A Study of the Relationship between the Microcatheter Shape and Stability by Numerical Simulation

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1 Journal of Neuroendovascular Therapy 2017; 11: Online March 27, 2017 DOI: /jnet.oa A Study of the Relationship between the Microcatheter Shape and Stability by Numerical Simulation Naoki Toma, 1 Koji Mori, 2 Kazuto Takashima, 3 Takanori Sano, 4 Yasuyuki Umeda, 5 Hidenori Suzuki, 1 and Takashi Saito 2 Objective: The stability of the microcatheter is important in safe coil deployment. However, quantitative information regarding the relationship between the microcatheter shape and stability is limited. We investigated deformation characteristics of the microcatheter by applying loads to the tip of a placed microcatheter using numerical analysis. Materials and Methods: Microcatheter models were prepared by connecting cylindrical rigid segments with coil springs. Four types of models with shapes mimicking parts of a blood vessel and three types of models with shapes that come into contact with the vascular wall on the opposite side of the aneurysm were prepared. Results: When the distance between the tip of the microcatheter and the position of its contact with the vascular wall exceeded 12 mm, the suppressive effect on displacement of the microcatheter tip was markedly reduced. When this distance was less than 6 mm, the microcatheter shape exerted a greater effect on the stability than the distance. Conclusion: These results suggest that not only the position of contact of the microcatheter with the vascular wall but also the shape of the microcatheter near the aneurysm plays an important role in securing the stability. Keywords simulator, stability, deformation, support distance, microcatheter shape Introduction Wire-like devices used in endovascular treatment such as the microguidewire and microcatheter are flexible and deform readily. They are delivered to the lesion by contacting 1 Department of Neurosurgery, Mie University Graduate School of Medicine, Tsu, Mie, Japan 2 Division of Mechanical Engineering, Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Ube, Yamaguchi, Japan 3 Department of Biological Functions Engineering, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka, Japan 4 Department of Neurosurgery, Ise Red Cross Hospital, Ise, Mie, Japan 5 Department of Neurosurgery, Mie Prefectural General Medical Center, Yokkaichi, Mie, Japan Received: August 31, 2016; Accepted: February 15, 2017 Corresponding author: Koji Mori. Division of Mechanical Engineering, Graduate School of Sciences and Technology for Innovation, Yamaguchi University, Tokiwadai, Ube, Yamaguchi , Japan kjmori@yamaguchi-u.ac.jp This work is licensed under a Creative Commons Attribution-NonCommercial- NoDerivatives International License The Japanese Society for Neuroendovascular Therapy multiple sites in a long and complicated vascular system. For this reason, it is difficult to accurately predict the behavior of the device during treatment. If deformation and behavior of devices in blood vessels can be clarified using a simulator, 1 6) it is expected to contribute to the selection of appropriate devices and their manipulation during endovascular procedures. This study was carried out by focusing on the stability of the microcatheter in endovascular treatment for cerebral aneurysms. Stability of the microcatheter is required for adequate coil placement in the aneurysm. The lack of stability leads to the escape of the microcatheter out of the aneurysm during coil placement. Stability of the microcatheter is determined by the relationship between the vascular morphology near the aneurysm and the shape of the selected microcatheter. More specifically, the stability is affected by whether or not the placed microcatheter is in contact with the vascular wall on the opposite side (whether or not the inserted microcatheter is supported by the vascular wall) and the position of contact (support position). 7) Namba et al. 8) reported that the microcatheter can be made more stable by preparing a 3D vessel model near the aneurysm using a 3D printer and shaping the microcatheter according to its shape. However, no quantitative information has been 333

2 Toma N, et al. Fig. 1 Concept of simulation models (microguidewire model, microcatheter model, and vessel model). Each model consists of rigid short bars (or pipes) and coil springs. A rigid short bar (pipe) was called a segment. obtained concerning the relationship between the microcatheter morphology and the support position or the effect of the support position on stability. Stability is considered to be closely related to deformation characteristics of the placed microcatheter, which is in contact with the vascular wall at multiple sites. In this study, a load was applied to the tip of a placed microcatheter, and displacement of the tip was investigated by numerical analysis. We prepared seven types of microcatheter models according to different shaping concepts. The microguidewire models were inserted into a vessel model with simplified morphology, and insertion of the microcatheter models along the microguidewire models was carried out. The objective of this study was to clarify deformation characteristics of placed microcatheters and evaluate their relationships with the microcatheter shape and support position. Materials and Methods Numerical microguidewire/microcatheter/vessel models and contact analysis In this section, the concepts of modeling of the microguidewire, microcatheter, and blood vessel necessary for numerical calculation are described. The models were prepared by splitting the microguidewire, microcatheter, and blood vessel longitudinally into short rigid cylindrical bodies (segments) and connecting them with coil springs (Fig. 1). 