Measurement of radial and axial forces of biliary self-expandable metallic stents

ORIGINAL ARTICLE: Clinical Endoscopy

Measurement of radial and axial forces of biliary self-expandable metallic stents Hiroyuki Isayama, MD, PhD, Yousuke Nakai, MD, PhD, Yoshihide Toyokawa, MT, Osamu Togawa, MD, Chimyon Gon, MT, Yukiko Ito, MD, Yoko Yashima, MD, Hiroshi Yagioka, MD, Hirofumi Kogure, MD, Takashi Sasaki, MD, Toshihiko Arizumi, MD, Saburo Matsubara, MD, Natsuyo Yamamoto, MD, PhD, Naoki Sasahira, MD, PhD, Kenji Hirano, MD, PhD, Takeshi Tsujino, MD, PhD, Nobuo Toda, MD, PhD, Minoru Tada, MD, PhD, Takao Kawabe, MD, PhD, Masao Omata, MD, PhD Tokyo, Japan

Background: Efforts to understand the properties of self-expandable metallic stents (SEMSs) through their mechanical properties have progressed. Among them, radial force (RF) is well known as an expanding force, but axial force (AF) has not been measured before. Correlations of these properties to clinical results are not well known. Objective: We measured RF and AF of 14 different SEMSs and discussed the results in terms of clinical implications. Design: Experimental study. Subjects: Measurement of RF and AF of 14 different covered and uncovered SEMSs. Methods: RF was measured with an RF measurement machine manufactured by Machine Solution, and AF was measured with in-house equipment. Results: Measurements of RF in the process of expansion showed characteristic patterns closely related to the structures and materials of SEMSs. Results of AF measurement can be classified into 3 groups: high, medium, and low AF, depending on the type of SEMS. AF decreased with an increase of the length of stents. A plot of RF against AF revealed 3 distinguished RF/AF combinations and indicated the importance of understanding the properties by not only RF or AF individually but also by RF/AF combination. Limitations: In vitro study using measurement equipment. Conclusion: It was demonstrated that a combination of RF and AF is more effective than RF or AF individually in understanding the clinical implications of SEMSs. More work is needed to correlate mechanical properties with clinical results by designing model experiments. (Gastrointest Endosc 2009;70:37-44.)

Endoscopic or transhepatic biliary stenting has become widely accepted as a standard procedure for the management of obstructed jaundice in patients with unresectable biliary malignancies.1,2 Recently, the self-expandable metallic stent (SEMS) has become recognized as an effective

Abbreviations: AF, axial force; e-PTFE, expanded polytetrafluoroethylene; RF, radial force; SEMS, self-expanding metallic stent. DISCLOSURE: All authors disclosed no financial relationships relevant to this publication. Copyright ª 2009 by the American Society for Gastrointestinal Endoscopy 0016-5107/$36.00 doi:10.1016/j.gie.2008.09.032

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standard biliary endoprosthesis because of its long patency.3,4 Covered SEMSs have been recently developed to prevent tumor ingrowth via stent mesh and have shown better results than the conventional uncovered type.5-11 Currently, many types of SEMSs with varying mechanical properties are commercially available. However, the relationship between mechanical properties of SEMSs and clinical outcomes is not well understood. The mechanical properties of SEMSs reported in the past include radial force (RF), chronic outward force (radial resistance force), flexibility, shortening ratio, radiopacity, and trackability.12-17 The first 3 properties are related to device behavior after stent deployment, and the last 3 are important factors during the insertion process. Volume 70, No. 1 : 2009 GASTROINTESTINAL ENDOSCOPY 37

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Capsule Summary

TABLE 1. SEMSs tested Metallic stents

Structure Material

Manufacturer

Uncovered metallic stents

What is already known on this topic d

In self-expandable metal stents (SEMSs), radial force, chronic outward force, and flexibility relate to device behavior, whereas shortening ratio, radiopacity, and trackability are factors in the insertion process.

Wallstent

Braided

Stainless steel

Microvasive

Diamond stent

Braided

Nitinol

Microvasive

Za-stent

Braided

Nitinol

Wilson Cook

SMART stent

Laser cut

Nitinol

Cordis

Sinus Super Flex Laser cut stent

Nitinol

OptiMed

Luminexx stent

Laser cut

Nitinol

Bard

SelfX stent

Laser cut

Nitinol

Abbott Vascular

Zilver stent

Laser cut

Nitinol

Wilson Cook

ZEO stent

Laser cut

Nitinol

Zeon Medical

cially available biliary SEMSs to better understand correlation with clinical results.

MATERIAL AND METHODS

What this study adds to our knowledge d

Measurement of the radial force and axial force of 14 covered and uncovered SEMSs demonstrated that the combination of the 2 forces is more effective than either individually in understanding the stent mechanical properties.

