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POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical.

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Presentazione sul tema: "POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical."— Transcript della presentazione:

1 POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical Engineering Division of Cardiovascular Biomechanics AN EXPERIMENTAL AND COMPUTATIONAL STUDY OF A NEW ENDOVASCULAR PROSTHESIS FOR THE TREATMENT OF ABDOMINAL AORTIC ANEURYSMS Supervisors: Prof. Gabriele DUBINI Prof. Frans N. Van de VOSSE MSc Thesis: Salvatore Luca FICCO

2 AIM OF THE PROJECT The study is about the possibility to realize a custom made prosthesis for the endovascular treatment of abdominal aortic aneuryms (AAA) Aim of the project COMPUTATIONAL ANALYSIS Structural analyses were carried out using the Finite Element Method REALIZATION OF A PROTOTYPE It was realized a prototype of the new prostheis afterwards it was tested in vitro by using an experimental set-up.

3 PATHOLOGY Aneuryms are permanent and localized dilatation of an artery. The abdominal aorta (the piece of the aorta between the renal arteries and the bifurcation of the femoral arteries) is considered aneurismatic if its diameter is greater than 5 cm. SANE AORTA ANEURYSM Pathology

4 During the treatment of AAA diagnostic and imaging techniques are very important mainly for two reasons: generally patients do not suffer any disease correlated with the dilatation of the abdominal aorta; the shape of an aneurysm is important in order to be able to operate in an appropriate way. Pathology

5 Aneurysms show tendency to grow untill wall rupture occurs in one or more sites Common sites of rupture 1.Behind the peritoneum 2.In the abdominal space 3.In the duodenum 4.Into the inferior vena cava Aneurysm formation and danger of rupture are well illustrated by Laplaces Law T = P r r: radius of the vessel T: wall tension necessary to withstand the blood pressure (P) Pathology

6 CURRENT SURGICAL TECHNIQUES ANEURSMECTOMY: It is the substitution of the aneurismatic piece with a vascular prosthesis very invasive operation Technique: (1) Incision(2) Opening and asportation of the thrombus (3) Insertion of the vascular prosthesis (4) Suture of the aortic wall ENDOVASCULAR SURGERY: It consists in the insertion of a stent-graft through one or two small incision(-s) in the femoral artery(-ies). Technique (bifurcated stent): (1) Catheter insertion(2) Stent release Surgical Techniques

7 DRAWBACKS & COMPLICATIONS ENDOVASCULAR SURGERYANEURISMECTOMY General anaesthesia Large incision Hypothermy Damages at the aneurysm necks (due to clamping procedure) Respiratory problems Significant blood and fluid loss Mobilization: prosthesis detaching at one or more attachment sites Endo-tension: transmission of pressure through thrombus or artheroma at the proximal attachment site Endo-leaks: four kinds of blood leakages Surgical Techniques

8 ENDOLINER ® : A NEW CONCEPT OF ENDOVASCULAR PROSTHESIS Distinctive characteristics The durability of the construction of an Endoliner ® is not necessarily a prerequisite. Its structure adapts entirely to the aneurysm wall from the proximal neck untill the bifurcation of the femoral arteries Working mechanism As an additional procedure during the treatment of intact aneurysms Occlusion of the collateral arteries and prevention of type II endoleaks As an emergency treatment of ruptured aneurysms Occlusion of leaks through emergency catheterization The Endoliner


10 MATERIALS:In order to realize the prototype it was chosen a nickel-titanium alloy showing shape memory behaviour. Solid-solid phase transformation Austenite (Hot shape) Martensite (Cold shape) Material tests after heat treatment 0.9 mm 0.17 mm 50 mm Sample Alloy: NiTinol alloy B 55.9% Ni, 43.9% Ti, C e O Traction to failure Load-Unload Materials

11 The Experimental Analysis THE EXPERIMENTAL SET-UP Transparency Sterility MRI proof Variability in lenght to visually follow the events occurring in the set-up for monitoring events inside the aneurysm in order to hold live aneurysms for different sizes of aneurysms For the realization of set-up different considerations were taken into account

