Ambroise Duprey

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.

In vitro analysis of localized aneurysm rupture

In this study, bulge inflation tests were used to characterize the failure response of 15 layers of human ascending thoracic aortic aneurysms (ATAA). Full field displacement data were collected during each of the mechanical tests using a digital image stereo-correlation (DIS-C) system. Using the collected displacement data, the local stress fields at burst were derived and the thickness evolution was estimated during the inflation tests. It was shown that rupture of the ATAA does not systematically occur at the location of maximum stress, but in a weakened zone of the tissue where the measured fields show strain localization and localized thinning of the wall. Our results are the first to show the existence of weakened zones in the aneurysmal tissue when rupture is imminent. An understanding these local rupture mechanics is necessary to improve clinical assessments of aneurysm rupture risk. Further studies must be performed to determine if these weakened zones can be detected in vivo using non-invasive techniques.

Endovascular treatment for carotid artery stenosis after neck irradiation

BACKGROUND To lower the risk of complications, carotid angioplasty and stenting (CAS) has been proposed as an alternative to open surgery for carotid artery stenosis after neck irradiation. However, there are little postoperative data to support the benefits of this strategy. This study evaluated the outcome of CAS in patients who had undergone neck irradiation. METHODS This retrospective study was conducted at 15 vascular surgery or interventional radiology centers in France between January 1998 and July 2006. A total of 135 patients (115 men) with a mean age of 67 +/- 8 years (range, 43-88) underwent CAS for 149 irradiation-induced lesions. The interval between irradiation and discovery of the lesions was 12 +/- 8 years. Mean diameter reduction was 81% (range, 50%-95%), and stenosis was symptomatic in 34%. Contralateral carotid lesions were observed in 48% of patients, including thrombosis in 18 and stenosis >50% in 53. RESULTS Technical failure occurred during CAS in three cases. The overall technical success rate was 98%. A cerebral protection device was used in 59%. No death, one transient ischemic attack, and two strokes occurred during the first postoperative month. Mean follow-up was 30 months. Six patients were lost to follow-up. Survival rates were 93.9% at 1 year and 75.3% at 3 years. Complications after the first postoperative month included neurologic events in six, carotid thrombosis in nine, and restenosis in 18. The rates of freedom from neurologic and anatomic events were, respectively, 96.2% and 93.2% at 1 year and 93.1% and 85.9% at 3 years. CONCLUSION The immediate outcome of CAS for irradiation-induced carotid artery stenosis was satisfactory. Medium-term neurologic outcome was acceptable, but the incidence of anatomic events such as thrombosis and restenosis was high. A randomized study is needed to confirm that the outcome of the endovascular and surgical therapy is comparable in this indication.

Experimental characterization of rupture in human aortic aneurysms using a full-field measurement technique

The present study aims at investigating biomechanical failure behaviour of human aneurismal aortic tissues so as to diagnose the rupture risk of aneurysms more accurately. An inflation test is performed on aneurismal aortic tissues up to failure and full-field measurements are achieved using stereo digital image correlation. Then, an appropriate constitutive model derived from histological structure of arteries is adopted to retrieve the Cauchy stress. The virtual fields method is used as an inverse procedure to identify material parameters. Next, the Cauchy stress components are calculated from the identified parameters and the measured Lagrange strain fields. Finally, an important stress parameter which can quantify the strength of aneurismal tissues is derived from the failure stress of aneurismal tissues.

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

Purpose 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 stent-grafts. Spiral and circular stents provide greater flexibility, as well as lower stress values than Z-stents, and thus better durability.

