닫기

Ex) Article Title, Author, Keywords

Review Article

Split Viewer

Journal of Digestive Cancer Research 2022; 10(2): 92-98

Published online December 1, 2022

https://doi.org/10.52927/jdcr.2022.10.2.92

© Korean Society of Gastrointestinal Cancer Research

Mechanical Property and Problems of the Self-expandable Metal Stent in Pancreaticobiliary Cancer


Thanawat Luangsukrerk1,2



1Division of General Internal Medicine, Department of Medicine, Faculty of Medicine, Chulalongkorn University and King Chulalongkorn Memorial Hospital, Thai Red Cross Society, 2Excellence Center for Gastrointestinal Endoscopy, King Chulalongkorn Memorial Hospital, Thai Red Cross Society, and Division of Gastroenterology, Department of Medicine, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand

Correspondence to :
Thanawat Luangsukrerk
E-mail: drthanawatl@gmail.com
https://orcid.org/0000-0002-5804-3068

Received: October 31, 2022; Accepted: November 16, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0). which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Self-expandable metal stent (SEMS) is effective for biliary drainage, especially in pancreaticobiliary cancer. The mechanical properties, material, and design of SEMS are important in preventing recurrent biliary obstruction and complication. Radial and chronic expansion forces play roles in preventing stent migration and collapse. Complications, such as stent impaction, cholecystitis, and pancreatitis, were related to the axial force. The nickel–titanium alloy shows more flexibility, conformability, and optimal axial force compared to previously used stainless steel. Additionally, the stent structure affected the mechanical properties of SEMS. Therefore, understanding the mechanical properties, material, and design of SEMS will provide the best outcome for biliary drainage, as well as better SEMS development.

KeywordsSelf expandable metal stents Pancreatic neoplasms Bile duct neoplasms Mechanical properties Recurrent biliary obstruction

The main problem of biliary drainage in malignant biliary obstruction is stent clogging. Stent diameter was considered an important factor. The self-expandable metal stent (SEMS) was first introduced for biliary stenting in the 1990s. The SEMS had a lower rate of recurrent biliary obstruction and re-intervention due to its larger diameter (10 mm) compared to conventional endoprostheses at that period [1,2]. However, the larger diameter of the stent, the more difficult to pass the stent through the tight stricture. In that era, the SEMS (Wallstent; Medinvent SA, Lausanne, Switzerland) was made using medical-grade stainless steel. It was braided into a tubular mesh. The stent was constrained on a small-diameter delivery catheter. After deployment, it returned to its original diameter by its expansion force. This development has been adopted from endovascular and urethral stents [3,4]. In the past 30 years, SEMS had been developed for many purposes for biliary drainage.

Mechanical properties of SEMS

Radial force and chronic expansion force

The radial force (RF) is the force to expand the SEMS against the tumor compression. The chronic expansion force is similar to RF, it is the force to maintain the expansion of SEMS after deployment [5]. These parameters are related to stent migration in case of the RF is lower than 4.0 N [6].

Axial force (AF)

The AF is the straightening force when the SEMS is bent. It also can be called an anti-bending force. The AF has a reverse correlation with the length of SEMS [7]. The AF affected the stent conformability and SEMS-related complications (e.g., pancreatitis, cholecystitis, stent impaction) [8].

Materials

Stainless steel

The medical-grade stainless steel was firstly used in the first biliary SEMS [1]. However, the discovery of a new nickel-titanium superelastic shape memorial alloy suited for SEMS. This alloy becomes more popular.

Nitinol

Nitinol was developed by The Naval Ordnance Laboratory to be used for the nose cone of a naval missile. This alloy is a mixture of nickel and titanium. Its name nitinol comes from Nickel Titanium Naval Ordnance Laboratory. Nitinol has a combination of properties that cannot be found in other materials. It has the property of shape memory, superelasticity, high radial force, corrosive resistance, and biocompatibility [9]. Compare to stainless steel in the same stent design, the nitinol SEMS has higher RF and lower AF. Therefore, the nitinol stent seems to have more conformability to the bile duct.

Stent structures

The Nitinol SEMS mainly made by nitinol wire-based and nitinol tube-based stent designs [9,10]. There were main 3 types of stent design; braided, specially braided/knitted stent and laser-cut (Fig. 1). The nitinol wire can be welded or knitted into a tubular shape to form the braided and specially braided/knitted stent. The nitinol tube was laser-cut into a pattern to form the laser-cur stent.

Fig. 1.Stent structure: (A) Laser-cut, (B) Braided, (C) Specially braided.

Laser-cut SEMS made from a nitinol tube and the tube was cut into the pattern. This SEMS has a tight junction between the mesh of the stent. This design has more AF than other types. The specially braided/knitted SEMS made from nitinol wire and knitted into a tubular structure. The loose joint made this type of SEMS has less AF compared to the others [5].

SEMS related complication

Tumor overgrowth

In malignant biliary obstruction. The tumor overgrowth can occur around 4–7% [11]. The tumor overgrowth was caused by a tumor that grew into the proximal or distal opening of the stent. This probably occur by the length of the stent across the tumor was too short. A longer stent should be inserted if this condition occurs [5].

Tumor ingrowth

Tumor ingrowth is caused by tumors that grow across the mesh of SEMS. The incidence of tumor ingrowth was 29% in uncovered SEMS. This increases the re-intervention rate in patient with uncovered SEMS [9]. From this information, cover material has been developed in the 2000s with various types of material such as polytetrafluoroethylene (PTFE) (Fig. 2) [12], silicone [13], and polyurethane [14]. The tumor in growth rate lowered in covered SEMS. In comparison between covered and uncovered SEMS, the covered can reduce the re-intervention rate and total cost of biliary drainage [11]. However, the covered biliary stent’s weakest point was the migration because the covered material did not allow ingrowth or epithelial hyperplasia that played a role to prevent migration

Fig. 2.Polytetrafluoroethylene covered self-expandable metal stent.

