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
Correspondence to :
Thanawat Luangsukrerk
E-mail: drthanawatl@gmail.com
https://orcid.org/0000-0002-5804-3068
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.
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].
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].
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 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.
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.
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].
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 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
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.
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 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.
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 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.
Bank and irregular cell width were similarly formed into irregular caliber to prevent migration.
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].
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 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 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 obstruction | Stent factor | Patient factor |
---|---|---|
Tumor overgrowth | Short stent length | Malignant stricture |
Tumor ingrowth | Uncovered/partially covered type | Malignant stricture |
Stent migration | Covered type | Benign stricture |
Low radial force | Chemotherapy | |
High axial force | ||
Lack of anti-migratory system | ||
Stent impaction/bile duct kinking | High axial force | Acute angle of bile duct |
Short stent length (causing to high axial force) | ||
Food impaction | Placement across papilla | Delayed duodenal motility |
Uncovered/partially covered type | Duodenal obstruction | |
Lack of anti-reflux system | ||
Complications | ||
Cholecystitis | High axial force | Tumor involvement to cystic duct opening |
Presence of gallstone | ||
Pancreatitis | High axial force | Non-pancreatic tumor |
Adapted from the article of Isayama et al. (Curr Gastroenterol Rep 2016;18:64) [5] with original copyright holder’s permission.
None.
No potential conflict of interest relevant to this article was reported.
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.
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
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.
Keywords: Self 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.
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].
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].
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 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.
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.
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].
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 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
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.
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 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.
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 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.
Bank and irregular cell width were similarly formed into irregular caliber to prevent migration.
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].
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 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 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 obstruction | Stent factor | Patient factor |
---|---|---|
Tumor overgrowth | Short stent length | Malignant stricture |
Tumor ingrowth | Uncovered/partially covered type | Malignant stricture |
Stent migration | Covered type | Benign stricture |
Low radial force | Chemotherapy | |
High axial force | ||
Lack of anti-migratory system | ||
Stent impaction/bile duct kinking | High axial force | Acute angle of bile duct |
Short stent length (causing to high axial force) | ||
Food impaction | Placement across papilla | Delayed duodenal motility |
Uncovered/partially covered type | Duodenal obstruction | |
Lack of anti-reflux system | ||
Complications | ||
Cholecystitis | High axial force | Tumor involvement to cystic duct opening |
Presence of gallstone | ||
Pancreatitis | High axial force | Non-pancreatic tumor |
Adapted from the article of Isayama et al. (Curr Gastroenterol Rep 2016;18:64) [5] with original copyright holder’s permission..
None.
No potential conflict of interest relevant to this article was reported.
Table 1 . Risk factors of recurrent biliary obstruction and complications caused by the self-expandable metal stent.
Recurrent biliary obstruction | Stent factor | Patient factor |
---|---|---|
Tumor overgrowth | Short stent length | Malignant stricture |
Tumor ingrowth | Uncovered/partially covered type | Malignant stricture |
Stent migration | Covered type | Benign stricture |
Low radial force | Chemotherapy | |
High axial force | ||
Lack of anti-migratory system | ||
Stent impaction/bile duct kinking | High axial force | Acute angle of bile duct |
Short stent length (causing to high axial force) | ||
Food impaction | Placement across papilla | Delayed duodenal motility |
Uncovered/partially covered type | Duodenal obstruction | |
Lack of anti-reflux system | ||
Complications | ||
Cholecystitis | High axial force | Tumor involvement to cystic duct opening |
Presence of gallstone | ||
Pancreatitis | High axial force | Non-pancreatic tumor |
Adapted from the article of Isayama et al. (Curr Gastroenterol Rep 2016;18:64) [5] with original copyright holder’s permission..