Types of Plastic Stents

Most of today’s plastic stents are made of a polyethylene material. These stents come in external diameters that range from 5 french to 11.5 french. The most popular stent caliber for distal biliary decompression is 10 french and the most desirable lengths are 7 cm and 10 cm. Both the straight Amsterdam style and double pigtail stents are commercially available for biliary drainage, but virtually all endoscopists choose the straight stents because of their better patency performance relative to the pigtail stents. A single flap near each end of the straight stents provides the anchorage necessary to hold them in place. Unsatisfied with the typical 3-month patency of the polyethylene stents, the Hamburg group introduced a Teflon stent to take advantage of its lower coefficient of friction, compared to common plastic stents, and smaller chance of clogging without any sideholes. This new product was thought to be the ideal plastic stent and the early results showed it to be patent for a median duration of 64 weeks. However, the enthusiasm toward this specially designed stent waned quickly as a randomized prospective study that compared the Tannenbaum Teflon stent to conventional polyethylene stents showed no difference in mean 90-day stent patency.

Plastic Stenting Technique

Plastic stenting begins with guidewire passage through the stricture. A biliary sphincterotomy is not typically required for insertion of a single plastic stent. Likewise, stricture dilation is not usually required for placement of a single plastic stent except when there is an extremely tight blockage; in that case a 4-mm or 6-mm over-the-wire balloon dilator may be used. The length of the stent is selected by estimating the distance between the top of the stricture and the papilla against a known dimension, such as the width of the duodenoscope, based on fluoroscopic assessment. An alternative is to measure the distance of a catheter that has been pulled back from the top of the stricture to the papilla. The plastic stent is then loaded onto the tip of a delivery device. There are three types of stent delivery devices, but they all have an inner guiding catheter and a shorter external pusher catheter. Once the inner catheter has securely passed the stricture, the pusher is advanced forward to push the stent up the bile duct until it reaches the desired location. The guiding catheter and the guidewire are then removed, leaving the stent in place. There are two commercially available modified stenting devices that allow simultaneous removal of the stent and device while leaving the guidewire in place, in case a decision is made to retrieve the stent.

Metallic Stenting

The first commercially available metal biliary stent was introduced in 1989. It was made of individual stainless steel wires that were twined together. One of the first studies reported effectiveness of the 10-mm caliber metal stents without significant problems. A comparison between metallic and plastic stents showed that the median duration of patency (273 days) of the metal stent was significantly longer than that of a polyethylene stent (126 days). However, the overall median survival was 149 days and did not differ significantly between the plastic and metal stent groups. Not only were metallic stents more patent over time than plastic stents, but they were also associated with less cholangitis, shorter duration of hospitalization, and less overall costs than plastic stents. A meta-analysis was performed to analyze seven randomized controlled trials that aimed to compare plastic and metal stents. In total, 724 patients were randomized into one of the treatment arms. The meta-analysis showed that metallic stents were associated with a significantly reduced risk of stent occlusion at 4 months and a lower overall risk of recurrent biliary obstruction. This analysis also showed that the metal stents were more cost effective, as the additional ERCP accounted for substantial cost. But a recent Korean study demonstrated that even in countries where ERCP costs were lower than those of metal stents, metal biliary stents were still the first-line treatment because they offered better palliation without adding a significant cost in palliating the malignant biliary obstruction. Hence, these studies provided strong support for the practice of metal stenting as the first-line palliative modality for malignant biliary obstruction that is typically caused by pancreatic carcinoma. However, these stents were appropriate only when treating definitively unresectable cancers because the strong adherence of these open mesh stents to the biliary tissue made them difficult to remove from the bile duct during dissection and creation of surgical anastomoses.

Early self-expanding metal stents had no covers to prevent tissue penetration through the wire mesh. Indeed, stent failures were noted to be caused by tumor ingrowth across the metal wires. Reactive hyperplastic reactions may also occur at either end of the stent where it is in close contact with the biliary or duodenal tissue. Creating a barrier to tissue ingrowth and, in turn, prolonging stent patency was the driving force that led to the development of partially covered stents. Today, several partially covered metal stents are available. The short, uncovered ends of these stents are designed to provide adherence to the biliary tissue whereas the large, covered portion provides protection from tissue penetration into the stent lumen.