Stent migration

The stent migration rate was 3–15% in covered SEMS [6,15,16]. This is the weakest point of covered SEMS because of lack of tumors in growth/epithelial hyperplasia. SEMS with low RF (< 4 N) was also associated with stent migration. The anti-migratory system was developed to improve stent patency rate.

Outer uncovered SEMS

This new type of stent was designed to have external uncovered wire by inserting PTFE membrane between two uncovered SEMS (ComVi; Taewoong Medical Inc., Seoul, Korea). This stent had lower migration rate compared to conventional covered SEMS (migration rate 2.1% vs. 17%) [17].

Flare end

Flare end of SEMS play important role in stent migration prevention in the ex-vivo study [18]. In vivo, SEMS with flare end (Fig. 3) seems to have lower stent migration compared to conventional SEMS [19,20]. However, the SEMS with flare end in previous study also had another type of anti-migratory system.

Fig. 3.Self-expandable metal stent with flare-end.

Flap

The flap was the structure that acted as the anchor of the SEMS. It showed 0–3% migration [21,22]. And the anchoring flap SEMS (MI Tech, Seoul, Korea) showed lower migration rate compared to flare-end SEMS in benign biliary stricture [21].

Bank/irregular cell width

Bank was the irregular stent diameter along caliber of stent. The SEMS with bank (modified Zeo stent; Zeon Medical Inc., Tokyo, Japan) had 0% migration rate compared to 70% of The SEMS without bank [19].

Irregular cell width (Fig. 4) SEMS (Bumpy; Taewoong Medical Inc., Seoul, Korea) provided different magnitudes of segmental RF depending on the cell sizes. There was no stent migration in benign pancreatic duct stricture during 3 months.

Fig. 4.Self-expandable metal stent with varying cell width.

Bank and irregular cell width were similarly formed into irregular caliber to prevent migration.

Bile duct kinking/stent impaction

This complication is related to poor stent conformability and AF. If SEMS had high AF, it will straighten the curve bile duct resulting in bile duct kinking at the end of the stent (Fig. 5). The risk factors of bile duct kinking were acute angulation of the bile duct and SEMS with high AF. The bile duct kinking or stent impaction can be reduced by using stent length as long as possible because longer SEMS demonstrated lower AF and higher conformability [10,23].

Fig. 5.Bile duct kinking/stent impaction (arrow) in patient with pancreatic cancer. CBD, common bile duct.

Cholecystitis

Acute cholecystitis occurred in 6.9% of patients with SEMS with median of 7 days from stent placement. Gallbladder drainages were required in 92% of patients. The risk factors of cholecystitis were tumor involvement of cystic duct opening, SEMS with high AF (≥ 0.4 N) and short stent length (≤ 60 mm) but the covered SEM was not associated with cholecystitis. Cholecystitis occurred 10.8% in high AF SEMS and 1.9% in low AF SEMS [8]. Recent developed flower-type SEMS which had five-petal-shape design with side groove showed less gallbladder blockage compared to conventional covered SEMS in animal model [24]. However, further clinical study was required to prove the concept of this SEMS.

Pancreatitis

Pancreatitis occurred in 6% of patients with SEMS. Most of patients had mild pancreatitis. This was caused by compression of pancreatic orifice from stent expansion and compression by SEMS’s axial force. The risk factors of pancreatitis were non-pancreatic cancer, SEMS with high AF (≥ 0.4 N) and pancreatic duct injection. Pancreatitis occurred 8.3% in high AF SEMS and 2.1% in low AF SEMS [25].

Food impaction

Food impaction can occur after SEMS placement across the ampulla because it bypassed sphincter of Oddi. This allowed food to reflux into bile duct. The risk factors of food impaction were irregular inner surface of SEMS, uncovered/partial-covered SEMS and duodenal obstruction [26]. The irregular inner surface or uncovered SEMS may serve as anchor for the food to attach and accumulate in the SEMS [17]. Duodenal obstruction induced food stasis in duodenum caused food reflux and attach to the SEMS. The anti-reflux system was developed to prevent this condition. Many types of anti-reflux valve showed low rate of food impaction in pilot studies such as dome with cross [27] and s-shaped valve [28]. However, some types of anti-reflux valve unable to prevent food reflux [29,30] and cause stent occlusion by valve malfunction. Hu et al. [31] conducted randomized study to compare partially covered SEMS and SEMS with anti-reflux valve. This study showed lower stent dysfunction rate in SEMS with anti-reflux valve group. However, the main cause of stent dysfunction was tumor ingrowth. Lee et al. [32] developed the wind sock anti-reflux valve and conduct randomized controlled trial. This study showed SEMS with wind sock anti-reflux valve had less duodenobiliary reflux and higher stent patency rate in comparison with covered SEMS.

Understanding the mechanical properties of SEMS will provide a better outcome in patients with malignant biliary obstruction. In summary (Table 1), the SEMS with high RF prevents stent migration. The stent with low AF prevents complications from SEMS such as acute cholecystitis, pancreatitis and stent impaction/bile duct kinking. The covered SEMS was designed to prevent tumor ingrowth but its weakest point is the stent migration. Thus, the anti-migratory system can prevent stent migration in covered SEMS. This is important knowledge in developing and using the SEMS. However, SEMS and patient factors should be considered together to prevent recurrent biliary obstruction and complications.

