POLITECNICO DI MILANO Facoltà di Ingegneria dei Sistemi Corso di Laurea in Ingegneria Biomedica

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. 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 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)

3. 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. ANEURYSM SANE AORTA Pathology PATHOLOGY Aneuryms are permanent and localized dilatation of an artery.

4. 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.

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

6. CURRENT SURGICAL TECHNIQUES ANEURSMECTOMY: 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: Technique (bifurcated stent): (1) Catheter insertion (2) Stent release Surgical Techniques It is the substitution of the aneurismatic piece with a vascular prosthesis It consists in the insertion of a stent-graft through one or two small incision(-s) in the femoral artery(-ies).

7. DRAWBACKS & COMPLICATIONS ANEURISMECTOMY • General anaesthesia • Large incision • Hypothermy • Damages at the aneurysm necks (due to clamping procedure) • Respiratory problems • Significant blood and fluid loss Surgical Techniques 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

8. Distinctive characteristics Occlusion of the collateral arteries and prevention of type II endoleaks Working mechanism Occlusion of leaks through emergency catheterization The Endoliner ENDOLINER®: A NEW CONCEPT OF ENDOVASCULAR PROSTHESIS • Thedurability of the construction of an Endoliner® is not necessarily a prerequisite. • Itsstructure 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 • As an emergency treatment of ruptured aneurysms

9. POSSIBLE GEOMETRIC CONFIGURATIONS Net Zigzag Spiral The Endoliner

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

11. For the realization of set-up different considerations were taken into account 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 The Experimental Analysis THE EXPERIMENTAL SET-UP Transparency MRI proof Sterility Variability in lenght

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

13. 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) Afterwards water-proof silk was hand sealed all around the structure. The Experimental Analysis the NiTi band was wound around it to procede with the heat training

14. 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.

15. By looking at the pressure characteristics it is possible to observe that: The prototype is not able to preserve the wall from high pressures Test Parameters Freezingeffect on the patient condition The Endoliner® can be an effective by-pass usable to contain the rupture 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. • In the case “with Endoliner®” pressures are a few inferior (1-2 mmHg) • Cardiacrate: 1.25 Hz (75 bpm) • Sampling: 128 samples per period (8 period) • 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 RESULTS

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

17. 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 Software The Computational Study • Geometrical approximation • The pre-load due to the blood pressure was not considered • 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. GEOMETRICAL MODEL Reduced model It is possible to consider only one coil The Computational Study • 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 • Section: 0.17 x 0.9 mm

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

20. 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 BOUNDARY CONDITIONS Releasing ANALYSIS STEPS “SMA” The Computational Study 1. To take into account the rest of the vessel 2. To avoid rigid body motion 3. To crimp the coil Crimping

21. MESHING THE MODELS 1 2 • 12464 for the thrombus (2) • 3420 for the wall (1) The Computational Study 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

22. RESULTS Unfolding of the coil Von Mises stresses

23. DISCUSSION The higher stress acting on the wall and due to the coil is about 0.04 MPa The single coil gives a very small contribute to the risk of rupture • Peak stress for an AAA [Fillinger, 2002] = 0.4 MPa • The nodal displacements are notelevate: 0.2 mm ( DSF = 10 ) The Computational Study It is 10 times less than the stress due to the pre-load only (0.3 MPa) • 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 • They can be comparated to the ones due to the pre-load only (0.14 mm)

24. C O N C L U S I O N S L A T E D E V E L O P M E N T S 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 • 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

25. The End

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

27. PRINCIPALI FATTORI DI RISCHIO Artereosclerosi ed ipertensione Carenza di collagene e\o elastina Età Razza Fumo Traumi alla parete vasale e infezioni Alterazioni dei sistemi di rilascio di ossigeno e nutrimenti alla parete Fattori genetici 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.

28. Vantaggi: costo ridotto, non invasiva, largamente diffusa. • Svantaggi: non adatta per pazienti obesi, poco oggettiva. • 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. • Vantaggi: identifica disturbi reno-vascolari e vasi anomali. • Svantaggi: Costi elevati, invasività, tolleranza del paziente. • Vantaggi: assenza di radiazioni, non invasiva. • Svantaggi: costi elevati, artefatti di movimento, disponibilità (SW e HW), claustrofobia del paziente. 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

29. Placca Spira 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