Increase in Circumferential Stress in Arterial Wall Due to Loss of Function of Elastin

Introduction Increase of mechanical stress in the arterial wall leads to an altered arterial function and structural degeneration. The study of circumferential stress profile across the arterial wall, and change of this profile by functional loss of components can enhance understanding of pathological events in arterial degeneration and disease [Avolio et al. 1998, Loree et al . 1992, Spina et al. 1976, Thubrikar . et al. 1988]. This investigation aimed to determine changing arterial wall circumferential stress due to altered elastic properties e.g. by aging. One major change in the arterial wall component is loss of function of elastin with age or diseases such as atherosclerosis. This may be due to physical removal of elastin fibres with age [Patel et al. 1995, Pearce et al. 1993] by elastolytic enzymes [Pearce et al. 1993, Robert et al. 1984], increase of collagen content and resultant decrease in elastin/collagen ratio [Cox 1976, Roach and Burton 1957 . 1990, Thubrikar M. et al. 1988] or calcification [Dobrin 1978, Yu and Blumenthal 1963]. Functional changes in the arterial media affects mechanical parameters of the arterial wall including elastic moduli [Cox 1980] which may result in change of stress levels. A possibility of increase in maximum circumferential stress may result in degeneration, rupture or ulceration of the tissue. Considering a non-linear stress-strain relationship for the arterial wall, the circumferential stress distribution across the wall is highly dependent on the elastic moduli [Fung 1997, Vaishnav and vossoughi 1984] with a highly non-linear profile along the media thickness, with a maximum value at intima and a minimum value at adventitia. There have been attempts to estimate arterial wall stress based on non-linear mechanics [Vaishnav and vossoughi 1984, Chuong and Fung 1983] but those studies did not include the lamellar structure of the media. The lamellar structure of the media is highly affected by age and arterial diseases and so needs to be considered in effects of circumferential stress changes by aging or arterial disease. Another effect of functional changes in the media may be on circumferential residual stress. The profile of circumferential residual stress (the stress in the artery when is free of external load) shows a compressive component on the inner wall and a tensile component on the outer side to satisfy balance of forces in the absence of external loads. Considering the high non-linear circumferential stress and the fact that the maximum tensile stress due to the luminal pressure occurs on the luminal edge, the compressive part reduces the maximum tensile stress on the luminal side. Therefore an implication of the residual stress in arteries is to make the stress distribution more uniform in the vessel wall under normal conditions [Liu and Fung 1989]. Change in the function of elastin may alter residual stress values and profile along the arterial thickness affecting the total circumferential stress profile. This study indicates that functional loss of elastin increases the maximum circumferential stress at lumen in two ways. Firstly by increase in the ratio of interlamellar/lamellar stiffness values and ultimately increase in maximum tensile stress caused by external force (blood pressure), and secondly by decrease in the compressive circumferential residual stress at lumen which compensate part of high tensile stress in that region.