POLITECNICO DI MILANO
Facoltà di Ingegneria dei Sistemi
TECHNISCHE UNIVERSITEIT
EINDHOVEN
Corso di Laurea in Ingegneria Biomedica
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
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.
Pathology
Aneurysms show tendency to grow untill wall rupture occurs in one or more sites
Common sites of rupture
Aneurysm formation and
danger of rupture are well
illustrated by Laplace’s Law
3
4
T=Pr
 r: radius of the vessel
 T: wall tension
necessary to withstand
the blood pressure (P)
1
2
1.
Behind the peritoneum
2.
In the abdominal space
3.
In the duodenum
4.
Into the inferior vena
cava
Surgical Techniques
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
(4) Suture of the aortic
prosthesis
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
DRAWBACKS & COMPLICATIONS
ENDOVASCULAR SURGERY
 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
ANEURISMECTOMY
 General anaesthesia
 Large incision
 Hypothermy
 Damages at the
aneurysm necks (due to
clamping procedure)
 Respiratory problems
 Significant blood and
fluid loss
The Endoliner
ENDOLINER®: A NEW CONCEPT OF ENDOVASCULAR PROSTHESIS
 The durability of the construction of an Endoliner® is not necessarily
a prerequisite.
Distinctive
characteristics
 Its structure adapts entirely to the aneurysm wall from the proximal neck
untill the bifurcation of the femoral arteries
 As an additional procedure
during the treatment of intact
aneurysms
Working
mechanism
 As an emergency
treatment of ruptured
aneurysms
Occlusion of the
collateral arteries and
prevention of type II
endoleaks
Occlusion of leaks
through
emergency
catheterization
The Endoliner
POSSIBLE GEOMETRIC CONFIGURATIONS
Net
Zigzag
Spiral
Materials
MATERIALS: In order to realize the prototype it was chosen a nickel-titanium alloy showing
shape memory behaviour.
Solid-solid phase transformation
Austenite
Martensite
(“Hot” shape)
(“Cold” shape)
Material tests after heat treatment
 Alloy: NiTinol alloy B 55.9% Ni,
43.9% Ti, C e O
Carico - Scarico
Trazione a rottura
Sample 1
0.9 mm
Sample 2
Sample 1
E1=12GPa
Sample 2
E2=10GPa
0.17 mm
 Load-Unload
Sforzo (MPa)
Sforzo (MPa)
 Traction to failure
50 mm
 Sample
1000
1600
900
1400
800
700
1200
600
1000
500
800
400
600
300
200
400
100
200
00
00
2
5
4
106
Deformazione(%)
Deformazione (%)
8
15
10 20
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
Modular structure
Electric motor
Volumetric pump
No-return valve
Variable resistence
Control
Resistence
Valve
Volumetric pump
Electric motor
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
By looking
were
at the
acquired
pressure
in the
characteristics
middle of the
it isaneurismatic
possible to observe
sac (withthat:
and without
Endoliner®) in order to study the ability of the prototype to avoid endoleakages and to
®” pressures
 In the case “with Endoliner
preserve
the aortic wall. The prototype is not able to
preserve the wall from high
are a few inferior
(1-2rate:
mmHg)
 Cardiac
1.25 Hz (75 bpm)
pressures
Test
 Sampling: 128 samples per period (8 period) Freezing effect on the patient
Parameters
 The insertion of the prototype does
 Signalfalls
for aortic
flow: systolic rise time (linear ) = 0.075 condition
s; diastolic decay time
not cause pressure
or peaks
(linear) = 0.225 s; diastolic time: rest of the cycle
The Endoliner® can be an effective by-pass usable to contain the rupture
RESULTS
Caratteristiche Pressioni
60
50
40
30
20
10
0
Pressione (mmHg)
Pressione (mmHg)
Caratteristiche Pressioni
0
200
400
600
800
Campionatura
Con Endoliner
Senza Endoliner
1000
60
50
40
30
20
10
0
0
50
100
150
200
Campioni (2 periodi)
Senza Endoliner
Con Endoliner
250
300
The Experimental Analysis: Images
By to
looking
at the
it isofpossible
to observe
that:
In order
estimate
the images
unfolding
the structure
and the
geometrical configuration of
the prototype images were acquired with a video camera connected to an endoscopic
 The prototype does
notand
adhere
device
coaxially inserted
into the Endoliner®.
