Post on 02-May-2015
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 AN EXPERIMENTAL AND COMPUTATIONAL STUDY OF A
NEW ENDOVASCULAR PROSTHESIS FOR THE NEW ENDOVASCULAR PROSTHESIS FOR THE
TREATMENT OF ABDOMINAL AORTIC ANEURYSMSTREATMENT OF ABDOMINAL AORTIC ANEURYSMS
Supervisors: Prof. Gabriele DUBINI
Prof. Frans N. Van de VOSSE
MSc Thesis:
Salvatore Luca FICCO
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 Aim of the projectproject
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.
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
PathologyPathology
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.
PathologyPathology
Aneurysms show tendency to grow untill wall rupture occurs in one or more sites
1
2
3
4
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 Laplace’s Law
T = P r r: radius of the vessel
T: wall tension necessary to withstand the blood pressure (P)
PathologyPathology
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 TechniquesSurgical 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
Surgical TechniquesSurgical Techniques
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 EndolinerThe Endoliner
POSSIBLE GEOMETRIC CONFIGURATIONS
Zigzag SpiralNet
The EndolinerThe Endoliner
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 m
m
Sample
Alloy: NiTinol alloy B 55.9% Ni, 43.9% Ti, C e O
Trazione a rottura
0200400600800
1000120014001600
0 5 10 15 20
Deformazione (%)
Sfo
rzo
(M
Pa)
Sample 1
Sample 2 Traction to failure
Carico - Scarico
0100200300400500600700800900
1000
0 2 4 6 8 10
Deformazione(%)
Sfo
rzo
(M
Pa) Sample 1
Sample 2
E1=12GPa
E2=10GPa
Load-Unload
MaterialsMaterials
The Experimental AnalysisThe 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
The Experimental AnalysisThe Experimental Analysis
TECHNICAL CHARACTERISTICS
Modular structure Electric motorVolumetric pump
Electric motor
Volumetric pump
No-return valve
Valve
Variable resistence
Control
Resistence
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 AnalysisThe Experimental Analysis
REALIZATION OF THE PROTOTYPE
PREPARATION OF AN ANEURYSM MODEL
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 AnalysisThe Experimental Analysis
The Experimental Analysis: Pressure AcquisitionThe 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 ) = 0.075 s; diastolic decay time (linear) = 0.225 s; diastolic time: rest of the cycle
Test Parameters
Caratteristiche Pressioni
0102030405060
0 50 100 150 200 250 300
Campioni (2 periodi)
Pre
ssio
ne (m
mH
g)
Senza Endoliner Con Endoliner
RESULTS
Caratteristiche Pressioni
0102030405060
0 200 400 600 800 1000
Campionatura
Pre
ssio
ne
(mm
Hg
)
Con Endoliner Senza Endoliner
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
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: ImagesThe Experimental Analysis: Images
The Computational StudyThe 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 ]
The Computational StudyThe 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
The Computational StudyThe 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)
ThrombusWall
Average mechanical
characteristics generated
by ABAQUS
The Computational StudyThe Computational Study
BOUNDARY CONDITIONS
ANALYSIS STEPS
Crimping Releasing “SMA”
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
• 723 for the coil
The Computational StudyThe 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 16607 in all:
• 12464 for the thrombus (2)
• 3420 for the wall (1)
1
2
Von Mises stresses Unfolding of the coil
RESULTS
The Computational StudyThe 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 MPaThe 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 simulate2. 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 DevelopmentsConclusions & 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
The EndThe End
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
50.000-60.000 di questi pazienti si sottopone ad un
intervento chirurgico
Il 10% della popolazione maschile manifesta dilatazioni
dell’aorta addominale
Fra i pazienti che presentano aneurismi aortici rotti
50%
Decede in breve tempo (prima di
raggiungere un’Unità di Pronto
Intervento)
Non sopravvive
alla chirurgia d’emergenza
25%Sopravvive
25%
La PatologiaLa Patologia
[Yano, 2000]
L’EZIOLOGIA
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
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 PatologiaLa Patologia
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 dell’aneurisma, localizza le estensioni prossimali dell’aneurisma.
Svantaggi: utilizzo di radiazioni, costi elevati, scarse informazioni circa l’anatomia dell’arteria.
Vantaggi: non invasiva, tempi di acquisizione ridotti.
Svantaggi: utilizzo di radiazioni, costi elevati, tecnologia.
HELICAL CT
La PatologiaLa Patologia
Lo Studio ComputazionaleLo 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