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The role of covered self-expandable metallic stents in malignant biliary strictures
EDITORIAL
The role of covered self-expandable metallic stents in malignant biliary strictures Endoscopic stent placement is an established method for the palliation of malignant obstructive jaundice in patients with unresectable pancreaticobiliary malignancies. Insertion of a plastic stent provides adequate drainage, with resolution of the obstructive jaundice, especially in patients with distal bile-duct obstruction.1 However, a major late complication with stent blockage from biliary infection and bacterial biofilm and sludge formation has limited the clinical benefits of plastic stents. Self-expandable metal stents (SEMS) provide a much larger internal diameter, with better drainage and a patency rate significantly longer than plastic stents. In prior randomized, prospective trials that compared uncovered metal biliary stents with plastic stents in malignant biliary strictures, the metal stents had longer stent patency rates.2,3 However, recurrent jaundice and cholangitis can occur as a result of tumor ingrowth and occlusion of the metal stents. This complication is more commonly encountered with the open-mesh stents, where expansion forces the wires into the tissue. Covered expandable metal stents on the other hand were designed to overcome this problem of tissue ingrowth, but early clinical data suggested that the covered stents were more prone to migration and dislocation. Furthermore, because of the limited length of the stents, blockage of the stents can still occur as a result of tumor overgrowth. There is also a concern that the covered stent may occlude the orifice of the cystic duct or the opposite branch duct when covered stents are placed in patients with hilar strictures. In the earlier reported series with covered SEMS, the complication of cholecystitis occurred in up to 3.5% of patients.4,5 In a recent report of 80 patients who received covered SEMS, no cases of pancreatitis, but 3 cases (3.8%) of cholecystitis were reported.6 The relative expense of SEMS may sometimes limit their application in patients, but it can be argued that an expandable metal stent with a longer patency may avoid repeat ERCP for stent exchange as in the case of plastic stents, thus offsetting the initial high cost. Also, for patients with terminal malignancies, it may be better to provide more definitive palliation than frequent stent exchanges, which impacts on the quality of remaining life for the patients. The
Copyright ª 2006 by the American Society for Gastrointestinal Endoscopy 0016-5107/$32.00 doi:10.1016/j.gie.2006.02.011
www.giejournal.org
choice of stents in patients with metastatic disease also poses a dilemma, because these patients may not survive long enough to enjoy the benefits of an expandable metal stent. The general consensus is that SEMS are more cost effective for patients who are likely to survive more than 4 to 6 months, and plastic stents may suffice in those who have a limited life expectancy of less than 3 to 4 months.7,8 However, much of the comparative data on the use of SEMS have come from early studies involving the use of open-mesh stents. The use of covered expandable metal stents may hold a different promise in terms of stent patency because
In malignant biliary obstruction, covered expandable metal stents have better patency rates than uncovered ones because they resist tumor ingrowth.
