Anisotropic and hyperelastic identification of in vitro human arteries from full-field optical measurements

In this paper, we present a new approach for the bi-axial characterization of in vitro human arteries and we prove its feasibility on an example. The specificity of the approach is that it can handle heterogeneous strain and stress distributions in arterial segments. From the full-field experimental data obtained in inflation/extension tests, an inverse approach, called the virtual fields method (VFM), is used for deriving the material parameters of the tested arterial segment. The obtained results are promising and the approach can effectively provide relevant values for the anisotropic hyperelastic properties of the tested sample

Characteristics of thoracic aortic aneurysm rupture in vitro

International audienceAscending thoracic aortic aneurysms (ATAAs) are focal dilatations in the aorta that are prone to rupture or dissection. To accurately evaluate the rupture risk, one must know the local mechanical conditions at the rupture site and understand how rupture is triggered in a layered fibrous media. A challenge facing experimental studies of ATAA rupture is that the ATAA tissue is highly heterogeneous; experimental protocols that operate under the premise of tissue homogeneity will have difficulty delineating the location conditions. In this work, we employed a previously established pointwise identification method to characterize wall stress, strain, and property distributions to a sub-millimeter resolution. Based on the acquired field data, we obtained the local mechanical properties at the rupture site in nine ATAA tissue samples. The rupture stress, ultimate strain, energy density, and the toughness of the tested samples were also reported. Our results show that the direction of the rupture is aligned with the direction of maximum stiffness, indicating that higher stiffness is not always related to higher strength. It was also found that the rupture generally occurs at a location of highest stored energy. As a higher stiffness and higher strain energy indicate a larger recruitment of collagen fibers in the tissue at the location and along the direction of rupture, the recruitment of collagen fibers in the deformation of the tissue is probably essential in ATAA rupture

Predictive Models with Patient Specific Material Properties for the Biomechanical Behavior of Ascending Thoracic Aneurysms

International audienceThe aim of this study is to identify the patient-specific material properties of ascending thoracic aortic aneurysms (ATAA) using preoperative dynamic gated Computed Tomography (CT) scans. The identification is based on the simultaneous minimization of two cost functions, which define the difference between model predictions and gated CT measurements of the aneurysm volume at respectively systole and cardiac mid-cycle. The method is applied on 5 patients who underwent surgical repair of their ATAA at the University Hospital Center of St. Etienne. For these patients, the aneurysms were collected and tested mechanically using an in vitro bench. For the sake of validation, the mechanical properties found using the in vivo approach and the in vitro bench were compared. We eventually performed finite-element stress analyses based on each set of material properties. Rupture risk indexes were estimated and compared, 2 showing promising results of the patient-specific identification method based on gated CT

Characterisation of failure in human aortic tissue using digital image correlation

International audienceAn aortic aneurism is a localized dilation of the aorta in a weakened area [1, 2]. The increase of aneurism size may result in rupture, which will be a life threatening emergency. The mechanism of failure in aneurysms is now relatively well understood. However, only limited research has provided quantitative values for the stresses that cause the failure of pathologic arterial tissue. The evaluation of the local failure stress remains an open problem. In this study we apply digital image correlation (DIC) to excised pieces of tissue that we test in a bulge inflation test. The tissue is taken from the ascending aorta in diseased patients requiring an excision for removing an aneurysm. All procedures are carried out in accordance with the guidelines of the Institutional Review Board of the University Hospital of Saint-Etienne, France

Biaxial rupture properties of ascending thoracic aortic aneurysms

International audienceAlthough hundreds of samples obtained from ascending thoracic aortic aneurysms (ATAA) of patients undergoing elective surgical repair have already been characterized biomechanically, their rupture properties were always derived from uniaxial tensile tests. Due to their bulge shape, ATAAs are stretched biaxially in vivo. In order to understand the biaxial rupture of ATAAs, our group developed a novel methodology based on bulge inflation and full-field optical measurements. The objective of the current paper is threefold. Firstly, we will review the failure properties (maximum stress, maximum stretch) obtained by bulge inflation testing on a cohort of 31 patients and compare them with failure properties obtained by uniaxial tension in a previously published study. Secondly, we will investigate the relationship between the failure properties and the age of patients, showing that patients below 55 years of age display significantly higher strength. Thirdly, we will define a rupture risk based on the extensibility of the tissue and we will show that this rupture risk is strongly correlated with the physiological elastic modulus of the tissue independently of the age, ATAA diameter or the aortic valve phenotype of the patient

