Medical Engineering and Physics 37 (2015) 840–844

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Influence of proximal drug eluting stent (DES) on distal bare metal stent (BMS) in multi-stent implantation strategies in coronary arteries Anqiang Sun1, Zhenze Wang1, Zhenmin Fan, Xiaopeng Tian, Fan Zhan, Xiaoyan Deng, Xiao Liu2,∗ Key Laboratory for Biomechanics and Mechanobiology of Ministry of Education, School of Biological Science and Medical Engineering, Beihang University, Beijing, China

a r t i c l e

i n f o

Article history: Received 7 November 2014 Revised 23 March 2015 Accepted 26 May 2015

Keywords: Drug eluting stent Bare metal stent Diffuse lesion Restenosis

a b s t r a c t The aim of this study was to investigate the drug distribution in arteries treated with DES-BMS stenting strategy and to analyze the influence of proximal DES on distal segments of BMS. A straight artery model (Straight Model) and a branching artery model (Branching Model) were constructed in this study. In each model, the DES was implanted at the proximal position and the BMS was implanted distally. Hemodynamic environments, drug delivery and distribution features were simulated and analyzed in each model. The results showed that blood flow would contribute to non-uniform drug distribution in arteries. In the Straight Model the proximal DES would cause drug concentration in BMS segments. While in the Branching Model the DES in the main artery has slight influence on the BMS segments in the branch artery. In conclusion, due to the blood flow washing effect the uniformly released drug from DES would distribute focally and distally. The proximal DES would have greater influence on the distal BMS in straight artery than that in branching artery. This preliminary study would provide good reference for atherosclerosis treatment, especially for some complex cases, like coronary branching stenting. © 2015 IPEM. Published by Elsevier Ltd. All rights reserved.

1. Introduction 1.1. Background and problem Drug eluting stents (DES) supports the narrow artery to its original section and gradually releases certain pharmacologic agent (drug) to interfere restenosis. DES have demonstrated excellent effects in prevention of angiographic restenosis and therefore brought percutaneous coronary intervention (PCI) to a new stage [1,2]. Currently, commercially available DES can reduce restenosis rate to approximately 15–30%, compared to 30–60% after balloon dilatation [2–4]. However the DES is still far from perfect. Although the restenosis rate was successfully reduced by the introduction of the DES, it is not completely diminished [4]. Stenting treatment of stenosed coronary would result in arterial injuries including severe damage of the endothelium and initiate a complex cascade of inflammatory processes, which may lead to the development of in-stent restenosis (ISR). Many



1 2

Corresponding author. Tel.: +86 10 82339962. E-mail address: [email protected] (X. Liu). These authors contributed equally to this work. Present address: Beihang University, Xueyuan Road 37#, Beijing 100191, China.

http://dx.doi.org/10.1016/j.medengphy.2015.05.016 1350-4533/© 2015 IPEM. Published by Elsevier Ltd. All rights reserved.

clinical and biological factors involved in the progression of restenotic lesions have been studied over the past few years. But the mystery behind the pathophysiological mechanisms is still unresolved [5–8]. Many studies have reported that postoperative problems after DES intervention, like restenosis and endothelial dysfunction, were likely to be focal [9]. But the problematic locations after stenting in specific cases were depended. Pedro A. Lemos and colleagues reported that after sirolimus-eluting stents (SES) intervention, edge restenosis occurred more frequently in the proximal than in the distal stent border [10]. However Shin et al. got opposite results that SES implantation may induce significant impairment of the endothelium-dependent vasomotor function in the distal and far distal portions of the treated vessel [11]. 1.2. Hypothesis The reasons for those focal problems are to be clarified. The authors hypothesized that flow induced non-uniformly distributed drugs (locally low dose or over dose) would conduct insufficient effect or impairment to local artery tissues. Although drug releases evenly from the stent, the drug deposition will be non-uniform due to the blood flow interference. The mechanic effect of longitudinal flow will deliver drugs to the distal segments or the distal branching

A. Sun et al. / Medical Engineering and Physics 37 (2015) 840–844

arteries [12]. This distally delivering effect (DDE) may cause drug distribution non-uniform, either insufficient or overdoes in arterial segments. But for diffuse lesions the DDE would become beneficial. Obata et al. [13] reported that less luminal lumen loss, greater minimal lumen area and less in-stent neointimal hyperplasia happened in BMS segments distal to a DES in DES–BMS group when compared with BMS–BMS group. This result was likely related to DDE of blood flow. Some numerical studies have been carried out to investigate hemodynamic features and drug delivery patterns at coronary bifurcations. For example, Demosthenes et al. investigated flow patterns at stented coronary bifurcations using ideal geometries models [37]; Chiastra et al. simulated hemodynamics of image-based stented coronary bifurcation [38]; Cutrì et al. investigated drug delivery patterns for different stenting techniques in coronary bifurcations [39]. However, to the authors’ knowledge, no study has focused on analyzing drug delivery features of DES–BMS stenting strategy previously.