5,6) Since their deformation is primarily bending deformation, longitudinal deformation was disregarded. Nodal points were set on both ends of each segment. The shape of the model can be expressed by the position of the nodal point q. The models are deformed by external forces, including contact force due to contact between models, and forced displacement of objects. It was assumed that the bending rigidity of these models was uniform at all positions. This differs compared with real devices, in which portions near the tip are more flexible. Such modeling was performed to examine the genuine relationship between the shape and stability of the device. In addition, the evaluation of the relationship between the shape and stability of the device was considered to be facilitated by eliminating the factor of localized change in the material characteristics. These models are stabilized when they are shaped in a manner that would minimize the potential energy of the coil spring U. 5) Therefore, the shape of the model at a given moment can be calculated by determining the position of the node q that minimizes the potential energy U. However, the potential energy U is dependent on the shape of the model, i.e., the position of the node q, at a given moment, the q at the next moment must be determined by repeating the calculation. Since q converges on a certain value by repeating the calculation, the solution obtained is called a convergence solution. The convergence solution cannot always be obtained. The calculation may not converge if there is large deformation or marked change in the surrounding environment such as external force and contact. The Lagrangian method was used for analysis of contact between models. 9) Using this method, the depth of penetration of the nodal point of one model into the wall of a segment of another model is calculated, and the condition that makes the depth zero is given to both models as forced displacement. In this study, contact between a 334

3 Relationship between the Microcatheter Shape and Stability Fig. 2 Schematics of vessel model and microcatheter models. microguidewire model and vessel model, between a microcatheter model and vessel model, and between the microguidewire model and microcatheter model were considered. Simulation of microguidewire/microcatheter insertion The vessel model used in this study is shown in Fig. 2 (upper left). It consisted of three curved parts (radius of curvature: 4 mm) and four linear parts. The vessel part was 4 mm in internal diameter, and the cerebral aneurysm part was 10 mm in internal diameter. The vessel model was assumed not to deform. Characteristic points were set on the vessel model (Fig. 2, upper right). A vertical line was drawn from the center of the aneurysm to the center line of the vessel, and the point of its intersection with the center line of the vessel was defined as the standard point (STD). Moreover, the points before and after each curved part were defined as FPn (n = 1 to 6). The distance between STD and FP1, calculated along the center line, was 12.1 mm, that between STD and FP2 was 17 mm, that between STD and FP4 was 27 mm, and that between STD and FP6 was 42 mm. The distance from STD to the orifice of the vessel at its left end was 62 mm. The shapes of the microcatheter models were determined according to two concepts (Fig. 2, below). In one group (Model FP1 to Model FP6), the shape of part of the blood vessel was mimicked. In Model FPn, part of the microcatheter shape mimicked the shape of the vessel model from STD to FPn (n = 1, 2, 4, 6) along the center line of the vessel model. The tips of these microcatheter models had a height of 6.0 mm and an angle of 60. In another group (Model R1 to Model R3), the position of contact with the vascular wall at placement was changed. The tips of the microcatheter models of this group were angled at 90 to make the position of their contact with the vascular wall (support position) shorter than in Model FPn when they were placed in a vessel. In Model Rn, the distance along the center line of a vessel between the tip of the microcatheter and the position of its contact with the vascular wall was expected to agree with the radius of curvature of the curved part located on the opposite side of the tip. For example, in Model R3, the distance was expected to be about 3 mm when it was placed in a blood vessel. 335