Mechanical characterization of the thoracic ascending aortae

International audienceIn this study the digital image correlation technique is used for characterizing the mechanical and fracture properties of aneurysmal tissues. The tissues which have been taken from the thoracic ascending aorta of diseased patients are tested in a bulge inflation test. The approach is original in the sense that it gives access to the local stress fields in the tissue and to local analysis of fracture. Applications to the dynamic behavior and fracture of vascular tissues are envisaged

Finite Element Analysis of the Mechanical Performances of 8 Marketed Aortic Stent-Grafts

International audiencePurpose: To assess numerically the flexibility and mechanical stresses undergone by stents and fabric of currently manufactured stent-grafts. Methods: Eight marketed stent-graft limbs (Aorfix, Anaconda, Endurant, Excluder, Talent, Zenith Flex, Zenith LP, and Zenith Spiral-Z) were modeled using finite element analysis. A numerical benchmark combining bending up to 180° and pressurization at 150 mmHg of the stent-grafts was performed. Stent-graft flexibility, assessed by the calculation of the luminal reduction rate, maximal stresses in stents, and maximal strains in fabric were assessed. Results: The luminal reduction rate at 90° was ‹<20% except for the Talent stent-graft. The rate at 180° was higher for Z-stented models (Talent, Endurant, Zenith, and Zenith LP; range 39%-78%) than spiral (Aorfix, Excluder, and Zenith Spiral-Z) or circular-stented (Anaconda) devices (range 14%-26%). At 180°, maximal stress was higher for Z-stented stent-grafts (range 370-622 MPa) than spiral or circular-stented endografts (range 177-368 MPa). At 90° and 180°, strains in fabric were low and did not differ significantly among the polyester stent-grafts (range 0.5%-7%), while the expanded polytetrafluoroethylene fabric of the Excluder stent-graft underwent higher strains (range 11%-18%). Conclusion: Stent design strongly influences mechanical performances of aortic stentgrafts. Spiral and circular stents provide greater flexibility, as well as lower stress values than Z-stents, and thus better durability

Biomechanics of ascending aortic aneurysms : towards a patient-specific prediction of rupture risk

Les anévrysmes de l'aorte ascendante sont un problème de santé publique qui peut menacer le pronostic vital par le risque de rupture ou de dissection. Actuellement, le seul critère permettant de décider d'une intervention est le diamètre maximal de l'anévrysme mesuré par le scanner. La recherche biomécanique a pour objectif de caractériser les propriétés élastiques de l'aorte et de proposer une approche patient-spécifique de l'évaluation du risque de complication. Le but de cette thèse était double. Dans une première partie, il était de déterminer les propriétés à rupture d'anévrysmes de l'aorte ascendante fraîchement excisés à partir d'un banc d'essai de tests en gonflement mis au point dans notre laboratoire. Les résultats ont permis de déterminer un index de risque de rupture basé sur l'extensibilité de l'aorte. Dans une deuxième partie, nous avons identifié les propriétés mécaniques des anévrysmes aortiques à l'aide d'un scanner dynamique préopératoire. Cette identification reposait sur la minimisation simultanée de deux fonctions coût, qui définissaient la différence entre les prédictions d"un modèle numérique et les mesures par le scanner du volume de l'anévrysme au milieu du cycle cardiaque et à la systole. Les résultats étaient corrélés à ceux des tests mécaniques en gonflement, montrant une application prometteuse du scanner dynamique pour l'identification du patient-scpécifique des propriétés mécaniques de l'aorte. Ce travail ouvre un peu plus la voie vers une évaluation patient-spécifique du risque de complication d'un anévrysme de l'aorte ascendante et vers une sélection plus affinée des patients pour la chirurgie.Aneurysms of the ascending aorta are a life-threatening desease by the risk of rupture or dissection. Currently, the only criterion for deciding an intervention is the maximum diameter of the aneurysm mesured from CT-scan. Biomechanical research aims to characterize the elastic properties of the aorta and to provide a patient-specific approach too assessing the risk of complications. The aim of this thesiswas twofold. In the first part, it was tocharacterize the rupture properties of freshly excised acsending aortic tissue from an inflation bench-test developed in our laboratory. The results were derived to determine a rupture risk index based on the extensibility of the aorta. In the second part, we identified the mechanical properties of aortic aneurysms using preoperative dynamic CT-scan. This identification was based on the simultaneous minimization of two cost functions, which defined the difference between the predictionsof a numerical model and the volumes of the aneurysms measured from the CT-scan in the middle of the cardiac cycle and the sytole. The results were correlated with those of the inflation tests, showing promising application of the dynamic scan for the patient-specific identification of the mechanical properties of the aorta. This work opens a little more the way for the patient-specific assessment of the risk of complications of an aneurysm of the ascending aorta and for a more refined selection of patients for surgery

