Plastic biliary endoprostheses have not changed much since their introduction more than 3 decades ago. Although their use has been challenged by the introduction of metal stents, plastic stents still remain commonly used. Much work has been done to improve the problem of stent obstruction but without substantial clinical success. In this review, the authors discuss the history of plastic biliary stent development and the current use of plastic stents for malignant biliary diseases.
Several malignancies can cause biliary obstruction at different levels with resultant jaundice. These malignancies include carcinoma of the papilla of Vater, pancreatic cancer, gallbladder cancer, distal cholangiocarcinoma, proximal cholangiocarcinoma (also known as Klatskin tumors), and metastatic disease involving pancreatic head and liver hilus. However, obstructive jaundice is often a manifestation of advanced disease, and curative surgery is often not possible. Consequently, palliative relief of common bile duct obstruction is an important part of patient management. Biliary stents can be placed not only to relieve obstructive jaundice for palliative treatment but also for preoperative biliary decompression. Endoscopic biliary stenting is now a well-established palliative treatment modality, and because of its lower risk and cost, this technique has almost completely replaced palliative surgery. Plastic endoscopic biliary prostheses were first used to treat malignant obstruction in 1979. Initially, 7F gauge plastic stents were used, but subsequently 10F stents were found to provide longer patency. Over the years, stents of different plastic materials, such as polyethylene, polyurethane, and polytetrafluoroethylene (Teflon), have been used. All plastic biliary stents clog within a few months after insertion ( Fig. 1 ). In the late 1980s, uncovered (bare metal) self-expandable metal endoprostheses were introduced. The larger-diameter metal stents still occlude because of tumor ingrowth and/or overgrowth. Furthermore, these uncovered metal stents cannot be removed endoscopically. More recently, fully covered expandable metals stents have become available, which although not designed for removability, can often be endoscopically removed. In this review, the authors discuss the history of plastic biliary stent development and the current use of plastic stents for malignant biliary diseases.
History
The first endoscopic biliary stent was placed in 1979, an intervention that had been thought to be impossible. In those days, the instrumentation channel of the available duodenoscopes only allowed for the insertion of a 7F catheter. The guidewires were at that time rather stiff and had a rough surface, which made stent placement technically difficult. The first endoprosthesis was cut off from an angiographic pigtail catheter. The high incidence of recurrent cholangitis in the first patients treated suggested that hindrance of bile flow in the stent caused occlusion, and the development of larger-diameter stents was deemed necessary. In 1981, Huibregtse and colleagues from the Amsterdam group were the first to describe the insertion of a newly developed straight 10F endoprosthesis with side flaps on both ends, which was passed through a forward-viewing large-channel gastroscope. The significantly prolonged patency and improved outcomes using these larger-diameter stents prompted development of side-viewing endoscopes with a larger-diameter working channel to allow insertion of a 10F endoprosthesis. In April 1981, the first prototype duodenoscope with a 3.7-mm channel was tested. In 1982, the first series of 30 patients with distal malignant biliary obstruction who underwent endoscopic insertion of a 10F stent was published. The technical success rate of endoscopic biliary stenting in distal and mid–common bile duct strictures improved and now exceeds 90% with low insertion-related complication rates. Over time, different stent diameters were tested, ranging from 7F to 12F. Any further increase in stent diameter longer than 10F to 11.5F or even 12F increased the technical difficulty of stent placement without improving stent patency. Therefore, a diameter of 10F is thought to be the best combination of patency and technically easy placement.