12 The Experimental Analysis TECHNICAL CHARACTERISTICS Modular structure Electric motor Volumetric pump Electric motor Volumetric pump No-return valve Valve Variable resistence ControlResistence

13 the NiTi band was wound around it to procede with the heat training After modelling a generic aneurysm shape by using gypsum powder, it was covered with some layers of latex leaving two small tubes for the insertion of the pressure wires. After the preparation of the aneurysm shape for the heat treatment (500 ° C per 10 min) Afterwards water-proof silk was hand sealed all around the structure. The Experimental Analysis REALIZATION OF THE PROTOTYPE PREPARATION OF AN ANEURYSM MODEL

14 Thus, the prosthesis prototype has a structure reproducing the geometry of the aneurysm model The Endoliner ® was than inserted coaxially into the delivery system and wound on itself. The Experimental Analysis

15 The Experimental Analysis: Pressure Acquisition Pressures were acquired in the middle of the aneurismatic sac (with and without Endoliner ® ) in order to study the ability of the prototype to avoid endoleakages and to preserve the aortic wall. Cardiac rate: 1.25 Hz (75 bpm) Sampling: 128 samples per period (8 period) Signal for aortic flow: systolic rise time (linear ) = s; diastolic decay time (linear) = s; diastolic time: rest of the cycle Test Parameters RESULTS By looking at the pressure characteristics it is possible to observe that: In the case with Endoliner ® pressures are a few inferior (1-2 mmHg) The insertion of the prototype does not cause pressure falls or peaks The prototype is not able to preserve the wall from high pressures Freezing effect on the patient condition The Endoliner ® can be an effective by-pass usable to contain the rupture

16 In order to estimate the unfolding of the structure and the geometrical configuration of the prototype images were acquired with a video camera connected to an endoscopic device and coaxially inserted into the Endoliner ®. RESULTS Fixed shot of the middle of the sac Pulling the camera Proximal neck Distal neck By looking at the images it is possible to observe that: A good unfolding of the prototype structure Endoleaks formation The prototype does not adhere completely to the wall at the proximal and distal attachment sites The Experimental Analysis: Images

17 The Computational Study AIM OF THE COMPUTATIONAL ANALYSIS Estimating the recovery of the memorized shape Studying the interaction between the aortic wall and the Nitinol structure Limits of the analysis Geometrical approximation The pre-load due to the blood pressure was not considered Software Rhinoceros: to create the models Gambit: to mesh the models ABAQUS: analysis code. It has been enriched by using a procedure to model the behaviour of shape memory alloys [Auricchio F., 2002 ]

18 The Computational Study Complex geometrical structure Long computational times Interaction between different materials Reduced model Lenght: 15 mm Ø: 49.5 mm s wall: 1.5 mm s thrombus: 4 mm GEOMETRICAL MODEL It is possible to consider only one coil Ø: 46 mm Pitch: 5 mm Section: 0.17 x 0.9 mm

19 The Computational Study MECHANICAL PROPERTIES OF THE MATERIALS Coil: the behaviour is described by the Auricchios procedure. Youngs moduls (10 GPa, 12 GPa) from the experimental tests. Thrombus: Hyperelastic model Strain Energy Function: Ogden N = 3 Wall: Hyperelastic model Strain Energy Function : Polynomial N = 2 From litterature uniaxial traction test data (executed on biological samples) ThrombusWall Average mechanical characteristics generated by ABAQUS

20 The Computational Study BOUNDARY CONDITIONS ANALYSIS STEPS CrimpingReleasingSMA AAA sections constrained along the longitudinal direction 1. To take into account the rest of the vessel AAA lateral surface constrained along the circumferential direction 2. To avoid rigid body motion Set of displacements along the radial direction assigned to the nodes of the inner coil surface 3. To crimp the coil

21 723 for the coil The Computational Study MESHING THE MODELS Hexahedral elements (each with 8 nodes) were chosen to mesh all the structures of the model. Therefore the elements were in all: for the thrombus (2) 3420 for the wall (1) 1 2

