Mechanical behavior of the abdominal aortic aneurysm assessed by biaxial tests in the rat xenograft model

Mechanical and histological characteristics of aortic dissection tissues

AIMS

This study investigated the association between the macroscopic mechanical response of aortic dissection (AoD) flap, its fibre features, and patient physiological features and clinical presentations.

METHODS

Uniaxial test was performed with tissue strips in both circumferential and longitudinal directions from 35 patients with (AoD:CC) and without (AoD:w/oCC) cerebral/coronary complications, and 19 patients with rheumatic or valve-related heart diseases (RH). A Bayesian inference framework was used to estimate the expectation of material constants (C₁, D₁, and D₂) of the modified Mooney-Rivlin strain energy density function. Histological examination was used to visualise the elastin and collagen in the tissue strips and image processing was performed to quantify their area percentages, fibre misalignment and waviness.

RESULTS

The elastin area percentage was negatively associated with age (p = 0.008), while collagen increased about 6% from age 40 to 70 (p = 0.03). Elastin fibre was less dispersed and wavier in old patients and no significant association was found between patient age and collagen fibre dispersion or waviness. Features of fibrous microstructures, either elastin or collagen, were comparable between AoD:CC and AoD:w/oCC group. Elastin and collagen area percentages were positively correlated with C₁ and D₂, respectively, while the elastin and collagen waviness were negatively correlated with C₁ and D₂, respectively. Elastin dispersion was negatively correlated to D₂. Multivariate analysis showed that D₂ was an effective parameter which could differentiate patient groups with different age and clinical presentations, as well as the direction of tissue strip.

CONCLUSION

Fibre dispersion and waviness in the aortic dissection flap changed with patient age and clinical presentations, and these can be captured by the material constants in the strain energy density function.

STATEMENT OF SIGNIFICANCE

Aortic dissection (AoD) is a severe cardiovascular disease. Understanding the mechanical property of intimal flap is essential for its risk evaluation. In this study, mechanical testing and histology examination were combined to quantify the relationship between mechanical presentations and microstructure features. A Bayesian inference framework was employed to estimate the expectation of the material constants in the modified Mooney-Rivlin constitutive equation. It was found that fibre dispersion and waviness in the AoD flap changed with patient age and clinical presentations, and these could be captured by the material constants. This study firstly demonstrated that the expectation of material constants can be used to characterise tissue microstructures and differentiate patients with different clinical presentations.

Loss of anisotropic properties in abdominal aorta aneurysm obtained from the xenograft rat model

BACKGROUND

Cellular treatments using mesenchymal stem cells (MSCs) cultured in 3D conditions constitute a solution to the classical surgery in treating abdominal aortic aneurysm (AAA). The recurrent question is: how this type of biotherapy changes the mechanical behavior of artery?

METHODS

Experiments measurements based on xenograft rat model showed that the proposed cellular treatment leads to a decreasing radius and length of the AAA during its growth. An inverse finite element method was used to investigate the mechanical hyperelastic behavior of the AAA in the untreated case compared to the treated one.

RESULTS

Although AAA leads a loss anisotropy while the cellular treatment does not restore it, it was shown that the stiffness of the arterial wall was improved. The numerical analysis of the stress distributions permitted to localize the stress concentration through the arterial wall and the probable zone of the rupture of the aneurysm developed from the xenograft rat model.

CONCLUSIONS

The treatment of AAA with MSCs cultured in a 3D conditions constitutes a new challenge. Based on xenograft rat model, this study shows the potential of this cellular treatment to reduce the variation of the growth, the stiffness and the stress distributions.