Plastic stents
When designing plastic stents, an endoprosthesis should ideally have all of the following characteristics: should be technically easy to insert, should effectively relieve biliary obstruction, should not occlude, and should not cause injury to the bile duct or duodenal wall. Several different materials, sizes, and shapes have been used to try and optimize these aspects. Different materials, such as polyethylene, polyurethane, and polytetrafluoroethylene (Teflon), have been used. In vitro studies showed a direct relation between the coefficient of friction and the amount of encrusted material within the stent lumen. Although Teflon had the lowest friction coefficient and greatest potential for prevention of clogging, it made the endoprostheses stiffer and a higher perforation risk was reported. Polyurethane tubing was also used but became brittle over time. Polyurethane endoprostheses fragmented during attempted retrieval, making removal difficult or even impossible. Polyethylene remains the preferred tubing material because it combines strength with relative softness, making injuries to the bile duct or duodenal wall rare. Although polyethylene has a higher coefficient of friction than Teflon, equal stent patency has been demonstrated. However, results from a meta-analysis showed that polyethylene stents provided superior patency in distal malignant biliary strictures.
Scanning electron microscopy of out-of-package biliary stents has shown that the inner surface smoothness of plastic stents is highly variable, which is probably inherent to the manufacturing process. The smoothness of the stent surface is considered to be an important factor in the occurrence of stent dysfunction. Scanning electron microscopy has shown that polyethylene stents have surface projections, whereas Teflon stents have the most irregular inner surface of all stents with multiple shallow pits and ridges. Only polyurethane stents were found to have a smooth surface.
The first endoprosthesis used for transpapillary endoscopic drainage had multiple side holes and measured 20 cm in length. The excessive length led to premature clogging and damage to the wall of the bile duct or duodenum. Shortening of the endoprosthesis led to a high incidence of proximal stent migration above the stricture. Placement of a second pigtail on the distal end prevented proximal migration, but the incidence of later cholangitis because of stent occlusion remained high. Thus, pigtail stents were replaced by straight endoprostheses with proximal and distal side flaps to prevent migration. These straight endoprosthesis were also easier to introduce. Most plastic stents are slightly curved to contour to the common bile duct ( Fig. 2 ). Multiple side holes at both ends of some stents maintain drainage even when the tip of the stent is impacted into the biliary or duodenal wall.
In 1995, a novel straight Teflon stent without side holes called the Tannenbaum prosthesis was introduced. This stent was designed because studies had shown that considerable sludge formation occurred around the side holes of straight stents. Furthermore, at that time, Teflon was thought to have the lowest friction coefficient. Initially, encouraging results were presented in a prospective nonrandomized study, but these results could not be confirmed in subsequent studies comparing the Tannenbaum and polyethylene stents. The addition of side holes to Tannenbaum stents, or a stainless steel mesh with polyamide outer layer, did not improve patency compared with standard polyethylene stents.
More recently, 2 stent designs have been used in an attempt to prolong patency of plastic stents. One is the use of a star-shaped stent with a limited central lumen. The other is the addition of an antireflux valve to prevent stent occlusion because of food and vegetable materials. Thus far, these stents have not definitively proven superior patency as compared with traditional plastic stents.
Plastic stents are typically available in lengths ranging from 5 to 19 cm, and custom-made models may be ordered from some manufacturers. Stent length usually indicates the distance between the proximal and distal flaps of the stent, but in some models, stent length represents the length of the complete stent. The length of an endoprosthesis is generally selected to allow the shortest length possible while simultaneously ensuring adequate drainage. Stents are usually inserted with their extremities protruding 1 to 2 cm above the proximal end of the biliary obstruction and 1 cm in the duodenum. A longer intraduodenal portion of the stent increases the risk of duodenal perforation. A technique that can be used to shorten plastic stents that extend too far into the duodenum involved snare resection with the aid of the metal sheath of a mechanical lithotripter.
