Finite Element Analysis Investigate Pulmonary Autograft Root and Leaflet Stresses to Understand Late Durability of Ross Operation

biomimetics Article Finite Element Analysis Investigate Pulmonary Autograft Root and Leaflet Stresses to Understand Late Durability of Ross Operation Francesco Nappi 1, *,† , Antonio Nenna 2,† , Francesca Lemmo 3 , Massimo Chello 2 , Juan Carlos Chachques 4 , Christophe Acar 5 and Domenico Larobina 6 1 2 3 4 5 6 * † Department of Cardiac Surgery, Centre Cardiologique du Nord de Saint-Denis, 93200 Paris, France Department of Cardiovascular Surgery, University Campus Bio-Medico of Rome, 00128 Rome, Italy; (A.N.); (M.C.) Faculty of Engineering, University of Turin, 10124 Turin, Italy; Department of Cardiovascular Surgery Carpentier Foundation, Pompidou Hospital, University Paris Descartes, 75015 Paris, France; Department of Cardiovascular Surgery, Hopital de la Salpetriere, 75013 Paris, France; Institute for Polymers, Composites, and Biomaterials, National Research Council of Italy, 00185 Rome, Italy; Correspondence: Francesco Nappi and Antonio Nenna contributed equally to the manuscript. Received: 21 June 2020; Accepted: 1 August 2020; Published: 3 August 2020



 Abstract: Ross operation might be a valid option for congenital and acquired left ventricular outflow tract disease in selected cases. As the pulmonary autograft is a living substitute for the aortic root that bioinspired the Ross operation, we have created an experimental animal model in which the vital capacity of the pulmonary autograft (PA) has been studied during physiological growth. The present study aims to determine any increased stresses in PA root and leaflet compared to the similar components of the native aorta. An animal model and a mathematical analysis using finite element analysis have been used for the purpose of this manuscript. The results of this study advance our understanding of the relative benefits of pulmonary autograft for the management of severe aortic valve disease. However, it launches a warning about the importance of the choice of the length of the conduits as mechanical deformation, and, therefore, potential failure, increases with the length of the segment subjected to stress. Understanding PA root and leaflet stresses is the first step toward understanding PA durability and the regions prone to dilatation, ultimately to refine the best implant technique. Keywords: pulmonary autograft; bioinspired Ross operation; pulmonary autograft expansion; pulmonary autograft biomechanical 1. Introduction International guidelines and position papers from professional societies recommend Ross operation as a valid option for congenital and acquired left ventricular outflow tract disease in selected cases [1–8]. Patients who benefit most from this procedure are children and young adults, women of childbearing age, and patients with contraindications to oral anticoagulants [9–13]. The advantages are related to the somatic growth of the cardiovascular structures and with the avoidance of anticoagulants that would be required lifelong in the case of conventional mechanical prostheses [14–18]. However, the incidence of pulmonary autograft (PA) expansion reported after Ross operation, without loss of integrity of the valve leaflets, varies from 20% to 40%, and reoperation is not uncommon [19–30]. Biomimetics 2020, 5, 37; doi:10.3390/biomimetics5030037 www.mdpi.com/journal/biomimetics Biomimetics 2020, 5, 37 2 of 17 As the pulmonary autograft is a living substitute for the aortic root that bioinspired the Ross operation, we have created an experimental animal model in which the vital capacity of the PA has been studied during physiological growth. Therefore, we have reinforced the PA with resorbable scaffolds or semi-absorbable composite prostheses capable of mediating a biomechanical effect and counteracting the abnormal process of the extracellular matrix disruption leading to PA dilatation when the conduit is subjected to systemic pressure. We further revealed the mechanisms of growth, remodeling, and stress shielding of the reinforced PA by means of an experimental large animal model supported by an ex vivo mathematical and physical model [31–38]. This study aims to integrate the animal pattern with mathematical models from biomechanics. In detail, the present study aims to determine any increased stresses in PA root and leaflet compared to the similar components of the native aorta. Understanding PA root and leaflet stresses is the first step toward understanding PA durability and the regions prone to dilatation, ultimately to refine the best implant technique. First, we assumed the nonlinear constitutive stress–strain relationship, as evidenced by the mechanical tests, to examine the mechanical differences between the two vessels along the circumferential and the longitudinal directions [35,39]. Second, a hexahedral regular mesh was generated, each finite element being associated with eight nodes with three translational degrees of freedom, to measure the expansion of PA [38]. Third, 3D ideal reinforced pulmonary autograft with a composite semi-resorbable device was designed to prevent degeneration and failure of PA [32–34,37]. Finally, above all, we tried to explain the relationship between the pathological process that occurs in the PA wall and the stress levels to which the pulmonary autograft is exposed. We thus explained the mechanisms underlying the structural integrity and flexibility of the PA, with particular regard to the balance between apoptosis and cell proliferation of vascular smooth muscle cells in conditions of high stress levels [36]. The final results of the regulatory remodeling pathways of the extracellular matrix within the PA reinforced with a semi-absorbable scaffold are described in the presence of high stress–strain condition both in valve leaflet and root [36]. The “Ross Experimental Project” We developed a “Ross experimental project” that is a European partnership of investigators who aim to provide the basis for studying how to prevent the expansion of pulmonary autograft used in aortic valve surgery. The project was initiated in January 2011 and required the collaboration of the Department of Cardiac Surgery of Centre Cardiologique du Nord, la Pitie Salpetriere Hospital and the Institute of Cardiovascular and Medical Sciences, University of Glasgow. The primary objective of the Ross experimental project was to combine the individual data of the experimental animal model by comparing nonreinforced and reinforced pulmonary autograft to provide an ideal substitute for aortic valve surgery. Using an experimental model of growing sheep based on the simulation of the Ross operation, the experimental project Ross estimated that the analysis of the results would have detected significant differences in the pulmonary autograft morpho-structure at the 6-month follow-up. The pulmonary autograft was inserted in the descending aorta, while the right ventricle outflow tract was reconstructed with a fresh homograft from another lamb of the sa (...truncated)