politecnico di milano facoltà di ingegneria dei sistemi corso di laurea in ingegneria biomedica...

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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 TREATMENT NEW ENDOVASCULAR PROSTHESIS FOR THE TREATMENT OF ABDOMINAL AORTIC ANEURYSMS OF ABDOMINAL AORTIC ANEURYSMS Supervisors: Prof. Gabriele DUBINI Prof. Frans N. Van de VOSSE MSc Thesis: Salvatore Luca FICCO

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Page 1: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 2: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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.

Page 3: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 4: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 5: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 6: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 7: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 8: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 9: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

POSSIBLE GEOMETRIC CONFIGURATIONS

Zigzag SpiralNet

The EndolinerThe Endoliner

Page 10: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 11: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 12: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

The Experimental AnalysisThe Experimental Analysis

TECHNICAL CHARACTERISTICS

Modular structure Electric motorVolumetric pump

Electric motor

Volumetric pump

No-return valve

Valve

Variable resistence

Control

Resistence

Page 13: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 14: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 15: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 16: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 17: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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 ]

Page 18: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 19: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 20: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 21: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

• 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

Page 22: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

Von Mises stresses Unfolding of the coil

RESULTS

Page 23: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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)

Page 24: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 25: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

The EndThe End

Page 26: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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]

Page 27: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 28: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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

Page 29: POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica TECHNISCHE UNIVERSITEIT EINDHOVEN Faculty of Biomedical

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