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Fig. 1. The Straight Model (up) and the Branching Model (bottom).

1.3. Objective

2.2. Assumptions

The objective of this study was to investigate the drug distribution in arteries treated with DES-BMS stenting strategy. We then numerically simulated drug delivery features of DES–BMS strategy in two typical models, namely the Straight Model and the Branching Model.

In the present study, blood was assumed to be homogeneous, incompressible Newtonian fluid [15–17]. Blood viscosity μ = 3.5 × 10-3 kg/m s and density ρ = 1050 kg/m3 .

1.4. Advantages and clinical significance The purpose of this study was to investigate the drug distribution in arteries treated with DES-BMS stenting strategy, particularly to investigate the influence of proximal DES on distal segments of BMS. The results would provide good reference for atherosclerosis treatment, especially for some complex cases, like diffuse lesions treatment and coronary branching stenting.

2. Methods 2.1. Models Two simplified 3D models were constructed using computeraided design software, SolidWorks, namely the Straight Model and the Branching Model. Both the two models were idealistic and gave the general features of stenting in straight and branching arteries. The two models represented the blood domains in the arteries after stenting. The stents in the two models were simplified and resembled general geometries rather than specific ones. All stent struts were supposed to be fixed on the arterial inner wall (outer wall of the two models for computation). The diameter of the main vessels in two models is 3mm, referring to the typical diameter of the left anterior descending coronary [14]. The side vessel in Branching Model was 2.5 mm in diameter. Two stents were deployed into each model. In the Straight Model, two stents (S1, S2) were implanted adjacently, with every stent 12 mm in length. In the Branching Model, one stent (B1) with 12 mm in length and 3 mm in diameter was implanted in the main vessel and the other stent (B2) with 9 mm in length and 2.5 mm in diameter was implanted distally to the branching. All stents in the two models had 0.15 mm × 0.15mm cross-sections. Both the main vessel and the side branching vessel were extended axially to ensure sufficient length for exit flow to be stabilized (Fig. 1). To quantitatively analyze the influence of DES on distal BMS, series of areas in the stenting segment were defined. In the Straight Model, series of faces along the artery were selected, including 6 faces (S11 to S1-6) in the DES and 6 faces (S2-1 to S2-6) in the BMS. In the Branching Model, also series of faces were selected, including 6 faces (B1-1 toB1-6) in the DES and 6 faces (B2-1 to B2-6) in the BMS.

2.3. Governing equations The numerical simulations were based on the three-dimensional incompressible Navier-Stokes equations:

   ρ u · ∇ u + ∇ p − μ u = 0 

∇ ·u=0

(1) (2)

and the mass transport equation:

∂ C   + u · ∇ C = D∇ 2C ∂t

(3)



where u and p represent fluid velocity vector and the pressure respectively. C is the concentration of the drug. D is the diffusion coefficient of the drug (sirolimus) in blood flow. 2.4. Boundary conditions for blood flow simulation (1) Inlet: Uniform inflow velocity profile for the axial velocity component and a zero transverse velocity component were used in the numerical simulation [40]. According to the study by Ofili et al. [14], we chose 273 as Reynolds number (Re) in the present simulation, and hence the inflow velocity applied at the inlet was 0.3 m/s. (2) Outlet: Outflow flow condition was used at the outlets of two models. For Branching Model, the flow ratio through the side branch was estimated as follows [18]:

qD2 = qD1



dD2 dD1

2.27

(4)

with qD1 and qD2 the flow rate through, dD1 and dD2 the diameters of the branches. Then the flow ratio through the main vessel and the side branching vessel was 3:2. (3) The vessel wall was assumed to be rigid and non-slip [19,20]. 2.5. Boundary conditions for drug transport simulation As drug deposition occurs less via contact between drug coating and the arterial wall than via flow-mediated deposition [21], we only simulated the drug transport in the flowing blood in the present study.