of their ability to resist tissue ingrowth. All of these issues continue to challenge the wisdom of biliary endoscopists. In the current issue of Gastrointestinal Endoscopy are 2 studies that have added further insight to the use of covered SEMS in malignant biliary obstruction: 1 study is a prospective comparison of plastic vs covered metal stents in patients with distal bile-duct strictures,9 and the other study is a retrospective analysis that compared uncovered metal vs covered metal stents.10 In both studies, the covered and noncovered stents used were Wallstents (Boston Scientific, Natick, Mass). Soderlund and Linder9 presented a single-center experience of a prospective randomized controlled study that compared the effectiveness and the patency rates of 10F polyethylene (plastic) stents vs 30F covered SEMS for the palliation of malignant jaundice because of common bile duct (CBD) strictures. Their study was carried out in 1 major Swedish hospital, with a surrounding catchment population of 0.6 million. Between August 2002 and March 2004, 100 patients were recruited: 49 were randomized to the covered SEMS group and the remaining 51 to the plastic stent group. There were no differences between the 2 groups in the overall patient characteristics at inclusion. In this study, the investigators reported more stent failures for the plastic stents than the covered SEMS group (22 vs 9, Volume 63, No. 7 : 2006 GASTROINTESTINAL ENDOSCOPY 1001
Editorial
Leung & Rahim
P Z .009), a longer time to stent failure with the covered SEMS group compared with plastic stents (3.5 vs 1.1 months, P Z.007), and a longer median patency for the covered SEMS vs plastic stents (3.6 vs 1.8 months, P Z.002) but no difference in the overall survival. In the Swedish setting, the use of covered SEMS did not incur more overall cost compared with plastic stent placement. They concluded that covered SEMS should be considered in patients with unresectable malignant CBD strictures without distant metastases. However, a major concern with this study was the much shorter stent patency for the covered stents compared with previous reports of uncovered SEMS.2,11 The investigators explained that this discrepancy was partly because of the shorter median survival of their patients and that they had measured and reported the median rather than the mean stent patency intervals. The overall difference did not change when the mean value was calculated. Although repeat ERCP was performed in case of suspected stent failure, the investigators did not state if routine post–stent-placement abdominal radiographs were taken to document full expansion of the covered SEMS. Open-mesh stents usually appear expanded on fluoroscopy, because the wires cut into the tissue. However, a tight stricture may resist full expansion of a covered stent because the covering membrane prevents tissue ingrowth. Thus, the covered stent lumen may remain restricted and is, therefore, more prone to obstruction by biliary sludge, similar to the smaller plastic stents, with a corresponding reduction in the overall stent patency. There were 2 cases of proximal stent migration attributed to improper technique in stent deployment across the stricture and a lack of a suitable length stent. In this study, the covered SEMS were easily removed by using extraction forceps, and this is very appealing in patients with benign tumors or a surgically resectable lesion when stent removal is a key issue. There was no report of any cases of cholecystitis or pancreatitis, a major improvement from previously reported studies.12,13 It is amazing to learn that no patient was lost to follow-up in this study, which may be a unique experience not seen in other clinical studies. Yoon et al10 conducted a retrospective analysis to compare the efficacy and complication rates of covered vs uncovered SEMS in patients with malignant biliary obstruction. The investigators collected data from patients with unresectable distal malignant biliary obstruction who underwent ERCP and placement of covered or uncovered Wallstents for the palliation of malignant jaundice. A total of 77 patients with SEMS were analyzed: 36 with covered and 41 with uncovered stents. They observed an increased but not statistically significant stent migration rate with the covered SEMS in 3 of 36 patients compared with 1 of 41 for the uncovered stent group (P Z not significant [NS]). Cholecystitis occurred in 1 of 36 patients with covered SEMS vs none in the uncovered stent group (P Z NS). Overall stent patency rates did not differ significantly, although there was a trend in favor of the covered SEMS. Stent occlusion occurred
