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Hindawi Journal of Healthcare Engineering Volume 2018, Article ID 9592513, 8 pages https://doi.org/10.1155/2018/9592513 Research Article The Wall Apposition Evaluation for a Mechanical Embolus Retrieval Device 1 2 3 Xuelian Gu , Yongxiang Qi, and Arthur G. Erdman Shanghai Institute for Minimally Invasive erapy, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China MicroPort Endovascular (Shanghai) Co., Ltd., 3399 Kangxin Rd., Shanghai 201318, China Department of Mechanical Engineering, University of Minnesota, 111 Church St. SE, Minneapolis, MN 55455, USA Correspondence should be addressed to Xuelian Gu; guxuelian@usst.edu.cn and Arthur G. Erdman; agerdman@umn.edu Received 4 November 2017; Revised 2 April 2018; Accepted 3 May 2018; Published 25 September 2018 Academic Editor: Shanshan Wang Copyright © 2018 Xuelian Gu et al. +is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A computational evaluation approach to the wall apposition of a cerebral mechanical emboli retrieval device (MERD) is presented. +e typical enclosed multilattice structure, manufactured from the thin-walled Nitinol tube, consists of repeated “V”-shaped unit cells. During interventional thrombectomy, the MERD system is delivered inside an artery stenosis segment to capture emboli and restore cerebral blood flow. +e wall apposition, which deteriorates during embolus capture, occurs during system migration along the tortuous intracranial vessel. +e commercial finite element analysis (FEA) solver ABAQUS 6.10 Standard and user subroutine (UMAT/Nitinol) are utilized to study the ability to remain in close contact with the curved vessel wall during migration. In this numerical analysis, the influence of the contacting interference loadings on structure deformation and strain field distribution is obtained and analyzed. +e results indicate that the middle segment of the MERD seriously contracts or collapses inside the curved vessel. In addition, the peak strain is in the apex flow-prone region and maintains at the safe range. Retrieval system (Concentric Medical, Mountain View, Cal- 1. Introduction ifornia, USA), the Penumbra Aspiration system and the Stent Stroke is a type of brain function disease which is due to the Retrievers, and others [5, 6]. +e latest generation of me- disturbance of blood supply to the brain tissue. Stroke has chanical thrombectomy devices is the ‘stent retriever’ family: already become the second leading cause of death in the world, Solitaire (ev3 Endovascular, Plymouth, Minnesota, USA), and approximately 88% is acute ischemic stroke (AIS) [1]. It is Trevo Pro (Stryker Neurovascular, Kalamazoo Michigan, USA), reported that the number of deep venous thrombosis cases has and arguably the Penumbra 3D separator (Penumbra, Alameda, increased to 250,000 per year in the United States, and the ratio California, USA) [7–9]. of morbidity or mortality is also fairly high [2]. During the past In clinical application, the MERD works according to the decade, mechanical embolus retrieval devices (MERDs) have following steps: been extensively used as an interventional neurovascular (1) Crimp the stent into a catheter and delivery to the device to treat AIS [3–6]. +e MERD is designed to capture the stenosed (blocked) vessel, as shown in Figure 1(a) embolus or thrombus, scaffold the afflicted artery, and restore blood flow. +is paper reports the analysis of the MERD which (2) Deploy to capture the embolus while the outer catheter is fabricated from the nickel-titanium alloy thin-walled tube by is removed, as shown in Figures 1(b) and 1(c) laser cutting. After electropolishing and heat treatment for the (3) Migrate the embolus removal device and extract tissue shape setting, the embolus capture device is shaped like an from the patient’s body, as shown in Figure 1(d). enclosed tubular multilattice structural “funnel.” MERD structures vary in complexity from a 3D helical coil to a laser- +e cerebral blood vessels have a relatively small profile cut nickel-titanium (Nitinol) tube. So far, several embolus and often small radius of curvature in the anatomy. After the retrieval devices have been used in clinics, such as the Merci MERD has been inserted and has migrated inside the 2 Journal of Healthcare Engineering Delivery (a) (b) Migration (d) (c) Figure 1: e numerical work‰ow of the MERD. (a) Crimp. (b) Deploy. (c) Extract. (d) Capture. tortuous