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(1) The drug concentration in the flowing blood at the inlet was set at 0. (2) The mass flux of drug at the arterial wall and at the outlet was set at 0. (3) The diffusion coefficient (D) of the drug in blood was assumed to be 10−7 m2 /s [22]. (4) The drug release rate Q¯ was assumed to be uniform with a rate of 1.05 × 10−10 kg/(m2 s) along the stent, which was calculated by taking the derivative of the release profile of sirolimus against time [22]. (5) Only the proximal stents in both models (stents S1 and B1) were assumed to be drug eluting stent, while the distal stents (stents S2 and B2) were assumed to be bare mental stent (boundary conditions as wall). 2.6. Meshing

Fig. 3. Drug concentration near the walls of Straight Model and Branching Model.

Meshes were generated using ICEM (ANSYS, Inc. Canonsburg, PA). The surfaces of the models were meshed using a mixture of tetrahedral and hexahedral volume meshes. The nodes number of Straight Model and Branching Model were 679,619 and 590,057 respectively. Refined meshes were used near the walls of models. 2.7. Numerical solutions Finite volume method was employed in the simulation, using commercial software Fluent 14.0 (ANSYS, Inc. Canonsburg, PA). Discretization of the pressure and momentum at each control volume was in a second-order scheme. The iterative process of computation was terminated when the residual of continuity and velocity were all less than the convergence criterion, 1.0 × 10−5 . 3. Results Due to the interference of stent struts to the blood flow, wall shear stress (WSS) on the artery wall was decreased, especially at areas distal to struts. Fig. 2 shows WSS distribution contours in two models. In both two models WSS in the stent areas was relatively lower, compared with non-stented vessel tubes adjacent. This WSS distribution pattern coincided with many previous hemodynamic studies on stent [23–25]. Drug distribution was obviously uneven along the flow direction (Fig. 3). In the DES areas (both in the Straight Model and Branching Model), drug concentration was relatively low in the stent heading areas and relatively high in the stent ending areas. Besides, in the

Fig. 2. WSS distribution on the wall of Straight Model and Branching Model.

Fig. 4. Area-weighted average of drug concentration in 12 faces of the Straight Model. These 12 faces are selected along the artery axial direction, including 6 faces (S1-1 to S1-6) in the DES and 6 faces (S2-1 to S2-6) in the BMS.

distal areas of DES the drug concentration was still at a high level and diminished gradually in a long distance. In the Straight Model, as the BMS was just adjacent to the ending of the DES, the drug concentration in the BMS area remained at a high level. While in the Branching Model the drug concentration was low, for the interval between the DES and the BMS as well as the low flow division rate to the branching vessel. In branching artery, the flow pattern was usually disturbed [26,27]. Therefore drug distribution (Branching Model) at artery branching would become uneven. At the outer side of branch the drug concentration was higher than that at the inner side. While in the Straight Model, drug concentration was almost axially symmetric. The area-weighted average of drug concentration in defined areas was calculated. The area-weighted average of drug concentration of Straight Model is shown in Fig. 4. The area-weighted average of drug concentration of Branching Model is shown in Fig. 5. Drug concentration results shown in Figs. 4 and 5 indicated that in the Straight Model, high quantity of drug accumulated in the adjacent distal BMS segments. In other words, the proximal DES has influence on the distal BMS areas. While in the Branching Model, drug concentration in the BMS segment was very low and uneven. Only the highest drug concentration, which appeared at top faces of BMS segments, can be comparable to that of BMS segments in Straight Model.

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Fig. 5. Area-weighted average of drug concentration in 12 faces of the Branching Model. These 12 faces are selected along the artery axial direction, including 6 faces (B1-1 to B1-6) in the upper of DES and 6 faces (B2-1 to B2-6) in the outer of BMS.

