Pressure-induced mechanical stress in the carotid artery bifurcation: a possible correlation to atherosclerosis.

A possible correlation between regions of high intramural wall stress and the development of atherosclerotic lesions in the carotid artery bifurcation is investigated. The bifurcation geometry is determined through in vivo studies, as well as the analysis of cadaver specimens. Having compiled accurate geometric data, two representative finite element models were created in order to determine the areas of localized stress concentrations that occur in the bifurcation. The artery is assumed isotropic and is mechanically loaded with an incremental pressure of 40 mmHg. A highly localized stress concentration of approximately 9 to 14 times the proximal circumferential wall stress occurs at the point of bifurcation. A lower stress concentration of approximately 3 to 4 times the proximal circumferential stress occurs over a large area of the sinus bulb. Acknowledging that these two regions of the carotid bifurcation are highly susceptible to atherosclerotic lesions, it appears possible that a correlation between wall stress and atherosclerosis may exist.

Study of stress concentration in the walls of the bovine coronary arterial branch.

The intramural stress concentration in the arterial wall is studied at the bovine circumflex coronary arterial branch. The material properties, geometry, and strains in the arterial branch are determined from experiments. The stresses are determined using a finite element analysis. The arterial branch is modeled as two interesecting thin cylindrical shells incorporating local variations in the branch geometry, thickness, and material properties. The artery is considered orthotropic and loaded with an incremental pressure of 40 mmHg. The highest intramural stresses are found to be localized at the proximal and distal regions of the ostium and are not significantly affected by the elastic properties. The stresses are 3 to 4 times greater in the branch at the inner surface than in the straight segment. The strains are twice as large at the branch than in the straight segment. We speculate that this stress concentration could injure the artery and make the branch region susceptible to atherosclerosis.

Stress analysis of porcine bioprosthetic heart valves in vivo.

During the normal functioning of aortic porcine bioprosthetic valves, the leaflets undergo complex configurational changes which can produce stresses large enough to damage the leaflets. Stress analyses of these valves in vivo have not been performed before. We investigated the behavior of aortic bioprostheses in vivo in calves by placing radiopaque markers on the valves and observing them under x-ray. Based upon the behavior of the leaflets, a method of stress analysis is proposed. Membrane stresses were associated with a pressure gradient across the leaflet and bending stresses with a change in the leaflet curvature. Total stresses were obtained by summation of the two stresses. A model of leaflet deformation at its attachment is proposed and the stresses determined. In diastole, the total stresses in the leaflet were tensile. In systole, the total stresses at the leaflet attachment were large and compressive on the aortic surface. Since the leaflet is unable to sustain compressive stresses, it is concluded that large compressive stresses cause structural damage at the leaflet attachment. This may explain the clinical observation that bioprosthetic leaflets detach or calcify in this region.

A method for analysis of bending and shearing deformations in biological tissue.

This paper describes a theoretical and experimental approach to the analysis of the deformations of a thin biological tissue. A biological tissue undergoes complex deformations during which normal and shearing strains are produced. These strains can be very large and yet be within the elastic range of the material. The procedure described is demonstrated for the pericardium used in making bioprosthetic heart valves. It is observed that the pericardium exhibits a directional property in which the shearing deformations occur in one direction but not in the opposite direction. By the application of the proposed method, modes of deformation can be determined and modes of failure predicted.