Even though the partially covered stents were designed to prevent tumor ingrowth, occasionally they are still difficult to remove. As neoadjuvant therapy for tumor downstaging and surgical resection becomes an increasingly common practice, the ease of removing these stents is an important consideration in the selection of a metal stent. Fully covered stents became available a few years ago and their removal at the time of a pancreaticoduodenectomy became feasible in virtually all patients.

Multiple studies have compared covered to uncovered metal stents in palliating malignant biliary obstruction. Yoon and colleagues found no significant differences in stent patency at 100, 200, 300, and 400 days between the two groups of metal stents. A randomized trial showed that the median time to recurrent biliary obstruction was 711 days for the uncovered and 357 days for the partially covered self-expanding metal stents. In addition, serious adverse events occurred significantly more frequently with the partially covered stents than with the uncovered stents (62% vs 44%, P <.05), mainly due to stent migration (12% vs 0, P <.01). Further, a European study that enrolled 400 patients and randomized them to covered or uncovered stents showed no significant differences in stent patency, patient survival, or complication rates between the two groups. The metal stents used in this study were made by a different manufacturer from the metal stents used in the other trials, but the conclusions of these independent studies were identical, disputing any potential advantage of the covered metal stents over their uncovered counterparts. Even though there are the suspicions that the stent cover would contribute to cholecystitis and pancreatitis, there is no evidence that it is the case.

Endoscopic Ultrasound-Guided Bile Duct Stenting

When the bile duct cannot be accessed through the papilla, or when the duodenum is completely obstructed by a large pancreatic tumor, an interventional radiologist or a surgeon is enlisted to perform a drainage or bypass procedure. However, endoscopic ultrasound (EUS) allows gastrointestinal interventionists to puncture the bile duct directly from the duodenal bulb or stomach. This allows the possibility to insert a guidewire pass the stricture and the major papilla to perform biliary stenting and this technique is commonly referred to as the endoscopic or EUS rendezvous procedure. Roughly two-thirds of the cases attempted in this manner were reported to be successful in draining the obstructed bile duct, ending with biliary stents crossing the major papillae. EUS-guided choledochoduodenostomy and EUS-guided hepatogastrostomy have been performed when the duodenum or major papilla cannot be accessed. Plastic or metal stents were inserted directly across the duodenum or stomach into the extrahepatic or intrahepatic bile duct. These highly invasive procedures demand great technical skills and clinical experience and should not be regarded as standard treatment options at the present time.

Drug-Eluting Biliary Stents

Taking biliary stenting one step further, experimental treatment of malignant stricture with drug-eluting metallic stents has been proposed to extend the duration of biliary patency. Paclitaxel, a chemotherapeutic agent, was mixed in a liquid form with polyurethane and tetrahydrofuran to create a stent membrane that slowly released paclitaxel. A low serum level of paclitaxel could be detected in patients stented for longer than 50 days. The mean patency of these covered, paclitaxel-eluting stents was 429 days in 21 patients with unresectable malignant biliary obstruction. This seemingly prolonged patency may serve as supporting evidence to further develop drug-eluting stents in the future.

Metal Stenting Technique

The technique of metal stent placement is similar to that for plastic stenting, although there are some key differences. The first step is to pass a wire across the stricture. As with plastic stenting, dilating a distal bile duct stricture is rarely necessary. Most commercially available metal stents have markings on the proximal and distal ends to guide deployment. Some even have a “point of no return” radio-opaque mark that denotes the position of stent release beyond which the stent cannot be recaptured in case it is necessary to adjust the stenting position. The stainless steel alloy material of some stents is radio-opaque and all wire elements can be seen on fluoroscopy. However, nitinol-based wires are not radio-dense and therefore are marked with radio-opaque spots that are typically placed on both ends of the stents to guide stent release. Virtually all these stents have the tendency to propel forward during deployment; therefore, it is crucial to open up the stent very slowly to avoid inadvertent placement of the entire stent above the stricture. Gradual pullback of the stent is typically needed to compensate for the upward thrust of these stents during deployment. Shortening of the stents is also common as they are allowed to expand. Virtually all metal stents show a temporary waist at the point of maximal obstruction that disappears gradually over the next few days as the stents expand to their full calibers. Although there is no scientific evidence of how the covered metal stents spontaneously migrate, it is logical to assume that the unidirectional expansive force contributes to stent movement. Therefore, if one side of the waist is larger and longer than the other side, there will be the tendency for the stent to shift toward the side that is already more expanded. If a partially or fully covered stent is used, it is best to place the stent such that the waist is located in the center of the stent.