Plastic stents
When designing plastic stents, an endoprosthesis should ideally have all of the following characteristics: should be technically easy to insert, should effectively relieve biliary obstruction, should not occlude, and should not cause injury to the bile duct or duodenal wall. Several different materials, sizes, and shapes have been used to try and optimize these aspects. Different materials, such as polyethylene, polyurethane, and polytetrafluoroethylene (Teflon), have been used. In vitro studies showed a direct relation between the coefficient of friction and the amount of encrusted material within the stent lumen. Although Teflon had the lowest friction coefficient and greatest potential for prevention of clogging, it made the endoprostheses stiffer and a higher perforation risk was reported. Polyurethane tubing was also used but became brittle over time. Polyurethane endoprostheses fragmented during attempted retrieval, making removal difficult or even impossible. Polyethylene remains the preferred tubing material because it combines strength with relative softness, making injuries to the bile duct or duodenal wall rare. Although polyethylene has a higher coefficient of friction than Teflon, equal stent patency has been demonstrated. However, results from a meta-analysis showed that polyethylene stents provided superior patency in distal malignant biliary strictures.
Scanning electron microscopy of out-of-package biliary stents has shown that the inner surface smoothness of plastic stents is highly variable, which is probably inherent to the manufacturing process. The smoothness of the stent surface is considered to be an important factor in the occurrence of stent dysfunction. Scanning electron microscopy has shown that polyethylene stents have surface projections, whereas Teflon stents have the most irregular inner surface of all stents with multiple shallow pits and ridges. Only polyurethane stents were found to have a smooth surface.
The first endoprosthesis used for transpapillary endoscopic drainage had multiple side holes and measured 20 cm in length. The excessive length led to premature clogging and damage to the wall of the bile duct or duodenum. Shortening of the endoprosthesis led to a high incidence of proximal stent migration above the stricture. Placement of a second pigtail on the distal end prevented proximal migration, but the incidence of later cholangitis because of stent occlusion remained high. Thus, pigtail stents were replaced by straight endoprostheses with proximal and distal side flaps to prevent migration. These straight endoprosthesis were also easier to introduce. Most plastic stents are slightly curved to contour to the common bile duct ( Fig. 2 ). Multiple side holes at both ends of some stents maintain drainage even when the tip of the stent is impacted into the biliary or duodenal wall.
In 1995, a novel straight Teflon stent without side holes called the Tannenbaum prosthesis was introduced. This stent was designed because studies had shown that considerable sludge formation occurred around the side holes of straight stents. Furthermore, at that time, Teflon was thought to have the lowest friction coefficient. Initially, encouraging results were presented in a prospective nonrandomized study, but these results could not be confirmed in subsequent studies comparing the Tannenbaum and polyethylene stents. The addition of side holes to Tannenbaum stents, or a stainless steel mesh with polyamide outer layer, did not improve patency compared with standard polyethylene stents.
More recently, 2 stent designs have been used in an attempt to prolong patency of plastic stents. One is the use of a star-shaped stent with a limited central lumen. The other is the addition of an antireflux valve to prevent stent occlusion because of food and vegetable materials. Thus far, these stents have not definitively proven superior patency as compared with traditional plastic stents.
Plastic stents are typically available in lengths ranging from 5 to 19 cm, and custom-made models may be ordered from some manufacturers. Stent length usually indicates the distance between the proximal and distal flaps of the stent, but in some models, stent length represents the length of the complete stent. The length of an endoprosthesis is generally selected to allow the shortest length possible while simultaneously ensuring adequate drainage. Stents are usually inserted with their extremities protruding 1 to 2 cm above the proximal end of the biliary obstruction and 1 cm in the duodenum. A longer intraduodenal portion of the stent increases the risk of duodenal perforation. A technique that can be used to shorten plastic stents that extend too far into the duodenum involved snare resection with the aid of the metal sheath of a mechanical lithotripter.
Malignant biliary strictures
Distal Strictures (Not Involving the Hilum)
Nonhilar malignant biliary obstruction is mainly caused by periampullary tumors (including carcinoma of the papilla of Vater, cancer of the pancreatic head, and distal cholangiocarcinoma), more proximal subhilar biliary cholangiocarcinoma, gallbladder carcinoma, and (rarely) metastatic cancer.
Distal or mid–bile duct obstruction due to pancreatic carcinoma, gallbladder cancer, cholangiocarcinoma, and lymph node metastases are common causes of jaundice and often present with advanced disease, and a minority of patients with these conditions are candidates for surgery.
