Residual strain and local strain distributions in the rabbit atherosclerotic aorta

The Contribution of Glycosaminoglycans/Proteoglycans to Aortic Mechanics in Health and Disease: A Critical Review

While elastin and collagen have received a lot of attention as major contributors to aortic biomechanics, glycosaminoglycans (GAGs) and proteoglycans (PGs) recently emerged as additional key players whose roles must be better elucidated if one hopes to predict aortic ruptures caused by aneurysms and dissections more reliably. GAGs are highly negatively charged polysaccharide molecules that exist in the extracellular matrix (ECM) of the arterial wall. In this critical review, we summarize the current understanding of the contributions of GAGs/PGs to the biomechanics of the normal aortic wall, as well as in the case of aortic diseases such as aneurysms and dissections. Specifically, we describe the fundamental swelling behavior of GAGs/PGs and discuss their contributions to residual stresses and aortic stiffness, thereby highlighting the importance of taking these polyanionic molecules into account in mathematical and numerical models of the aorta. We suggest specific lines of investigation to further the acquisition of experimental data to complement simulations and solidify our current understanding. We underscore different potential roles of GAGs/PGs in thoracic aortic aneurysm (TAAD) and abdominal aortic aneurysm (AAA). Namely, we report findings according to which the accumulation of GAGs/PGs in TAAD causes stress concentrations which may be sufficient to initiate and propagate delamination. On the other hand, there seems to be no clear indication of a relationship between the marked reduction in GAG/PG content and the stiffening and weakening of the aortic wall in AAA.

Variation of Axial Residual Strains Along the Course and Circumference of Human Aorta Considering Age and Gender

Our understanding of aortic biomechanics is customarily limited by lack of information on the axial residual stretches of the vessel in both humans and experimental animals that would facilitate the identification of its actual zero-stress state. The aim of this study was thus to acquire hitherto unreported quantitative knowledge of axial opening angle and residual stretches in different segments and quadrants of the human aorta according to age and gender. Twenty-three aortas were harvested during autopsy from the aortic root to the iliac bifurcation and were divided into ≥12 segments and 4 quadrants. Morphometric measurements were taken in the excised/curled configuration of rectangular strips considered to be under zero-stress using image-analysis software to study the axial/circumferential variation of axial opening angle, internal/external residual stretch, and thickness of the aortic wall. The measured data demonstrated: (1) an axial opening angle peak at the arch branches, decreasing toward the ascending and to a near-constant value in the descending thoracic aorta, and increasing in the abdominal aorta; (2) the variation of residual stretches resembled that of opening angle, but axial differences in external residual stretch were more prominent; (3) wall thickness showed a progressive diminution along the vessel; (4) the highest opening angle/residual stretches were found in the inner quadrant and the lowest in the outer quadrant; (5) the anterior was the thinnest quadrant throughout the aorta; (6) age caused thickening but greatly reduced axial opening angle/residual stretches, without differences between males and females.

Multi-sector approximation method for arteries: the residual stresses of circumferential rings with non-trivial openings

The opening angle method is a popular choice in biomechanics to estimate residual stresses in arteries. Experimentally, it means that an artery is cut into rings; then the rings are cut axially or radially allowing them to open into sectors; finally, the corresponding opening angles are measured to give residual stress levels by solving an inverse problem. However, for many tissues, for example in pathological tissues, the ring does not open according to the theory into a neat single circular sector, but rather creates an asymmetric geometry, often with abruptly changing curvature(s). This phenomenon may be due to a number of reasons including variation in thickness, microstructure, mechanical properties, etc. As a result, these samples are often eliminated from studies relying on the opening angle method, which limits progress in understanding and evaluating residual stresses in real arteries. With this work, we propose an effective approach to deal with these non-trivial openings of rings. First, we digitize pictures of opened rings to split them into multiple, connected circular sectors. Then we measure the corresponding opening angles for each sub-sector. Subsequently, we can determine the residual stresses for individual sectors in a closed-ring configuration and, thus, approximate the circumferential residual bending effects.