Types of Plastic Stents

Most of today’s plastic stents are made of a polyethylene material. These stents come in external diameters that range from 5 french to 11.5 french. The most popular stent caliber for distal biliary decompression is 10 french and the most desirable lengths are 7 cm and 10 cm. Both the straight Amsterdam style and double pigtail stents are commercially available for biliary drainage, but virtually all endoscopists choose the straight stents because of their better patency performance relative to the pigtail stents. A single flap near each end of the straight stents provides the anchorage necessary to hold them in place. Unsatisfied with the typical 3-month patency of the polyethylene stents, the Hamburg group introduced a Teflon stent to take advantage of its lower coefficient of friction, compared to common plastic stents, and smaller chance of clogging without any sideholes. This new product was thought to be the ideal plastic stent and the early results showed it to be patent for a median duration of 64 weeks. However, the enthusiasm toward this specially designed stent waned quickly as a randomized prospective study that compared the Tannenbaum Teflon stent to conventional polyethylene stents showed no difference in mean 90-day stent patency.

Plastic Stenting Technique

Plastic stenting begins with guidewire passage through the stricture. A biliary sphincterotomy is not typically required for insertion of a single plastic stent. Likewise, stricture dilation is not usually required for placement of a single plastic stent except when there is an extremely tight blockage; in that case a 4-mm or 6-mm over-the-wire balloon dilator may be used. The length of the stent is selected by estimating the distance between the top of the stricture and the papilla against a known dimension, such as the width of the duodenoscope, based on fluoroscopic assessment. An alternative is to measure the distance of a catheter that has been pulled back from the top of the stricture to the papilla. The plastic stent is then loaded onto the tip of a delivery device. There are three types of stent delivery devices, but they all have an inner guiding catheter and a shorter external pusher catheter. Once the inner catheter has securely passed the stricture, the pusher is advanced forward to push the stent up the bile duct until it reaches the desired location. The guiding catheter and the guidewire are then removed, leaving the stent in place. There are two commercially available modified stenting devices that allow simultaneous removal of the stent and device while leaving the guidewire in place, in case a decision is made to retrieve the stent.

Metallic Stenting

The first commercially available metal biliary stent was introduced in 1989. It was made of individual stainless steel wires that were twined together. One of the first studies reported effectiveness of the 10-mm caliber metal stents without significant problems. A comparison between metallic and plastic stents showed that the median duration of patency (273 days) of the metal stent was significantly longer than that of a polyethylene stent (126 days). However, the overall median survival was 149 days and did not differ significantly between the plastic and metal stent groups. Not only were metallic stents more patent over time than plastic stents, but they were also associated with less cholangitis, shorter duration of hospitalization, and less overall costs than plastic stents. A meta-analysis was performed to analyze seven randomized controlled trials that aimed to compare plastic and metal stents. In total, 724 patients were randomized into one of the treatment arms. The meta-analysis showed that metallic stents were associated with a significantly reduced risk of stent occlusion at 4 months and a lower overall risk of recurrent biliary obstruction. This analysis also showed that the metal stents were more cost effective, as the additional ERCP accounted for substantial cost. But a recent Korean study demonstrated that even in countries where ERCP costs were lower than those of metal stents, metal biliary stents were still the first-line treatment because they offered better palliation without adding a significant cost in palliating the malignant biliary obstruction. Hence, these studies provided strong support for the practice of metal stenting as the first-line palliative modality for malignant biliary obstruction that is typically caused by pancreatic carcinoma. However, these stents were appropriate only when treating definitively unresectable cancers because the strong adherence of these open mesh stents to the biliary tissue made them difficult to remove from the bile duct during dissection and creation of surgical anastomoses.

Early self-expanding metal stents had no covers to prevent tissue penetration through the wire mesh. Indeed, stent failures were noted to be caused by tumor ingrowth across the metal wires. Reactive hyperplastic reactions may also occur at either end of the stent where it is in close contact with the biliary or duodenal tissue. Creating a barrier to tissue ingrowth and, in turn, prolonging stent patency was the driving force that led to the development of partially covered stents. Today, several partially covered metal stents are available. The short, uncovered ends of these stents are designed to provide adherence to the biliary tissue whereas the large, covered portion provides protection from tissue penetration into the stent lumen.