4. Discussion Drug eluting stent (DES) is a milestone in the history of interventional cardiology. Gradually released therapeutic agent reduces the artery neointimal growth induced by stent implantation. As many percentages of released drug defuses to the flow blood, the local hemodynamics will mainly rule drug transportation and localization in the artery. Many studies by Chiastra, Morlacchi, et al., have discussed the general relationship between local blood flow patterns and drug delivery features [30–33]. In the present study, we focused on drug delivery features of DES–BMS multi-stent implantation strategy in coronary arteries with diffuse lesions. We numerically investigated the drug distribution characteristics of DES–BMS strategy, especially the influence of proximal DES on distal BMS in two cases: straight artery and branching artery. Numerical studies revealed that due to the blood flow washing, the uniformly released drug distributed non-uniformly in artery segments. Near the proximal part of DES the drug concentration was only half of the drug concentration near the distal part. As much drug defused into blood and was carried downstream to distal segments, the drug concentration in the distal BMS segments maintained a relatively high level. The drug concentration in the BMS segments was negatively related to the distance to DES. For the Straight Model, the proximal DES had distinct effect on the BMS. This result coincided with Obata JE’s study [13] which reported that the in-stent restenosis was inhibited in a BMS implanted distally to a sirolimus-eluting stent. This finding may has two sides. On one side, current guidelines for diffuse long lesions are suggested to be reconsidered or modified, because due to the influence of DES on distal segments, the distal DES may become unnecessary or may be replaced by BMS to cut operation time or cost. On the other side, if the lesion locates at a short segment, the distally distributed drug would be toxic and detrimental to adjacent normal segments, which should be seriously evaluated too. For the Branching Model, the DES in the main artery had very limited effect on the BMS in the side branch. In the branching flow field, recirculation usually occurs near the outer side wall of the branch. Only in this area a little bit of drug can accumulate. Then drug would play very weak role in the branch artery. This result partly attributes to the distance between DES and BMS, and also for the

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shunting effect that small part of blood flows into the branching artery. When compared Straight Model with Branching Model, the results found that the drug concentration in DES segments of Branching Model was higher than that in the Straight Model (concentration in B1 was higher than that in B2). The difference mainly attributed to the geometric structure difference of the two models. In the Branching Model, due to the presence of branching geometry, the flow velocity near the top areas of DES (B1-6) (especially in the distal part of DES) was relatively low, compared with that near bottom areas. Therefore the weaker convection contributed to higher drug concentration near the top area of DES in the Branching Model. The results figures, which revealed that drug concentration in the distal part of the DES was higher in the Branching Model than that in the Straight Model, were only used to describe and compare drug concentration at top areas of main arteries in each model. For the drug concentration in the BMS segments, although the difference of average drug concentration in regions from S2-2 to S2-6 and regions from B2-2 to B2-6 was not marked, there was still an obvious difference at the proximal regions of BMS (S2-1 vs B2-1). What was the worse, the drug concentration features in the Straight Model was almost axisymmetric, while in the Branching Model the drug concentration was non-uniform with high concentration appeared at the top regions of BMS (from B2-1 to B2-6). So generally, the drug concentration at BMS segments in Straight Model was relatively higher than that in the Branching Model. Although stent has been commercially available for more than one decade, some problems are still unsolvable. Restenosis is inevitable in some cases and many studies even suggested a higher rate of thrombotic occlusion with drug-eluting stents than with bare-metal stents [28,29]. Different drug eluting type, stent structure design, biomaterial application, et al. are suggested to be investigated under specific hemodynamic conditions. The present study added information to the understanding of stenting for diffuse lesions artery using DES and would provide good reference for atherosclerosis treatment. Numerical simulations were affected by a number of limitations. Some assumptions were used in this simulation, including Newtonian blood fluid, steady flow condition and rigid wall of arteries. For Newtonian fluid assumption, our group has previously investigated the influence of Newtonian and Non-Newtonian on drug delivery of stent and found that ‘Newtonian assumption of blood can be used to replace its non-Newtonian one for the numerical simulation of drug transport in the DES implanted coronary artery’ [16]. For steady flow assumptions, the pulsatile characteristics will lose in the results. But according to Kolachalama’s study [36], simulation under steady flow conditions can still provide useful insights and has its significance for study of drug delivery of stents. For uniform velocity profile inlet boundary condition, due to the technical difficulties to obtain the in vivo blood flow profile as well as the individual difference in anatomy, the authors can’t use a very precise profile as the inlet boundary condition. On the other hand, the previous studies [40] have reported that ‘The flow is not fully developed near some of the origins of arteries.’ and ‘The velocity profiles are blunt near the center’. Based on these reasons, a uniform velocity profile was applied as inlet boundary condition. For solid wall boundary condition with no slip, this assumption also has limited influence since stents are typically more rigid than the artery into which they are placed. The arterial bed will be rendered less compliant, stiffer, and less viscous, as compared with such features of normal arterial walls [34,35]. In conclusion, this study numerically investigated the drug distribution characteristics of DES-BMS strategy. Due to the blood flow washing effect, the uniformly released drug from DES will distributed focally and distally. The DES would have more influence on the distal BMS in straight artery than branching artery. This study thus drew attention to the treatment of diffuse lesions in arteries using multistent implantation strategies and gave suggestions to further DES design.