Multiphoton microscopy observations of 3D elastin and collagen fiber microstructure changes during pressurization in aortic media

Elastin and collagen fibers play important roles in the mechanical properties of aortic media. Because knowledge of local fiber structures is required for detailed analysis of blood vessel wall mechanics, we investigated 3D microstructures of elastin and collagen fibers in thoracic aortas and monitored changes during pressurization. Using multiphoton microscopy, autofluorescence images from elastin and second harmonic generation signals from collagen were acquired in media from rabbit thoracic aortas that were stretched biaxially to restore physiological dimensions. Both elastin and collagen fibers were observed in all longitudinal-circumferential plane images, whereas alternate bright and dark layers were observed along the radial direction and were recognized as elastic laminas (ELs) and smooth muscle-rich layers (SMLs), respectively. Elastin and collagen fibers are mainly oriented in the circumferential direction, and waviness of collagen fibers was significantly higher than that of elastin fibers. Collagen fibers were more undulated in longitudinal than in radial direction, whereas undulation of elastin fibers was equibiaxial. Changes in waviness of collagen fibers during pressurization were then evaluated using 2-dimensional fast Fourier transform in mouse aortas, and indices of waviness of collagen fibers decreased with increases in intraluminal pressure. These indices also showed that collagen fibers in SMLs became straight at lower intraluminal pressures than those in EL, indicating that SMLs stretched more than ELs. These results indicate that deformation of the aorta due to pressurization is complicated because of the heterogeneity of tissue layers and differences in elastic properties of ELs, SMLs, and surrounding collagen and elastin.

Effects of mechanical properties and atherosclerotic artery size on biomechanical plaque disruption - mouse vs. human

Mouse models of atherosclerosis are extensively being used to study the mechanisms of atherosclerotic plaque development and the results are frequently extrapolated to humans. However, major differences have been described between murine and human atherosclerotic lesions and the determination of similarities and differences between these species has been largely addressed recently. This study takes over and extends previous studies performed by our group and related to the biomechanical characterization of both mouse and human atherosclerotic lesions. Its main objective was to determine the distribution and amplitude of mechanical stresses including peak cap stress (PCS) in aortic vessels from atherosclerotic apoE(-/-) mice, in order to evaluate whether such biomechanical data would be in accordance with the previously suggested lack of plaque rupture in this model. Successful finite element analysis was performed from the zero-stress configuration of aortic arch sections and mainly indicated (1) the modest role of atherosclerotic lesions in the observed increase in residual parietal stresses in apoE(-/-) mouse vessels and (2) the low amplitude of murine PCS as compared to humans. Overall, the results from the present study support the hypothesis that murine biomechanical properties and artery size confer less propensity to rupture for mouse lesions in comparison with those of humans.

Novel biaxial tensile test for studying aortic failure phenomena at a microscopic level

Background

An aortic aneurysm is a local dilation of the aorta, which tends to expand and often results in a fatal rupture. Although larger aneurysms have a greater risk of rupture, some small aneurysms also rupture. Since the mechanism of aortic rupture is not well understood, clarification of the microstructure influencing the failure to rupture is important. Since aortic tissues are stretched biaxially in vivo, we developed a technique to microscopically observe the failure of an aortic rupture during biaxial stretch.

Methods

A thinly sliced porcine thoracic aortic specimen was adhered to a circular frame and pushed onto a cylinder with a smaller diameter to stretch the specimen biaxially. To induce failure to rupture at the center, the specimen was thinned at the center of the hole as follows: the specimen was frozen while being compressed with metal plates having holes, which were 3 mm in diameter at their centers; the specimen was then sliced at 50-μm intervals and thawed.

Results

The ratio of the thickness at the center to the peripheral area was 99.5% for uncompressed specimens. The ratio decreased with an increase in the compression ratio ε_c and was 47.3% for specimens with ε_c = 40%. All specimens could be stretched until failure to rupture. The probability for crack initiation within the cylinder was <30% and 100% for specimens with ε_c <10% and ε_c >30%, respectively. Among specimens ruptured within the cylinder, 93% of those obtained from the mid-media showed crack initiation at the thin center area.

Conclusions

Aortic tissues were successfully stretched biaxially until failure, and their crack initiation points were successfully observed under a microscope. This could be a very useful and powerful method for clarifying the mechanism of aortic rupture. We are planning to use this technique for a detailed investigation of events occurring at the point of failure when the crack initiates in the aortic aneurysm wall.

Osmotic swelling and residual stress in cardiovascular tissues

Osmotic swelling (OS) and residual stress (RS) significantly affect the function of cardiovascular (CVS) tissues and organs. The physical mechanisms of OS and RS are reviewed and analyzed with focus on the theoretical background and related experimental evidence. It will be shown that swelling of CVS tissues stems from the presence of charged proteoglycan macro-molecules in these tissues, and that this swelling is a key determinant of RS. In view of OS and RS functional significance in mechanical function, modeling attempts which incorporate them in CVS stress analysis will be presented and discussed.

