World Journal of Mechanics, 2012, 2, 216-223 doi:10.4236/wjm.2012.24026 Published Online August 2012 (http://www.SciRP.org/journal/wjm) The Effect of Carotid Plaque Morphology on Longitudinal Fibrous Cap Stress Levels Samuel A. Thrysøe 1,2 , Anders F. Stegmann 1 , Nikolaj Eldrup 2 , Anette Klærke 2 , William Paaske 2 , Won Yong Kim 1,3 , Jens V. Nygaard 4 1 MR-Center, Aarhus University Hospital, Aarhus, Denmark 2 Department of Cardiothoracic and Vascular Surgery T, Aarhus University Hospital, Aarhus, Denmark 3 Department of Cardiology, Aarhus University Hospital, Aarhus, Denmark 4 Aarhus School of Engineering, Aarhus University, Aarhus, Denmark Email: samuel@mr.au.dk Received May 29, 2012; revised June 30, 2012; accepted July 11, 2012 ABSTRACT Background and Purpose: Rupture of vulnerable carotid atherosclerotic plaques is a major cause of stroke. Stress levels may reflect risk of rupture in patients with carotid atherosclerotic plaques. Features thought to influence the risk of plaque rupture include the degree of stenosis, lipid-rich necrotic core (LR-NC) size, and thickness of the protective fi- brous caps. We used computational models to investigate the effect of these variables on fibrous cap stress levels. Methods: Two-way coupled fluid-structure interaction longitudinal 2D simulations were performed on a bifurcation model based on idealized geometry derived from a symptomatic patient. Models with varying degrees of stenosis (50% - 95%), fibrous cap thicknesses (0.05 - 1 mm), and LR-NC sizes (2 × 1 mm - 6 × 3 mm) were simulated. The stress dis- tribution for each model was calculated and peak principal stresses extracted. Regression analysis was used for assess- ing the relationship between the variables and stress levels. Results: Mechanical stresses increased with decreasing fi- brous cap thicknesses (ß = –0.905, p < 0.001) and increasing LR-NC sizes (ß = 0.262, p < 0.001). The degree of steno- sis (ß = 0.024, p = 0.344) and LR-NC placement (ß = –0.001, p = 0.979) had insignificant effects on mechanical stress levels. Conclusions: Thin-capped plaques with large atheromas, known predictors of plaque vulnerability, were shown to exhibit the greatest mechanical stress levels. Keywords: Atherosclerosis; Carotid Artery; Fluid Structure Interaction; Plaque Rupture; Stroke 1. Introduction Rupture of vulnerable carotid atherosclerotic plaques is a major source of strokes and transient ischemic attacks [1-4]. Carotid endarterectomy (CEA) has been proven beneficial in symptomatic patients with high degrees of stenosis [5-8]. It is clinical practice to estimate the risk of carotid plaque rupture by measuring the degree of lu- minal stenosis using ultrasonic Doppler blood velocity measurements [9,10]. Unfortunately, the degree of steno- sis may not truly reflect the plaque burden since athero- sclerotic plaque growth is characterized by an initial ex- pansion. This may result in normal luminal size belying substantial plaque volumes, a process known as outward remodeling [11,12]. Histopathological examinations sug- gest morphological plaque features such as large lipid- rich necrotic cores and thin protective fibrous caps to be more predictive of rupture prone vulnerable plaques [13]. Detailed in vivo morphological characterization of ca- rotid plaques is possible through high resolution imaging techniques such as magnetic resonance imaging (MRI) [5], intravascular ultrasound [14], and optical coherence tomography [15]. The morphological images may be used for constructing geometrical models for the purpose of computational simulations of mechanical stresses and plaque rupture. Given the traumatic nature of plaque rup- ture, biomechanical properties of atherosclerotic plaques have long been suspected to influence the probability of rupture [1,4,16]. Intra-plaque stress levels have been de- termined to differ between symptomatic and asympto- matic patients with carotid atherosclerosis [4], and be- tween ruptured and non-ruptured coronary atheroscle- rotic lesions [1]. In this study, we performed 2D fluid-structure interac- tion simulations of an idealized carotid artery based on the geometry of a symptomatic patient. We investigated the impact of different degrees of luminal stenosis, fi- brous cap thicknesses, lipid-rich necrotic core (LR-NC) size, and LR-NC position to determine their effect on plaque stress levels and possible risk of plaque rupture. Copyright © 2012 SciRes. WJM