4 Toma N, et al. Fig. 3 Snapshots of the placed microcatheter model. The figure in the snapshot shows support distance. The support distance is defined as the length projected to the center line of the vessel model from the tip of the microcatheter to the contact point on the vessel wall. The material characteristics were the same in all microcatheter models. Young s modulus was 2 GPa, the internal diameter was 0.42 mm, and the external diameter was 0.56 mm. The total length of the microcatheter models ranged from to mm. The number of segments was 200 to 311. As a model of the microguidewire used for insertion of the microcatheter models, one with the same shape as Model FP2 was adopted. Young s modulus was 20 GPa, and the diameter was 0.3 mm. The microguidewire was divided into 199 segments 0.5 mm long. Calculation conditions The vessel model was arranged by placing its left end at the origin. As the conditions of constraint, the microguidewire model/microcatheter model was required to be linear in the section from 1 mm to the positive side to 5 mm to the negative side at the orifice of the vessel model (X = 0 mm). Forced displacement was applied to the segment of the microguidewire model/microcatheter model nearest the orifice of the vessel model. The calculation procedure consisted of the following 4 steps: Step 1: Insertion of the microguidewire model The microguidewire model is inserted from the orifice of the vessel model. In Fig. 2 (upper right), the line between the center of the aneurysm (O) and STD was defined as the finish line. Analysis of insertion of the microguidewire model was ended when its tip crossed the finish line. Step 2: Insertion of the microcatheter model The microcatheter model was inserted along the microguidewire model. It was inserted until it reached the tip of the microguidewire model. Step 3: Withdrawal of the microguidewire model (placement of the microcatheter model) The microguidewire model was withdrawn until its tip reached the orifice of the vessel model. Step 4: Finally, forces of N to 0.1 N were applied to the tip of the inserted microcatheter model to simulate coil insertion. The forces were applied in the axial or normal directions of the tip of the microcatheter model. The measured force directly applied to the coil in coil insertion was about N. 10) In this study, no converged solution was obtained in any of the microcatheter models at loads exceeding 0.1 N. The above loading range was determined for these reasons. Results Figure 3 shows the shapes of all microcatheter models after Step 3. Concerning the microcatheter models that obtained support, the distance from the tip of the model to the support position, calculated along the center line of the vessel model, is shown. This figure showed that the length of the part of the microcatheter mimicking the shape of the blood vessel must be 17 mm or less if support is necessary. 336

5 Relationship between the Microcatheter Shape and Stability Fig. 4 Load condition (left) and displacement at the microcatheter tip versus load (center and right). Figure 4 shows the direction of the load applied to the microcatheter model placed in the blood vessel and displacement of its tip under the load. The displacement curve of Model FP2 closely resembled that of Model FP4 without support. Generally, displacement of the tip was found to be smaller in Models R1, R2, and R3 than in Models FP1 to FP6 even at large loads. When a load was applied to the tip of the microcatheter that obtained support, the tip bent with the support position as the fulcrum point. This suggests that tip displacement is affected by the moment of loading of the tip of the microcatheter model exerted to the support position. The moment is calculated as the product of the length of the line between the support position and the tip of the microcatheter model to which the load is applied and the vertical load applied to this line. Therefore, the moment can be an index unaffected by the apparent difference in shape such as the tip angle or apparent direction of loading (axial or normal direction). Figure 5 shows the relationship between the tip displacement and moment in microcatheter models that obtained support. On the whole, the tip displacement was in proportion to the moment. However, in Models FP1 and FP2, the linear relationship between the tip displacement and moment disappeared if the tip displacement exceeded 2 mm, or it was affected by the direction of the load. This suggests that these microcatheter models show marked non-linearity and that their tip displacement is difficult to predict. In these graphs, the reciprocal of the slope of the approximated line passing the origin reflects the resistance of the microcatheter model to deformation (less likely to deform and more stable as the value is higher). Therefore, we calculated it by regression analysis regardless of the direction of loading. The coefficient of determination of the line ranged from (Model FP1) to (Model R1). Figure 6 shows a graph of the distance from the tip of the microcatheter model and support position and the calculated reciprocal of the slope of the approximated line. In each group (Models FPn and Models Rn), the reciprocal of the slope of the approximated line tended to decrease with increases in support distance. This indicates that the resistance of microcatheters of each group to deformation is related to the support distance. However, when the support distance and this index were compared by disregarding the 337