Continuum modeling of biological tissue growth by cell division, and alteration of intracellular osmolytes and extracellular fixed charge density

A framework is formulated within the theory of mixtures for continuum modeling of biological tissue growth that explicitly addresses cell division, using a homogenized representation of cells and their extracellular matrix (ECM). The model relies on the description of the cell as containing a solution of water and osmolytes, and having a porous solid matrix. The division of a cell into two nearly identical daughter cells is modeled as the doubling of the cell solid matrix and osmolyte content, producing an increase in water uptake via osmotic effects. This framework is also generalized to account for the growth of ECM-bound molecular species that impart a fixed charge density (FCD) to the tissue, such as proteoglycans. This FCD similarly induces osmotic effects, resulting in extracellular water uptake and osmotic pressurization of the ECM interstitial fluid, with concomitant swelling of its solid matrix. Applications of this growth model are illustrated in several examples.

Mechanisms of residual stress in soft tissues

Although the importance of knowing the magnitude of residual stress (RS) and its functional significance are widely recognized, there is still disagreement and confusion regarding the nature of physical mechanisms giving rise to RS in tissues and organs. Here an attempt is made to examine the various mechanisms which may be involved in producing RS, and to estimate their roles and significance based on previously published experimental observations. Two concepts are introduced. The first establishes a hierarchy of different possible RS producing mechanisms from the micro (local) level of the tissue space, through the meso-level of the whole tissue, to the macro (organ) one. Whereas micro-level RS seem to be present in all soft tissues, the existence of macro- and meso-level mechanisms are tissue and organ specific. The second concept introduced highlights the significance of tissue swelling as an RS producing mechanism in the local micro-level. The implications of RS mechanism hierarchy are discussed regarding the interpretations of commonly used experimental methods aimed to study RS or to estimate its magnitude. Of the three categories of RS mechanisms, the local micro-RS is the least understood. It is analyzed here in terms of the tissue's multiconstituent structure, in the framework of mixture theory. It is shown that the micro-RS can stem either from interactions between the solid tissue constituents or between its solids and its fluidlike matrix. The latter mode is associated with osmotic-driven tissue swelling. The feasibility of these two mechanisms is analyzed based on published observations and measured data. The analysis suggests that under conditions not too remote from the in vivo homeostatic one, osmotic-driven tissue swelling is a predominant RS producing mechanism. The analysis also suggests that a true stress-free configuration can be obtained only if all RS producing mechanisms are relieved, and outlines a manner by which this may be achieved.

Altered structural and mechanical properties in decellularized rabbit carotid arteries

Recently, major achievements in creating decellularized whole tissue scaffolds have drawn considerable attention to decellularization as a promising approach for tissue engineering. Decellularized tissues are expected to have mechanical strength and structure similar to the native tissues from which they are derived. However, numerous studies have shown that mechanical properties change after decellularization. Often, tissue structure is observed by histology and electron microscopy, but the structural alterations that may have occurred are not always evident. Here, a variety of techniques were used to investigate changes in tissue structure and relate them to altered mechanical behavior in decellularized rabbit carotid arteries. Histology and scanning electron microscopy revealed that major extracellular matrix components were preserved and fibers appeared intact, although collagen appeared looser and less crimped after decellularization. Transmission electron microscopy confirmed the presence of proteoglycans (PG), but there was decreased PG density and increased spacing between collagen fibrils. Mechanical testing and opening angle measurements showed that decellularized arteries had significantly increased stiffness, decreased extensibility and decreased residual stress compared with native arteries. Small-angle light scattering revealed that fibers had increased mobility and that structural integrity was compromised in decellularized arteries. Taken together, these studies revealed structural alterations that could be related to changes in mechanical properties. Further studies are warranted to determine the specific effects of different decellularization methods on the structure and performance of decellularized arteries used as vascular grafts.

Effect of osmolarity on the zero-stress state and mechanical properties of aorta

Some pathological conditions may affect osmolarity, which can impact cell, tissue, and organ volume. The hypothesis of this study is that changes in osmolarity affect the zero-stress state and mechanical properties of the aorta. To test this hypothesis, a segment of mouse abdominal aorta was cannulated in vivo and mechanically distended by perfusion of physiological salt (NaCl) solutions with graded osmolarities from 145 to 562 mosM. The mechanical (circumferential stress, strain, and elastic modulus) and morphological (wall thickness and wall area) parameters in the loaded state were determined. To determine the osmolarity-induced changes of zero-stress state, the opening angle was observed by immersion of the sectors of mouse, rat, and pig thoracic aorta in NaCl solution with different osmolarities. Wall volume and tissue water content of the rings were also recorded at different osmolarities. Our results show that acute aortic swelling due to low osmolarity leads to an increase in wall thickness and area, a change in the stress-strain relationship, and an increase in the elastic modulus (stiffness) in mouse aorta. The opening angle, wall volume, and water content decreased significantly with increase in osmolarity. These findings suggest that acute aortic swelling and shrinking result in immediate mechanical changes in the aorta. Osmotic pressure-induced changes in the zero-stress state may serve to regulate mechanical homeostasis.