Biomécanique des anévrysmes de l'aorte thoracique ascendante : vers une prédiction personnalisée du risque de rupture

Aneurysms of the ascending aorta are a life-threatening desease by the risk of rupture or dissection. Currently, the only criterion for deciding an intervention is the maximum diameter of the aneurysm mesured from CT-scan. Biomechanical research aims to characterize the elastic properties of the aorta and to provide a patient-specific approach too assessing the risk of complications. The aim of this thesiswas twofold. In the first part, it was tocharacterize the rupture properties of freshly excised acsending aortic tissue from an inflation bench-test developed in our laboratory. The results were derived to determine a rupture risk index based on the extensibility of the aorta. In the second part, we identified the mechanical properties of aortic aneurysms using preoperative dynamic CT-scan. This identification was based on the simultaneous minimization of two cost functions, which defined the difference between the predictionsof a numerical model and the volumes of the aneurysms measured from the CT-scan in the middle of the cardiac cycle and the sytole. The results were correlated with those of the inflation tests, showing promising application of the dynamic scan for the patient-specific identification of the mechanical properties of the aorta. This work opens a little more the way for the patient-specific assessment of the risk of complications of an aneurysm of the ascending aorta and for a more refined selection of patients for surgery.Les anévrysmes de l'aorte ascendante sont un problème de santé publique qui peut menacer le pronostic vital par le risque de rupture ou de dissection. Actuellement, le seul critère permettant de décider d'une intervention est le diamètre maximal de l'anévrysme mesuré par le scanner. La recherche biomécanique a pour objectif de caractériser les propriétés élastiques de l'aorte et de proposer une approche patient-spécifique de l'évaluation du risque de complication. Le but de cette thèse était double. Dans une première partie, il était de déterminer les propriétés à rupture d'anévrysmes de l'aorte ascendante fraîchement excisés à partir d'un banc d'essai de tests en gonflement mis au point dans notre laboratoire. Les résultats ont permis de déterminer un index de risque de rupture basé sur l'extensibilité de l'aorte. Dans une deuxième partie, nous avons identifié les propriétés mécaniques des anévrysmes aortiques à l'aide d'un scanner dynamique préopératoire. Cette identification reposait sur la minimisation simultanée de deux fonctions coût, qui définissaient la différence entre les prédictions d"un modèle numérique et les mesures par le scanner du volume de l'anévrysme au milieu du cycle cardiaque et à la systole. Les résultats étaient corrélés à ceux des tests mécaniques en gonflement, montrant une application prometteuse du scanner dynamique pour l'identification du patient-scpécifique des propriétés mécaniques de l'aorte. Ce travail ouvre un peu plus la voie vers une évaluation patient-spécifique du risque de complication d'un anévrysme de l'aorte ascendante et vers une sélection plus affinée des patients pour la chirurgie

Pseudoaneurysms postangioplasty of the renal artery: case reports and review of the literature.

International audiencePseudoaneurysms postangioplasty of the renal artery are a life-threatening complication but are poorly known due to their rarity. The aim of this report is to study this complication through three personal case reports and 10 cases found in a literature review. The data analysis led us to highlight important aspects about four main elements: initial procedure events, time to presentation, symptoms leading to diagnosis, and treatment