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Conflict of interest None conflict of interest. Ethical approval Not required. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no.: 11472031, 11102014, 11202016, 61190123), the 111 Project (B13003) and National Key Technology R&D Program of the Ministry of Science and Technology of China (2012BAI14B04). References [1] Tiroch K, Mehilli J, Byrne RA, Schulz S, Massberg S, Laugwitz KL, et al. Impact of coronary anatomy and stenting technique on long-term outcome after drugeluting stent implantation for unprotected left main coronary artery disease. JACC Cardiovasc Interv 2014;7(1):29–36. [2] Simsekyilmaz S, Liehn EA, Militaru C, Vogt F. Progress in interventional cardiology: Challenges for the future. Thromb Haemost 2015;113(3):464–72. [3] Stone GW, Ellis SG, Cox DA, Hermiller J, O’Shaughnessy C, Mann JT, et al. A polymer-based, paclitaxel eluting stent in patients with coronary artery disease. N Engl J Med 2004;350:221–31. [4] Ishikawa K, Aoyama Y, Hirayama H. Management of Drug-Eluting Stent Restenosis. J Invasive Cardiol 2012;24(4):178–82. [5] Sorkin GC, Dumont TM, Eller JL, Mokin M, Hopkins LN, Snyder KV, et al. Instent restenosis after carotid stenting: treatment using an off-label cardiac scoring balloon. J Vasc Interv Neurol 2014;7(1):29–34. [6] Fuke S, Maekawa K, Kawamoto K, Saito H, Sato T, Hioka T, et al. Impaired endothelial vasomotor function after sirolimus-eluting stent implantation. Circ J 2007;71:220–5. [7] Togni M, Räber L, Cocchia R, Wenaweser P, Cook S, Windecker S, et al. Local vascular dysfunction after coronary paclitaxel-eluting stent implantation. Int J Cardiol 2007;120:212–20. [8] Kubo S, Kadota K, Otsuru S, Hasegawa D, Habara S, Tada T, et al. Everolimuseluting stent implantation versus repeat paclitaxel-coated balloon angioplasty for recurrent in-stent restenosis lesion caused by paclitaxel-coated balloon failure. Eur Intervent 2015;10(9):e1–8. [9] Corbett SJ1, Cosgrave J, Melzi G, Babic R, Biondi-Zoccai GG, Godino C, et al. Patterns of restenosis after drug-eluting stent implantation: insights from a contemporary and comparative analysis of sirolimus- and paclitaxel-eluting stents. Eur Heart J 2006;27:2330–7. [10] Lemos PA, Saia F, Ligthart JM, Arampatzis CA, Sianos G, Tanabe K, et al. Coronary restenosis after sirolimus-eluting stent implantation: morphological description and mechanistic analysis from a consecutive series of cases. Circulation 2003;108:257–60. [11] Shin DI, Seung KB, Kim PJ, Chang K, Choi JK, Jeon DS, et al. Long-term coronary endothelial function after zotarolimus-eluting stent implantation. A 9 month comparison between zotarolimus-eluting and sirolimus-eluting stents. Int Heart J 2008;49:639–52. [12] Kolachalama VB, Levine EG, Edelman ER. Luminal flow amplifies stent-based drug deposition in arterial bifurcations. PLoS ONE 2009;4:e8105. [13] Obata JE, Kitta Y, Takano H, Kodama Y, Nakamura T, Mende A, et al. Sirolimuseluting stent implantation aggravates endothelial vasomotor dysfunction in the infarct-related coronary artery in patients with acute myocardial infarction. J Am Coll Cardiol 2007;50:1305–9. [14] Ofili EO, Kern MJ, St Vrain JA, Donohue TJ, Bach R, al-Joundi B, et al. Differential characterization of blood flow, velocity, and vascular resistance between proximal and distal normal epicardial human coronary arteries: analysis by intracoronary Doppler spectral flow velocity. Am Heart J 1995;130:37–46. [15] Morbiducci U, Ponzini R, Grigioni M, Redaelli A. Helical flow as fluid dynamic signature for atherogenesis risk in aortocoronary bypass. A numeric study. J Biomech 2007;40:519–34.

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Influence of proximal drug eluting stent (DES) on distal bare metal stent (BMS) in multi-stent implantation strategies in coronary arteries.

The aim of this study was to investigate the drug distribution in arteries treated with DES-BMS stenting strategy and to analyze the influence of prox...
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