Patient-specific finite element simulation of the insertion of guidewires during an EVAR procedure: towards clinically relevant indicators

Ce travail présente une méthode de simulation par éléments finis explicite pour le calcul des déformations d'une structure vasculaire aorto-iliaque induites par l'insertion de guides endovasculaires de type « extra-stiff » pour le traitement des anévrismes abdominaux. Le modèle mécanique prend en compte le comportement non-linéaire de la paroi vasculaire, l'effet de précontrainte induit par la pression artérielle et le support mécanique dû aux organes et structures environnants. Les résultats de simulation sont confrontés à des données d'imagerie 3D acquises au cours de la procédure chirurgicale sur 24 patients. Ces résultats sont ensuite utilisés afin de déduire des quantités utiles d'un point de vue clinique, comme le raccourcissement de segments artériels, la longueur des prothèses à déployer, le déplacement de points anatomiques importants

Simulation spécifique patient de la réponse mécanique de la structure vasculaire à l'insertion d'outils lors d'une chirurgie EVAR

Endovascular Aneurysm Repair (EVAR) is a mini-invasive technique that is commonly used to treat Abdominal Aortic Aneurysms (AAA). It relies on the exclusion of the aneurysm sac by introducing one or more stent-grafts through the femoral arteries and deploying them inside the aneurysm. During the procedure, several tools of varying stiffness are introduced to enable the delivery of the stent graft to its deployment site. During this process, the vascular structure undergoes major deformations. Usually, these have no consequence on the smooth progress of the procedure. However, in some instances, particularly when the patient presents an unfavorable anatomical profile (major tortuousness or angulation, deep calcification, long length of the common and external iliac arteries), the deformation caused by the insertion of stiff guidewires can have major consequences. Today, their prediction relies mainly on the surgeon’s experience. Numerical simulation appears to be an appropriate tool to give the practitioner more objective and more useful indicators when planning the procedure: guiding the surgical act and making it safer using such an approach would potentially reduce the risks of intraoperative and postoperative complications. In the first step of the work, we developed a mechanical model of the aorto-iliac vascular structure and a simulation methodology to answer the mechanical problem. This patient-specific model has been parametrized based on available preoperative data. Then the second step of the work consisted in the validation of this model by confronting the simulation results to real intraoperative 3D data that were collected on 28 cases of patients operated at the University Hospital of Rennes. All the methods that were developed during this PhD were integrated in demonstration module of EndoSize® software (Therenva, France).Dans ce travail de thèse, on s’intéresse au traitement endovasculaire de l’anévrisme de l’aorte abdominale (EVAR). Cette technique mini-invasive couramment utilisée et connaît une croissance importante depuis 10 ans. Elle repose sur l’exclusion du sac anévrismal par le déploiement au niveau de l’anévrisme d’une ou plusieurs endoprothèses introduites par voies fémorales. Au cours de l’intervention, plusieurs types d’outils de rigidité variable sont introduits pour permettre la navigation de l’endoprothèse. La structure vasculaire subit alors des déformations importantes. Ces déformations sont en général sans incidence sur le bon déroulement de l’intervention. Cependant dans certains cas, notamment pour les patients présentant des anatomies défavorables (fortes tortuosités ou angulations, important degré de calcification, longueur importante des artères iliaques communes et externes) les déformations produites par l’insertion des guides rigides peuvent avoir des conséquences sur le déroulement de l’intervention. Actuellement leur anticipation repose principalement sur l’expérience du chirurgien. La simulation mécanique semble être un outil adapté pour fournir des indicateurs plus objectifs et utiles au praticien lors du planning de son intervention : cette pratique permettrait en guidant et sécurisant le geste chirurgical de diminuer potentiellement les risques de complications peropératoires et postopératoires. La première étape du travail a consisté à développer un modèle mécanique de la structure aorto-iliaque et une méthode de simulation permettant de répondre au problème mécanique posé. Ce modèle a été paramétré de façon patient-spécifique à partir des données préopératoires disponibles. Puis la deuxième étape du travail a consisté à valider la modélisation développée en la confrontant à des données peropératoires réelles obtenus sur 28 cas de patients opérés au CHU Rennes. L’ensemble des méthodes développées à enfin été intégré à un module de démonstration du logiciel EndoSize® (Therenva, France)

Patient-specific simulation of the mecanical response of the vascular structure under the insertion of tools during EVAR

