A histology-informed numerical structural and fluid mechanical investigation of pericardial patches for treatment of pulmonary artery stenosis

Pulmonary artery stenosis (PAS) is a narrowing of the pulmonary artery reducing blood flow from the right ventricle to the lungs. Severe cases, requiring intervention, are diagnosed shortly after birth and often occur in association with congenital heart disease. Treatment is done by balloon angioplasty, stenting or surgical patch augmentation. For the latter, autologous, allogenic, bovine pericardium or synthetic materials are used. Collapses of patched pulmonary arteries are reported causing restenosis and the need for reoperation, while the pathomechanisms remain unclear. We hypothesize, that the root cause for this complication is of combined structural and fluid mechanical nature: a mismatch of mechanical properties between native artery tissue and the patch material might lead to unfavorable stress and strain distributions and unphysiological changes in artery shape. Changes in lumen shape and blood vessel structural stiffness affect the flow profile of the blood, interacting again with the structural behavior of the vessel. This might lead to instabilities, i.e. collapse of the artery. Although the prevalence is low, accounting for 2-3% of all congenital heart anomalies, severe PAS is associated with severe suffering and risk for the newborn, and an enormous burden for the affected families. Thus, there is a strong motivation to better understand and resolve the problem of pulmonary artery collapse.

Preliminary mechanical tests on human pericardium tissue showed local strain inhomogeneities, unexpected shear strains, a large variability of the macroscopic biaxial stiffnesses, and biaxial stiffness ratios between the two loading directions lacking explainability (Figure). These observations reveal the role of the yet unknown pericardial microstructure, i.e. the orientation distribution of collagen and elastin networks and the lack of knowledge of the principal directions of strain. While there is a lot of research on the microstructure of arteries, its implications on the mechanical behavior and physiological function, little is known about the pericardium – which is used to augment arterial tissue.

The longterm goal of the collaborative project between BUTE and ZHAW is to improve the current state of the art for treatment of severe PAS by a micromechanics-informed selection of a patch for artery augmentation. This will be achieved by a detailed study of the histology of pericardium patches directly correlated to their mechanical behavior in uniaxial and biaxial loading conditions. Numerical simulations of both the structural mechanical behavior of the augmented artery and fluid-structure interactions between artery and blood flow shall explain the sensitivity of the system behavior to microstructural changes of the patch. They shall confirm our hypothesis of a structural and fluid mechanics reasoning explaining the occurrence of post-patching pulmonary artery collapse.