Covered metallic stents Silicone-covered Wallstent

Braided

Stainless steel

Microvasive

PU-covered Wallstent*

Braided

Stainless steel

Microvasive

PU-covered Diamond stent*

Braided

Nitinol

Microvasive

e-PTFE-covered Viabil stent

Specially braided

Nitinol

Gore-tex

e-PTFE covered ComVi type

Specially braided

Nitinol

Taewoong

Identification of commercially available SEMSs tested in this study. PU, Polyurethane. *Covered in house.

We are most interested in the first 3 properties, namely, RF, chronic outward force, and flexibility, because they are believed to have an effect on the clinical performances such as patency and complications. Among these properties, RF is obviously the most important because in SEMSs radial force maintains and expands the luminal patency at the stricture once the SEMS is deployed. Unfortunately, details of its proper value are not known. Flexibility is usually measured as a force to bend the stent.13 However, it does not represent the force to recover to a straight position after bending. The latter force, defined as axial force (AF) in this article, has not been reported before. AF is important because it is the force required to keep stents straight. If AF is too high, the stent may exert forces to change its shape, which could result in adverse clinical events in a bile duct. This article deals with a measurement of 2 important mechanical properties, RF and AF, of a variety of commer38 GASTROINTESTINAL ENDOSCOPY Volume 70, No. 1 : 2009

Samples We evaluated 5 covered and 9 uncovered SEMSs, as shown in Table 1, in which their structures, materials, cover materials, and manufacturers are listed. As shown, there are 3 types of structures, namely, braided, specially braided, and laser-cut types. Among them, the specially braided SEMSs (ie, ComVi and Viabil stents) have unique structures and may need further explanation. ComVi stents consist of 2 uncovered SEMSs overlapping each other with an expanded polytetrafluoroethylene (e-PTFE) membrane sandwiched between them. As a result of this structure, the 2 wires are not attached to each other, and there is no bonding between the wire and the membrane. Viabil stents, on the other hand, consist of one spiral Z-shaped wire with an e-PTFE membrane attached to it. There are two types of materials, nitinol and stainless steel, and 3 types of cover membranes: silicone, polyurethane, and e-PTFE. Most of the samples are commercially available, except the polyurethane-covered Wallstent and Diamond stents, which were covered in house by the first author of this article as reported elsewhere.9,10,18 All stents are 10 mm  8 cm, and 2 samples each were measured to get a mean value.

Measurement method of RF RF was measured by using an RF measurement machine (RX 500, Machine Solutions, Flagstaff, Ariz) in an oven at 37 C. An SEMS sample in a fully expanded state was placed in the cylindric space of the machine (Fig. 1), and the cylinder was contracted to shrink the SEMS to its minimum size of 2 mm. Then the force on the cylinder was reversed by an expansion force of the SEMS until it www.giejournal.org

Isayama et al

Figure 1. The cylinder of the RF measurement machine, RX 500, supplied by Machine Solution, Fragstaff, Arizona. The stent samples are inserted with the use of a mandrel in the center space of the cylinder shown.

achieved its fully expanded state of 10 mm in diameter. The forces to contract and expand the SEMS were continuously recorded by a force gauge deployed inside the cylinder.

Measurement method of AF A section of an SEMS sample was placed over a plastic rod and then inserted in a glass tube, which was then clamped by a vise as shown in Figure 2. The rod and glass tube were tightly fitted so that there was no space between them. The lower portion was fixed in a vise, and the upper portion was left flexible to simulate the condition of a stent in a bile duct. The intrapancreas portion of a bile duct is usually fixed, and the portion between hepatic hilum and the upper edge of pancreas usually moves freely. To measure the AF, a portion of stent was pushed perpendicularly by a force gauge (model DPX-5TR, Imada, Tokyo, Japan) until the angle became 60 degrees, as shown in Figure 2, and the force to keep it at the place was recorded. The measurement was made in an oven at 37 C and for 3 distances from the bending point: 20, 40, and 60 mm.

Radial and axial force in biliary metallic stents

Figure 2. AF measurement setup. A force is applied to a part of the free end of the SEMS until it bends at a 60-degree angle; the force to keep it there is recorded. The distance from the bending point to the point that the force is applied is changed to 20, 40, and 60 mm.

in RF with the increase in diameter. All the nitinol SEMSs exhibited this characteristic (Figs. 3B-I and 4C-E), whereas the SEMS other than nitinol showed a straight reduction to 0 value at full expansion (Figs. 3A, 4A, and B). The second characteristic is a very high RF at the start of expansion, followed by a sharp drop. This characteristic was observed in all braided SEMSs (Figs. 3A-C and 4A-E). The force measured during the contraction process resulted in relatively higher values than that of the expansion curve.