cerebral vessel, the embolus retrieval performance e axial ‰exibility of the stent is studied by applying is of great importance [3, 5]. e wall apposition, which de- moment in [14]. Grogan et al. designed CoCr stents and compared the mechanical properties of the deployment, scribes the stent or MEDR’s ability to remain in close contact with the adjacent vessel wall [3], is a signi�cant mechanical radial force, longitudinal resistance, and ‰exibility. e dif- ference between a rigid expanding tool and a polymer balloon requirement of a MERD system while it is deployed in a curved vessel and will a ect the e ectiveness and accuracy of embolus is demonstrated in [15]. retrieval. After being delivered to the targeted artery wall, Nitinol So far, few researches have been published to study the stents expand and contact with the artery wall. e tubular mechanical behavior of a thrombectomy device with a nu- stent is designed to be embedded in the stenosed blood vessel merical modeling method. However, the mechanical per- segment to avoid movement. e multilattice MERD, formance of multilattice tubular devices, such as vascular however, is manipulated so that it migrates inside the blood stents, has been studied already [3, 10–15]. Krischek et al. lumen to capture and retrieve the plaque blockage, as shown proposed the mechanical features for a MERD, including the in Figures 1(c) and 1(d). Poor wall apposition inside tor- tuous vessels may result in undesired clinical accidents and radial force, wall apposition, conformability, and Gator backing (Gator backing describes a stent’s tendency to ‰air even failure to retrieve plaque. So far, the numerical sim- its struts outward, forming protrusions, when the stent is ulation process of embolus retrieval in tortuous vessel walls placed around a bend) and carried out experimental tests [3]. has not been reported. In interventional thrombectomy, the Kleinstreuer et al. simulated crimping, deployment, and Nitinol MERD functions as a disposable surgical instrument cyclic-loading procedures. A high-cycle fatigue prediction rather than a permanent implant. Prediction of structural method for mean strain/alternating strain in Nitinol ma- failure depends on the peak strain value. Finally, the unique terial was established in [10]. Kate et al. compared nonstent design of the distal and proximal structure constructed is retriever and stent retriever mechanical thrombectomy demonstrated. e paper develops a scienti�c numerical devices for the endovascular treatment of acute ischemic analysis work‰ow of the MERD for the expansion capability stroke. Stent retriever mechanical thrombectomy devices and wall apposition. e analysis method provides a struc- tural optimization scheme for MERD design. In clinical use, achieve higher recanalization rates than nonstent retriever devices in acute ischemic stroke with improved clinical and an operational guideline for embolus retrieval and migration radiographic outcomes and safety [11]. Azaouzi et al. ana- of the MERD is demonstrated, which will help optimize lyzed the deployment of a self-expanding stent inside an a speci�c procedure. In conclusion, the study of the MERD artery by FEA, and the results could be used to assess the for the wall apposition inside a tortuous blood vessel is impact of the stent on the artery and the in‰uence of the provided to improve the device’s mechanical capability as artery on the deformation �eld within the stent [12]. Gu et al. well as to aid in surgical training. presented a numerical analysis of a semienclosed tubular In this paper, a typical tubular enclosed mesh-like MERD mechanical embolus retrieval device (MERD) for the treat- model is built to study the in‰uence of shape setting and migration. ABAQUS 6.10/Standard (DS SIMULIA, RI, USA) ment of AIS, and the FEA methodology is used to evaluate mechanical performance and provide suggestions for opti- commercial FE code and its user material subroutine (UMAT/ Nitinol) are employed to simulate the procedure and the mizing the geometric design [13]. Wu et al. generated a �nite element (FE) model of a vascular stent with tetra-elements. MERD/artery contact interaction mechanism. Journal of Healthcare Engineering 3 In the shape-setting step, a radial outward displacement is Our study presents the following critical measurement of performance: (1) large strain distribution and the highest