. 1989b). In particular, large eccentric lipid cores are of mechanical disadvan-
tage since circumferential tensile stresses are configured in such a way that fibrous
caps have a tendency to rupture most of the time (Cheng et al. 1993a). This gives
rise to the relationship between plaque rupture and the critical stress acting on the
fibrous cap.
Autopsies of patients that are diagnosed of cardiac ischemia showed that the
level of macrophages is high, smooth muscle cells are reduced, the proportion of
crescentic acellular mass for a lipid core is significant, and the fibrous cap is thin
(Fayad and Fuster 2001; Moreno et al. 1994b). For plaque rupture, a 65 μm thick
fibrous cap with an infiltrate of macrophages is defined as the threshold after histo-
logical analysis (Burke et al. 1997). This can give guidance to critical risk analysis
of plaque condition.

2.3.3 Aneurysm

Aneurysm is a blood filled dilation that is caused by atherosclerotic disease of blood
vessel walls and is common near the branches of the intracranial arteries (espe-
cially at the anterior cerebral artery, and the internal carotid artery), and also at
the abdominal aortic artery (distal to the origin of the renal arteries at the infrare-
nal abdominal aorta) as shown in Fig. 2.18 . Aneurysms at the cerebral arteries are

Fig. 2.18 Cerebral and
abdominal aortic aneurysms.
Aneurismal dilatations can
occur at different arteries
and locations. For a cerebral
aneurysm, stenting is per-
formed at the aneurysm neck
to support coiling within a
wide-neck aneurysm and to
prevent the coil from falling
into the artery. For abdominal
aortic aneurismal stenting,
the placement of an endo-
vascular stent graft within an
aortic aneurysm is via a cath-
eter which is inserted up to
the abdominal aorta in order
to release the stent. (Adapted
from images by Moore 1996)