Table 1 . Risk factors of recurrent biliary obstruction and complications caused by the self-expandable metal stent

Recurrent biliary obstructionStent factorPatient factor
Tumor overgrowthShort stent lengthMalignant stricture
Tumor ingrowthUncovered/partially covered typeMalignant stricture
Stent migrationCovered typeBenign stricture
Low radial forceChemotherapy
High axial force
Lack of anti-migratory system
Stent impaction/bile duct kinkingHigh axial forceAcute angle of bile duct
Short stent length (causing to high axial force)
Food impactionPlacement across papillaDelayed duodenal motility
Uncovered/partially covered typeDuodenal obstruction
Lack of anti-reflux system
Complications
CholecystitisHigh axial forceTumor involvement to cystic duct opening
Presence of gallstone
PancreatitisHigh axial forceNon-pancreatic tumor

Adapted from the article of Isayama et al. (Curr Gastroenterol Rep 2016;18:64) [5] with original copyright holder’s permission.


No potential conflict of interest relevant to this article was reported.

  1. Neuhaus H, Hagenmüller F, Classen M. Self-expanding biliary stents: preliminary clinical experience. Endoscopy 1989;21:225-228. https://doi.org/10.1055/s-2007-1012954.
    Pubmed CrossRef
  2. Huibregtse K, Cheng J, Coene PP, Fockens P, Tytgat GN. Endoscopic placement of expandable metal stents for biliary strictures--a preliminary report on experience with 33 patients. Endoscopy 1989;21:280-282. https://doi.org/10.1055/s-2007-1012969.
    Pubmed CrossRef
  3. Sigwart U, Puel J, Mirkovitch V, Joffre F, Kappenberger L. Intravascular stents to prevent occlusion and restenosis after transluminal angioplasty. N Engl J Med 1987;316:701-706. https://doi.org/10.1056/NEJM198703193161201.
    Pubmed CrossRef
  4. Milroy EJ, Chapple CR, Cooper JE, et al. A new treatment for urethral strictures. Lancet 1988;1:1424-1427. https://doi.org/10.1016/s0140-6736(88)92238-6.
    Pubmed CrossRef
  5. Isayama H, Nakai Y, Hamada T, Matsubara S, Kogure H, Koike K. Understanding the mechanical forces of self-expandable metal stents in the biliary ducts. Curr Gastroenterol Rep 2016;18:64. https://doi.org/10.1007/s11894-016-0538-5.
    Pubmed CrossRef
  6. Nakai Y, Isayama H, Kogure H, et al. Risk factors for covered metallic stent migration in patients with distal malignant biliary obstruction due to pancreatic cancer. J Gastroenterol Hepatol 2014;29:1744-1749. https://doi.org/10.1111/jgh.12602.
    Pubmed CrossRef
  7. Isayama H, Nakai Y, Toyokawa Y, et al. Measurement of radial and axial forces of biliary self-expandable metallic stents. Gastrointest Endosc 2009;70:37-44. https://doi.org/10.1016/j.gie.2008.09.032.
    Pubmed CrossRef
  8. Nakai Y, Isayama H, Kawakubo K, et al. Metallic stent with high axial force as a risk factor for cholecystitis in distal malignant biliary obstruction. J Gastroenterol Hepatol 2014;29:1557-1562. https://doi.org/10.1111/jgh.12582.
    Pubmed CrossRef
  9. Stoeckel D, Pelton A, Duerig T. Self-expanding nitinol stents: material and design considerations. Eur Radiol 2004;14:292-301. https://doi.org/10.1007/s00330-003-2022-5.
    Pubmed CrossRef
  10. Jeong S. Basic knowledge about metal stent development. Clin Endosc 2016;49:108-112. https://doi.org/10.5946/ce.2016.029.
    Pubmed KoreaMed CrossRef
  11. Isayama H, Komatsu Y, Tsujino T, et al. A prospective randomised study of "covered" versus "uncovered" diamond stents for the management of distal malignant biliary obstruction. Gut 2004;53:729-734. https://doi.org/10.1136/gut.2003.018945.
    Pubmed KoreaMed CrossRef
  12. Saito H, Sakurai Y, Takamura A, Horio K. [Biliary endoprosthesis using Gore-Tex covered expandable metallic stents: preliminary clinical evaluation]. Nihon Igaku Hoshasen Gakkai Zasshi 1994;54:180-182. Japanese.
    Pubmed CrossRef
  13. Silvis SE, Sievert CE Jr, Vennes JA, Abeyta BK, Brennecke LH. Comparison of covered versus uncovered wire mesh stents in the canine biliary tract. Gastrointest Endosc 1994;40:17-21. https://doi.org/10.1016/s0016-5107(94)70004-4.
    Pubmed CrossRef
  14. Thurnher SA, Lammer J, Thurnher MM, Winkelbauer F, Graf O, Wildling R. Covered self-expanding transhepatic biliary stents: clinical pilot study. Cardiovasc Intervent Radiol 1996;19:10-14. https://doi.org/10.1007/BF02560140.
    Pubmed CrossRef
  15. Kullman E, Frozanpor F, Söderlund C, et al. Covered versus uncovered self-expandable nitinol stents in the palliative treatment of malignant distal biliary obstruction: results from a randomized, multicenter study. Gastrointest Endosc 2010;72:915-923. https://doi.org/10.1016/j.gie.2010.07.036.
    Pubmed CrossRef
  16. Telford JJ, Carr-Locke DL, Baron TH, et al. A randomized trial comparing uncovered and partially covered self-expandable metal stents in the palliation of distal malignant biliary obstruction. Gastrointest Endosc 2010;72:907-914. https://doi.org/10.1016/j.gie.2010.08.021.
    Pubmed CrossRef
  17. Isayama H, Kawabe T, Nakai Y, et al. Management of distal malignant biliary obstruction with the ComVi stent, a new covered metallic stent. Surg Endosc 2010;24:131-137. https://doi.org/10.1007/s00464-009-0537-9.
    Pubmed CrossRef
  18. Minaga K, Kitano M, Imai H, et al. Evaluation of anti-migration properties of biliary covered self-expandable metal stents. World J Gastroenterol 2016;22:6917-6924. https://doi.org/10.3748/wjg.v22.i30.6917.
    Pubmed KoreaMed CrossRef
  19. Isayama H, Kawakubo K, Nakai Y, et al. A novel, fully covered laser-cut nitinol stent with antimigration properties for nonresectable distal malignant biliary obstruction: a multicenter feasibility study. Gut Liver 2013;7:725-730. https://doi.org/10.5009/gnl.2013.7.6.725.
    Pubmed KoreaMed CrossRef
  20. Moon SH, Kim MH, Park DH, et al. Modified fully covered self-expandable metal stents with antimigration features for benign pancreatic-duct strictures in advanced chronic pancreatitis, with a focus on the safety profile and reducing migration. Gastrointest Endosc 2010;72:86-91. https://doi.org/10.1016/j.gie.2010.01.063.
    Pubmed CrossRef
  21. Park DH, Lee SS, Lee TH, et al. Anchoring flap versus flared end, fully covered self-expandable metal stents to prevent migration in patients with benign biliary strictures: a multicenter, prospective, comparative pilot study (with videos). Gastrointest Endosc 2011;73:64-70. https://doi.org/10.1016/j.gie.2010.09.039.
    Pubmed CrossRef
  22. Mangiavillano B, Manes G, Baron TH, et al. The use of double lasso, fully covered self-expandable metal stents with new "anchoring flap" system in the treatment of benign biliary diseases. Dig Dis Sci 2014;59:2308-2313. https://doi.org/10.1007/s10620-014-3158-7.
    Pubmed CrossRef
  23. Yang KY, Ryu JK, Seo JK, et al. A comparison of the Niti-D biliary uncovered stent and the uncovered Wallstent in malignant biliary obstruction. Gastrointest Endosc 2009;70:45-51. https://doi.org/10.1016/j.gie.2008.10.029.
    Pubmed CrossRef
  24. Woo YS, Lee KH, Lee JK, et al. Novel flower-type covered metal stent to prevent cholecystitis: experimental study in a pig model. Surg Endosc 2016;30:1141-1145. https://doi.org/10.1007/s00464-015-4317-4.
    Pubmed CrossRef
  25. Kawakubo K, Isayama H, Nakai Y, et al. Risk factors for pancreatitis following transpapillary self-expandable metal stent placement. Surg Endosc 2012;26:771-776. https://doi.org/10.1007/s00464-011-1950-4.
    Pubmed CrossRef
  26. Isayama H, Nakai Y, Hamada T, Yamamoto N, Koike K. Development of an ideal self-expandable metallic stent design. Gastrointest Interv 2015;4:46-49. https://doi.org/10.1016/j.gii.2015.03.002.
    CrossRef
  27. Hu B, Wang TT, Shi ZM, et al. A novel antireflux metal stent for the palliation of biliary malignancies: a pilot feasibility study (with video). Gastrointest Endosc 2011;73:143-148. https://doi.org/10.1016/j.gie.2010.08.048.
    Pubmed CrossRef
  28. Lee KJ, Chung MJ, Park JY, et al. Clinical advantages of a metal stent with an S-shaped anti-reflux valve in malignant biliary obstruction. Dig Endosc 2013;25:308-312. https://doi.org/10.1111/j.1443-1661.2012.01382.x.
    Pubmed CrossRef
  29. Kim DU, Kwon CI, Kang DH, Ko KH, Hong SP. New antireflux self-expandable metal stent for malignant lower biliary obstruction: in vitro and in vivo preliminary study. Dig Endosc 2013;25:60-66. https://doi.org/10.1111/j.1443-1661.2012.01324.x.
    Pubmed CrossRef
  30. Hamada T, Isayama H, Nakai Y, et al. Antireflux metal stent as a first-line metal stent for distal malignant biliary obstruction: a pilot study. Gut Liver 2017;11:142-148. https://doi.org/10.5009/gnl15579.
    Pubmed KoreaMed CrossRef
  31. Hu B, Wang TT, Wu J, Shi ZM, Gao DJ, Pan YM. Antireflux stents to reduce the risk of cholangitis in patients with malignant biliary strictures: a randomized trial. Endoscopy 2014;46:120-126. https://doi.org/10.1055/s-0034-1364872.
    Pubmed CrossRef
  32. Lee YN, Moon JH, Choi HJ, et al. Effectiveness of a newly designed antireflux valve metal stent to reduce duodenobiliary reflux in patients with unresectable distal malignant biliary obstruction: a randomized, controlled pilot study (with videos). Gastrointest Endosc 2016;83:404-412. https://doi.org/10.1016/j.gie.2015.08.084.
    Pubmed CrossRef