imposed on the expanding cylinder to achieve the expanded shape size. Besides, it is also constrained axially and cir- peak value of maximum principal strain (MPS) in the mi- crostructure and (2) wall apposition performance along the cumferentially, that is, preventing the transitional and rota- tortuous artery in the macrostructure. tional movements. Meanwhile, the shape-setting cylinder is +e analysis results are used to assess the safety and restrained in all DOF to avoid offset. A single node of MERD efficiency of the MERD. +e wall apposition performance instance is fixed axially to restrict movement. In the migration evaluation methodology is a scientific numerical method to step, the reference point of a rigid artery is restrained in all analyze preexisting structural flaws and offer approaches for DOFs under a global rectangular coordinate system. Negative design optimization. axial displacement, imposed on the guiding-wire side, is used to pull the MERD along the desired tortuous artery. +e master/slave contact pair algorithm is utilized to build 1.1. Methods. A 3D finite element model has been generated to study the effects of MERD expansion and migration. +e the contact interface between the expanding/shape-setting cylinder and the MERD. Meanwhile, the arterial inner sur- numerical simulation processes are achieved with the con- tact interactions of a MERD/shape-setting cylinder, MERD/ face is taken as the master surface, and the MERD outer expanding cylinder, and MERD/artery at a body temperature surface is set as the slave surface. +e penalty contact method of 37 C [10]. +e geometry and mesh model of the embolus is employed. It is notable that self-contact interaction should removing device are shown in Figure 2. also be considered to avoid struts overlapping during the In the numerical analysis, the MERD is assumed to be migration step. In addition, a damping factor could be a homogeneous isotropic incompressible deformed body in employed discreetly to stabilize the contact-induced vibration the absence of residual stress. +e shape-setting cylinder and behavior, improve convergence, and reduce computational expanding cylinder are designed to be a semirigid movable expense in contacting interaction domains. cylinder shell. +e tortuous cerebral vessel is modeled as a “C”-shaped discrete rigid shell. +e Nitinol material con- 2. Results stitutive model is characterized by the ABAQUS 6.10 software (UMAT/Nitinol) user subroutine. Figure 3 provides the 2.1. Validation. To validate the finite element model and stress-strain and stress-temperature curves for the template of computational algorithm, a diamond-shaped pattern is Nitinol alloy based on ABAQUS software, while Table 1 lists generated for the numerical simulation and experimental the specified parameters of Nitinol material from the previous test. +e comparison between test and FEA radial forces is research [10]. +ese parameters demonstrate the distinct used to predict the accuracy and reliability. In the experi- mechanical behavior of Nitinol during loading and unloading mental test, the measurement system applies displacement conditions with specific temperature. on several metal slices to compress the multilattice pattern. +is mesh-like tubular MERD comprises five “column” +e electrical force sensor (RX500, Machine Solutions Inc.), cells in the axial direction. Each “column” cell consists of shown in Figure 5, obtains and reflects the real-time support four “V”-like wave rings, and each “column” cell is con- force value. For the radial force (RF) curve plots, the outer nected with connected bridges circumferentially. +e strut diameter of the MERD is designed as the horizontal x-axis, thickness, width, original outer diameter, nominal final while the vertical axis prescribes the balanced force value. outer diameter, artery centerline radius, and artery cross- In the progress of numerical simulation, the crimping slice profile diameter are 0.07, 0.07, 2.00, 4.00, 5.00, and 3.00 mm, is replaced by a rigid removable cylinder to compress the original respectively. +e element types C3D8I (three-dimensional pattern. +e curves for the test and numerical process are eight-node Stress Hex incompatible element), SFM3D4 shown in Figure 6. (three-dimensional four-node quadrilateral surface ele- +e result indicates that the test and simulation curve ment), and R3D4 (three-dimensional linear four-node