Influence of residual stress/strain on the biomechanical stability of vulnerable coronary plaques: potential impact for evaluating the risk of plaque rupture

In a vulnerable plaque (VP), rupture often occurs at a site of high stress within the cap. It is also known that vessels do not become free of stress when all external loads are removed. Previous studies have shown that such residual stress/strain (RS/S) tends to make the stress distribution more uniform throughout the media of a normal artery. However, the influence of RS/S on the wall stress distribution in pathological coronaries remains unclear. The aim of this study was to investigate the effects of RS/S on the biomechanical stability of VPs. RS/S patterns were studied ex vivo in six human vulnerable coronary plaque samples. Because the existence of RS/S can only be assessed by releasing it, the opening angle technique was the experimental approach used to study the geometrical opening configurations of the diseased arteries, producing an arterial wall in a near-zero stress state. Reciprocally, these opening geometries were used in finite element simulations to reconstruct the RS/S distributions in closed arteries. It was found that the RS/S 1) is not negligible, 2) dramatically affects the physiological peak stress amplitude in the thin fibrous cap, 3) spotlights some new high stress areas, and 4) could be a landmark of the lipid core's developmental process within a VP. This study demonstrates that plaque rupture is not to be viewed as a consequence of intravascular pressure alone, but rather of a subtle combination of external loading and intraplaque RS/S.

Layer-Specific 3D Residual Deformations of Human Aortas with Non-Atherosclerotic Intimal Thickening

Data relating to residual deformations in human arteries are scarce. In this paper we investigate three-dimensional residual deformations for intact strips and for their separate layers from human aortas in their passive state. From 11 abdominal aortas with identified anamnesis, 16 pairs of rings and axial strips were harvested, and the rings cut open. After 16 h images of the resulting geometries were recorded, and the strips were separated into their three layers; after another 6 h images were again recorded. Image processing and analysis was then used to quantify residual stretches and curvatures. For each specimen histological analysis established that the intima, media and adventitia were clearly separated, and the separation was atraumatic. Axial in situ stretches were determined to be 1.196+/-0.084. On separation, the strips from the adventitia and media shortened (between 4.03 and 8.76% on average), while the intimal strips elongated on average by 3.84% (circumferential) and 4.28% (axial) relative to the associated intact strips. After separation, the adventitia from the ring sprang open by about 180 degrees on average, becoming flat, the intima opened only slightly, but the media sprang open by more than 180 degrees (as did the intact strip). The adventitia and intima from the axial strips remained flat, while the media (and the intact strip) bent away from the vessel axis. This study has shown that residual deformations are three dimensional and cannot be described by a single parameter such as 'the' opening angle. Their quantification and modeling therefore require consideration of both stretching and bending, which are highly layer-specific and axially dependent.

Changes of opening angle in hypertensive and hypotensive arteries in 3-day organ culture

To study the effect of pressure changes on the opening angle of arteries in organ culture, tubular segments of porcine common carotid arteries were cultured with pulsatile flow perfusion under hypertensive (150+/-20 mmHg), normotensive (100+/-20 mmHg), or hypotensive (30+/-10 mmHg) pressure while maintaining the arteris at a physiological wall shear stress of approximately 15 dyn/cm(2) for up to 3 days. Arteries were then cut into short ring segments by sections perpendicular to the axis and then cut open radially to observe the opening angle in aerated phosphate buffered saline solution (37 degrees C). Norepinephrine (NE, 10 microM), carbacol (CCh, 100 microM), and sodium nitroprusside (SNP, 10 microM) were added after the radial cut at 30, 20, and 30 min intervals, the opening angles were measured, respectively. Results show that hypertensive arteries developed a significantly larger opening angle than normotensive and hypotensive arteries, associated with a significant increase in cell proliferation. In addition, with smooth muscle contraction activated by NE, the opening angle decreases significantly in hypertensive arteries but has little change in hypotensive and normotensive arteries, indicating an enhancement of smooth muscle contraction on the lumen side of the hypertensive arterial wall. In comparison, hypotensive pressure has little effect on arterial opening angle and cell proliferation.

