Nitinol biomedical devices: Design analysis

Transcription

1 Biomedical Engineering Nitinol biomedical devices: Design analysis M. Conti Dipartimento di Meccanica Strutturale, Università degli Studi di Pavia, Italy Università degli Studi di Pavia - Structural Mechanics Department 1

2 Summary Nitinol biomedical devices SMA and biomedical applications: why? Carotid artery stenting: an example of SMA application 2

3 SMA and medical application: why? (1) External stimuli (load, temperature) Minimally invasive intervention SMArt devices Superelastic Effect (SE) Or Shape Memory Effect (SME) Innovative MEDICAL devices Stent: a common SMA medical application T SME SE 3

4 SMA and medical application: why? (2) Minimally invasive intervention: Percutaneous operation Less trauma than invasive traditional surgery Care cost reduction Treatment of high risk patients Use of miniaturized devices: Low profile Capability to expand/change shape Remote-control manipulation of instruments Actuation Indirect observation of the surgical field (endoscope or similar device) capability to approach tortuous anatomy Coronary stenting: a classical example of minimally invasive approach Shape Memory Alloy based devices address many of these issues 4

5 Design of medical device (Generic consideration) Safety Material Biocompatibility Medical Device Design Required mechanical properties Superficial treatments/ Coating Radiopacity/ Visibility Fatigue Geometrical features 5

6 Summary Nitinol biomedical devices SMA and biomedical applications: why? Carotid artery stenting: an example of SMA application 6

7 Carotid Artery Carotid arteries (CA) Extremely important function: Supply brain Neck region: several degree of freedom 7

8 Carotid Stenosis and its treatments CAROTID STENOSIS Atherosclerotic plaque Risks Blood flow blockage Thrombo-emboli STROKE Chronic endothelial injury Healthy vessel Atherosclerotic vessel 8

9 Carotid Stenosis and its treatments CAROTID STENOSIS Atherosclerotic plaque Risks Blood flow blockage Thrombo-emboli STROKE 2 clinical approaches Surgical carotid Endarterectomy (CEA) Carotid artery Stenting (CAS) 9

10 Carotid endarterectomy (CEA), the surgical approach CEA Surgical invasive procedure Clamping CA Artery incision Peeling out the plaque Prevent thromboembolic strokes Many drawbacks related to surgical operation in high risk patients 10

11 CAS: a minimally invasive approach with advantages CAS VESSEL DILATION and STENT IMPLANT (as in coronary arteries) ADVANTAGES NON-INVASIVE technique Treatment of HIGH RISK PATIENTS Before After 11

12 Carotid stent design (specific consideration) Low Profile During the procedure Trackability Radiopacity System flexibility Courtesy of Tedesco Contourability* Stent retension Fatigue resistance * Ability to respect the original anatomy of the vessel 12

13 Carotid stents: current designs Laser cut nitinol stents Crush resistance Braided wirestent 13

14 CAS: a minimally invasive approach with also disadvantages CAS VESSEL DILATION and STENT IMPLANT (as in coronary arteries) ADVANTAGES NON-INVASIVE technique Treatment of HIGH RISK PATIENTS DISADVANTAGES DEBRIS FORMATION DURING PROCEDURE EMBOLIC PROTECTION DEVICES EPD Stent fracture In stent restenosis 14

15 Embolic protection devices (EPDs): an adjunction to CAS EPDs: 3 Main Families BALLOON OCCLUSION SYSTEMS (A) Low profile design Occlusion ICA blood flow A FILTER DEVICES (B) Allow Filtration and Blood Flow Bad Positioning/ Tortuous CA anatomy B RETROGRADE FLOW DEVICES (C) Low Rate of Intraprocedural Emboli Patient Intolerance to Balloon Occlusion C 15

16 Carotid stenting: many device designs available on the market SPIDERx (EV3) AccuNET Rx (Guidant) Emboshield (Abbott) Several Stent designs FilterWire (Boston Scientific) Angioguard (J&J) Trap NFS (Microvena)? Interceptor (Medtronic) Rubicon (Rubicon-medica) 16

17 Carotid artery stenting: a technique requiring improvements Stent and filter design has effect on: FILTER STENT Ability to cross tortuous stenosis/flexibility Contour-ability Filtering performance Easy retrieval/no post debris release Vessel injury Flow dynamics Position above carotid bifurcation In stent restenosis and stenting failure Clinical studies + ENGINEERING TOOLS: Experiments Simulations CFD FEA 17

