LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS www.labsmech.polimi.it FLUID MECHANICAL PERTURBATIONS INDUCED BY STENT IMPLANTATION: A NUMERICAL STUDY Rossella Balossino, Francesca Gervaso, Francesco Migliavacca, Gabriele Dubini LaBS, Department of Structural Engineering, Politecnico di Milano, ITALY
INTRODUCTION 2
INTRODUCTION 3 A vascular stent is a small metal tube, which is inserted into an artery at the site of a narrowing to act as an internal scaffolding or a support to the blood vessel.
MOTIVATION 4 IN-STENT RISTENOSIS Intimal thickening following a stent implantation with progressive lumen reduction [Mehran R., 2002] Three phases (Edelman e Rogers, 1998): INFLAMMATION during implantation + PROLIFERATION first 3 weeks + REMODELING 10/12 months HYPOTHESIS: non physiological stress state field responsible for restenosis.
STATE OF THE ART 5 QUANTITIES OF INTEREST Effect of wire spacing, wire diameter, vessel diameter and flow conditions Stent design: number, thickness and width of the strut Deployment ratio Comparison of resting or maximal vasodilatation condition Foreshortening Changes in vascular geometry after stent deployment [Moore et al.,2002] [LaDisa et al.,2003-2004-2005] Effect of vessel curvature [Seo et al., 2005] Non-Newtonian condition [Soulis et al.,2002; Seo et al.,2005; Bernard et al.,2004] QUANTITATIVELY OBSERVED PARAMETERS wall shear stress (WSS) distribution velocity vectors recirculation length velocity profiles
THE PROBLEM 6 FROM SIMPLIFIED MODELS TO PLAQUE MODEL Migliavacca et al., Proceedings of 2005 Summer Bioengineernig ASME Conference Healthy artery Artery with plaque Expansion under displacement control until a diameter of 3 mm was reached The stent geometry was modelled as shell elements Cordis BX Velocity (Johnson & Johnson Interventional System, Warren, NJ, USA)
METHODS 7 1. Preliminary step: structural analysis This step is necessary to obtain the correct configuration for the fluid dynamics simulations: fluid domain
METHODS 8 First step: creation of the fluid domain 1 Point cloud of the deformed configuration 2 Creation of the curves and surfaces 3 Creation of each volume 4 Substraction and creation of the final fluid domain
METHODS 9 Second step: Boundary conditions OUTLET Constant fixed pressure ASSUMPTION: - rigid vessel wall - Newtonian fluid: Viscosity = 0.0035 kg/(m s) Density = 1060 kg/m 3 0.2 0.16 0.12 LaDisa et al. (2005) WALL [m/s] 0.08 0.04 No slip condition 0 0 0.1 0.2 0.3 0.4 0.5 0.6 Time [s] 4 cardiac cycles pulse period = 0.54 s INLET Velocity profile: parabolic and transient Fluent (Fluent Inc., Lebanon, NH, USA)
OBSERVATIONS 10 STENTED REGION 0.16 s dynes/cm 2 HEALTHY MODEL 50 PLAQUE MODEL 25 0 The highest WSS magnitude can be noticed on the stent
OBSERVATIONS 11 ARTERIAL REGION INSIDE STENT STRUTS 0.16 s dynes/cm 2 18 9 0 HEALTHY MODEL PLAQUE MODEL high WSS in the regions between the stent struts low WSS were localized around stent struts
AIM OF THE STUDY 12?? Is it correct to ignore the presence of an atherosclerotic plaque four different stent designs previously expanded against the same stented artery Cordis BX Velocity stent like (Johnson & Johnson Interventional System, Warren, NJ, USA) Jostent Flex stent like (JOMED AB, Helsingborg, Sweden) Sorin Carbostent stent like (Sorin Biomedica S.p.A., Saluggia (VC), Italy) Palmaz-Schatz stent like (Johnson & Johnson Interventional System, Warren, NJ, USA) transient simulation for each model comparison of the WSS magnitude distribution during time