Article

Review Article

Journal of Digestive Cancer Research 2022; 10(2): 92-98

Published online December 1, 2022 https://doi.org/10.52927/jdcr.2022.10.2.92

Copyright © Korean Society of Gastrointestinal Cancer Research.

Mechanical Property and Problems of the Self-expandable Metal Stent in Pancreaticobiliary Cancer

Thanawat Luangsukrerk1,2

1Division of General Internal Medicine, Department of Medicine, Faculty of Medicine, Chulalongkorn University and King Chulalongkorn Memorial Hospital, Thai Red Cross Society, 2Excellence Center for Gastrointestinal Endoscopy, King Chulalongkorn Memorial Hospital, Thai Red Cross Society, and Division of Gastroenterology, Department of Medicine, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand

Correspondence to:Thanawat Luangsukrerk
E-mail: drthanawatl@gmail.com
https://orcid.org/0000-0002-5804-3068

Received: October 31, 2022; Accepted: November 16, 2022

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0). which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Self-expandable metal stent (SEMS) is effective for biliary drainage, especially in pancreaticobiliary cancer. The mechanical properties, material, and design of SEMS are important in preventing recurrent biliary obstruction and complication. Radial and chronic expansion forces play roles in preventing stent migration and collapse. Complications, such as stent impaction, cholecystitis, and pancreatitis, were related to the axial force. The nickel–titanium alloy shows more flexibility, conformability, and optimal axial force compared to previously used stainless steel. Additionally, the stent structure affected the mechanical properties of SEMS. Therefore, understanding the mechanical properties, material, and design of SEMS will provide the best outcome for biliary drainage, as well as better SEMS development.