6 Toma N, et al. Fig. 5 Displacement at the tip of the microcatheter model with support versus moment. Most plots were linear to moment regardless of load conditions. The linearity of each microcatheter shows deformation resistance of the microcatheter. Discussion Fig. 6 Relationship between the reciprocal of the slope of the microcatheter model with support and support distance. The reciprocal of the slope reflects the deformation resistance of each microcatheter model regardless of axial or lateral load conditions. group, the value of this index was about 35% smaller in Model FP2 (0.127) with the longest distance between the tip and the support position compared with other microcatheter models. When microcatheter models with a support distance of 6 mm or less were compared, the value of the index was larger in Model FP1 (0.214) than in Model R1 (0.205) with the shortest support distance, indicating greater resistance to deformation (higher stability). This index was shown not to be simply in inverse proportion to the support distance. In this study, deformation characteristics of microcatheter models were investigated as a basic research to clarify factors that increase the stability of placed microcatheters. When the results obtained in this study are fed back to clinical evaluation, they have the following implications. For a microcatheter to secure stability, it must obtain support. It is necessary to note that the microcatheter fails to gain support in its placement if it is given exactly the same shape as the center line of the blood vessel from its tip. In this study, the microcatheters shaped exactly as the center line of the blood vessel over a length of 17 mm or longer gained no support from the vessel model. Generally, the stability is higher as the support distance is shorter. However, this relationship may not apply if the support distance decreases below a certain level (Fig. 6). In the vessel model used in this study, this relationship was not observed when the support distance was smaller than 6 mm. This suggests that the microcatheter shape may exert a greater effect than we initially expected on the stability. For the future, studies focused on the shape of the microcatheter tip are considered necessary. In this study, hypotheses and simplification were derived by abstracting the clinical environment to carry out numerical calculations. Therefore, despite limitations, the above results are considered to be generally applicable to clinical situations. However, from the viewpoint of faithfully reproducing the clinical environment, such hypotheses and simplification mean inadequacies of reproduction. Limitations of this study are discussed later. 338

7 Relationship between the Microcatheter Shape and Stability The objective of this study resembles that of studies addressing the relationship between the back-up force and shape of the guiding catheter in the field of coronary artery intervention ) The tip of the guiding catheter is fixed at the orifice of the coronary artery, and the balloon catheter is inserted through it. The guiding catheter is pushed out from the orifice of the coronary artery due to the kickback of the device. To prevent this, the guiding catheter is brought into contact with the aortic wall (the aortic wall supports the guiding catheter). Methods to plan the guiding catheter shape effective for gaining this support were studied. According to a theory (back-up theory 12) ), a shorter support distance is generally considered to be more desirable (the catheter is less likely to be pushed out). In this study, however, the tip of the microcatheter model was not restricted, constituting the greatest difference compared with the above studies. According to the results of our present study, when Model FP1, in which the support distance was 6 mm or less, was compared with Model Rn (n = 1, 2, 3), the reciprocal of the slope of the approximated line was smaller, indicating greater deformability of Model Rn. This suggests that, if the support distance is short, marked stability may be obtained from not only the support distance but also the shape of the microcatheter, particularly, its tip. In this sense, a support distance below a threshold level may play a minor role in reducing the tip displacement. In this study, the upper limit of the load was set at 0.1 N, because no converged solution was obtained as mentioned above. The objective of this study was to evaluate the stability of the microcatheter, which prevents its escape from the aneurysm during coil insertion. We considered that a displacement of 5 mm, which was the radius of the aneurysm in the vessel model used, or greater would be necessary to estimate the index of resistance to deformation (reciprocal of the slope of the approximated line) shown in Fig. 6. The maximum displacement of the tip of the microcatheter models ranged from about 3 mm to 7 mm, and not all models showed a displacement of 5 mm or greater (Fig. 5). Therefore, we consider that the values shown in Fig. 6 contain uncertainty. However, if a greater load was applied, most of the part distal to the support position came into contact with the vascular wall in some microcatheter models. We speculate that it became impossible to obtain converged solutions due to the rapid increase in contact node points. The possibility of obtaining converged solutions under a greater load may be increased by modifications such as gradual application of loads. The implementation of consistent calculations under high load conditions is one of the research themes for the future. In this study, the stability was evaluated using the resistance of the microcatheter tip to deformation. However, the stability required in actual clinical situations is no escape of the microcatheter from the aneurysm during coil placement. By such a definition, it can be readily inferred that not