Even though the partially covered stents were designed to prevent tumor ingrowth, occasionally they are still difficult to remove. As neoadjuvant therapy for tumor downstaging and surgical resection becomes an increasingly common practice, the ease of removing these stents is an important consideration in the selection of a metal stent. Fully covered stents became available a few years ago and their removal at the time of a pancreaticoduodenectomy became feasible in virtually all patients.

Multiple studies have compared covered to uncovered metal stents in palliating malignant biliary obstruction. Yoon and colleagues found no significant differences in stent patency at 100, 200, 300, and 400 days between the two groups of metal stents. A randomized trial showed that the median time to recurrent biliary obstruction was 711 days for the uncovered and 357 days for the partially covered self-expanding metal stents. In addition, serious adverse events occurred significantly more frequently with the partially covered stents than with the uncovered stents (62% vs 44%, P <.05), mainly due to stent migration (12% vs 0, P <.01). Further, a European study that enrolled 400 patients and randomized them to covered or uncovered stents showed no significant differences in stent patency, patient survival, or complication rates between the two groups. The metal stents used in this study were made by a different manufacturer from the metal stents used in the other trials, but the conclusions of these independent studies were identical, disputing any potential advantage of the covered metal stents over their uncovered counterparts. Even though there are the suspicions that the stent cover would contribute to cholecystitis and pancreatitis, there is no evidence that it is the case.

Endoscopic Ultrasound-Guided Bile Duct Stenting

When the bile duct cannot be accessed through the papilla, or when the duodenum is completely obstructed by a large pancreatic tumor, an interventional radiologist or a surgeon is enlisted to perform a drainage or bypass procedure. However, endoscopic ultrasound (EUS) allows gastrointestinal interventionists to puncture the bile duct directly from the duodenal bulb or stomach. This allows the possibility to insert a guidewire pass the stricture and the major papilla to perform biliary stenting and this technique is commonly referred to as the endoscopic or EUS rendezvous procedure. Roughly two-thirds of the cases attempted in this manner were reported to be successful in draining the obstructed bile duct, ending with biliary stents crossing the major papillae. EUS-guided choledochoduodenostomy and EUS-guided hepatogastrostomy have been performed when the duodenum or major papilla cannot be accessed. Plastic or metal stents were inserted directly across the duodenum or stomach into the extrahepatic or intrahepatic bile duct. These highly invasive procedures demand great technical skills and clinical experience and should not be regarded as standard treatment options at the present time.

Drug-Eluting Biliary Stents

Taking biliary stenting one step further, experimental treatment of malignant stricture with drug-eluting metallic stents has been proposed to extend the duration of biliary patency. Paclitaxel, a chemotherapeutic agent, was mixed in a liquid form with polyurethane and tetrahydrofuran to create a stent membrane that slowly released paclitaxel. A low serum level of paclitaxel could be detected in patients stented for longer than 50 days. The mean patency of these covered, paclitaxel-eluting stents was 429 days in 21 patients with unresectable malignant biliary obstruction. This seemingly prolonged patency may serve as supporting evidence to further develop drug-eluting stents in the future.

Metal Stenting Technique

The technique of metal stent placement is similar to that for plastic stenting, although there are some key differences. The first step is to pass a wire across the stricture. As with plastic stenting, dilating a distal bile duct stricture is rarely necessary. Most commercially available metal stents have markings on the proximal and distal ends to guide deployment. Some even have a “point of no return” radio-opaque mark that denotes the position of stent release beyond which the stent cannot be recaptured in case it is necessary to adjust the stenting position. The stainless steel alloy material of some stents is radio-opaque and all wire elements can be seen on fluoroscopy. However, nitinol-based wires are not radio-dense and therefore are marked with radio-opaque spots that are typically placed on both ends of the stents to guide stent release. Virtually all these stents have the tendency to propel forward during deployment; therefore, it is crucial to open up the stent very slowly to avoid inadvertent placement of the entire stent above the stricture. Gradual pullback of the stent is typically needed to compensate for the upward thrust of these stents during deployment. Shortening of the stents is also common as they are allowed to expand. Virtually all metal stents show a temporary waist at the point of maximal obstruction that disappears gradually over the next few days as the stents expand to their full calibers. Although there is no scientific evidence of how the covered metal stents spontaneously migrate, it is logical to assume that the unidirectional expansive force contributes to stent movement. Therefore, if one side of the waist is larger and longer than the other side, there will be the tendency for the stent to shift toward the side that is already more expanded. If a partially or fully covered stent is used, it is best to place the stent such that the waist is located in the center of the stent.