Identification of the material parameters of soft tissues in the compressed leg

International audienceElastic compression (EC) is highly recommended in prophylaxis and treatment of venous disorder of the human leg. However, the exact mechanisms of its action are not enough understood and the response of internal tissues to the external pressure are still partially unknown. A 3D biomechanical FE model for simulating the effect of EC on the human leg has been developed based on the actual geometry of a female leg, obtained from 3D CT-scan images. The model is made up of soft tissues (fat and muscles) and rigid bones. A 2D FE model, reconstructed from MRI, is used to identify the elastic properties of soft tissues by an inverse method. The pressure applied by EC increases linearly from ankle to knee. The results show a non homogeneous pressure field (more than 35 % of discrepancies in a cross section of the leg) bringing evidence that the socks should be adapted to the diseased vein location

Identification des propriétés des tissus mous de la jambe sous compression élastique

National audienceLa compression élastique (CE) est largement utilisée pour le traitement et la prévention des insuffisances veineuses. Pour connaître la réponse des tissus internes à une pression externe, un modèle EF 3D d'une jambe humaine a été développé. Le modèle est constitué des tissus mous et des os dont les géométries sont obtenues à partir d'images scanner 3D. Une méthode inverse basée sur les images de la jambe avec et sans CE est utilisée pour identifier les propriétés des tissus mous. La méthode consiste à minimiser l'écart entre l'image de la jambe déformée par le modèle EF et l'image de la jambe sous CE

Biomechanical response of varicose veins to elastic compression: A numerical study.

International audienceA patient-specific finite-element (FE) model of the human leg is developed to model the stress distribution in and around a vein wall in order to determine the biomechanical response of varicose veins to compression treatment. The aim is to investigate the relationship between the local pressure on the soft tissues induced by wearing the compression garment and the development and evolution of varicose veins and various skin-related diseases such as varicose veins and ulcers. Because experimental data on the mechanical properties of healthy superficial veins and varicose veins are scarce in literature, ultrasound images of in vivo varicose veins are acquired and analysed to extract the material constants using Finite Element Model Updating. The decrease in trans-mural pressure, which conditions the effectiveness of compressive treatments, is computed from the simulation results. This constitutes the original added value of the developed model as decrease in trans-mural pressures cannot be assessed experimentally by any other means. Results show that external compression is effective in decreasing the trans-mural pressure, thereby having a positive effect in the control and treatment of vein-related diseases

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

Mechanical identification of hyperelastic anisotropic properties of mouse carotid arteries

International audienceThe role of mechanics is known to be of primary order in many arterial diseases; however determining mechanical properties of arteries remains a challenge. This paper discusses the identifiability of a Holzapfel-type material model for a mouse carotid artery, using an inverse method based on a finite element model and 3D digital image correlation measurements of the surface strain during an inflation/extension test. Layer-specific mean fiber angles are successfully determined using a five parameter constitutive model, demonstrating good robustness of the identification procedure. Importantly, we show that a model based on a single thick layer is unable to render the biaxial mechanical response of the artery tested here. On the contrary, difficulties related to the identification of a seven parameter constitutive model are evidenced; such a model leads to multiple solutions. Nevertheless, it is shown that an additional mechanical test, different in nature with the previous one, solves this proble

Patient-specific simulation of stent-graft deployment within an abdominal aortic aneurysm

In this study, finite element analysis is used to simulate the surgical deployment procedure of a bifurcated stent-graft on a real patient's arterial geometry. The stent-graft is modeled using realistic constitutive properties for both the stent and most importantly for the graft. The arterial geometry is obtained from pre-operative imaging exam. The obtained results are in good agreement with the post-operative imaging data. As the whole computational time was reduced to less than 2 hours, this study constitutes an essential step towards predictive planning simulations of aneurysmal endovascular surger

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

Mechanical identification of layer-specific properties of mouse carotid arteries using 3D-DIC and a hyperelastic anisotropic constitutive model

The role of mechanics is known to be of primary order in many arterial diseases; however, determining mechanical properties of arteries remains a challenge. This paper discusses the identifiability of the passive mechanical properties of a mouse carotid artery, taking into account the orientation of collagen fibres in the medial and adventitial layers. On the basis of 3D digital image correlation measurements of the surface strain during an inflation/extension test, an inverse identification method is set up. It involves a 3D finite element mechanical model of the mechanical test and an optimisation algorithm. A two-layer constitutive model derived from the Holzapfel model is used, with five and then seven parameters. The five-parameter model is successfully identified providing layer-specific fibre angles. The seven-parameter model is over parameterised, yet it is shown that additional data from a simple tension test make the identification of refined layer-specific data reliable.Comment: PB-CMBBE-15.pd

Biomechanics of porcine renal arteries and role of axial stretch.

International audienceIt is known that arteries experience significant axial stretches in vivo. Several authors have shown that the axial force needed to maintain an artery at its in vivo axial stretch does not change with transient cyclical pressurization over normal ranges. However, the axial force phenomenon of arteries has never been explained with microstructural considerations. In this paper we propose a simple biomechanical model to relate the specific axial force phenomenon of arteries to the predicted load-dependent average collagen fiber orientation. It is shown that (a) the model correctly predicts the authors' experimentally measured biaxial behavior of pig renal arteries and (b) the model predictions are in agreement with additional experimental results reported in the literature. Finally, we discuss the implications of the model for collagen fiber orientation and deposition in arteries

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