only resistance of the microcatheter tip to deformation but also the tip morphology including the tip angle and tip position exert considerable effects on the stability. The preparation of an evaluation method by taking multiple factors of stability in consideration is another theme for the future. In this study, a simplified vascular shape was used by disregarding the friction between the microcatheter and microguidewire or between these devices and blood vessel. Bending deformation or deformation in the radial direction of the vessel model was ignored, and the vascular shape was two-dimensional unlike clinical cases. Material characteristics of the devices were assumed to be uniform. Generally, the ability of the devices to memorize the shape is attenuated by repeated deformation. However, such material characteristics were not considered in the calculations in this study. In addition, the effects of inertia or velocity were disregarded. The effects of the blood flow were also disregarded. Because of such simplification, the simulation does not accurately reflect the real phenomena. Furthermore, we have not validated the agreement between the calculation methods employed in this study and actual deformation except under particular conditions. 6) The relationship between the results of calculations in this study and actual stability must also be evaluated by experiments. 14) Since the stability is performance representing the deformation caused by interaction between the coil and microcatheter, simulation of coil insertion through a microcatheter must be carried out for more precise evaluation. Also, as only one type of vessel model was used in this study, the results obtained must be validated using vessel models with a wide variety of shapes. These must be approached for the future. Conclusion This study, in which deformation behavior of placed microcatheters was evaluated, showed that microcatheters mimicking the vascular shape over a distance of 17 mm or more do not gain support. In addition, the suppressive effect on the microcatheter tip displacement was markedly reduced when the support distance exceeded 12 mm. Also, the microcatheter shape exerted a greater effect on microcatheter tip 339

8 Toma N, et al. deformation than the support distance if the support distance was 6 mm or less. These results suggest that not only the position of contact of the microcatheter with the vascular wall but also the microcatheter shape near the aneurysm plays a major role in securing the stability. We consider that these findings provide useful guidelines in evaluating microcatheter shapes for preventing the microcatheter from slipping out of the aneurysm during coil placement. Funding This work was supported by JSPS KAKENHI Grant Number 15K Disclosure Statement The first author and coauthors have no conflicts of interest. References 1) Wang Y, Chui C, Lim H, et al: Real-time interactive simulator for percutaneous coronary revascularization procedures. Comput Aided Surg 1998; 3: ) Otani T, Ii S, Shigematsu T, et al: Computational model of coil placement in cerebral aneurysm with using realistic coil properties. J Biomech Sci Eng 2015; 10: ) Konigs KM, van de Kraats EB, Alderlisten T, et al: Analytical guide wire motion algorithm for simulation of endovascular interventions. Med Biol Eng Comput 2003; 41: ) Duriez C, Cotin S, Lenoir J, et al: New approaches to catheter navigation for interventional radiology simulation. Comput Aided Surg 2006; 11: ) Yamamura N, Fukasaku K, Himeno R, et al: [Development of catheter simulator]. Proceedings of Riken Symposium on Computational Biomechanics 2003: comp-bio.riken.jp/1/download/files/yamamura_naoto- NAIYOU.pdf. (accessed March, 13, 2017) (in Japanese) 6) Takashima K, Tsuzuki S, Ooike A, et al: Numerical analysis and experimental observation of guidewire motion in a blood vessel model. Med Eng Phys 2014; 36: ) Kwon BJ, Im SH, Park JC, et al: Shaping and navigating methods of microcatheters for endovascular treatment of paraclinoid aneurysms. Neurosurgery 2010; 67: 34 40; discussion 40. 8) Namba K, Higaki A, Kaneko N, et al: Microcatheter shaping for intracranial aneurysm coiling using the 3-dimensional printing rapid prototyping technology: preliminary result in the first 10 consecutive cases. World Neurosurg 2015; 84: ) Chaudhary BA and Bathe KJ: A solution method for static and dynamic analysis of three-dimensional contact problems with friction. Comput Struct 1986; 24: ) Matubara N, Miyauchi S, Nagano Y, et al: [Experimental study of generation pattern of coil insertion force using an force sensor system: investigation of friction state between coil and aneurysm wall determined by difference of coil insertion method and insertion speed]. JNET 2010; 4: (in Japanese) 11) Voda J: Long-tip guiding catheter: successful and safe for left coronary artery angioplasty. Cathet Cardiovasc Diagn 1992; 27: ) Ikari Y, Nagaoka M, Kim JY, et al: The physics of guiding catheters for the left coronary artery in transfemoral and transradial interventions. J Invasive Cardiol 2005; 17: ) Ikari Y, Ochiai M, Hangaishi M, et al: Novel guide catheter for left coronary intervention via a right upper limb approach. Cathet Cardiovasc Diagn 1998; 44: ) Yoshida Y, Kobayasi E, Adachi A, et al: [Estimation of kickback of micro catheter in coiling using high precision pressure gage]. JNET 2015; 9: S444. P425 (in Japanese) 340