Patient specific stress and rupture analysis of ascending thoracic aneurysms

An ascending thoracic aortic aneurysm (ATAA) is a serious medical condition which, more often than not, requires surgery. Aneurysm diameter is the primary clinical criterion for determining when surgical intervention is necessary but, biomechanical studies have suggested that the diameter criterion is insufficient. This manuscript presents a method for obtaining the patient specific wall stress distribution of the ATAA and the retrospective rupture risk for each patient. Five human ATAAs and the preoperative dynamic CT scans were obtained during elective surgeries to replace each patients aneurysm with a synthetic graft. The material properties and rupture stress for each tissue sample were identified using bulge inflation tests. The dynamic CT scans were used to generate patient specific geometries for a finite element (FE) model of each patients aneurysm. The material properties from the bulge inflation tests were implemented in the FE model and the wall stress distribution at four different pressures was estimated. Three different rupture risk assessments were compared: the maximum diameter, the rupture risk index, and the overpressure index. The peak wall stress values for the patients ranged from 28% to 94% of the ATAAs failure stress. The rupture risk and overpressure indices were both only weakly correlated with diameter (ρ=-0.29, both cases). In the future, we plan to conduct a large experimental and computational study that includes asymptomatic patients under surveillance, patients undergoing elective surgery, and patients who have experienced rupture or dissection to determine if the rupture risk index or maximum diameter can meaningfully differentiate between the groups.

Biaxial rupture properties of ascending thoracic aortic aneurysms.

UNLABELLED Although 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 55years 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. STATEMENT OF SIGNIFICANCE Despite their medical importance, rupture properties of ascending thoracic aortic aneurysms (ATAA) subjected to biaxial tension were inexistent in the literature. In order to address this lack, our group developed a novel methodology based on bulge inflation and full-field optical measurements. Here we report rupture properties obtained with this methodology on 31 patients. It is shown for the first time that rupture occurs when the stretch applied to ATAAs reaches the maximum extensibility of the tissue and that this maximum extensibility correlates strongly with the elastic properties. The outcome is a better detection of at-risk individuals for elective surgical repair.

First human totally endoscopic aortic valve replacement: An early report

FIGURE 1. A, Preoperative computed tomographic images of patient 2 demonst patients with insufficient periaortic working space (<2 cm between the inferior mar distance between the aortic clamp, the cardioplegia needle, and the aortotomy, an should also be excluded (the central line of the ascending aorta should be longer t avoided; the central axis of the proximal aortic root (bottom left, red line), togethe should create anangle not exceeding45 , to avoidany tractiononto themargins of th The last panel (bottom right) shows a 3-dimensional reconstruction of the computed operative photograph of patient 2 shows operating field and trocar positioning. The ative trocar (15mm) is in the third, a percutaneous transthoracic aortic clamp is in th vein venting line and purse-string with carbon dioxide insufflation line are in the fi From the Department of Cardiovascular Surgery, Centre Hospitalier Universitaire de SaintEtienne, Saint-Etienne, France. Disclosures: Marco Vola reports consulting and lecture fees from Medtronic. JeanFrancois Fuzellier reports lecture fees from Medtronic. All other authors have nothing to disclose with regard to commercial support. Received for publication Aug 12, 2013; revisions received Sept 29, 2013; accepted for publication Oct 6, 2013; available ahead of print Dec 2, 2013. Address for reprints: Marco Vola, MD, PhD, Cardiovascular Surgery Unit, Centre Hospitalier Universitaire de SaintEtienne Nord, 42055 Saint-Etienne cedex, France (E-mail: marco.vola@univ-st-etienne.fr). J Thorac Cardiovasc Surg 2014;147:1091-3 0022-5223/

Local mechanical properties of human ascending thoracic aneurysms

36.00 Copyright 2014 by The American Association for Thoracic Surgery http://dx.doi.org/10.1016/j.jtcvs.2013.10.010

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

Ascending thoracic aortic aneurysms (ATAAs) are focal, asymmetric dilatations of the aortic wall which are prone to rupture. To identify potential rupture locations in advance, it is necessary to consider the inhomogeneity of the ATAA at the millimeter scale. Towards this end, we have developed a combined experimental and computational approach using bulge inflation tests, digital image correlation (DIC), and an inverse membrane approach to characterize the pointwise stress, strain, and hyperelastic properties of the ATAA. Using this approach, the pointwise hyperelastic material properties were identified on 10 human ATAA samples collected from patients undergoing elective surgery to replace their ATAAs with a graft. Our method was able to capture the varying levels of heterogeneity in the ATAA from regional to local. It was shown for the first time that the material properties in the ATAA are unmistakably heterogeneous at length scales between 1mm and 1cm, which are length scales where vascular tissue is typically treated as homogeneous. The distributions of the material properties for each patient were also examined to study the inter- and intra-patient variability. Large inter-subject variability was observed in the elastic properties.