Transmural strain distribution in the blood vessel wall

The transmural distributions of stress and strain at the in vivo state have important implications for the physiology and pathology of the vessel wall. The uniform transmural strain hypothesis was proposed by Takamyzawa and Hayashi (Takamizawa K and Hayashi K. J Biomech 20: 7–17, 1987; Biorheology 25: 555–565, 1988) as describing the state of arteries in vivo. From this hypothesis, they derived the residual stress and strain at the no-load condition and the opening angle at the zero-stress state. However, the experimental evidence cited by Takamyzawa and Hayashi ( J Biomech 20: 7–17, 1987; and Biorheology 25: 555–565, 1988) to support this hypothesis was limited to arteries whose opening angles (θ) are <180°. It is well known, however, that θ > 180° do exist in the cardiovascular system. Our hypothesis is that the transmural strain distribution cannot be uniform when θ is >180°. We present both theoretical and experimental evidence for this hypothesis. Theoretically, we show that the circumferential stretch ratio cannot physically be uniform across the vessel wall when θ exceeds 180° and the deviation from uniformity will increase with an increase in θ beyond 180°. Experimentally, we present data on the transmural strain distribution in segments of the porcine aorta and coronary arterial tree. Our data validate the theoretical prediction that the outer strain will exceed the inner strain when θ > 180°. This is the converse of the gradient observed when the residual strain is not taken into account. Although the strain distribution may not be uniform when θ exceeds 180°, the uniformity of stress distribution is still possible because of the composite nature of the blood vessel wall, i.e., the intima-medial layer is stiffer than the adventitial layer. Hence, the larger strain at the adventitia can result in a smaller stress because the adventitia is softer at physiological loading.

Residual strains in conduit arteries

Residual strains and stresses are those that exist in a body when all external loads are removed. Residual strains in arteries can be characterized by the opening angle of the sector-like cross-section which arises when an unloaded ring segment is radially cut. A review of experimental methods for measuring residual strains and the main results about the variation of the opening angle with arterial localization, age, smooth muscle activity, mechanical environment and certain vascular pathologies are presented and discussed. It is shown that, in addition to their well-established ability to homogenize the stress field in the arterial wall, residual strains make arteries more compliant and thereby improve their performance as elastic reservoirs and ensure more effective local control of the arterial lumen by smooth muscle cells. Finally, evidence that, in some cases, residual strains remain in arteries even after they have been cut radially is discussed.

Local elastic modulus of atherosclerotic lesions of rabbit thoracic aortas measured by pipette aspiration method

Changes in mechanical properties of arteries during atherogenesis remain controversial. One of the reasons could be that they have been evaluated with parameters measured in a whole vessel, although the lesions are localized. The local elastic modulus of atherosclerotic lesions was measured by the pipette aspiration method in thoracic aortas of rabbits fed a cholesterol diet for 8, 16, 24 and 28 weeks. The global elastic modulus of the whole aorta was measured by the pressure-diameter test. The local modulus decreased from that of the normal tissue in 8 weeks and then increased during the cholesterol feeding period. The global modulus did not change until 24 weeks and increased by 28 weeks. Histological observation revealed that the initial soft lesion was mainly composed of foam cells, and the stiffening accompanied first the appearance of smooth muscle cells in the top layer of the hyperplastic intima and then calcification in its bottom layer. The global elastic modulus did not change until marked calcification occurred in the tissue. These results suggest that change in mechanical properties of atherosclerotic lesion is not simple and has a close correlation with its histology. Assessment of local mechanical properties is important for studying mechanical properties of atherosclerotic arteries.

Uniform stress state in bone structure with residual stress

Residual stress and strain in living tissues have been investigated from the viewpoint of mechanical optimality maintained by adaptive remodeling. In this study, the residual stresses in the cortical-cancellous bone complex of bovine coccygeal vertebrae were examined. Biaxial strain gages were bonded onto the cortical surface, so that the gage axes were aligned in the cephalocaudal and circumferential directions. Strains induced by removal of the end-plate and the cancellous bone were recorded sequentially. The results revealed the existence of compressive residual stress in the cortical bone and tensile residual stress in the cancellous bone in both the cephalocaudal and the circumferential direction. The observed strains were examined on the basis of the uniform stress hypothesis using a three-bar model for the cephalocaudal direction and a three-layered cylinder model for the circumferential direction. In this model study, the magnitude of effective stresses, which is defined as the macroscopic stress divided by the area fraction of bone material, was found not to differ significantly between cephalocaudal and circumferential directions, although they were evaluated using independent models. These results demonstrate that the uniform stress state of the cortical-cancellous bone structure is consistent with results obtained in the cutting experiment when the existence of residual stress is taken into account.