after a mean of 398 days for the covered SEMS group vs 319 days for the uncovered stent group (P O .05). The investigators concluded that although covered Wallstents are safe, they did not offer significantly longer patency rates. However, a difference of almost 80 days in the mean stent patency interval may contribute to an improved quality of life in this group of patients with a relatively short survival. Because this was only a retrospective analysis, further prospective randomized trials comparing covered vs uncovered Wallstents and other SEMS are needed to better define the role of covered metal stents in malignant biliary obstruction. Similar to the Swedish study, the investigators did not observe any cases of pancreatitis. There were no cases with stent occlusion because of tumor ingrowth among the covered SEMS cohort as was reported in another study.4 However, in the absence of autopsy or surgical removal of the stents, it may be difficult to prove that there was no actual breakdown of the covering material and resultant tissue ingrowth. The investigators did observe the occurrence of tumor overgrowth (n Z 5) and sludge formation (n Z 2) with the covered stents. Although most malignant CBD strictures are distal, potential risks of distal and proximal stent migration exist. Prior animal studies demonstrated that the wire mesh of the uncovered stents caused tissue necrosis and embedded themselves in the tissue, a feature absent for the covered stents.14 However, the current covered stent is not fully covered but has a 0.5-cm rim of uncovered mesh on each end of the stent. In addition, unlike the previous covered stents design in which the coating was located on the outer surface of stent, the current covered stents have the cover coating on the inside (luminal) surface, allowing some anchoring of the stent against the tissue to resist migration. A recent Japanese paper that compared covered vs uncovered (nitinol) SEMS found no cases of stent migration with the covered stents.12 Although the covered SEMS provide a larger internal luminal diameter and less chance for biliary sludge formation and blockage, in areas of very tight strictures, where the luminal diameter is narrowed, the risk of sludge formation, which parallels that of plastic stents, may pose a potential risk of stent blockage. This raises 2 rather important questions: (1) Should we routinely perform an abdominal radiograph to document full expansion of the stent, because the persistence of a waist would indicate incomplete stent expansion? (2) How should we define stent blockagedclinically or radiologically? Because most of us would use recurrent jaundice or cholangitis as a surrogate for stent blockage, this may not truly reflect stent occlusion, because stent migration may be complicated by stasis and infection. The permanence of an open-mesh stent may compromise subsequent curative surgical resection, but placement of a covered SEMS that can be removed offers a different approach to the management of malignant jaundice. In both reported studies, removal of the covered SEMS was possible
1002 GASTROINTESTINAL ENDOSCOPY Volume 63, No. 7 : 2006
www.giejournal.org
Leung & Rahim
Editorial
by using similar stent extraction devices for removal of plastic stents. Successful stent removal was also reported in a prior study.15 The advantage of using a covered SEMS for initial drainage of malignant jaundice is that, if the patient is deemed unresectable, the initial placement of a covered stent would obviate the need for repeat ERCP to exchange the plastic stent for a metal stent. However, the low patency rates in the Swedish study9 and the lack of a significantly improved stent patency over uncovered stents in the Korean study10 have certainly dampened some of the initial enthusiasm about the role of covered stents for preoperative drainage in malignant biliary obstruction. However, despite these findings, further studies involving the use of covered metal stents are warranted. In fact, the enthusiasm with the use of covered SEMS in malignant biliary strictures has raised the option of using covered stents for management of benign bile duct strictures because they are removable. The use of covered SEMS may decrease the number of stent-placement sessions and provide an appealing alternative to regular stent exchanges every 3 to 4 months when plastic stents are used for benign strictures.16
DISCLOSURE Both authors do not have any conflict of interest to disclose in relation to authorship of this editorial. Joseph Leung, MD, FRCP, FACP, FACG Mr. & Mrs. C. W. Law Professor of Medicine Division of Gastroenterology Davis School of Medicine University of California Chief, Section of Gastroenterology VA Northern California Health Care System Sacramento, California, USA Nazir Rahim, MD Clinical Fellow Division of Gastroenterology UC Davis Medical Center Sacramento, California, USA
www.giejournal.org
REFERENCES 1. Smith AC, Dowsett JF, Russell RC, et al. Randomized trial of endoscopic stenting vs. surgical bypass in malignant low bile duct obstruction. Lancet 1994;344:1655-60. 2. Davids PH, Groen AK, Rauws EA, et al. Randomized trial of self-expanding metal stents versus polyethylene stents for distal malignant biliary obstruction. Lancet 1992;340:1488-92. 3. Knyrim K, Wagner HJ, Pausch J, et al. A prospective, randomized, controlled trial of metal stents for malignant obstruction of the common bile duct. Endoscopy 1993;25:207-12. 4. Kahaleh M, Tokar J, Conaway MR, et al. Efficacy and complications of covered Wallstents in malignant distal biliary obstruction. Gastrointest Endosc 2005;61:528-33. 5. Hausegger KA, Thurner S, Bodendo¨rfer G, et al. Treatment of malignant biliary obstruction with polyurethane-covered Wallstents. AJR Am J Roentgenol 1998;170:403-8. 