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

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

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

Hybrid treatment of an ascending aortic pseudoaneurysm following multiple sternotomies

Ascending aortic pseudoaneurysm following prior cardiac or aortic surgery is a rare entity that requires reoperation. Surgical repair is a complex procedure associated with high operative mortality. We report the case of a 76-year-old male patient with an ascending aortic pseudoaneurysm developing from distal anastomosis of a Dacron aorto-aortic prosthesis. This high-risk patient had previously undergone multiple cardiovascular operations and was treated by performing an extra-anatomic bypass between the descending thoracic aorta and supra-aortic vessels, followed by endovascular stent graft placement, avoiding median re-sternotomy

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

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

Identification of the in vivo elastic properties of common carotid arteries from MRI: a study on subjects with and without atherosclerosis.

International audienceThe stiffness of the arterial wall, which is modified by many cardiovascular diseases such as atherosclerosis, is known to be an indicator of vulnerability. This work focuses on the in vivo quantification of the stiffness of the common carotid artery (CCA) by applying the Magnitude Based Finite Element Model Updating (MB-FEMU) method to 13 healthy and diseased volunteers aged from 24 to 76 years old. The MB-FEMU method is based on the minimisation of the deviation between the image of a deformed artery and a registered image of this artery deformed by means of a finite elements analysis. Cross sections of the neck of each subject at different times of the cardiac cycle are recorded using a Phase Contrast cine-MRI. Applanation tonometry is then performed to obtain the blood pressure variations in the CCA throughout a heart beat. First, a time averaged elastic modulus of each CCA between diastole and systole is identified and a stiffening of the artery with age and disease is observed. Second, four elastic moduli are identified during a single heart beat for each artery, highlighting the nonlinear mechanical behaviour of the artery. A stiffening of the artery is observed and quantified at systole in comparison to diastole

The Leadership Processes of Pacific Public Servants in Aotearoa, New Zealand

This dissertation presents research focused on leadership processes among Pacific public servants at multiple levels in the New Zealand Public Service. The current study was guided by this research question: What are the leadership processes currently employed by Pacific public servants in the New Zealand Public Service? This study also explored participants' views on the effect of Pacific cultural backgrounds and organisational contexts on their current experience of leadership processes. The exploration of the topic was developed within a post-positivist research paradigm, using phenomenological methodology to examine the leadership processes of Pacific public servants. It employs qualitative case studies of two New Zealand Public Service organisations in the Wellington region. I employed two data collection tools in these case studies. The first was the use of in-depth interviews, and the second was an analysis of relevant organisational documents. A total of sixteen Pacific public servants participated in my study, eight from each case organisation. The findings indicated that the Pacific participants understood leadership as a social process of collective influence within a context. Participants perceived participating, networking and relationship building, learning about leadership from cultural contexts, and practising the Pacific value of va as important leadership processes for their performance in the organisations in which they were working. This study also found that the organisations' key roles and leadership values, which are embedded in Pacific cultures, shaped participants' experiences of the leadership processes. The findings also highlight some factors that contribute to and constrain the Pacific public servants' leadership processes. This emphasises the need for diverse policies to encompass leadership development. This study also highlights the need for leadership support for Pacific public servants at all levels in their New Zealand organisations. Practical and future research recommendations gained from the findings are discussed. The study contributes to the field of leadership research on Pacific public servants in New Zealand, and provides a different perspective on leadership processes in general leadership theory

Role of Impurities on CO2 Injection: Experimental and Numerical Simulations of Thermodynamic Properties of Water-salt-gas Mixtures (CO2 + Co-injected Gases) Under Geological Storage Conditions

International audienceRole of impurities on CO 2 injection: experimental and numerical simulations of thermodynamic properties of water-salt-gas mixtures (CO 2 + co-injected gases) under geological storage conditions Abstract Regarding the hydrocarbon source and CO 2 capture processes, fuel gas from boilers may be accompanied by so-called "annex gases" which could be co-injected in a geological storage. These gases, such as SOx, NOx, or oxygen for instance, are likely to interact with reservoir fluids and rocks and well materials (casing and cement) and could potentially affect the safety of the storage. However, there are currently only few data on the behaviour of such gas mixtures, as well as on their chemical reactivity, especially in the presence of water. One reason for this lack comes from the difficulty in handling because of their dangerousness and their chemical reactivity. Therefore, the purpose of the Gaz Annexes was to develop new experimental and analytical protocols in order to acquire new thermodynamic data on these annex gases, in fine for predicting the behaviour of a geological storage of CO 2 + co-injected gases in the short, medium and long terms. This paper presents Gaz Annexes concerning acquisition of PVT experimental and pseudo-experimental data to adjust and validate thermodynamic models for water / gas / salts mixtures as well as the possible influence of SO 2 and NO on the geological storage of CO 2. The Gaz Annexes s new insights for the establishment of recommendations concerning acceptable content of annex gases

Immunosuppressive therapy after solid-organ transplantation: does the INTERMED identify patients at risk of poor adherence?

Lack of adherence to medication is a trigger of graft rejection in solid-organ transplant (SOT) recipients. This exploratory study aimed to assess whether a biopsychosocial evaluation using the INTERMED instrument before transplantation could identify SOT recipients at risk of suboptimal post-transplantation adherence to immunosuppressant drugs. We hypothesized that complex patients (INTERMED&gt;20) might have lower medication adherence than noncomplex patients (INTERMED≤20). Each patient eligible for transplantation at the University Hospital of Lausanne, Switzerland, has to undergo a pre-transplantation psychiatric evaluation. In this context the patient was asked to participate in our study. The INTERMED was completed pre-transplantation, and adherence to immunosuppressive medication was monitored post-transplantation by electronic monitors for 12 months. The main outcome measure was the implementation and persistence to two calcineurin inhibitors, cyclosporine and tacrolimus, according to the dichotomized INTERMED score (&gt;20 or ≤20). Among the 50 SOT recipients who completed the INTERMED, 32 entered the study. The complex (N=11) and noncomplex patients (N=21) were similar in terms of age, sex and transplanted organ. Implementation was 94.2% in noncomplex patients versus 87.8% in complex patients (non-significant p-value). Five patients were lost to follow-up: one was non-persistent, and four refused electronic monitoring. Of the four patients who refused monitoring, two were complex and withdrew early, and two were noncomplex and withdrew later in the study. Patients identified as complex pre-transplant by the INTERMED tended to deviate from their immunosuppressant regimen, but the findings were not statistically significant. Larger studies are needed to evaluate this association further, as well as the appropriateness of using a nonspecific biopsychosocial instrument such as INTERMED in highly morbid patients who have complex social and psychological characteristics