bi- match well; that is, all of the numerical outcomes are relatively linear rigid quadrilateral element) are utilized for the MERD, accurate and could therefore provide a reliable performance tortuous vessel, and expanding/shape-setting cylinder, analysis and structure optimization report. separately. To minimize the influences of mesh density, two layer elements for the wall thickness [16] and 16 elements along the fillet edge are used. +e element and node numbers 2.2. Shape Setting. +e original laser-cut tubular multilattice of the MERD FE model are 40,000 and 20,000, respectively. structural MERD is expanded by imposing an outward radial In addition, to form an enclosed thrombus-capturing “cage,” displacement on a cylinder surface. Once the final shape is “funnel”-shaped tails are meshed and connected to the accomplished, the mechanical behavior of cross-profile ex- MERD’s FE principal body after expansion and annealing. pansion and axial length shrinkage is presented in Figure 7. At the end of shape setting, a Nitinol tortuous guiding wire is During shape setting, the obtained radial outer diameter tied to the “funnel”-shaped head for the pulling/pushing ranges from 2.00 to 4.00 mm, and the axial length ranges from simulation. A flare-shaped tube is connected to the “C”-shaped 42.9 to 40.0 mm. In contrast to traditional metallic material, cerebral vessel. +e novel structural design offers smooth the Nitinol material fracture and fatigue failure are strain and stable surface interaction to avoid computational contact induced [10]. Structural damage can occur from outright divergence. +e boundary conditions for shape setting and fracture during the expansion step. Figure 7 presents the migration are described in Figure 4. strain field contour plots of the expanded MERD. It indicates 4 Journal of Healthcare Engineering L = 43.0 mm 11.0 mm 2.1 mm Fillet region mesh Struts cross section “Column” cells Connecting bridge mesh 0.06 mm Wave strut mesh 0.07 mm Figure 2: Geometry parameters and mesh model of the MERD. Loading S S σ σ σ σ L L U δσ Unloading δT δσ ε L δT σ cL Figure 3: Nitinol material properties (from ABAQUS Nitinol UMAT) [10]. Table 1: Material parameters of Nitinol. for Nitinol of 12% [10]. erefore, no issues of cracks or Parameters Values fractures in the structure are likely in the shape-setting step. Austenite elasticity, E (MPa) 51700 Austenite Poisson’s ratio, ] 0.3 Martensite elasticity, E (MPa) 47800 2.3. Migrating. MERD struts may seriously contract or Martensite Poisson’s ratio, ] 0.3 collapse due to contact-dominant bending in the tortuous Transformation strain, ε 0.063 blood vessel. e wall apposition performance is a signi�cant Start of transformation loading, σ (MPa) 600 criterion to evaluate the device e ectiveness during mi- End of transformation loading, σ (MPa) 670 gration and embolus capture. Figure 8 illustrates a signi�- cant cross-pro�le contraction of the middle segment struts while they are being pulled along a curved path. that high-magnitude strains are always located in the strut’s e formula for the cross-pro�le reduction ratio is inner �llet tensile side and its vicinity. e strain distributions calculated and proposed as below: across the MERD middle segments are negligible. e peak D − L strain located in the “‰ow-prone” region achieves a value of (1) R , 6.1%, which is much lower than the critical strain threshold d = 3.0 mm Journal of Healthcare Engineering 5 Nitinol guiding wire Shape-setting cylinder Axial displacement Expanding cylinder 3 mm MERD Rigid cerebral vessel Radial displacement 22 mm Reference point Funnel-shaped tail Funnel-shaped head Junction point Super-elastic Nitinol MERD Flare-shaped tube (a) (b) Figure 4: Schematic points of assembly boundary conditions. (a) Expansion for shape setting. (b) Pulling and migrating inside the tortuous cerebral vessel. 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 Figure 5: e electrical force sensor (RX500, Machine Solutions Inc.). 