Shijia Zhao
Department of Mechanical&
Materials Engineering,
University of Nebraska-Lincoln,
Lincoln,NE,68588-0656
Xiangyi(Cheryl)Liu Dassault Systemes Simulia Corporation,
166Valley Street,
Providence,RI,02909-2499
Linxia Gu1
Department of Mechanical&
Materials Engineering,
laffisUniversity of Nebraska-Lincoln,
四级阅读分值Lincoln,NE,68588-0656; Nebraska Center for Materials and Nanoscience,
Lincoln,NE,68588-0656The Impact of Wire Stent Fabrication Technique on the Performance of Stent Placement Braided wire stents demonstrate distinct characteristics compared to welded ones.In this study,both braided and welded wire stents with the same nominal dimensions were crimped inside a sheath and then deployed into a stenod artery usingfinite element analysis.The braided wire stent was generated by overlapping wires to form crisscross shape.A welded wire stent was created by welding the interction points of wires to avoid sliding between wires.The effect of fabrication technique on mechanical behavior of Nitinol wire stents was evaluated.The results showed that relative sliding between wires reduced the deformation of the braided stent,which led to less radial strength than the welded one;therefore,the deployed braided stent was more conformed to the ana-tomic shape of the lesion and much less efficient for restoring the patency of the stenotic artery.Post balloon-dilation was commonly ud to improve its performance in terms of lumen gain and deployed shape of the stent.On the contrary,the welded wire stent exhib-ited a high capacity for pushing the occlusion outward.It reached an approximately uni-form shape after deployment.The welded joints caud larger deformation and high strain on the stent struts,which indicate a potential
earlier failure for the welded stent.In addition,higher contact pressure at the stent-lesion interface and higher arterial stress were obrved in the artery supported by the welded stent.The peak stress concentration may increa the occurrence of neointimal hyperplasia.[DOI:10.1115/1.4005788] Keywords:Nitinol stent,braiding,welding,finite element method,stress analysis,post balloon-dilation,strain energy
1Introduction
Self-expandable wire stents are either braided or welded to-gether[1].Specifically,braided wire stents are manufactured in various crisscross patterns and materials;if the interction points of Nitinol wires are welded together at the nominal dimension, they become welded wire stents.There is a paucity of numerical models regarding the performance of wire stents in a stenod ar-tery,even though thefinite element method has demonstrated its effectiveness in evaluating the performance of lar-cut stents dur-ing their deployment[2–5].This may be due to the difficulty in developing braided wire stent models or the complexity of wire interactions.Kim et al.[6,7]simulated the mechanical behavior of lf-expandable braided stents and investigated the effect of num-ber of wires and braiding angle on the radial strength of stent.De Beule et al.[8]optimized the braided wire stent geometry to reduce foreshortening by20%while maintaining the radial stiff-ness.Additional investigati
ons of the wire stent-artery interaction are needed to understand the outcomes after wire stent deploy-ment and improve the stent design accordingly.
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In this study,a braided wire stent was developed using Aba-qus=Explicit6.10(Dassault Syste`mes Simulia Corp.,Providence, RI,USA),and its performance in a moderate stenod artery was investigated and compared with its welded form.
2Material and Methods
The braided Nitinol lf-expandable Expander TM stent(Medi-corp S.A.,Villers les Nancy,France)was studied.It was braided with20Nitinol wires in a helical form with pitch angle of30deg. The wire diameter is0.14mm.The nominal dimension of the stent is10mm in diameter,and36mm in length.The3D model of braided wire stent is demonstrated in Fig.1.The stent was meshed with6400linear beam elements(B31),which account for large axial strains as well as transver shear strains.A welded wire version of the same configuration was created for the com-parative study.The distinct features of the two types of stents are depicted in Fig.2.
The nonlinear superelasticity of Nitinol was modeled as a pre-defined ur material with nine critical parameters to define the stress induced pha transformation between the Austenite and Martensite
phas[9].Initially,the Nitinol exhibits Austenite’s crystal structure with Young’s modulus of46.728GPa and a Pois-son’s ratio of0.33.When loaded such that the stent is crimped into the sheath,the Nitinol material transforms from Austenite to Martensite.The transformation is initiated from the stress of 358.2MPa and completed by the stress value of437.8MPa into a complete Martensite pha with a Young’s modulus of25.199 GPa and Poisson’s ratio of0.33.The Martensite begins to revert to Austenite once the stress is decread to124.25MPa,such as when the sheath is removed.The original Austenite pha will be totally recovered when the stress is further reduced to17.75MPa. The transformation strain is4.26%.The material parameters were implemented in the Abaqus ur material subroutine(VUMAT). Both braided and welded wire stent were crimped into a sheath through uniform radial displacement boundary conditions,and then confined by the sheath(Fig.3).The deployment of the stent was achieved by applying a linear ramping velocity on the sheath to remove it axially.The sheath is defined by a surface obtained by extending a circle ction along the axial direction,which was meshed with38four-node surface elements with reduced integra-tion(SFM3D4R).
