On the mechanical behaviour of carotid artery plaques: The influence of curve-fitting experimental data on numerical model results

Computational models of diseased arteries are advancing rapidly, and a need exists to develop more accurate material models of human atherosclerotic plaques. However, intact samples for in vitro mechanical testing are not readily available. Most plaque samples are harvested from carotid endarterectomies where the geometries are not suitable for the boundary parameters necessary for classical uniaxial tensile testing. Experimental studies of biological tissue, particularly human plaque tissue, have not specified the minimum width-to-length (WL) ratio necessary for appropriate tensile testing. This study proposes either tensile or planar shear testing on whole specimen samples depending on the WL ratio. However, a "grey-area" of WL ratios exists which are unsuitable for either test, between 0.5:1 and 4:1 WL ratio. Eighteen plaque samples are investigated in this study, and according to classical approaches, two of the plaque samples have WL ratios suitable for tensile testing and four are suitable for planar shear testing. The remaining twelve samples fall in the grey-area of WL ratio. The study analyses which test method is suitable for the samples in this grey-area and what effect using the incorrect test method has on results from a computational model. The study highlights that tissues above a WL ratio of 2:1 are suitable for planar shear testing, and samples below 1:1 are more suited for tensile testing. Therefore, the "grey-area" can be reduced with certain limitations applied by the minor strain assumption which need to be taken into account during experimental testing. This study also demonstrates the influence of curve-fitting experimental results using tensile- and planar shear-based boundary parameters from eighteen plaque samples.