Postsurgical changes of the opening angle of canine autogenous vein graft

The opening angles of 30 canine autogenous vein grafts were measured to determine the postsurgical change of residual strain in the vein graft. Canine femoral veins were grafted to femoral arteries in the end-to-end anastomosis fashion. When harvested, the vein grafts were cut into short segments and the segments were cut open radially. The opened-up configurations were taken as the zero-stress states of the vessels. Opening angle, defined as the angle between the two lines from the middle point to the tips of the inner wall, was used to describe the zero-stress states. Results show that the opening angles (mean +/- SD) are 63.0 +/- 30.6 deg for normal femoral veins, and -0.4 +/- 4.6, 6.1 +/- 19.4, 25.4 +/- 20.1, and 47.8 +/- 11.4 deg for vein grafts at 1 day, 1 week, 4 and 12 weeks postsurgery, respectively. The postsurgical changes in opening angle reveal nonuniform transmural tissue remodeling in the vascular wall. The relations between the changes in opening angle and the changes in the morphology of the vein grafts are discussed. Intimal hyperplasia is correlated to the opening angle and is suggested to be the main factor for the postsurgical increase in opening angle. The longitudinal strain in the vein graft is found to decrease postsurgically.

Altered compliance and residual strain precede angiographically detectable early atherosclerosis in low-density lipoprotein receptor deficiency

BACKGROUND

This study was performed to detect changes in vascular biomechanical properties early in atherogenesis.

METHODS AND RESULTS

Age- and weight-matched LDL-receptor deficient Watanabe hypercholesterolemic male rabbits (Group I: n = 11) and normal rabbits (Group II: n = 11) were studied. Fasting plasma lipoprotein concentrations, aortic angiography and intravascular ultrasound, in vivo aortic compliance evaluation, ex vivo aortic residual strain measurements, aortic lipid content and histopathology were determined. Plasma cholesterol was increased 9.8 fold and aortic cholesterol content was increased from 20 to 43 fold in Group I compared to Group II, respectively (P < .00005). Angiography revealed no stenoses in either group, whereas intravascular ultrasound and histological studies of Group I showed small circumferential plaques with < 10% cross-sectional area involvement. The residual strain in Group I was significantly increased in the ascending thoracic aorta (22.1 +/- 6.9% versus 10.4 +/- 3.2% in Group II, P < .0001), descending thoracic aorta (15.7 +/- 7.2% versus 4.8 +/- 1.3% in Group II, P < .0001), and abdominal aorta (18.0 +/- 4.8% versus 8.3 +/- 6.3% in Group II, P < .005). Changes in residual strain were inversely correlated with the aortic cholesterol content in the ascending thoracic aorta (r = -.72; P = -.001), descending thoracic aorta (r = -.95; P < .001), and abdominal aorta (r = -.51; P = .019).

CONCLUSIONS

Early atherosclerosis in LDL-receptor deficient rabbits, undetectable by angiography yet observed by intravascular ultrasound imaging and histology, is associated with marked changes in ex vivo residual strain. Alterations in vascular biomechanical properties, associated with changes in cholesterol content, may have physiologic consequences and may be useful in detecting and quantitating early atherosclerosis.

Tensile property of atheromatous plaque and an analysis of stress in atherosclerotic wall

To know the contributions of blood pressure induced stress to the progression of atherosclerosis and the rupture of atheroma, cross-sectional distributions of stresses in atherosclerotic walls were calculated by finite element method, using stress-strain data of aortic walls and atherosclerotic plaques obtained from hypercholesterolemic rabbits. In the aortic wall attached by a local plaque, circumferential tensile stress was maximal in the wall near plaque edge, and the stress value was very high compared to the tensile strength of wall media. This local stress concentration may be a contributing factor to the progression of atherosclerotic disease. In the aortic wall uniformly lined with atheroma, circumferential stress was high in the innermost layer of atheroma, and its magnitude was comparable to the tensile strength of plaque. Stress gradient at the interface between atheroma and wall was also high. These results indicate a possibility of rupture of atheroma near the intraluminal surface of atheroma or at the junction of atheroma with wall intima.