0.00 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 Outer diameter (mm) where D is the artery cross-pro�le diameter, L is the radial TEST RF contraction lengths, and R is the cross-pro�le reduction FEA RF ratios. As a result, D obtains a value of 3.0 mm, L 0.8 mm, Figure 6: Experimental validation of FEA. and R 73.3%. e strain �eld distribution contour plots of the deformed MERD are displayed in Figure 9. Figure 9 indicates that high-magnitude MPS is always located in the shoulder of the constrained strut’s apex. In the 3. Discussion migration step, a structure-induced resultant force is as- is paper explores the biomechanical concept of the wall sumed to be applied to stretch and straighten the apex struts. e peak strain located on the ‘funnel’-shaped head reaches apposition performance for tubular MERD migration simulation. is behavior is regarded as re‰ecting the safety a value of 7.0%. Compared to a Nitinol ultimate tensile strength (UTS) critical threshold of 12% [10], there is no risk of pulling migration as well as the e ects of embolus re- moval. Structure designers can simulate and analyze the of crack or fracture failure during the migration step. All other areas appear to have relatively low strain. ey are also virtual prototypes. Various topology and dimension opti- mization methods could also be used to provide closer con�rmed to stay in the safe domain of Nitinol alloy. It has been speculated that the former highest peak MPS appears contact with the tortuous vessel. As a vital mechanical index, on the basis of unsmooth geometry [17] and a tiny con- the numerical wall apposition performance could o er surgeons a guideline for a reasonable MERD choice. necting section in the microstructure. Radial force (N) 6 Journal of Healthcare Engineering LE, max. principal (avg: 75%) +6.1e – 02 +5.6e – 02 +5.1e – 02 +4.6e – 02 +4.1e – 02 +3.6e – 02 +3.0e – 02 A-A +2.5e – 02 +2.0e – 02 Maximum expanding strain is 6.1% +1.5e – 02 +1.0e – 02 +5.1e – 03 +4.7e – 05 Figure 7: Maximum principal strain contour plots of MERD expansion: global and detailed schematic views. R =(D – L )/D 1 1 Figure 8: Wall apposition performance of a MERD segment in parametrical schematic. LE, max. principal (avg: 75%) +7.0e – 02 +6.4e – 02 +5.8e – 02 +5.2e – 02 +4.6e – 02 +4.1e – 02 +3.5e – 02 +2.9e – 02 +2.3e – 02 +1.7e – 02 Maximum principal strain is 7.0% +1.2e – 02 +5.8e – 03 +2.4e – 06 Figure 9: MPS �eld contour plots of MERD migration along a tortuous artery: global and detailed schematic views. Journal of Healthcare Engineering 7 +e contraction or collapse behavior is predicted to be performance of the delivery system insertion and the effec- induced by the bend resistance deficiency of the tubular axial tiveness of embolus retrieval [13]. structure. +e key factors of the desired mechanical per- formance include the strut width, strut thickness, wave axial Conflicts of Interest length, unit cell number, and connecting-bridge topology. +e authors declare no conflicts of interest. +e wall apposition numerical analysis of the MERD has been displayed in the absence of an arterial constitutive Authors’ Contributions model and anatomical geometry. +e simulation process of thrombus capture is also not presented. +e simulation Xuelian Gu and Yongxiang Qi contributed equally to this work. results can be applied to provide a reference for the design of stent retriever mechanical thrombectomy devices which Acknowledgments achieve higher recanalization rates than nonstent retriever devices in acute ischemic stroke with improved clinical and Xuelian Gu was partially supported by the National Natural radiographic outcomes and safety [11]. Science Foundation of China (no. 11502146). Arthur Erdman Based on the analysis above, a number of recommen- was partially supported by the National Science Founda- dations are suggested for engineers for optimization of MERD tion (IIS-1251069) and the National Institutes of Health structure. For example, a shorter independent support unit (1R01EB018205-01). cell could undergo and resist higher contact-induced axial bending loading. +erefore, an approach of axial length References shortening can be utilized to improve the structure perfor- mance and minimize cross-sectional collapse effects. +e [1] M. Desai, Development of Asymmetric Stent Designs and geometry of the former highest peak MPS region should be Validation through Finite Element Analysis Simulations, State optimized. To reduce the strut’s apex strain, the strut’s ex- University of New York at Buffalo, New York, NY, USA, 2006. ternal side (the so-called “shoulder” region) and internal side [2] H. J. Shi, Y. H. Huang, T. 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Journal of Healthcare Engineering – Hindawi Publishing Corporation
Published: Sep 25, 2018
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