STENT MODELS 13 CORDIS JOSTENT RADIUS a fter exp a nsio n LENGTH a fter exp a nsio n THICKNESS CORDIS 1.5 3.53 0.1 JOSTENT 1.5 2.30 0.1 PALMAZ 1.5 2.97 0.1 SORIN 1.55 3.50 0.1 SORIN PALMAZ Length: 11.68 mm Internal diameter: 2.15 mm Thickness: 0.5 mm Length: 3.68 mm Internal diameter: 1.25 mm Thickness: 0.45 mm
% of cells RESULTS: WALL SHEAR STRESSES 14 WSS < 5 dynes/cm 2 correlated with sites of intima thickening and smooth muscle cells migration locations where stagnation of blood occurs prone to thrombus formation and platelet accumulation 100 95 0.16 s 0.32 s 0.44 s 90 0 s 0.4 s 85 0 s 0.16 s 0.32 s 0.4 s 0.44 s
% of cells RESULTS: LOW WSS 15 CORDIS WSS < 5 dynes/cm 2 SORIN 0 s 100 95 90 85 0 s 0.16 s 0.32 s 0.4 s 0.44 s JOSTENT [dynes/cm 2 ] 5 2.5 0 PALMAZ
% of cells RESULTS: LOW WSS 16 CORDIS WSS < 5 dynes/cm 2 0.16 s SORIN 100 95 90 85 0 s 0.16 s 0.32 s 0.4 s 0.44 s JOSTENT [dynes/cm 2 ] 5 2.5 0 PALMAZ
RESULTS: MAXIMUM WSS ON STENT 17 [dynes/cm 2 ] 50 40 30 20 10 0 0 s 0.16 s 0.32 s 0.4 s 0.44 s CORDIS PALMAZ JOSTENT SORIN
RESULTS: MAXIMUM WSS 18 [dynes/cm 2 ] 50 40 30 20 10 0 0.16 s
RESULTS: MAXIMUM WSS ON THE ARTERIAL WALL 19 [dynes/cm 2 ] 20 15 10 5 0 0 s 0.16 s 0.32 s 0.4 s 0.44 s CORDIS PALMAZ JOSTENT SORIN
LIMITATIONS AND ASSUMPTIONS 20 Rigid wall: valid in the stented region Newtonian fluid Straight vessel: neglecting the curvature of the coronary artery Post implant condition Single strut Symmetric and hyperelastic plaque
WORKS IN PROGRESS 21 Rigid wall: valid in the stented region Newtonian fluid Straight vessel: neglecting the curvature of the coronary artery Post implant condition Single strut Symmetric and hyperelastic plaque 2 2 n Carreau model: 1 [Seo et al., 2005] 0 1 S
WORKS IN PROGRESS 22 Rigid wall: valid in the stented region Newtonian fluid Straight vessel: neglecting the curvature of the coronary artery Post implant condition Single strut Symmetric and hyperelastic plaque Influence of the stent length:
WORKS IN PROGRESS 23 Influence of the strut thickness: comparison of different stent design with same thickness 0.15 mm 0.1 mm CORDIS JOSTENT SORIN PALMAZ
CONCLUSIONS 24 In each stent model the WSS distribution is similar: the maximum values are located over the stent strut the arterial wall portion delimited by the links and the stent strut showed an increasing WSS value from the zones near the stent to the centre WSS values change during the cardiac cycle, showing an oscillatory behaviour The comparison among the four stent models indicates that: Jostent shows the lowest WSS value during the whole cardiac cycle the best model in terms of minimal neointima thickening is the Cordis stent the maximum WSS on the stent and the arterial wall occurs in the Cordis stent at the systolic peak CFD techniques have the advantages of producing accurate information on local flow variables very close to the arterial wall CFD can thus provide a research tool by complementing experimental studies, especially where experimental measurements are difficult to perform and affected by uncertainties.
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