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LABORATORY OF BIOLOGICAL STRUCTURE MECHANICS 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

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Balossino R. MPF INTRODUCTION

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Balossino R. MPF 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. INTRODUCTION

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Balossino R. MPF IN-STENT RISTENOSIS Intimal thickening following a stent implantation with progressive lumen reduction HYPOTHESIS: non physiological stress state field responsible for restenosis. Three phases (Edelman e Rogers, 1998): + REMODELING 10/12 months + PROLIFERATION first 3 weeks INFLAMMATION during implantation MOTIVATION [Mehran R., 2002]

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Balossino R. MPF STATE OF THE ART Effect of wire spacing, wire diameter, vessel diameter and flow conditions [Moore et al.,2002] Stent design: number, thickness and width of the strut Deployment ratio Comparison of resting or maximal vasodilatation condition [LaDisa et al., ] Foreshortening Changes in vascular geometry after stent deployment 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 QUANTITIES OF INTEREST

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Balossino R. MPF THE PROBLEM Expansion under displacement control until a diameter of 3 mm was reached The stent geometry was modelled as shell elements FROM SIMPLIFIED MODELS … TO PLAQUE MODEL Healthy arteryArtery with plaque Migliavacca et al., Proceedings of 2005 Summer Bioengineernig ASME Conference Cordis BX Velocity (Johnson & Johnson Interventional System, Warren, NJ, USA)

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Balossino R. MPF This step is necessary to obtain the correct configuration for the fluid dynamics simulations: fluid domain METHODS 1.Preliminary step: structural analysis

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Balossino R. MPF First step: creation of the fluid domain METHODS Point cloud of the deformed configuration Creation of the curves and surfaces Creation of each volume Substraction and creation of the final fluid domain

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Balossino R. MPF Second step: Boundary conditions METHODS 4 cardiac cycles pulse period = 0.54 s INLET OUTLET WALL Velocity profile: parabolic and transient Constant fixed pressure No slip condition ASSUMPTION:- rigid vessel wall - Newtonian fluid: Viscosity = kg/(ms) Density = 1060 kg/m 3 Fluent (Fluent Inc., Lebanon, NH, USA) LaDisa et al. (2005)

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Balossino R. MPF s dynes/cm 2 OBSERVATIONS STENTED REGION The highest WSS magnitude can be noticed on the stent HEALTHY MODEL PLAQUE MODEL

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Balossino R. MPF dynes/cm s OBSERVATIONS ARTERIAL REGION INSIDE STENT STRUTS high WSS in the regions between the stent struts low WSS were localized around stent struts HEALTHY MODELPLAQUE MODEL

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Balossino R. MPF AIM OF THE STUDY 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

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Balossino R. MPF CORDISJOSTENT SORINPALMAZ STENT MODELS RADIUS afterexpansion LENGTH afterexpansion THICKNESS CORDIS JOSTENT PALMAZ SORIN Length: mm Internal diameter: 2.15 mm Thickness: 0.5 mm Length: 3.68 mm Internal diameter: 1.25 mm Thickness: 0.45 mm

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Balossino R. MPF 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 RESULTS: WALL SHEAR STRESSES 0 s0.16 s0.32 s0.4 s0.44 s % of cells 0 s 0.16 s 0.32 s 0.4 s 0.44 s

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Balossino R. MPF CORDIS JOSTENT SORIN PALMAZ [dynes/cm 2 ] RESULTS: LOW WSS 0 s WSS < 5 dynes/cm 2 0 s 0.16 s 0.32 s 0.4 s 0.44 s % of cells

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Balossino R. MPF RESULTS: LOW WSS WSS < 5 dynes/cm 2 0 s 0.16 s 0.32 s 0.4 s 0.44 s % of cells [dynes/cm 2 ] CORDIS JOSTENT SORIN PALMAZ 0.16 s

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Balossino R. MPF CORDIS JOSTENT SORIN PALMAZ s0.16 s0.32 s0.4 s0.44 s [dynes/cm 2 ] RESULTS: MAXIMUM WSS ON STENT

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Balossino R. MPF [dynes/cm 2 ] s RESULTS: MAXIMUM WSS

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Balossino R. MPF CORDIS JOSTENT SORIN PALMAZ 0 s0.16 s0.32 s0.4 s0.44 s [dynes/cm 2 ] RESULTS: MAXIMUM WSS ON THE ARTERIAL WALL

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Balossino R. MPF LIMITATIONS AND ASSUMPTIONS 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

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Balossino R. MPF 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 Carreau model: [Seo et al., 2005]

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Balossino R. MPF 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 Influence of the stent length:

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Balossino R. MPF WORKS IN PROGRESS Influence of the strut thickness: comparison of different stent design with same thickness CORDISJOSTENT 0.15 mm SORINPALMAZ 0.1 mm

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Balossino R. MPF 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. CONCLUSIONS

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Balossino R. MPF THANK YOU

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