Several comparative trials have shown that self-expandable metal biliary stents provide superior patency and reduce the need for reintervention for stent occlusion compared with plastic stents in patients with malignant distal common bile duct obstruction. In a systemic review and meta-analyses, the median time to stent occlusion ranged from 111 to 273 days for metal stents versus 62 to 165 days for plastic stents. The median patient survival ranged from 99 to 175 days, suggesting that many patients die long before stent occlusion occurs. No significant difference was seen between the metal and plastic stents in terms of technical success, therapeutic success, 30-day mortality, or complications. The prolonged patency of metal stents is offset by a substantially higher cost compared with plastic stents. Therefore, predicting the prognosis of a patient before endoscopic retrograde cholangiopancreatography (ERCP) may allow selection of patients most likely to benefit. Two prospective studies in 213 patients concluded that tumor size and presence of liver metastases were independent prognostic factors for shorter survival.
Stent insertion in patients with malignant distal bile duct obstruction for temporary preoperative drainage before pancreaticoduodenectomy is not uncommon because it provides relief from jaundice while allowing assessment of operability. In addition, it has long been proposed that reduction in serum bilirubin level may reduce postoperative complications. In this clinical situation, plastic stents are usually inserted rather than metal stents because they usually remain in place for only a short duration and do not interfere with subsequent surgical resection. However, preoperative biliary drainage remains a subject of debate. A recent large prospective randomized controlled trial comparing endoscopic preoperative biliary drainage using 10F plastic stents with surgery performed 4 to 6 weeks later with surgery alone within 1 week of diagnosis in patients with malignant distal biliary obstruction demonstrated that preoperative biliary drainage increased the overall complication rate. Both the complication rate of the initial ERCPs as well as the need for reintervention were high, and no improvement in postoperative complications was found when preoperative drainage was performed. Stent occlusion accounted for 15 of the 27 patients who suffered cholangitis.
Hilar Strictures
Malignant hilar biliary obstruction can be caused by a heterogeneous group of tumors that include primary bile duct cancer (the so-called Klatskin tumor), cancers that directly extend into the bifurcation (eg, gallbladder cancer), and metastatic cancer. Hilar cancers have a poor prognosis with less than 10% of patients surviving 5 years after the diagnosis and most patients dying in the first year. Although surgery is the only chance for cure, resectability rates are commonly less than 20%. For most patients, palliation is therefore the goal of treatment.
The extent of duct involvement by perihilar tumors may be classified according to Bismuth and Corlette ( Fig. 3 ). Type I tumors are completely below the confluence of the left and right hepatic ducts; the right and left ductal systems communicate. Type II tumors reach the confluence but do not involve the left or right segmental hepatic ducts, and only the left and right hepatic ducts are separated. Type III tumors occlude the common hepatic duct and either the right (IIIa) or left (IIIb) segmental hepatic ducts. In type IV tumors, the hilum is obstructed with tumor extension into both left and right segmental bile ducts. Palliation with biliary drainage through endoprostheses in patients with malignant hilar stenoses poses particular difficulties, especially in advanced lesions (type III and IV lesions). In these patients, the risk of incomplete drainage after contrast injection into the intrahepatic bile ducts leads to a high incidence of post-ERCP cholangitis. Retention of contrast and subsequent segmental cholangitis are risks associated with endoscopic attempts to treat advanced hilar lesions. The success rate of plastic stent insertion for hilar obstruction is lower than that of distal obstruction, although relief of symptoms with increase in quality of life can be achieved in nearly all patients successfully stented.
Related posts:
Pancreaticobiliary and Gastrointestinal Stents
Expandable Metal Stents: Principles and Tissue Responses
Expandable Stents for Benign Esophageal Disease
Expandable Metal Stents for Benign Biliary Disease
Self-Expandable Metal Stents for Malignant Distal Biliary Strictures
Expandable Metal Stents for Malignant Hilar Biliary Obstruction
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