AF measurement Figure 5 illustrates the change of AF values with an increase of the distance from the bending point to the point that the force is applied, from 20 to 40 and to 60 mm. AF values rapidly decrease with the increase of the distance. Table 2 lists the AF values when the distance between the bending point and the point the force is applied is 20 mm, together with RF at 4 mm in diameter. It is observed that the AF values of all Wallstents are higher than any other type (0.66-0.95 N) and those of ComVi, Viabil, SelfX, Zilver, and ZEO stents are all very low (0.04-0.22 N). All other types are between those extremes (0.29-0.51 N).

RESULTS A plot of RF versus AF RF measurement The results of RF measurement during expansion and contraction processes are exhibited in Figures 3A to 3I for the uncovered SEMS and in Figures 4A to 4E for the covered SEMS. Special attention was made to the expansion curve because the expansion process was designed to simulate device behavior during deployment. There are characteristics observed in the expansion curves. One is a 2-step reduction www.giejournal.org

To study the relationships between RF and AF, these 2 numbers are plotted against each other, as shown in Figure 6. These plots revealed that a combination of AF and RF can be classified into 3 categories, which we call groups 1, 2, and 3, as circled in Figure 6. Group 1 represents medium AF and high to moderate RF values, typically represented by SMART stents, whereas group 2 has very high AF and very low RF values, as observed in all Volume 70, No. 1 : 2009 GASTROINTESTINAL ENDOSCOPY 39

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Figure 3. RF recorded against the diameter of the SEMS in expansion and contraction processes for uncovered SEMSs with the radial force measurement machine. A, Wallstent. B, Diamond stent. C, Za stent. D, SMART stent. E, Sinus Super Flex stent. F, Luminexx stent. G, SelfX stent. H, Zilver stent. I, ZEO stent.

Wallstents. Group 3 SEMSs represent very low AF and medium RF values exhibited by ComVi and several other stents. We would like to stress here that the mechanical properties of SEMSs must be understood in terms of not only RF or AF individually, but also in combination, because both properties are important in understanding clinical implications, as discussed later.

DISCUSSION Our first discussions are centered on RF observations. As described before, 2 characteristics were observed in the RF versus diameter curves in Figures 3 and 4. The first characteristic is a 2-step reduction in RF, which is typically observed in all nitinol stents regardless of whether they are braided or laser cut. This could be attributed to the unique nature of nitinol metal as discussed by Stoeckel et al,16 who reported a similar phenomenon and attributed it to ‘‘superelasticity’’ of nitinol metal. The second characteristic is a high RF at the beginning of expansion 40 GASTROINTESTINAL ENDOSCOPY Volume 70, No. 1 : 2009

and a sharp drop with an increase of diameter. This was observed in all braided SEMSs. This initial high RF value may be due to a wire crossover structure of braided SEMSs. It is assumed that this crossover wire causes friction in an initial expansion motion to make the RF values high. As the diameter increases, the crossover points are released and RF sharply drops, followed by a straight decline of RF to 0 at fully expanded state. It is interesting to note that by this RF measurement the type and material of SEMSs can be identified. Our second discussion is centered on AF observations. Manufacturers of stents usually pay more attention to RF with almost no regard for AF, as evidenced by the abundant literature on RF and the existence of excellent commercial RF measurement machines, contrasted with the dearth of literature and no commercial measurement machines for AF. It is our belief that the current article is the first attempt to measure and demonstrate the importance of AF. We defined AF as a recovery force to keep the stent straight after it was bent. In this study, we found that all Wallstents had quite high AF values. On www.giejournal.org

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Radial and axial force in biliary metallic stents

Figure 4. RF recorded against the diameter of the SEMS in expansion and contraction processes for covered SEMSs with the radial force measurement machine. A, Silicon-covered Wallstent. B, Polyurethane-covered Wallstent. C, Polyurethane-covered Diamond stent. D, Viabil stent. E, ComVi stent.

the other hand, specially braided nitinol SEMSs such as ComVi and Viabil stents had very low AF, most likely because they did not have crossover structure of wires, as explained previously. There are 4 clinical events to which AF values may be related and that we would like to discuss. These are kinking, biliary wall damage, sludge formation, and migration. Tendencies to kinking have been reported in Wallstents,3,4,8-11 which exhibited the highest AF values among the SEMSs measured in this study. It is also known that no kinking was reported in ComVi,19 Viabil,20,21 Diamond,10,22 SelfX,23 and Zilver stents,24 all of which had very low AF values. Although further clarification is necessary, it is reasonable to assume that kinking tendencies may be attributed to high AF values. It is also reasonable to speculate that high AF stents may not fit and stay well in a biliary duct, thus causing biliary wall damage, sludge formation, and migration. In coronary arterial stenting, Gyongyosi et al25 reported that stent-straightening effect after deployment was a significant predictor of major adverse events. These results www.giejournal.org