Dans ce travail de thèse, on s’intéresse au traitement endovasculaire de l’anévrisme de l’aorte abdominale (EVAR). Cette technique mini-invasive couramment utilisée et connaît une croissance importante depuis 10 ans. Elle repose sur l’exclusion du sac anévrismal par le déploiement au niveau de l’anévrisme d’une ou plusieurs endoprothèses introduites par voies fémorales. Au cours de l’intervention, plusieurs types d’outils de rigidité variable sont introduits pour permettre la navigation de l’endoprothèse. La structure vasculaire subit alors des déformations importantes. Ces déformations sont en général sans incidence sur le bon déroulement de l’intervention. Cependant dans certains cas, notamment pour les patients présentant des anatomies défavorables (fortes tortuosités ou angulations, important degré de calcification, longueur importante des artères iliaques communes et externes) les déformations produites par l’insertion des guides rigides peuvent avoir des conséquences sur le déroulement de l’intervention. Actuellement leur anticipation repose principalement sur l’expérience du chirurgien. La simulation mécanique semble être un outil adapté pour fournir des indicateurs plus objectifs et utiles au praticien lors du planning de son intervention : cette pratique permettrait en guidant et sécurisant le geste chirurgical de diminuer potentiellement les risques de complications peropératoires et postopératoires. La première étape du travail a consisté à développer un modèle mécanique de la structure aorto-iliaque et une méthode de simulation permettant de répondre au problème mécanique posé. Ce modèle a été paramétré de façon patient-spécifique à partir des données préopératoires disponibles. Puis la deuxième étape du travail a consisté à valider la modélisation développée en la confrontant à des données peropératoires réelles obtenus sur 28 cas de patients opérés au CHU Rennes. L’ensemble des méthodes développées à enfin été intégré à un module de démonstration du logiciel EndoSize® (Therenva, France).Endovascular Aneurysm Repair (EVAR) is a mini-invasive technique that is commonly used to treat Abdominal Aortic Aneurysms (AAA). It relies on the exclusion of the aneurysm sac by introducing one or more stent-grafts through the femoral arteries and deploying them inside the aneurysm. During the procedure, several tools of varying stiffness are introduced to enable the delivery of the stent graft to its deployment site. During this process, the vascular structure undergoes major deformations. Usually, these have no consequence on the smooth progress of the procedure. However, in some instances, particularly when the patient presents an unfavorable anatomical profile (major tortuousness or angulation, deep calcification, long length of the common and external iliac arteries), the deformation caused by the insertion of stiff guidewires can have major consequences. Today, their prediction relies mainly on the surgeon’s experience. Numerical simulation appears to be an appropriate tool to give the practitioner more objective and more useful indicators when planning the procedure: guiding the surgical act and making it safer using such an approach would potentially reduce the risks of intraoperative and postoperative complications. In the first step of the work, we developed a mechanical model of the aorto-iliac vascular structure and a simulation methodology to answer the mechanical problem. This patient-specific model has been parametrized based on available preoperative data. Then the second step of the work consisted in the validation of this model by confronting the simulation results to real intraoperative 3D data that were collected on 28 cases of patients operated at the University Hospital of Rennes. All the methods that were developed during this PhD were integrated in demonstration module of EndoSize® software (Therenva, France)

A structural model of passive skeletal muscle shows two reinforcement processes in resisting deformation

Passive skeletal muscle derives its structural response from the combination of the titin filaments in the muscle fibres, the collagen fibres in the connective tissue and incompressibility due to the high fluid content. Experiments have shown that skeletal muscle tissue presents a highly asymmetrical three-dimensional behaviour when passively loaded in tension or compression, but structural models predicting this are not available. The objective of this paper is to develop a mathematical model to study the internal mechanisms which resist externally applied deformation in skeletal muscle bulk. One cylindrical muscle fibre surrounded by connective tissue was considered. The collagenous fibres of the endomysium and perimysium were grouped and modelled as tension-only oriented wavy helices wrapped around the muscle fibre. The titin filaments are represented as non-linear tension-only springs. The model calculates the force developed by the titin molecules and the collagen network when the muscle fibre undergoes an isochoric along-fibre stretch. The model was evaluated using a range of literature based input parameters and compared to the experimental fibre-direction stress-stretch data available. Results show the fibre direction non-linearity and tension/compression asymmetry are partially captured by this structural model. The titin filament load dominates at low tensile stretches, but for higher stretches the collagen network was responsible for most of the stiffness. The oblique and initially wavy collagen fibres account for the non-linear tensile response since, as the collagen fibres are being recruited, they straighten and re-orient. The main contribution of the model is that it shows that the overall compression/tension response is strongly influenced by a pressure term induced by the radial component of collagen fibre stretch acting on the incompressible muscle fibre. Thus for along-fibre tension or compression the model predicts that the collagen network contributes to overall muscle stiffness through two different mechanisms: (1) a longitudinal force directly opposing tension and (2) a pressure force on the muscle fibres resulting in an indirect longitudinal load. Although the model presented considers only a single muscle fibre and evaluation is limited to along-fibre loading, this is the first model to propose these two internal mechanisms for resisting externally applied deformation of skeletal muscle tissu