Keywords: Self expandable metal stents, Pancreatic neoplasms, Bile duct neoplasms, Mechanical properties, Recurrent biliary obstruction

INTRODUCTION

The main problem of biliary drainage in malignant biliary obstruction is stent clogging. Stent diameter was considered an important factor. The self-expandable metal stent (SEMS) was first introduced for biliary stenting in the 1990s. The SEMS had a lower rate of recurrent biliary obstruction and re-intervention due to its larger diameter (10 mm) compared to conventional endoprostheses at that period [1,2]. However, the larger diameter of the stent, the more difficult to pass the stent through the tight stricture. In that era, the SEMS (Wallstent; Medinvent SA, Lausanne, Switzerland) was made using medical-grade stainless steel. It was braided into a tubular mesh. The stent was constrained on a small-diameter delivery catheter. After deployment, it returned to its original diameter by its expansion force. This development has been adopted from endovascular and urethral stents [3,4]. In the past 30 years, SEMS had been developed for many purposes for biliary drainage.

MAIN SUBJECTS

Mechanical properties of SEMS

Radial force and chronic expansion force

The radial force (RF) is the force to expand the SEMS against the tumor compression. The chronic expansion force is similar to RF, it is the force to maintain the expansion of SEMS after deployment [5]. These parameters are related to stent migration in case of the RF is lower than 4.0 N [6].

Axial force (AF)

The AF is the straightening force when the SEMS is bent. It also can be called an anti-bending force. The AF has a reverse correlation with the length of SEMS [7]. The AF affected the stent conformability and SEMS-related complications (e.g., pancreatitis, cholecystitis, stent impaction) [8].

Materials

Stainless steel

The medical-grade stainless steel was firstly used in the first biliary SEMS [1]. However, the discovery of a new nickel-titanium superelastic shape memorial alloy suited for SEMS. This alloy becomes more popular.

Nitinol

Nitinol was developed by The Naval Ordnance Laboratory to be used for the nose cone of a naval missile. This alloy is a mixture of nickel and titanium. Its name nitinol comes from Nickel Titanium Naval Ordnance Laboratory. Nitinol has a combination of properties that cannot be found in other materials. It has the property of shape memory, superelasticity, high radial force, corrosive resistance, and biocompatibility [9]. Compare to stainless steel in the same stent design, the nitinol SEMS has higher RF and lower AF. Therefore, the nitinol stent seems to have more conformability to the bile duct.

Stent structures

The Nitinol SEMS mainly made by nitinol wire-based and nitinol tube-based stent designs [9,10]. There were main 3 types of stent design; braided, specially braided/knitted stent and laser-cut (Fig. 1). The nitinol wire can be welded or knitted into a tubular shape to form the braided and specially braided/knitted stent. The nitinol tube was laser-cut into a pattern to form the laser-cur stent.

Figure 1. Stent structure: (A) Laser-cut, (B) Braided, (C) Specially braided.

Laser-cut SEMS made from a nitinol tube and the tube was cut into the pattern. This SEMS has a tight junction between the mesh of the stent. This design has more AF than other types. The specially braided/knitted SEMS made from nitinol wire and knitted into a tubular structure. The loose joint made this type of SEMS has less AF compared to the others [5].

SEMS related complication

Tumor overgrowth

In malignant biliary obstruction. The tumor overgrowth can occur around 4–7% [11]. The tumor overgrowth was caused by a tumor that grew into the proximal or distal opening of the stent. This probably occur by the length of the stent across the tumor was too short. A longer stent should be inserted if this condition occurs [5].

Tumor ingrowth

Tumor ingrowth is caused by tumors that grow across the mesh of SEMS. The incidence of tumor ingrowth was 29% in uncovered SEMS. This increases the re-intervention rate in patient with uncovered SEMS [9]. From this information, cover material has been developed in the 2000s with various types of material such as polytetrafluoroethylene (PTFE) (Fig. 2) [12], silicone [13], and polyurethane [14]. The tumor in growth rate lowered in covered SEMS. In comparison between covered and uncovered SEMS, the covered can reduce the re-intervention rate and total cost of biliary drainage [11]. However, the covered biliary stent’s weakest point was the migration because the covered material did not allow ingrowth or epithelial hyperplasia that played a role to prevent migration

Figure 2. Polytetrafluoroethylene covered self-expandable metal stent.

Stent migration

The stent migration rate was 3–15% in covered SEMS [6,15,16]. This is the weakest point of covered SEMS because of lack of tumors in growth/epithelial hyperplasia. SEMS with low RF (< 4 N) was also associated with stent migration. The anti-migratory system was developed to improve stent patency rate.

Outer uncovered SEMS

This new type of stent was designed to have external uncovered wire by inserting PTFE membrane between two uncovered SEMS (ComVi; Taewoong Medical Inc., Seoul, Korea). This stent had lower migration rate compared to conventional covered SEMS (migration rate 2.1% vs. 17%) [17].

Flare end

Flare end of SEMS play important role in stent migration prevention in the ex-vivo study [18]. In vivo, SEMS with flare end (Fig. 3) seems to have lower stent migration compared to conventional SEMS [19,20]. However, the SEMS with flare end in previous study also had another type of anti-migratory system.

Figure 3. Self-expandable metal stent with flare-end.

Flap

The flap was the structure that acted as the anchor of the SEMS. It showed 0–3% migration [21,22]. And the anchoring flap SEMS (MI Tech, Seoul, Korea) showed lower migration rate compared to flare-end SEMS in benign biliary stricture [21].