6. Shah RJ, Howell DA, Desilets DJ, et al. Multicenter randomized trial of the spiral Z-stent compared with the Wallstent for malignant biliary obstruction. Gastrointest Endosc 2003;57:830-6. 7. Yeoh KG, Zimmerman MJ, Cunningham JT, et al. Comparative costs of metal versus plastic biliary stent strategies for malignant obstructive jaundice by decision analysis. Gastrointest Endosc 1999;49:466-71. 8. Prat F, Chapat O, Ducot B, et al. A randomized trial of endoscopic drainage methods for inoperable malignant strictures of the common bile duct. Gastrointest Endosc 1998;47:1-7. 9. Soderlund C, Linder S. Covered metal versus plastic stents for malignant common bile duct stenosis: a prospective, randomized, controlled trial. Gastrointest Endosc 2006;63:986-95. 10. Yoon WJ, Lee KJ, Lee KH, et al. A comparison of covered and uncovered Wallstents for the management of distal malignant biliary obstruction. Gastrointest Endosc 2006;63:996-1000. 11. Lammer J, Hausegger K, Fluckiger F, et al. Common bile duct obstruction due to malignancy: treatment with plastic versus metal stents. Radiology 1996;201:167-72. 12. 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;53:729-34. 13. Carr-Locke DL. Metal stents for distal biliary malignancy: have we got you covered? Gastrointest Endosc 2005;61:534-5. 14. Silvis SE, Sievert CE, Vennes JA, et al. Comparison of covered versus uncovered wire mesh stents in the canine biliary tract. Gastrointest Endosc 1994;40:17-21. 15. Familiari P, Bulajic M, Mutignani M, et al. Endoscopic removal of malfunctioning biliary self-expandable metallic stents. Gastrointest Endosc 2005;62:903-10. 16. Bergman JJ, Burgemeister L, Bruno M, et al. Long-term follow-up after biliary stent placement for postoperative bile duct stenosis. Gastrointest Endosc 2001;54:154-61.
Volume 63, No. 7 : 2006 GASTROINTESTINAL ENDOSCOPY 1003
The role of covered self-expandable metallic stents in malignant biliary strictures Endoscopic stent placement is an established method for the palliation of malignant obstructive jaundice in patients with unresectable pancreaticobiliary malignancies. Insertion of a plastic stent provides adequate drainage, with resolution of the obstructive jaundice, especially in patients with distal bile-duct obstruction.1 However, a major late complication with stent blockage from biliary infection and bacterial biofilm and sludge formation has limited the clinical benefits of plastic stents. Self-expandable metal stents (SEMS) provide a much larger internal diameter, with better drainage and a patency rate significantly longer than plastic stents. In prior randomized, prospective trials that compared uncovered metal biliary stents with plastic stents in malignant biliary strictures, the metal stents had longer stent patency rates.2,3 However, recurrent jaundice and cholangitis can occur as a result of tumor ingrowth and occlusion of the metal stents. This complication is more commonly encountered with the open-mesh stents, where expansion forces the wires into the tissue. Covered expandable metal stents on the other hand were designed to overcome this problem of tissue ingrowth, but early clinical data suggested that the covered stents were more prone to migration and dislocation. Furthermore, because of the limited length of the stents, blockage of the stents can still occur as a result of tumor overgrowth. There is also a concern that the covered stent may occlude the orifice of the cystic duct or the opposite branch duct when covered stents are placed in patients with hilar strictures. In the earlier reported series with covered SEMS, the complication of cholecystitis occurred in up to 3.5% of patients.4,5 In a recent report of 80 patients who received covered SEMS, no cases of pancreatitis, but 3 cases (3.8%) of cholecystitis were reported.6 The relative expense of SEMS may sometimes limit their application in patients, but it can be argued that an expandable metal stent with a longer patency may avoid repeat ERCP for stent exchange as in the case of plastic stents, thus offsetting the initial high cost. Also, for patients with terminal malignancies, it may be better to provide more definitive palliation than frequent stent exchanges, which impacts on the quality of remaining life for the patients. The
Copyright ª 2006 by the American Society for Gastrointestinal Endoscopy 0016-5107/$32.00 doi:10.1016/j.gie.2006.02.011
www.giejournal.org
choice of stents in patients with metastatic disease also poses a dilemma, because these patients may not survive long enough to enjoy the benefits of an expandable metal stent. The general consensus is that SEMS are more cost effective for patients who are likely to survive more than 4 to 6 months, and plastic stents may suffice in those who have a limited life expectancy of less than 3 to 4 months.7,8 However, much of the comparative data on the use of SEMS have come from early studies involving the use of open-mesh stents. The use of covered expandable metal stents may hold a different promise in terms of stent patency because
In malignant biliary obstruction, covered expandable metal stents have better patency rates than uncovered ones because they resist tumor ingrowth.
