The paper aims at evaluation of mechanical tests of soft tissues and creation of their representative stress-strain responses and respective constitutive models. Interpretation of sets of experimental results depends highly on the approach to the data analysis. Their common representation through mean and standard deviation may be misleading and give non-realistic results. In the paper, raw data of 7 studies consisting of 11 experimental data sets (concerning carotid wall and atheroma tissues) are re-analysed to show the importance of their rigorous analysis. The sets of individual uniaxial stress-strain curves are evaluated using three different protocols: stress-based, stretch-based and constant-based, and the population-representative response is created by their mean or median values. Except for nearly linear responses, there are substantial differences between the resulting curves, being mostly the highest for constant-based evaluation. But also the stretch-based evaluation may change the character of the response significantly. Finally, medians of the stress-based responses are recommended as the most rigorous approach for arterial and other soft tissues with significant strain stiffening.
Biomechanical computational models used more and more in medical research as well as in clinical applications need mathematical description of soft tissues with their very complex mechanical behaviour. They show large deformations with stress softening during cyclic loading, being moreover time-dependent due to their viscoelasticity, anisotropic due to fibrous structure and dependent on tissue excitation in case of muscular tissues, including smooth muscle cells in arterial walls. They are capable to adapt to the acting load (tissue remodelation) and to self-repair (healing) in case of damage or failure (rupture of fibres, etc.) This paper deals with constitutive description of passive elastic response of soft tissues to the acting load and with the ways, how the anisotropic structure of the tissue can be reflected. For this purpose, the spatial arrangement of fibres in the tissue needs to be detected and mathematically described. Lack of data on fibre arrangement in soft tissues and their interpatient variability is a major limiting factor for anisotropic constitutive description of soft tissues. Some problems related to transformation of results of mechanical tests and histological analyses of arterial tissues into parameters applicable in constitutive models are addressed as well.
The paper deals with the impact of chosen geometric and material factors on maximal stresses in carotid atherosclerotic plaque calculated using patient-specific finite element models. These stresses are believed to be decisive for the plaque vulnerability but all applied models suffer from inaccuracy of input data, especially when obtained in vivo only. One hundred computational models based on ex vivo MRI are used to investigate the impact of wall thickness, MRI slice thickness, lipid core and fibrous tissue stiffness, and media anisotropy on the calculated peak plaque and peak cap stresses. The investigated factors are taken as continuous in the range based on published experimental results, only the impact of anisotropy is evaluated by comparison with a corresponding isotropic model. Design of Experiment concept is applied to assess the statistical significance of these investigated factors representing uncertainties in the input data of the model. The results show that consideration of realistic properties of arterial wall in the model is decisive for the stress evaluation; assignment of properties of fibrous tissue even to media and adventitia layers as done in some studies may induce up to eightfold overestimation of peak stress. The impact of MRI slice thickness may play a key role when local thin fibrous cap is present. Anisotropy of media layer is insignificant, and the stiffness of fibrous tissue and lipid core may become significant in some combinations.
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