Bank/irregular cell width

Bank was the irregular stent diameter along caliber of stent. The SEMS with bank (modified Zeo stent; Zeon Medical Inc., Tokyo, Japan) had 0% migration rate compared to 70% of The SEMS without bank [19].

Irregular cell width (Fig. 4) SEMS (Bumpy; Taewoong Medical Inc., Seoul, Korea) provided different magnitudes of segmental RF depending on the cell sizes. There was no stent migration in benign pancreatic duct stricture during 3 months.

Figure 4. Self-expandable metal stent with varying cell width.

Bank and irregular cell width were similarly formed into irregular caliber to prevent migration.

Bile duct kinking/stent impaction

This complication is related to poor stent conformability and AF. If SEMS had high AF, it will straighten the curve bile duct resulting in bile duct kinking at the end of the stent (Fig. 5). The risk factors of bile duct kinking were acute angulation of the bile duct and SEMS with high AF. The bile duct kinking or stent impaction can be reduced by using stent length as long as possible because longer SEMS demonstrated lower AF and higher conformability [10,23].

Figure 5. Bile duct kinking/stent impaction (arrow) in patient with pancreatic cancer. CBD, common bile duct.

Cholecystitis

Acute cholecystitis occurred in 6.9% of patients with SEMS with median of 7 days from stent placement. Gallbladder drainages were required in 92% of patients. The risk factors of cholecystitis were tumor involvement of cystic duct opening, SEMS with high AF (≥ 0.4 N) and short stent length (≤ 60 mm) but the covered SEM was not associated with cholecystitis. Cholecystitis occurred 10.8% in high AF SEMS and 1.9% in low AF SEMS [8]. Recent developed flower-type SEMS which had five-petal-shape design with side groove showed less gallbladder blockage compared to conventional covered SEMS in animal model [24]. However, further clinical study was required to prove the concept of this SEMS.

Pancreatitis

Pancreatitis occurred in 6% of patients with SEMS. Most of patients had mild pancreatitis. This was caused by compression of pancreatic orifice from stent expansion and compression by SEMS’s axial force. The risk factors of pancreatitis were non-pancreatic cancer, SEMS with high AF (≥ 0.4 N) and pancreatic duct injection. Pancreatitis occurred 8.3% in high AF SEMS and 2.1% in low AF SEMS [25].

Food impaction

Food impaction can occur after SEMS placement across the ampulla because it bypassed sphincter of Oddi. This allowed food to reflux into bile duct. The risk factors of food impaction were irregular inner surface of SEMS, uncovered/partial-covered SEMS and duodenal obstruction [26]. The irregular inner surface or uncovered SEMS may serve as anchor for the food to attach and accumulate in the SEMS [17]. Duodenal obstruction induced food stasis in duodenum caused food reflux and attach to the SEMS. The anti-reflux system was developed to prevent this condition. Many types of anti-reflux valve showed low rate of food impaction in pilot studies such as dome with cross [27] and s-shaped valve [28]. However, some types of anti-reflux valve unable to prevent food reflux [29,30] and cause stent occlusion by valve malfunction. Hu et al. [31] conducted randomized study to compare partially covered SEMS and SEMS with anti-reflux valve. This study showed lower stent dysfunction rate in SEMS with anti-reflux valve group. However, the main cause of stent dysfunction was tumor ingrowth. Lee et al. [32] developed the wind sock anti-reflux valve and conduct randomized controlled trial. This study showed SEMS with wind sock anti-reflux valve had less duodenobiliary reflux and higher stent patency rate in comparison with covered SEMS.

CONCLUSION

Understanding the mechanical properties of SEMS will provide a better outcome in patients with malignant biliary obstruction. In summary (Table 1), the SEMS with high RF prevents stent migration. The stent with low AF prevents complications from SEMS such as acute cholecystitis, pancreatitis and stent impaction/bile duct kinking. The covered SEMS was designed to prevent tumor ingrowth but its weakest point is the stent migration. Thus, the anti-migratory system can prevent stent migration in covered SEMS. This is important knowledge in developing and using the SEMS. However, SEMS and patient factors should be considered together to prevent recurrent biliary obstruction and complications.

Table 1 . Risk factors of recurrent biliary obstruction and complications caused by the self-expandable metal stent.

Recurrent biliary obstructionStent factorPatient factor
Tumor overgrowthShort stent lengthMalignant stricture
Tumor ingrowthUncovered/partially covered typeMalignant stricture
Stent migrationCovered typeBenign stricture
Low radial forceChemotherapy
High axial force
Lack of anti-migratory system
Stent impaction/bile duct kinkingHigh axial forceAcute angle of bile duct
Short stent length (causing to high axial force)
Food impactionPlacement across papillaDelayed duodenal motility
Uncovered/partially covered typeDuodenal obstruction
Lack of anti-reflux system
Complications
CholecystitisHigh axial forceTumor involvement to cystic duct opening
Presence of gallstone
PancreatitisHigh axial forceNon-pancreatic tumor

Adapted from the article of Isayama et al. (Curr Gastroenterol Rep 2016;18:64) [5] with original copyright holder’s permission..


FUNDING

None.

CONFLICTS OF INTEREST

No potential conflict of interest relevant to this article was reported.

Fig 1.

Figure 1.Stent structure: (A) Laser-cut, (B) Braided, (C) Specially braided.
Journal of Digestive Cancer Research 2022; 10: 92-98https://doi.org/10.52927/jdcr.2022.10.2.92

Fig 2.

Figure 2.Polytetrafluoroethylene covered self-expandable metal stent.
Journal of Digestive Cancer Research 2022; 10: 92-98https://doi.org/10.52927/jdcr.2022.10.2.92

Fig 3.

Figure 3.Self-expandable metal stent with flare-end.
Journal of Digestive Cancer Research 2022; 10: 92-98https://doi.org/10.52927/jdcr.2022.10.2.92

Fig 4.