22 Von Mises stressesUnfolding of the coil RESULTS

23 The Computational Study DISCUSSION It is 10 times less than the stress due to the pre-load only (0.3 MPa) The higher stress acting on the wall and due to the coil is about 0.04 MPa Peak stress for an AAA [Fillinger, 2002] = 0.4 MPa The single coil gives a very small contribute to the risk of rupture The coil does not recover completely its shape, mainly for two reasons: 1. The biomechanical behaviour of the thrombus is very difficult to simulate 2. The single coil cannot develop a force able to deform enough the thrombus The nodal displacements are not elevate: 0.2 mm ( DSF = 10 ) They can be comparated to the ones due to the pre-load only (0.14 mm)

24 Conclusions & Late Developments C O N C L U S I O N S The experimental study showed that it is possible to realize a prototype of the Endoliner ® and the experimental set-up resulted suitable for those kind of tests. From the computational analysis it came out that a prosthesis like the Endoliner ® does not overload the aorta, therefore it can be a good supporting structure for the aneurismatic sac The analysis of the pressures revealed a freezing effect of the Endoliner ® that can be useful during the stabilization phase L A T E D E V E L O P M E N T S Tests on biological samples of AAA Construction of an attacching system for the prototype Implementation of complex models for the thrombus without axial simmetry Development of different geometries for the prototype Analysis of the behaviour of two or more coils Different approaches to the computational problem

25 The End

26 Pochi dati statistici sono sufficienti a sottolineare lincidenza di questa patologia: Ogni anno negli Stati Uniti sono diagnosticati circa casi di aneurismi aortici addominali di questi pazienti si sottopone ad un intervento chirurgico Il 10% della popolazione maschile manifesta dilatazioni dellaorta addominale Fra i pazienti che presentano aneurismi aortici rotti 50% Decede in breve tempo (prima di raggiungere unUnità di Pronto Intervento) Non sopravvive alla chirurgia demergenza 25% Sopravvive 25% La Patologia [Yano, 2000]

27 LEZIOLOGIA Nonostante i numerosi studi a tal proposito, lesatta causa che porta allinsorgenza di un aneurisma aortico è tuttora sconosciuta. PRINCIPALI FATTORI DI RISCHIO Fumo Traumi alla parete vasale e infezioni Artereosclerosi ed ipertensione Carenza di collagene e\o elastina Alterazioni dei sistemi di rilascio di ossigeno e nutrimenti alla parete Razza Fattori genetici Età La Patologia

28 Le principali tecniche di imaging si differenziano per: qualità, costo, tempi di acquisizione. Quelle maggiormente utilizzate sono: ULTRASUONOGRAFIA Vantaggi: costo ridotto, non invasiva, largamente diffusa. Svantaggi: non adatta per pazienti obesi, poco oggettiva. AORTOGRAFIA Vantaggi: identifica disturbi reno- vascolari e vasi anomali. Svantaggi: Costi elevati, invasività, tolleranza del paziente. RISONANZA MAGNETICA (MRI) Vantaggi: assenza di radiazioni, non invasiva. Svantaggi: costi elevati, artefatti di movimento, disponibilità (SW e HW), claustrofobia del paziente. TOMOGRAFIA COMPUTERIZZATA (CT) Vantaggi: non invasiva, buona stima delle dimensioni dellaneurisma, localizza le estensioni prossimali dellaneurisma. Svantaggi: utilizzo di radiazioni, costi elevati, scarse informazioni circa lanatomia dellarteria. Vantaggi: non invasiva, tempi di acquisizione ridotti. Svantaggi: utilizzo di radiazioni, costi elevati, tecnologia. HELICAL CT La Patologia

29 Lo Studio Computazionale LINTERAZIONE DI CONTATTO PLACCA/SPIRA Contatto fra le due superfici gestito da ABAQUS ® tramite lalgoritmo master-slave PlaccaSpira Modello di contatto: soft esponenziale

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