TECHNISCHE UNIVERSITEIT EINDHOVEN 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

Aim of the project 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) 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. COMPUTATIONAL ANALYSIS Structural analyses were carried out using the Finite Element Method

Aneuryms are permanent and localized dilatation of an artery. Pathology 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 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.

Common sites of rupture Pathology Aneurysms show tendency to grow untill wall rupture occurs in one or more sites 1 2 3 4 Common sites of rupture Behind the peritoneum In the abdominal space In the duodenum Into the inferior vena cava Aneurysm formation and danger of rupture are well illustrated by Laplace’s Law T = P r r: radius of the vessel T: wall tension necessary to withstand the blood pressure (P)

CURRENT SURGICAL TECHNIQUES ANEURSMECTOMY: It is the substitution of the aneurismatic piece with a vascular prosthesis ENDOVASCULAR SURGERY: 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 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

DRAWBACKS & COMPLICATIONS Surgical Techniques DRAWBACKS & COMPLICATIONS ENDOVASCULAR SURGERY ANEURISMECTOMY 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

ENDOLINER®: A NEW CONCEPT OF ENDOVASCULAR PROSTHESIS The Endoliner 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 Occlusion of leaks through emergency catheterization As an emergency treatment of ruptured aneurysms

POSSIBLE GEOMETRIC CONFIGURATIONS The Endoliner POSSIBLE GEOMETRIC CONFIGURATIONS Net Zigzag Spiral

Solid-solid phase transformation Materials 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 Alloy: NiTinol alloy B 55.9% Ni, 43.9% Ti, C e O 0.9 mm 0.17 mm 50 mm Sample Traction to failure Load-Unload

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

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

The Experimental Analysis PREPARATION OF AN ANEURYSM MODEL 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. REALIZATION OF THE PROTOTYPE After the preparation of the aneurysm shape for the heat treatment (500°C per 10 min) the NiTi band was wound around it to procede with the heat training Afterwards water-proof silk was hand sealed all around the structure.

The Experimental Analysis 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: 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. By looking at the pressure characteristics it is possible to observe that: The prototype is not able to preserve the wall from high pressures In the case “with Endoliner®” pressures are a few inferior (1-2 mmHg) Test Parameters Cardiac rate: 1.25 Hz (75 bpm) Sampling: 128 samples per period (8 period) Freezing effect on the patient condition The insertion of the prototype does not cause pressure falls or peaks Signal for aortic flow: systolic rise time (linear ) = 0.075 s; diastolic decay time (linear) = 0.225 s; diastolic time: rest of the cycle The Endoliner® can be an effective by-pass usable to contain the rupture RESULTS

The Experimental Analysis: Images By looking at the images it is possible to observe that: 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®. The prototype does not adhere completely to the wall at the proximal and distal attachment sites Endoleaks formation A good unfolding of the prototype structure RESULTS Fixed shot of the middle of the sac Pulling the camera Proximal neck Distal neck

The Computational Study AIM OF THE COMPUTATIONAL ANALYSIS Studying the interaction between the aortic wall and the Nitinol structure Estimating the recovery of the memorized shape 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 ]

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

The Computational Study MECHANICAL PROPERTIES OF THE MATERIALS Coil: the behaviour is described by the Auricchio’s procedure. Young’s 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) Wall Thrombus Average mechanical characteristics generated by ABAQUS

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

The Computational Study MESHING THE MODELS 12464 for the thrombus (2) 3420 for the wall (1) 1 2 Hexahedral elements (each with 8 nodes) were chosen to mesh all the structures of the model. Therefore the elements were 16607 in all: 723 for the coil

RESULTS Unfolding of the coil Von Mises stresses

The Computational Study DISCUSSION The higher stress acting on the wall and due to the coil is about 0.04 MPa It is 10 times less than the stress due to the pre-load only (0.3 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)

Conclusions & Late Developments 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. The analysis of the pressures revealed a freezing effect of the Endoliner® that can be useful during the stabilization phase 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 L A T E D E V E L O P M E N T S 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 Tests on biological samples of AAA Different approaches to the computational problem Construction of an attacching system for the prototype

The End

Fra i pazienti che presentano aneurismi aortici rotti La Patologia Pochi dati statistici sono sufficienti a sottolineare l’incidenza di questa patologia: Ogni anno negli Stati Uniti sono diagnosticati circa 200.000 casi di aneurismi aortici addominali Il 10% della popolazione maschile manifesta dilatazioni dell’aorta addominale Fra i pazienti che presentano aneurismi aortici rotti 50.000-60.000 di questi pazienti si sottopone ad un intervento chirurgico 50% Decede in breve tempo (prima di raggiungere un’Unità di Pronto Intervento) Non sopravvive alla chirurgia d’emergenza 25% Sopravvive 25% [Yano, 2000]

L’EZIOLOGIA La Patologia Nonostante i numerosi studi a tal proposito, l’esatta causa che porta all’insorgenza di un aneurisma aortico è tutt’ora sconosciuta. PRINCIPALI FATTORI DI RISCHIO Traumi alla parete vasale e infezioni Artereosclerosi ed ipertensione Età Fattori genetici Razza Fumo Alterazioni dei sistemi di rilascio di ossigeno e nutrimenti alla parete Carenza di collagene e\o elastina

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

Lo Studio Computazionale L’INTERAZIONE DI CONTATTO PLACCA/SPIRA Contatto fra le due superfici gestito da ABAQUS® tramite l’algoritmo master-slave Placca Spira Modello di contatto: soft esponenziale