Figure 4.Self-expandable metal stent with varying cell width.
Journal of Digestive Cancer Research 2022; 10: 92-98https://doi.org/10.52927/jdcr.2022.10.2.92

Fig 5.

Figure 5.Bile duct kinking/stent impaction (arrow) in patient with pancreatic cancer. CBD, common bile duct.
Journal of Digestive Cancer Research 2022; 10: 92-98https://doi.org/10.52927/jdcr.2022.10.2.92

Table 1 . Risk factors of recurrent biliary obstruction and complications caused by the self-expandable metal stent.

Recurrent biliary obstructionStent factorPatient factor
Tumor overgrowthShort stent lengthMalignant stricture
Tumor ingrowthUncovered/partially covered typeMalignant stricture
Stent migrationCovered typeBenign stricture
Low radial forceChemotherapy
High axial force
Lack of anti-migratory system
Stent impaction/bile duct kinkingHigh axial forceAcute angle of bile duct
Short stent length (causing to high axial force)
Food impactionPlacement across papillaDelayed duodenal motility
Uncovered/partially covered typeDuodenal obstruction
Lack of anti-reflux system
Complications
CholecystitisHigh axial forceTumor involvement to cystic duct opening
Presence of gallstone
PancreatitisHigh axial forceNon-pancreatic tumor

Adapted from the article of Isayama et al. (Curr Gastroenterol Rep 2016;18:64) [5] with original copyright holder’s permission..