The anisotropic mechanical behaviour of passive skeletal muscle tissue subjected to large tensile strain

The passive mechanical properties of muscle tissue are important for many biomechanics applications. However, significant gaps remain in our understanding of the three-dimensional tensile response of passive skeletal muscle tissue to applied loading. In particular, the nature of the anisotropy remains unclear and the response to loading at intermediate fibre directions and the Poisson's ratios in tension have not been reported. Accordingly, tensile tests were performed along and perpendicular to the muscle fibre direction as well as at 30°, 45° and 60° to the muscle fibre direction in samples of Longissimus dorsi muscle taken from freshly slaughtered pigs. Strain was measured using an optical non-contact method. The results show the transverse or cross fibre (TT') direction is broadly linear and is the stiffest (77 kPa stress at a stretch of 1.1), but that failure occurs at low stretches (approximately λ=1.15). In contrast the longitudinal or fibre direction (L) is nonlinear and much less stiff (10 kPa stress at a stretch of 1.1) but failure occurs at higher stretches (approximatelyλ=1.65). An almost sinusoidal variation in stress response was observed at intermediate angles. The following Poisson's ratios were measured: VLT=VLT'=0.47, VTT'=0.28 and VTL=0.74. These observations have not been previously reported and they contribute significantly to our understanding of the three dimensional deformation response of skeletal muscle tissu

Influencing factors of sac shrinkage after endovascular aneurysm repair

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Interest of fusion imaging and modern navigation tools with hybrid rooms in endovascular aortic procedures

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Use of Numerical Simulation to Predict Iliac Complications During Placement of an Aortic Stent Graft

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Patient-specific simulation of tools insertion before stent-graft deployment during EVAR and results comparison to 3D introperative data

International audienceEndovascular aneurysm repair (EVAR) is a mini-invasive technique used to treat abdominal aortic aneurysms. It relies on the exclusion of the aneurysmal site by deployment of one or several stent-grafts introduced via femoral and iliac arteries. During the intervention, insertion of several endovascular tools is required to offer support and stability to the stent-graft delivery system. Among them, insertion of stiff guidewires often leads to the straightening of vascular structure. In complex anatomies, these arterial deformations may be related to difficulties to deliver the grafts or discrepancies in arterial lengths calculated from the preoperative CT-scan. Anticipation of such complications involves several factors like arteries tortuosity and angulation, level of calcification, or type of devices used and is currently based on surgical experience. We believe that mechanical finite-element simulation is a promising tool to predict these deformations and understand their effects in order to provide surgeons with objective data that would help interventions 'planning. We developed a semi-automatic process to build a biomechanical model of the vascular structure from preoperative CT-scan data. The biomechanical model takes into account nonlinear properties of arterial wall, loading due to arterial pressure and external support provided by external tissues and bones structures. This model is then used to run an explicit finite element simulation of endovascular tools insertion with Ansys Ls-Dyna® software. Two types of tools are modeled, soft catheters and stiff guidewires. The method is presented on the case of one patient who underwent EVAR at the University Hospital of Rennes, France. During the intervention, two 3D cone-beam-CT acquisitions were performed:-After the insertion of two soft angiographic catheters-After the additional insertion of two stiff guidewires Then rigid registration based on bone structures was applied to the acquired data to align with the preoperative CT-scan. 3D tools structures and arterial wall calcified plaques were segmented to allow for position measurement and comparison to simulation results. The shape of catheters and guidewires predicted by the simulation is visually very close to the one observed on intraoperative images. 3D position error was measured along the length of the tools, it encompasses the error due to rigid registration and the simulation error. Qualitative comparison between the vascular structure deformed mesh given by the simulation and 2D angiographic images shows very similar shapes, in particular on common iliac arteries segments that underwent the largest intraoperative displacements. Arterial wall displacement is measured at several points of interest located on calcification plaques that are easily identifiable on intraoperative images. The 3D error between the position predicted by the simulation and the intraoperative data is calculated at these points Results presented here show the feasibility of finite-element biomechanical simulations to predict the deformed position of tool and arterial wall during EVAR before stent-graft deployment for a particularly tortuous patient. A parameters sensitivity study and more patient cases are now necessary as next steps towards a patient-specific parameterization of the model

Finite element simulation of the insertion of guidewires during an EVAR procedure: example of a complex patient case, a first step toward patient-specific parameterized models

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