of their ability to resist tissue ingrowth. All of these issues continue to challenge the wisdom of biliary endoscopists. In the current issue of Gastrointestinal Endoscopy are 2 studies that have added further insight to the use of covered SEMS in malignant biliary obstruction: 1 study is a prospective comparison of plastic vs covered metal stents in patients with distal bile-duct strictures,9 and the other study is a retrospective analysis that compared uncovered metal vs covered metal stents.10 In both studies, the covered and noncovered stents used were Wallstents (Boston Scientific, Natick, Mass). Soderlund and Linder9 presented a single-center experience of a prospective randomized controlled study that compared the effectiveness and the patency rates of 10F polyethylene (plastic) stents vs 30F covered SEMS for the palliation of malignant jaundice because of common bile duct (CBD) strictures. Their study was carried out in 1 major Swedish hospital, with a surrounding catchment population of 0.6 million. Between August 2002 and March 2004, 100 patients were recruited: 49 were randomized to the covered SEMS group and the remaining 51 to the plastic stent group. There were no differences between the 2 groups in the overall patient characteristics at inclusion. In this study, the investigators reported more stent failures for the plastic stents than the covered SEMS group (22 vs 9, Volume 63, No. 7 : 2006 GASTROINTESTINAL ENDOSCOPY 1001
Editorial
Leung & Rahim
P Z .009), a longer time to stent failure with the covered SEMS group compared with plastic stents (3.5 vs 1.1 months, P Z.007), and a longer median patency for the covered SEMS vs plastic stents (3.6 vs 1.8 months, P Z.002) but no difference in the overall survival. In the Swedish setting, the use of covered SEMS did not incur more overall cost compared with plastic stent placement. They concluded that covered SEMS should be considered in patients with unresectable malignant CBD strictures without distant metastases. However, a major concern with this study was the much shorter stent patency for the covered stents compared with previous reports of uncovered SEMS.2,11 The investigators explained that this discrepancy was partly because of the shorter median survival of their patients and that they had measured and reported the median rather than the mean stent patency intervals. The overall difference did not change when the mean value was calculated. Although repeat ERCP was performed in case of suspected stent failure, the investigators did not state if routine post–stent-placement abdominal radiographs were taken to document full expansion of the covered SEMS. Open-mesh stents usually appear expanded on fluoroscopy, because the wires cut into the tissue. However, a tight stricture may resist full expansion of a covered stent because the covering membrane prevents tissue ingrowth. Thus, the covered stent lumen may remain restricted and is, therefore, more prone to obstruction by biliary sludge, similar to the smaller plastic stents, with a corresponding reduction in the overall stent patency. There were 2 cases of proximal stent migration attributed to improper technique in stent deployment across the stricture and a lack of a suitable length stent. In this study, the covered SEMS were easily removed by using extraction forceps, and this is very appealing in patients with benign tumors or a surgically resectable lesion when stent removal is a key issue. There was no report of any cases of cholecystitis or pancreatitis, a major improvement from previously reported studies.12,13 It is amazing to learn that no patient was lost to follow-up in this study, which may be a unique experience not seen in other clinical studies. Yoon et al10 conducted a retrospective analysis to compare the efficacy and complication rates of covered vs uncovered SEMS in patients with malignant biliary obstruction. The investigators collected data from patients with unresectable distal malignant biliary obstruction who underwent ERCP and placement of covered or uncovered Wallstents for the palliation of malignant jaundice. A total of 77 patients with SEMS were analyzed: 36 with covered and 41 with uncovered stents. They observed an increased but not statistically significant stent migration rate with the covered SEMS in 3 of 36 patients compared with 1 of 41 for the uncovered stent group (P Z not significant [NS]). Cholecystitis occurred in 1 of 36 patients with covered SEMS vs none in the uncovered stent group (P Z NS). Overall stent patency rates did not differ significantly, although there was a trend in favor of the covered SEMS. Stent occlusion occurred