Endoleaks
completely to the wall at the
formation
proximal and distal attachment sites
 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
 Geometrical approximation
Limits of the
analysis
 The pre-load due to the blood pressure was
not considered
• Rhinoceros: to create the models
Software
• 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
 Wall: Hyperelastic model
Strain Energy Function: Ogden N = 3
Strain Energy Function : Polynomial N = 2
From litterature uniaxial traction test data (executed on biological samples)
Average mechanical
characteristics generated
by ABAQUS
Wall
Thrombus
The Computational Study
BOUNDARY CONDITIONS
1. To take into account the rest of the vessel
2. To avoid rigid body motion
3. To crimp the coil
AAA sections constrained along the
longitudinal direction
AAA lateral surface constrained along the
circumferential direction
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
Hexahedral elements (each with 8 nodes) were
chosen to mesh all the structures of the model.
Therefore the elements were 16607 in all:
1
2
• 723 for the coil
• 12464 for the thrombus (2)
• 3420 for the wall (1)
RESULTS
Unfolding
Von Misesofstresses
the coil
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
CONCLUSIONS
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
LATE DEVELOPMENTS
• Development of different
geometries for the prototype
• Implementation of complex models
for the thrombus without axial simmetry
• Tests on biological samples
of AAA
• Analysis of the behaviour of two or
more coils
• Construction of an attacching
system for the prototype
• Different approaches to the
computational problem
The End
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
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%
25%
25%
Decede in breve
Sopravvive
tempo (prima di
Non
raggiungere un’Unità
sopravvive
di Pronto Intervento) alla chirurgia
d’emergenza
[Yano, 2000]
La Patologia
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
Artereosclerosi
ed ipertensione
Età
Razza
Traumi alla parete
vasale e infezioni
Fattori
genetici
Fumo
Carenza di
collagene
e\o elastina
Alterazioni dei sistemi di
rilascio di ossigeno e
nutrimenti alla parete
La Patologia
Le principali tecniche di imaging si differenziano per: qualità, costo, tempi di
acquisizione. Quelle maggiormente utilizzate sono:
 ULTRASUONOGRAFIA
 AORTOGRAFIA
 RISONANZA MAGNETICA (MRI)
 TOMOGRAFIA COMPUTERIZZATA (CT)
 HELICAL CT
 Vantaggi: costo ridotto, non
largamente
diffusa.
 Vantaggi: non invasiva,invasiva,
stima
delle dimensioni
buona
Vantaggi:
identifica
disturbi renodell’aneurisma, localizzavascolari
leSvantaggi:
estensioni
prossimali
dell’aneurisma.
e vasi
anomali.
non
adattadiper
 Vantaggi:
non
invasiva,
tempi
acquisizione
Vantaggi: assenza di radiazioni, non
invasiva.
pazienti
obesi,
poco
oggettiva.
 Svantaggi: utilizzo
di radiazioni,
costiCosti
elevati,
scarse
informazioni
 Svantaggi:
elevati,
invasività,
ridotti.
Svantaggi:
costi elevati,
artefatti di movimento,
circa l’anatomia dell’arteria.
tolleranza
del paziente.
disponibilità
Svantaggi: utilizzo
di
radiazioni,
costidel
elevati,
(SW e HW), claustrofobia
paziente.
tecnologia.
Lo Studio Computazionale
L’INTERAZIONE DI CONTATTO PLACCA/SPIRA
 Contatto fra le due superfici gestito da ABAQUS® tramite l’algoritmo master-slave
 Modello di contatto: soft esponenziale
Placca
Spira