Figure 5. Dependence of AF on the distance from bending point to the point that the force is applied: 20, 40, and 60 mm.

seem to indicate that lower AF is preferred from clinical standpoint. Volume 70, No. 1 : 2009 GASTROINTESTINAL ENDOSCOPY 41

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TABLE 2. RF and AF of each metallic stent

RF (No.) (4 mm)

AF (No.) 60 degrees (20 mm)

Wallstent

2.08

0.66

Diamond stent

4.42

0.29

Za-stent

4.14

0.30

SMART stent

14.1

0.46

Sinus Super Flex stent

8.59

0.44

Luminexx stent

6.55

0.51

SelfX stent

5.32

0.19

Zilver stent

4.34

0.22

ZEO stent

5.75

0.18

Silicone-covered Wallstent

3.41

0.95

Polyurethane-covered Wallstent

2.19

0.73

Polyurethane-covered Diamond stent

6.67

0.46

Viabil biliary stent

9.67

0.14

ComVi stent

7.67

0.04

Uncovered metallic stent

Covered metallic stent

AF at 20-mm distance from bending point, together with RF at 4-mm diameter of SEMSs.

At this point, we must discuss a related issue about stent length. It was demonstrated in this article that the longer the stent is, the lower the AF values. One may be inclined to conclude that a longer stent is preferred to prevent those adverse effects described above resulting from a lower AF value. However, before making such an assertion, one must consider that unrelated problems caused by, for example, closing bifurcations and covering cystic ducts, could become an issue. Our final discussion focuses on RF-AF plots for all the SEMSs in this study, as shown in Figure 6. As pointed out in the previous section, there are 3 typical groups identified among the samples. Group 1 has a combination of high to moderate RF values with a medium level of AF, as typically represented by SMART stent. It is true from clinical experiences that a rather high RF is necessary to maintain patency. However, an RF value that is too high may be a problem because it could exert an excessively high stress to the biliary wall, causing an increase of inflammation and blocking of orifice of side branches, the cystic duct, and the pancreatic duct. Group 2 represents SEMSs with very low RF and with very high AF. Typical samples are all Wallstents. We believe these stents may not be well accepted because AF is too high and RF could be too low to maintain patency. 42 GASTROINTESTINAL ENDOSCOPY Volume 70, No. 1 : 2009

Group 3 is believed to be the most balanced from a clinician’s standpoint. The group 3 stents provide very low AF to prevent kinking and other adverse effects, yet have a medium level of RF that is strong enough to keep the domain open for a smooth flow of bile but not too high as to cause damage to the biliary duct wall. As a clinician, the first author prefers ComVi stents because they maintain appropriately high values of RF for lesser migration but exhibit extremely low AF, most likely because of their unique doubly overlapped structure. Ingrowth is prevented by a thin e-PTFE membrane sandwiched between the 2 stents without being fixed to the wires. The best way to determine the most preferred level of RF and AF combination for sure is to prepare model samples that are made by the same stent materials and structures with varying RF and AF values. To do so, cooperation between clinicians and manufacturers is vitally necessary. We hope that some manufacturers will be encouraged to make such an effort.

CONCLUSION We measured both RF and AF of 14 different SEMSs. The RF and AF of SEMSs are strongly influenced by the www.giejournal.org

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Radial and axial force in biliary metallic stents

Figure 6. A plot of RF against AF for all the SEMSs tested. Group 1 showed high to moderate RF and moderate AF. Group 2 showed low RF and high AF. Group 3 showed moderate RF and low AF.

stent materials and structures. A discussion was made to correlate such mechanical properties with actual clinical experiences and to identify a preferred level of RF and AF combination. We believe that our efforts represent an initial stab at driving conclusions because not enough information on clinical/mechanical property relationships is currently available. It is imperative that more clinical data be accumulated with well-identified SEMSs.

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Received May 7, 2008. Accepted September 17, 2008. Current affiliations: Departments of Gastroenterology (H.I., Y.N., O.T., Y.I., Y.Y., H.Y., J.L., T.S., T.A., S.M., N.Y., N.S., K.H., T.T., N.T., M.T., M.O.) and Endoscopy and Endoscopic Surgery (T.K., M.O.), Faculty of Medicine, University of Tokyo, Research and Development Center, Zeon Medical Inc (Y.T.), Tokyo, Japan. Reprint requests: Hiroyuki Isayama, MD, PhD, Department of Gastroenterology, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. If you want to chat with an author of this article, you may contact him at [email protected].

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