An atherosclerotic plaque is an intimal lesion consisting of various degrees of fibrous tissue, fat deposition, and calcification. Plaque formation causes partial or total occlusion of the arterial lumen leading to hemorrhage, stroke, myocardial infarction, gangrene in the lower limbs, or death. Balloon angioplasty is a commonly used mechanical method for the revascularization of the occluded arteries, however, recoil occurs in 3-5% of the patients and restenosis occurs in 25- % of all the cases within six months. Angioplasty can be interpreted as a complex contact boundary value problem, and the solution depends on boundary conditions, vessel geometry, and the material properties. Knowledge of the material properties of atherosclerotic plaque is scant and most of the available information is on plaque caps. Hence, the general objective of this study was to investigate the biomechanical behavior of the atherosclerotic plaque.

We developed a computer-controlled Biological Materials Testing System specifically to test soft biological tissue specimens. The salient features of the experimental system were on-line data acquisition and display, high resolution load and displacement measurements, a temperature controlled physiological solution bath, and software controlled experimental protocols. Whole human atherosclerotic plaque specimens were harvested from the aorto-iliac region. Two finalized test protocols were executed - (i) a multiple cyclic compressive loading protocol, executed in two phases separated by a non-loaded rest period, and (ii) a stress relaxation protocol, executed in three phases with a non-loaded rest period between consecutive loading phases. Histological analysis using specialized stains was performed on the mechanically tested specimens to identify plaque structural composition.

The multiple cyclic compression data showed that plaques exhibit composition dependent non-linear and inelastic responses under finite deformations. The multiple cyclic protocol distinguished three types of mechanical responses of different plaques based on qualitative and quantitative characteristics of the response. Similarly, the stress relaxation data also revealed the composition dependent viscoelastic responses in terms of the degree of relaxation and the saturation (asymptotic) response. The composition dependent nature of mechanical behavior of atherosclerotic plaque indicates that the outcome of angioplasty is likely lesion dependent.