after a mean of 398 days for the covered SEMS group vs 319 days for the uncovered stent group (P O .05). The investigators concluded that although covered Wallstents are safe, they did not offer significantly longer patency rates. However, a difference of almost 80 days in the mean stent patency interval may contribute to an improved quality of life in this group of patients with a relatively short survival. Because this was only a retrospective analysis, further prospective randomized trials comparing covered vs uncovered Wallstents and other SEMS are needed to better define the role of covered metal stents in malignant biliary obstruction. Similar to the Swedish study, the investigators did not observe any cases of pancreatitis. There were no cases with stent occlusion because of tumor ingrowth among the covered SEMS cohort as was reported in another study.4 However, in the absence of autopsy or surgical removal of the stents, it may be difficult to prove that there was no actual breakdown of the covering material and resultant tissue ingrowth. The investigators did observe the occurrence of tumor overgrowth (n Z 5) and sludge formation (n Z 2) with the covered stents. Although most malignant CBD strictures are distal, potential risks of distal and proximal stent migration exist. Prior animal studies demonstrated that the wire mesh of the uncovered stents caused tissue necrosis and embedded themselves in the tissue, a feature absent for the covered stents.14 However, the current covered stent is not fully covered but has a 0.5-cm rim of uncovered mesh on each end of the stent. In addition, unlike the previous covered stents design in which the coating was located on the outer surface of stent, the current covered stents have the cover coating on the inside (luminal) surface, allowing some anchoring of the stent against the tissue to resist migration. A recent Japanese paper that compared covered vs uncovered (nitinol) SEMS found no cases of stent migration with the covered stents.12 Although the covered SEMS provide a larger internal luminal diameter and less chance for biliary sludge formation and blockage, in areas of very tight strictures, where the luminal diameter is narrowed, the risk of sludge formation, which parallels that of plastic stents, may pose a potential risk of stent blockage. This raises 2 rather important questions: (1) Should we routinely perform an abdominal radiograph to document full expansion of the stent, because the persistence of a waist would indicate incomplete stent expansion? (2) How should we define stent blockagedclinically or radiologically? Because most of us would use recurrent jaundice or cholangitis as a surrogate for stent blockage, this may not truly reflect stent occlusion, because stent migration may be complicated by stasis and infection. The permanence of an open-mesh stent may compromise subsequent curative surgical resection, but placement of a covered SEMS that can be removed offers a different approach to the management of malignant jaundice. In both reported studies, removal of the covered SEMS was possible
1002 GASTROINTESTINAL ENDOSCOPY Volume 63, No. 7 : 2006
www.giejournal.org
Leung & Rahim
Editorial
