We used quasi-physiologic conditions and a closed loop flow system to measure the pressure-diameter relationship in segments of thoracic aortas from a New Zealand rabbit model of induced atherosclerosis. After 8 weeks of a 4% high-cholesterol diet without additional atherogenic components, we found that the aortic elastic modulus decreased as the atherosclerosis became more advanced, specifically with the appearance of intermediate (type III) and early advanced (type IV) atherosclerotic lesions. Histologically, these lesions were fatty, composed mainly of foam cells filled with lipid (RAM 11-positive cells), and with a lesser number of SMCs rich in solid-like actin (HHF-35 positive cells); while fibrous change was notably absent. Especially, in type IV atherosclerotic lesions, the usual intimal smooth muscle cells and the intercellular matrix of the deep intima are dispersed and replaced by accumulated particles of extracellular lipid. Between the lipid core and the endothelial surface, the intima contains macrophages and smooth muscle cells with and without lipid droplet inclusions. Much of the tissue between the core and the surface endothelium corresponds to the proteoglycan-rich layer of the intima, although infiltrated with the cells as macrophages, foam cells, and lymphocytes are more densely concentrated in the lesion periphery . Taken together, these factors could account for the observed decreases in stiffness. The potential clinical significance of decreased stiffness can be defined considering the histological basis of atherosclerotic lesions. Taking into account, that the region between the lipid core and the lesion surface contains proteoglycans and macrophage foam cells and only isolated smooth muscle cells and minimal collagen (potential mechanism of decreased stiffness), it may be susceptible to formation of fissures, intramural hematomas, aortic dilatation or even dissection. Thus, the periphery of advanced lesions, particularly type IV, may be vulnerable to rupture because macrophages are generally abundant in this location.
Our results are in accordance with previous studies of the mechanical properties of arterial segments with atherosclerosis. Firstly, Newman et al  reported that structural stiffness and wall elasticity of abdominal aorta was decreased during the earliest stages of atherosclerosis in cockerels fed an atherogenic diet. The decreased stiffness accompanying early atherosclerosis was attributed to the weakening of medial interlamellar elastic tissue-collagen network, probably due to lipid infiltration, whereas in advanced disease with calcification and fibrosis, aortic structural stiffness was higher than the control aortas. Nichol  reported that atherosclerosis reduced aortic stiffness at pressures below 70 mmHg, due to the destruction of elastic tissues. In a recent study designed to represent early and intermediate atherosclerosis, Hayashi and Imai  demonstrated that the force-deformation characteristics of atherosclerotic specimens of denuded thoracic rabbit aorta were less stiff than controls. Moreover, Hamilton et al  used intravascular ultrasound (IVUS) imaging, 3D reconstruction, and finite element analysis (FEA) to examine alterations in denuded femoral artery wall of Yucatan pigs fed a high cholesterol diet. The elastic modulus of the non-denuded femoral arteries decreased significantly on the high cholesterol diet, in agreement with our results. The decrease in elastic modulus with early/intermediate (fatty and fibrofatty lesions) atheroma was reversed and then increased as the lesions became fibrotic. Similarly, Vonesh et al  used IVUS image data with FEA to perform 3D reconstruction of human atherosclerotic segments of iliac and femoral arteries respectively, and found that the elastic modulus of non-atherosclerotic tissue regions was greater than early lipidous atherosclerotic regions for transmural pressure load at normal and hypertensive pressures (80-160 mmHg). De Korte et al  also used IVUS elastography of diseased human femoral and coronary arteries at 80 and 100 mmHg, and then compared the differences in strain between normal and diseased tissue. They reported that the pressure-strain modulus of fibrous tissue was double the modulus of fatty tissue, indicating the vulnerability of fatty lesions with only a thin fibrous cap. A recent study  used IVUS elastography to analyze denuded iliac and femoral arteries of Yacatan pigs on an atherogenic diet for 9-10 months. The mean strain value of arteries with early fat lesions (0.46) was greater that non diseased arteries (0.21) or diseased arteries with fibrous lesions (0.24), in agreement with our findings. Finally, Matsumoto et al  used pipette aspiration to measure the local elastic modulus of rabbit thoracic aortas fed a 1% cholesterol diet for 8-28 weeks. Similar to our results, the local elastic modulus of vessels with early lesions after 8 weeks of the atherogenic diet were significantly lower than normal tissue, although the modulus significantly increased after 24-28 weeks. The local elastic modulus appeared to decrease concurrently with the formation of early atherosclerotic lesions (intimal hyperplasia filled with foam cells), and increased gradually, coincident with the appearance of SMCs and calcification.
Our results conflict with a large number of studies that have used a variety of methods to examine the elastic properties of atherosclerotic artery walls in monkeys on high cholesterol diets for between 18-38 months. However, the contradiction with our findings is reasonable, as the duration of high cholesterol diet for atherosclerosis induction was much longer. Contrary to our findings, these studies found that arterial wall stiffness increases and decreases with the progression or regression of the atherosclerosis [28–30], respectively. It is possible that the increased stiffness after years of high cholesterol diet was due to the presence of highly fibrotic and calcified atherosclerotic lesions that would have an elastic modulus comparable to that of bone . Others have found similar changes in short duration studies of rabbits or rats [31–34]. Interestingly, Hayashi et al.  found that arterial stiffness in rabbits fed a high cholesterol diet for 4-32 weeks did not increase unless there was also considerable calcification and wall thickening, even if the atherosclerosis was highly advanced. Finally, Richter and Mittermayer  observed that the modulus of volume elasticity of autopsied human aortas was higher in more advanced stages of atherosclerosis. It is possible that the tangential strips of aorta used in some studies [31, 32] may behave differently in tests of elasticity than intact cylindrical arterial segments, such as we have used here, since it is difficult to set up isolated strips of tissue in a mechanical state that is comparable to the physiological loading conditions in vivo.
These conflicting results might also be attributed to different definitions or measurement of stiffness, testing methodologies, or stage of atherosclerosis examined. Although stiffness is generally described as a reflection of the arterial wall tissue rigidity, there are significant differences in how it is defined. While wall elasticity involves direct or indirect measurement of wall thickness, expressing the material properties of wall; stiffness, expressed as more or less similar to Peterson modulus EP, treats the vessel as a whole geometric-material structure. Thus, the different in vitro methods of physiological loading such as closed- or open-ended tube, with or without restriction of axial movement may also contribute to variability in the mechanical characteristics of the arterial wall. Determination of wall stiffness instead of wall elasticity in conjunction with the fixed ends of the aortic specimens during mechanical testing, that constrict the axial deformation occurred in vivo, could be regarded as limitations of our in vitro model.