After the stent was crimped inside a3.4mm in diameter sheath, it was then delivered into the target lesion.The artery has an inner diameter of9mm,wall thickness of1mm and length of70mm.
1Corresponding author.e-mail:lgu2@unl.edu
Manuscript received August15,2011;final manuscript received December20, 2011;published online March12,2012.Assoc.Editor:Danny Bluestein.
The plaque producing 50%stenosis at the narrowest occlusion,categorized as moderate stenosis,with the length of 30mm was attached to the middle of the artery.
The mechanical properties of artery,from a commercial 10mm CryoValve V R
aortic root (CryoLife Inc.,Kennesaw,GA,USA),were obtained from uniaxial tensile tests conducted on an uniaxial tensile tester (TestResources Inc.,Shakopee,MN,USA)in our laboratory.The reduced polynomial hyperelastic constitutive
equation U ¼
九九宿舍网四六级成绩查询P
N i ¼1
C i 0ðI 1À3Þi was ud to fit experimental data as shown in Fig.4.The obtained material coefficients C i 0for the ar-tery are C 10¼0.0104673MPa,C 20¼0.0194098MPa and
C 30¼0.0109830MPa.
A hyperelastic-perfect plastic model was ud to describe the mechanical respon for plaque as shown in Fig.4.The correspond-ing coefficients for the polynomial hyperelastic model were adopted from publication [2]as C 10¼0.04MPa,C 02¼0.003MPa,and C 03¼0.02976MPa.The plaque plasticity is assumed to initiate at 34%strain [10].The material properties of balloon ud in post balloon-dilation are assumed as E ¼940MPa and ¼0.4[11].The artery and plaque were all discretized with reduced-integration eight-node linear brick elements (C3D8R),and the numbers of elements are 10,500and 4200,respectively.The gen-eral contact algorithm,which can handle the contact problem between surfaces including element-bad surfaces effectively,was applied to the stent wire sliding,as well as stent-artery inter-face with a friction coefficient of 0.05[12].
3Results
The stent crimping process led to different behaviors for both the braided and welded wire stents.When the 10mm in diameter stent was totally confined by the sheath of 3.4mm in diameter,the
stent length was incread from original 36mm to 70.46mm and 65.62mm for the braided wire stent and welded wire stent,
respectively.For the braided wire stent,the pitch angle,shown as b in Fig.5,incread to 73.92deg from its initial value of 30deg,and the corresponding pitch decread from 1.58mm to 0.84mm.For the welded wire stent,the wire underwent large deformation due to the constraints at the welding spots between crossing wires.Under the same confined diameter,the peak principal logarithm strain on the strut was 0.86%for welded wire stent,which is 43%higher than that of the braided wire stent.It is clear that the strain distribution is more uniform for the braided wire stent.The strain energy stored in the welded wire stent is approximately 180times of the one in the braided wire stent,indicating a higher stiffness for the welded wire stent.When both stents were compresd between two flat plates at the same distance during a stent com-pression test,the resultant force was 5.713N for the welded wire stent and 0.615N for the braided wire stent,respectively.
After the crimped stent was delivered to the target lesion and the sheath was pulled away,both stents lf-expanded toward their nominal dimensions.For the braided wire stent,the lumen diameter at the narrowest occlusion incread from 4.5mm to 4.82mm,and the corresponding residual stenosis was 46.44%,which did not satisfy the desired residual stenosis standard of 30%or less [13].
Meanwhile,the deployed stent exhibited a dogbone shape with a diameter of 4.63mm,9.54mm and 9.68mm at cen-ter,proximal and distal ends,respectively (Fig.6).The dogboning effect,defined as one minus the ratio of central diameter versus average diameter of two ends,was 51.82%.Poor performance of the braided wire stent in terms of acute lumen gain was consistent with the experimental obrvation of the Expander TM stent by Tanaka et al.[14].It is recommended that a post
balloon-dilation
Fig.2Stent configurations fabricated with braided wire (left)and corresponding welded wire
(right)
Fig.3Crimped braided wire stent in target stenotic
artery
Fig.4Stress-strain curves for artery and
plaque
jedwardFig.5Principal logarithm strain distributions on one unit of wire stent:braided wire stent (top)and weld
ed wire stent
(bottom)
Fig.1Solid model of braided wire stent
is performed to enhance lumen gain and wall apposition.The post balloon-dilation was conducted in
floccinaucinihilipilification
our computational model through the inrtion of a cylindrical balloon,which is controlled by displacement boundary conditions.It is radially forced to reach the reference dimension of the artery and then unloaded and extracted.The plastic deformation of plaque and the scaffolding support of the braided wire stent led to a final lumen diameter of 6.84mm,which corresponds to a residual stenosis of 24%.The length of deployed stent was 49.12mm compared with its nominal value of 36mm,and the dogboning effect of the stent after bal-loon dilation was decread from 51.82%to 30.28%.