by using similar stent extraction devices for removal of plastic stents. Successful stent removal was also reported in a prior study.15 The advantage of using a covered SEMS for initial drainage of malignant jaundice is that, if the patient is deemed unresectable, the initial placement of a covered stent would obviate the need for repeat ERCP to exchange the plastic stent for a metal stent. However, the low patency rates in the Swedish study9 and the lack of a significantly improved stent patency over uncovered stents in the Korean study10 have certainly dampened some of the initial enthusiasm about the role of covered stents for preoperative drainage in malignant biliary obstruction. However, despite these findings, further studies involving the use of covered metal stents are warranted. In fact, the enthusiasm with the use of covered SEMS in malignant biliary strictures has raised the option of using covered stents for management of benign bile duct strictures because they are removable. The use of covered SEMS may decrease the number of stent-placement sessions and provide an appealing alternative to regular stent exchanges every 3 to 4 months when plastic stents are used for benign strictures.16
DISCLOSURE Both authors do not have any conflict of interest to disclose in relation to authorship of this editorial. Joseph Leung, MD, FRCP, FACP, FACG Mr. & Mrs. C. W. Law Professor of Medicine Division of Gastroenterology Davis School of Medicine University of California Chief, Section of Gastroenterology VA Northern California Health Care System Sacramento, California, USA Nazir Rahim, MD Clinical Fellow Division of Gastroenterology UC Davis Medical Center Sacramento, California, USA
www.giejournal.org
REFERENCES 1. Smith AC, Dowsett JF, Russell RC, et al. Randomized trial of endoscopic stenting vs. surgical bypass in malignant low bile duct obstruction. Lancet 1994;344:1655-60. 2. Davids PH, Groen AK, Rauws EA, et al. Randomized trial of self-expanding metal stents versus polyethylene stents for distal malignant biliary obstruction. Lancet 1992;340:1488-92. 3. Knyrim K, Wagner HJ, Pausch J, et al. A prospective, randomized, controlled trial of metal stents for malignant obstruction of the common bile duct. Endoscopy 1993;25:207-12. 4. Kahaleh M, Tokar J, Conaway MR, et al. Efficacy and complications of covered Wallstents in malignant distal biliary obstruction. Gastrointest Endosc 2005;61:528-33. 5. Hausegger KA, Thurner S, Bodendo¨rfer G, et al. Treatment of malignant biliary obstruction with polyurethane-covered Wallstents. AJR Am J Roentgenol 1998;170:403-8. 6. Shah RJ, Howell DA, Desilets DJ, et al. Multicenter randomized trial of the spiral Z-stent compared with the Wallstent for malignant biliary obstruction. Gastrointest Endosc 2003;57:830-6. 7. Yeoh KG, Zimmerman MJ, Cunningham JT, et al. Comparative costs of metal versus plastic biliary stent strategies for malignant obstructive jaundice by decision analysis. Gastrointest Endosc 1999;49:466-71. 8. Prat F, Chapat O, Ducot B, et al. A randomized trial of endoscopic drainage methods for inoperable malignant strictures of the common bile duct. Gastrointest Endosc 1998;47:1-7. 9. Soderlund C, Linder S. Covered metal versus plastic stents for malignant common bile duct stenosis: a prospective, randomized, controlled trial. Gastrointest Endosc 2006;63:986-95. 10. Yoon WJ, Lee KJ, Lee KH, et al. A comparison of covered and uncovered Wallstents for the management of distal malignant biliary obstruction. Gastrointest Endosc 2006;63:996-1000. 11. Lammer J, Hausegger K, Fluckiger F, et al. Common bile duct obstruction due to malignancy: treatment with plastic versus metal stents. Radiology 1996;201:167-72. 12. 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;53:729-34. 13. Carr-Locke DL. Metal stents for distal biliary malignancy: have we got you covered? Gastrointest Endosc 2005;61:534-5. 14. Silvis SE, Sievert CE, Vennes JA, et al. Comparison of covered versus uncovered wire mesh stents in the canine biliary tract. Gastrointest Endosc 1994;40:17-21. 15. Familiari P, Bulajic M, Mutignani M, et al. Endoscopic removal of malfunctioning biliary self-expandable metallic stents. Gastrointest Endosc 2005;62:903-10. 16. Bergman JJ, Burgemeister L, Bruno M, et al. Long-term follow-up after biliary stent placement for postoperative bile duct stenosis. Gastrointest Endosc 2001;54:154-61.
Volume 63, No. 7 : 2006 GASTROINTESTINAL ENDOSCOPY 1003