References

  1. Neuhaus H, Hagenmüller F, Classen M. Self-expanding biliary stents: preliminary clinical experience. Endoscopy 1989;21:225-228. https://doi.org/10.1055/s-2007-1012954.
    Pubmed CrossRef
  2. Huibregtse K, Cheng J, Coene PP, Fockens P, Tytgat GN. Endoscopic placement of expandable metal stents for biliary strictures--a preliminary report on experience with 33 patients. Endoscopy 1989;21:280-282. https://doi.org/10.1055/s-2007-1012969.
    Pubmed CrossRef
  3. Sigwart U, Puel J, Mirkovitch V, Joffre F, Kappenberger L. Intravascular stents to prevent occlusion and restenosis after transluminal angioplasty. N Engl J Med 1987;316:701-706. https://doi.org/10.1056/NEJM198703193161201.
    Pubmed CrossRef
  4. Milroy EJ, Chapple CR, Cooper JE, et al. A new treatment for urethral strictures. Lancet 1988;1:1424-1427. https://doi.org/10.1016/s0140-6736(88)92238-6.
    Pubmed CrossRef
  5. Isayama H, Nakai Y, Hamada T, Matsubara S, Kogure H, Koike K. Understanding the mechanical forces of self-expandable metal stents in the biliary ducts. Curr Gastroenterol Rep 2016;18:64. https://doi.org/10.1007/s11894-016-0538-5.
    Pubmed CrossRef
  6. Nakai Y, Isayama H, Kogure H, et al. Risk factors for covered metallic stent migration in patients with distal malignant biliary obstruction due to pancreatic cancer. J Gastroenterol Hepatol 2014;29:1744-1749. https://doi.org/10.1111/jgh.12602.
    Pubmed CrossRef
  7. Isayama H, Nakai Y, Toyokawa Y, et al. Measurement of radial and axial forces of biliary self-expandable metallic stents. Gastrointest Endosc 2009;70:37-44. https://doi.org/10.1016/j.gie.2008.09.032.
    Pubmed CrossRef
  8. Nakai Y, Isayama H, Kawakubo K, et al. Metallic stent with high axial force as a risk factor for cholecystitis in distal malignant biliary obstruction. J Gastroenterol Hepatol 2014;29:1557-1562. https://doi.org/10.1111/jgh.12582.
    Pubmed CrossRef
  9. Stoeckel D, Pelton A, Duerig T. Self-expanding nitinol stents: material and design considerations. Eur Radiol 2004;14:292-301. https://doi.org/10.1007/s00330-003-2022-5.
    Pubmed CrossRef
  10. Jeong S. Basic knowledge about metal stent development. Clin Endosc 2016;49:108-112. https://doi.org/10.5946/ce.2016.029.
    Pubmed KoreaMed CrossRef
  11. Isayama H, Komatsu Y, Tsujino T, et al. A prospective randomised study of "covered" versus "uncovered" diamond stents for the management of distal malignant biliary obstruction. Gut 2004;53:729-734. https://doi.org/10.1136/gut.2003.018945.
    Pubmed KoreaMed CrossRef
  12. Saito H, Sakurai Y, Takamura A, Horio K. [Biliary endoprosthesis using Gore-Tex covered expandable metallic stents: preliminary clinical evaluation]. Nihon Igaku Hoshasen Gakkai Zasshi 1994;54:180-182. Japanese.
    Pubmed CrossRef
  13. Silvis SE, Sievert CE Jr, Vennes JA, Abeyta BK, Brennecke LH. Comparison of covered versus uncovered wire mesh stents in the canine biliary tract. Gastrointest Endosc 1994;40:17-21. https://doi.org/10.1016/s0016-5107(94)70004-4.
    Pubmed CrossRef
  14. Thurnher SA, Lammer J, Thurnher MM, Winkelbauer F, Graf O, Wildling R. Covered self-expanding transhepatic biliary stents: clinical pilot study. Cardiovasc Intervent Radiol 1996;19:10-14. https://doi.org/10.1007/BF02560140.
    Pubmed CrossRef
  15. Kullman E, Frozanpor F, Söderlund C, et al. Covered versus uncovered self-expandable nitinol stents in the palliative treatment of malignant distal biliary obstruction: results from a randomized, multicenter study. Gastrointest Endosc 2010;72:915-923. https://doi.org/10.1016/j.gie.2010.07.036.
    Pubmed CrossRef
  16. Telford JJ, Carr-Locke DL, Baron TH, et al. A randomized trial comparing uncovered and partially covered self-expandable metal stents in the palliation of distal malignant biliary obstruction. Gastrointest Endosc 2010;72:907-914. https://doi.org/10.1016/j.gie.2010.08.021.
    Pubmed CrossRef
  17. Isayama H, Kawabe T, Nakai Y, et al. Management of distal malignant biliary obstruction with the ComVi stent, a new covered metallic stent. Surg Endosc 2010;24:131-137. https://doi.org/10.1007/s00464-009-0537-9.
    Pubmed CrossRef
  18. Minaga K, Kitano M, Imai H, et al. Evaluation of anti-migration properties of biliary covered self-expandable metal stents. World J Gastroenterol 2016;22:6917-6924. https://doi.org/10.3748/wjg.v22.i30.6917.
    Pubmed KoreaMed CrossRef
  19. Isayama H, Kawakubo K, Nakai Y, et al. A novel, fully covered laser-cut nitinol stent with antimigration properties for nonresectable distal malignant biliary obstruction: a multicenter feasibility study. Gut Liver 2013;7:725-730. https://doi.org/10.5009/gnl.2013.7.6.725.
    Pubmed KoreaMed CrossRef
  20. Moon SH, Kim MH, Park DH, et al. Modified fully covered self-expandable metal stents with antimigration features for benign pancreatic-duct strictures in advanced chronic pancreatitis, with a focus on the safety profile and reducing migration. Gastrointest Endosc 2010;72:86-91. https://doi.org/10.1016/j.gie.2010.01.063.
    Pubmed CrossRef
  21. Park DH, Lee SS, Lee TH, et al. Anchoring flap versus flared end, fully covered self-expandable metal stents to prevent migration in patients with benign biliary strictures: a multicenter, prospective, comparative pilot study (with videos). Gastrointest Endosc 2011;73:64-70. https://doi.org/10.1016/j.gie.2010.09.039.
    Pubmed CrossRef
  22. Mangiavillano B, Manes G, Baron TH, et al. The use of double lasso, fully covered self-expandable metal stents with new "anchoring flap" system in the treatment of benign biliary diseases. Dig Dis Sci 2014;59:2308-2313. https://doi.org/10.1007/s10620-014-3158-7.
    Pubmed CrossRef
  23. Yang KY, Ryu JK, Seo JK, et al. A comparison of the Niti-D biliary uncovered stent and the uncovered Wallstent in malignant biliary obstruction. Gastrointest Endosc 2009;70:45-51. https://doi.org/10.1016/j.gie.2008.10.029.
    Pubmed CrossRef
  24. Woo YS, Lee KH, Lee JK, et al. Novel flower-type covered metal stent to prevent cholecystitis: experimental study in a pig model. Surg Endosc 2016;30:1141-1145. https://doi.org/10.1007/s00464-015-4317-4.
    Pubmed CrossRef
  25. Kawakubo K, Isayama H, Nakai Y, et al. Risk factors for pancreatitis following transpapillary self-expandable metal stent placement. Surg Endosc 2012;26:771-776. https://doi.org/10.1007/s00464-011-1950-4.
    Pubmed CrossRef
  26. Isayama H, Nakai Y, Hamada T, Yamamoto N, Koike K. Development of an ideal self-expandable metallic stent design. Gastrointest Interv 2015;4:46-49. https://doi.org/10.1016/j.gii.2015.03.002.
    CrossRef
  27. Hu B, Wang TT, Shi ZM, et al. A novel antireflux metal stent for the palliation of biliary malignancies: a pilot feasibility study (with video). Gastrointest Endosc 2011;73:143-148. https://doi.org/10.1016/j.gie.2010.08.048.
    Pubmed CrossRef
  28. Lee KJ, Chung MJ, Park JY, et al. Clinical advantages of a metal stent with an S-shaped anti-reflux valve in malignant biliary obstruction. Dig Endosc 2013;25:308-312. https://doi.org/10.1111/j.1443-1661.2012.01382.x.
    Pubmed CrossRef
  29. Kim DU, Kwon CI, Kang DH, Ko KH, Hong SP. New antireflux self-expandable metal stent for malignant lower biliary obstruction: in vitro and in vivo preliminary study. Dig Endosc 2013;25:60-66. https://doi.org/10.1111/j.1443-1661.2012.01324.x.
    Pubmed CrossRef
  30. Hamada T, Isayama H, Nakai Y, et al. Antireflux metal stent as a first-line metal stent for distal malignant biliary obstruction: a pilot study. Gut Liver 2017;11:142-148. https://doi.org/10.5009/gnl15579.
    Pubmed KoreaMed CrossRef
  31. Hu B, Wang TT, Wu J, Shi ZM, Gao DJ, Pan YM. Antireflux stents to reduce the risk of cholangitis in patients with malignant biliary strictures: a randomized trial. Endoscopy 2014;46:120-126. https://doi.org/10.1055/s-0034-1364872.
    Pubmed CrossRef
  32. Lee YN, Moon JH, Choi HJ, et al. Effectiveness of a newly designed antireflux valve metal stent to reduce duodenobiliary reflux in patients with unresectable distal malignant biliary obstruction: a randomized, controlled pilot study (with videos). Gastrointest Endosc 2016;83:404-412. https://doi.org/10.1016/j.gie.2015.08.084.
    Pubmed CrossRef

Journal Info

JDCR
Vol.12 No.2
August 20, 2024
eISSN : 2950-9505
pISSN : 2950-9394
Frequency: Triannual

open access

Article Tools

Stats or Metrics

Share this article on

  • line

Journal of Digestive Cancer Research

eISSN 2950-9505
pISSN 2950-9394

  • 2021
  • 2022
  • 2023
  • 2024