For the welded wire stent,the acute lumen diameter at the nar-rowest occlusion incread from 4.50mm to 9.94mm after its lf-expansion,and no residual stenosis occurred.The welded wire stent expanded to a uniform shape and reached the length of 35.32mm,which is very clo to the nominal dimensions.This indicates a high rigidity of the welded wire stent,which is consist-ent with the obrvations by Stoeckel et al.[1].
nba什么意思Larger contact pressure between the welded wire stent and underlying tissue was expected.The maximum contact pressure between stent and the underlying tissue is 0.045MPa for the braided wire stent and 0.065MPa for the welded wire stent,
respectively.The stent-induced peak principal stress were obrved on the central ction of the artery which contacts with the thinner side of the plaque as shown in Fig.7.This may indi-cate the plaque could rve as a cushion.The peak arterial princi-pal stress is 0.196MPa for the welded wire stent,while it is only 0.037MPa for the braided wire stent.
4Discussion
This work examined the effect of fabrication techniques of Niti-nol wire stents on their performance in treating a stenod artery.The behavior of the braided wire stent was studied and compared to a welded wire stent with identical nominal geometry.Before stents were delivered to the target lesion,they need to be confined into a sheath.The crimping process caud distinct behaviors of the stents in terms of changes of length,unit geometry,radial strength and wire stress.The braided wire stent tends to be longer after crimping than the welded wire stent.The crisscross configuration was maintained for the braided wire stent;however,the pitch angle was incread with a reduced pitch as demonstrated in Fig.3.The braided fabrication technique allows the relative sliding and rota-tion between wires,corresponding to the adaptation of pitch and the pitch angle to altered stent geometry.This way the deformation of wire is reduced.On the contrary,the integral configuration of the welded wire stent constrained the wire joints at the cost of large deformation on the wire as show
n in Fig.5.Larger stress and strains in our welded stent agree with the obrvation by Zahora et al.[15],which is highly associated with its radial strength and the extent of stent spring back [16].For the welded wire stent with all junctions welded together,a much higher radial strength was expected,which was predicted by the higher resultant force during a compression test of 5.713N.This is 9.3times more than the strength of the braided wire stent.The radial strength of the braided wire stent could be improved by the welded ends [7,17],the num-ber of wires,and the braiding pitch angle [6].
After the deployment of the braided wire stent,an apparent dogbone shape was obrved,which might be explained by the non-uniform plaque profile along the longitudinal direction [18],the low stiffness of Nitinol material [3],and the braided structure configurations.This also indicates a good conformity of the stent to the lumen shape,along with the incomplete expansion of the stent and insufficient lumen gain.The shortcomings could be relieved by post balloon-dilation.The welded wire stent
学习外语的网站exhibits
Fig.6Performance of wire stents in a stenod artery:braided wire stent (top),braided wire stent with post balloon-dilation (middle)and welded wire stent
东北林业大学怎么样(bottom)
Fig.7Stent-induced stress distribution on artery wall:braided wire stent with post balloon-dilation (top)and welded wire stent (bottom)
great performance on instant lumen gain and nearly full recovery to its nominal dimension.This implies higher stiffness leading to overstretching of the artery wall especially at the site of narrowest occlusion.
Better performance in terms of lumen gain for the welded wire stent induced more stretching on the artery wall than the braided wire stent and hence resulted in a higher stress level on the artery as shown in Fig.7.Large arterial stress may trigger neointimal pro-liferation,and increa the possibility of restenosis[19],but might alleviate stent migration[20].It is speculated that stent migration is associated with the contact pressure between the tissue and its scaf-folding stent[21];thus,higher contact pressure between the welded wire stent and the underlying tissue might anchor the stent and reduce the possibility of stent migration.On the other hand,such sudden overpressure on the artery during expansion of the welded wire stent might injure the vesl wall[7].
Even though both artery and plaque exhibit inhomogeneous, anisotropic and viscoelastic behaviors[
22,23],a homogeneous and isotropic material model is justified for a comparative study between a braided wire stent and a welded wire stent.Perfect plas-ticity was also ud to characterize the behavior of plaque[24].In spite of the simplifications,this work provides a fundamental understanding of the distinct behaviors of both braided and welded wire stents,and their impact on the stenod artery. Acknowledgment
The authors are grateful for funding from the National Science Foundation under Grant No.0926880.
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