18 Angioguard XP: the object of our investigation Filter system: inch guidewire (a) Nitinol basket frame (b) Markers (c) Porous polymeric membrane (d) d c a b Free to move along the guide wire Fixed 18

19 Angioguard XP: EXPERIMENTAL investigation Experiments: Filter Filter Flow Dynamics Pressure drop measurement Various flow rate Inputs for CFD 19

20 Angioguard XP: EXPERIMENTAL investigation X-ray tube Experiments: X-ray detector Micro-CT scan Expanded Filter Filter within catheter Filter deployment Accurate geometry and validation for FEA Object manipulator Piezo positioning 20

21 Angioguard XP: Finite Element Analysis Geometrical Tool Virtual design loop Traditional approach Parametric adaptable CAD model Alternative approach pyformex Parametric adaptable mesh Mesh generation FEA FEA ADVANTAGES: - two steps instead of three - can be fully automated 21

22 Angioguard XP: Finite Element Analysis Geometry (1) y=f_2(x) r 2 L 2 L 1 r 1 x y=f_1(x) y L 2 L 1 r 1 r 2 r 2 REAL GEOMETRY BY MicroCT SCAN SINGLE STRUT GEOMETRY (PYFORMEX) CENTER LINES OF STRUTS 22

23 Angioguard XP: Finite Element Analysis Geometry (2) Wire cross section definition Sweeping section along a path (one filter strut) Geometrical manipulations pyformex Real device Parametric model 23

24 Angioguard XP: Finite Element Analysis Geometry (3) Model Parameters Filter diameter Filter length Strut shape Strut section diameter Strut distribution Number of struts Membrane coverage Marker position... Filter diam:4 mm 60% covered 6 struts 70% covered Filter diam:6 mm 80% covered Filter diam:8 mm 10 struts 24

25 Angioguard XP: Finite Element Analysis Materials and Methods Finite Element Solver ABAQUS EXP 6.8 Material properties Struts C3DR VUMAT Pelton et al. (2000) Min. Invas. Ther. & Allied Tech. Membrane M3D3 or M3D4R Superelastic Nitinol Nylon DeBeule et al. (2008) J Biomech Marker C3D8R Tantalum 25

26 Nitinol superelasticity modeling User subroutine in the finite element solver (Abaqus) Pelton et al (experimental data of nitinol wires) Identification of Model parameters 26

27 Angioguard XP: Finite Element Analysis Model validation FILTER DEPLOYING OUT THE DELIVERY SHEATH Micro-CT VS Finite Element Analysis Micro-CT GOOD QUALITATIVE Simulation AGREEMENT 27

28 Angioguard XP: FEA Vessel/Filter interaction Simulation 2 steps Filter Insertion in the catheter + Vessel pressuration and prestretch Filter deployment 28

29 Angioguard XP: FEA Vessel/Filter interaction Vessel: Hyperelastic material Lally et al. (2005) Hypothesis: Pressure acting on membrane == Pressure drop Radial Stiffness Filter << Radial stiffness Vessel Vessel == Rigid surface GAP Finol et al. JEVT

30 Angioguard XP: FEA Effect of vessel asymmetry Clinical investigation: tid%20artery%20stenosis.pdf Numerical results of filter/vessel wall apposition. Gap [% of vessel lumen] SIMULATION: 4mm size filter deployed in a 3mm circular vessel (on the left) and in 3 mm vessel having 0.75 ovality (on the right). Filter size [mm] Ovality Vessel shape influence the filte/vessel wall apposition 30

31 FEA of nitinol carotid stent (Whei et al. 2007) Modified design: shorter struts 31

32 FEA of nitinol carotid stent (Whei et al. 2007) Impact of design on stent apposition Impact of design on stent performance 32

33 Conclusions + further scenarios Guidewire Conclusion Shape memory alloys have a great prospective in clinical applications Engineering tools support design improvements and clinical outcome improvements Tortuous vessel Further scenarios More accurate material modeling Wirestent vs Nitinol lasercut stent Patient specific models Simulation of complete carotid stenting procedure (STENT + FILTER) Stent Filter 33

34 THANKS More info: Acknowledgments : Matthieu De Beule 1, Peter Mortier 1, Denis Van Loo 2, Frank Vermassen 3 1 IBiTech, Institute Biomedical Technology, Ghent University, Ghent, Belgium 2 UGCT, Ghent University, Ghent Belgium 3 Department of Vascular Surgery, Ghent University Hospital, Ghent, Belgium 34