Material property alterations with early atheroma in an animal model

Atherosclerosis is a diffuse and highly variable disease process that is distinguished by the subintimal accumulation of varying amounts of extracellular lipid, fibrous tissue, smooth muscle cells, and calcium. Atherosclerosis changes arterial wall morphology (Farrar et al. (1978)) and alters its mechanical properties (Kitney et al. (1989), Lee et al. (1992)). This pathologically altered morphology of diseased arterial tissue results in a complex structure that is geometrically irregular, structurally inhomogeneous, anisotropic, incompressible, non-linearly viscoelastic, subject to large strain deformations, and defies straightforward rheological characterization (Born and Richardson (1990), Cheng et al. (1993), Salunke and Topoleski (1997)). Even though the stress-strain characteristics of the arterial wall have been demonstrated to be non-linear (Bergel (1961), Dobrin (1986)) has suggested a linear relationship in deformation in the physiological pulse pressure range. Previous attempts to describe the mechanical behavior of arterial tissue have used various methods (strain gauge, linear differential transformer, sonomicrometer, angiography, ultrasound imaging) to measure arterial tissue deformation in response to an applied transmural pressure load (Buntin and Silver (1990), Farrar et al. (1978), Hayashi (1993), Hudetz et al. (1981)). Lee et al. (1992) employed an intravascular ultrasound technique to predict biomechanical parameters of human atheroma components. They classified specimens of human atheroma caps obtained from the abdominal aortas of patients at autopsy as non-fibrous, fibrous, or calcified based on intravascular ultrasound appearance. By measuring static strain by in vitro mechanical testing, uniaxial static stiffness was determined and compared with the imaged data. The results revealed that the atheroma components had three quantitatively distinct static stiffnesses - nonfibrous, fibrous and calcified with 41.2 ± 18.8, 81.7±33.2, 354.5±245.4kPa, respectively (p = 0.0002 by analysis of variance). This indicated that plaque appearance by intravascular ultrasound was related to the static stiffness. There have been many studies to investigate the change of the material properties of animal and human arterial walls with induced atherosclerosis by high cholesterol-fed and/or endothelial cells-denuded methods. The diffuse and variable nature of atherosclerosis is manifested in the conflicting results reported by different investigators on the constitutive relationships of arterial tissue. Hayashi et al. (1994) and Pynadath and Mukherjee (1977) reported an increase in the elastic modulus with atherosclerosis, whereas others (Farrar et al. (1980)) have reported a decrease in stiffness with atheroma formation. Hayashi (1993), in a review of the previous studies on the alterations of arterial wall material properties with atheroma, reported that inconsistent results from the previous studies might be ascribed to the many factors affecting the initiation and progression of atherosclerosis such as the species, and vascular site used for the studies, stage of lesion development, and the experimental techniques employed. In particular, he suggested that additional work is necessary on the correlation between the stage of atherosclerosis and the mechanical property alterations in the arterial wall. Hayashi proposed that atherosclerotic plaque might be stiffer and lipidous lesions softer than the normal arterial wall, respectively, the combination of which might yield inconclusive results. He has also indicated that it is most likely that atherosclerosis is accompanied by wall thickening, which might be the main reason that provokes the conflicting study results of elastic modulus with the disease. He pointed out that a clear distinction must be made between material properties and structural properties of the vascular wall as atherosclerosis is often accompanied by an increase in wall thickness. Material properties refer to the so-called elastic modulus which expresses the elastic properties inherent to the material. On the other hand, structural stiffness is determined by both the material elasticity and material dimensions. Hayashi et al. (1994) induced atheroma in a rabbit model by endothelial cell denudation and high cholesterol diet. The aortic segments were excised at various times after denudation followed by high cholesterol diet and the pressure-diameter relationship was obtained in order to characterize alterations in wall material properties and structural stiffness. They found that denudation of endothelial cells alone did not result in lipid deposition on the luminal surface of the aortic wall. Cholesterol feeding alone also did not change the structural stiffness, elastic modulus, or wall thickness of the aortic wall. Combination of the denudation of endothelial cells and high cholesterol diet, however, induced significantly greater changes in structural stiffness, elastic modulus and wall thickness. The technique of employing the pressure-diameter relationship for the assessment of alterations in the wall material property will measure the average change in the material stiffness, but does not provide information on the regional alterations in wall material properties due to the effect of diffuse atheroma lesions with varying degree of lipidous, fibrous, and calcific components. In a subsequent study employing a rabbit model for induced atheroma, Hayashi and Imai (1997) studied normal and 12-week atheromatous segments subjected to physiological pressures. The results showed that the elastic stiffness reduced in specimens with lesions compared to the normal segments. Here the lesion presumably corresponded to early lipidous lesions even though no histological results were presented. A common drawback of conventional methods applied to intact vascular segments is an inference of the mechanical behavior of an entire segment by using only discrete measurements in a localized area of the artery (e.g., external diameter at a specific axial location). These methods have severe limitations especially when applied to atherosclerotic vessels, in which composition and structure vary as a function of both longitudinal and circumferential position within the artery (Born and Richardson (1990)). A method to assess threedimensional regional variability in material properties is essential to determine the extent and location of atherosclerotic lesions in vivo. The de Korte group employed intravascular elastography to characterize the various components in atherosclerotic lesions (de Korte et al. (2000)). They performed ultrasound imaging of excised atherosclerotic human femoral and coronary arteries and correlated the ultrasound elastograms with the histological data. Their studies demonstrated the local strains in arterial wall regions with fatty lesions were higher (indicating reduction in arterial stiffness) corresponding to normal segments. With fibro-fatty and fibrous lesions, the local strains decreased compared to those for fatty lesions, indicating an increase in stiffness with lesion development. Their study also revealed that the local strains depended on the composition of the lesion, but not on the arterial segment studied (coronary vs. femoral). More recently, their group investigated atherosclerotic plaque components using ultrasound elastography in vivo in a Yucatan miniswine atherosclerotic model (de Korte et al. (2002)). Their results showed that higher mean strain values were found in regions corresponding to early fatty lesions compared to lesion-free arterial segments. However, the average strain in the lesion-free segments was similar to those with fibrous and advanced fibrous lesions. They were able to prove that elastography has a high sensitivity and specificity for identification of the local mechanical properties of the arterial wall and plaque. Even though the work by the de Korte group demonstrated the variability in the material properties with varying composition of the lesions, information on the threedimensional regional variations in the arterial wall material properties affected by the atherosclerotic lesion was not presented in this study. In order to describe the regional alterations in arterial wall material property with atheromatic lesions in vivo, we have developed a novel methodology to evaluate three-dimensional regional alterations in arterial wall material properties. We have demonstrated the feasibility of assessment of material property alterations with induced lesions in an animal model. We employed intravascular ultrasound (IVUS) imaging and a three-dimensional reconstruction of vascular segments with and without atheroma formation and performed finite element and optimization analysis to identify regional alterations in arterial wall material property and correlated the same with histology. Our discussion focuses on the benefits of these methodologies to evaluate atheroma progression in vivo and compares our findings in the material property alterations between the femoral and carotid arterial beds.