thirty - seven subjects were enrolled in this study ; there were 19 men and 18 women with a mean age of 70 years ( age range : 47 - 85 years ) . the study protocol was approved by the institutional review board of our university and also by the ethics committee of the institute for medical science . the procedure was explained in detail to all the prospective patients , and a written consent was obtained from all the subjects before the procedure . the study inclusion criteria were ( i ) the presence of malignant obstruction of the common bile duct below the hilar confluence , and this was caused by unresectable malignant tumor or reocclusion of a previously inserted self - expandable uncovered biliary stent , and ( ii ) obtaining an informed consent . the exclusion criteria included surgical resectability of the tumor , hilar obstruction or postoperative anastomotic site obstruction due to malignant recurrence , or the presence of a generally fatal patient condition such as sepsis or disseminated intracoagulopathy . thirty - seven patients had percutaneous biliary drainage performed , of which 34 were treated with a covered stent as the first stent , and three were treated for failure of a standard uncovered stent . six patients underwent sphincterotomy via endoscopic guidance before stent insertion . in our study , 14 patients had biopsy - confirmed adenocarcinomas and 23 patients lacked a confirming biopsy , but these 23 patients had been diagnosed by the findings on abdominal spiral computed tomography ( ct ) , magnetic resonance ( mr ) imaging , percutaneous cholangiography and endoscopic retrograde cholangiopancreatography ( ercp ) . thirteen patients had common bile duct carcinoma , eight patients had pancreatic head carcinoma , four patients had gallbladder carcinoma , six patients had periampullary carcinoma and six patients had gastric carcinoma . the site of obstruction was the proximal common bile duct ( cbd ) in three patients , the middle cbd in six patients and at the distal cbd in 28 patients . the stents used in this study were self - expanding biliary ptfe covered niti - s stents ( taewoong medical corporation , seoul , korea ) , as are described in figure 1 . the niti - s stent we used was made of 0.007-inch single nitinol wire , which has excellent biocompatibility and expansile force , and the stent was also self - expanding without any sharp spurs at either end ( low trauma ends ) . to prevent stent migration , a ptfe membrane covered the stent on the inside of the metal wire . the stents were 5 - 8 cm long when fully expanded to a maximum diameter of 10 mm , and they were positioned using a 9-fr introducer system . the stent was held , compressed and elongated on the delivery catheter by a cylindrical rolling membrane ( the outer sheath ) . the delivery catheter had three markers : one at a first - proximal portion ( the proximal marker ) , the second at the actual stent length position ( the stent length marker ) , and the third at the distal position of the loaded stent ( the stent marker ) . the devised niti - s stent became shortened by 51% after deployment , which is similar to that of wallstents ( 50% ) ; the nitis - s stent had tungsten markers on its proximal and distal ends that helped with manipulation during deployment ( 13 ) . stent placement was accomplished within two weeks after performing percutaneous transhepatic biliary drainage ( ptbd ) . stent placement was accomplished in three patients during the follow - up tubogram after protracted ptbd or stent placement . a right intercostal and/or left epigastric approach was chosen depending on the extent of tumor . stent insertion was performed over an extra stiff amplatz guidewire ( 180 cm , 0.035 inch , curved tip ; cook inc . , care was taken to precisely locate the covered mid - portion of the stent exactly at the stricture . balloon dilation after stent insertion was not performed in any of our patients ; instead , an 8.5-fr or 10.2-fr biliary drainage catheter was left in place for seven days . this catheter was used only for flushing and maintaining access to the biliary tree , and not for drainage . thereafter , imaging was repeated via the drainage catheter to check the stent 's patency , function and position . the catheter was removed if free bile flowing into the duodenum was documented or if the bilirubin level was normalized or markedly reduced . all the patients received 200 mg of prophylactic antibiotics that were administered intravenously over a period of 12 hours ( flomoxef sodium ; flumarin , ildong , ansung , korea ; or fosfomycin sodium ; fonofos , chong keun dang , chunan , korea ) before and usually 48 - 72 hours after the procedure . coagulation tests were performed before the procedure , and if a patient had disseminated intravascular coagulopathy , then stent placement was not performed . sonography of the upper abdomen was performed , and the bilirubin levels were checked at each examination . the patients were also followed by conducting telephone interviews with the referring physicians or with the patients when the patients failed to appear at the follow - up examinations . if stent occlusion was suspected , then the stent was revised percutaneously using a 10-fr drainage catheter . finally , the cause of death was ascertained for all the patients who died within the observation period . successful decompression of the biliary system was defined by a decrease of the serum bilirubin level of more than 30% versus the baseline value within a week of stent insertion ( 10 ) . we measured the distance from the bifurcated line between the right and left intrahepatic ducts to the end of the biliary stent . proximal stent migration was defined as a stent that migrated into the common hepatic duct , and distal stent migration was defined as a stent that migrated into the duodenum or out of the patient 's body . the extents of these migrations were established by comparing the stent ends with the stent positions seen on the initial images . early migration was defined as migration that occurred within one week of deployment , and delayed migration was defined as migration that occurred from one week to one month after stent placement . for the cases of recurrent jaundice , any stent occlusion was confirmed by sonography or ct , which showed dilation of the intrahepatic duct and the presence of soft tissue density inside the stent . recurrent episodes of inexplicable fever were defined as cholangitis . at the time of death , a stent was assumed to be patent if the patient had normal or only mildly elevated serum bilirubin levels ( < 3 mg / dl ) . if the patients had obvious jaundice or if they had a higher bilirubin level than that before stent placement , then the stent was assumed to be obstructed . echogenic materials in the stents , as were determined by us , were considered to represent sludge , and floating material in the stent , as determined by percutaneous transhepatic cholangiography , was also considered to represent sludge formation . the bilirubin and amylase / lipase levels before and during follow - up were statistically analyzed by using the wilcoxon signed - rank test . the stent patency rate and the patient survival rate were calculated by performing kaplan - meier survival ( life - table ) analysis . the duration of primary patency was defined as the interval between stent placement and the recurrence of obstructive jaundice . if obstruction was not evident during a patient 's life , then the patency period was considered to be equal to the survival duration , but this data was censored . thirty - seven subjects were enrolled in this study ; there were 19 men and 18 women with a mean age of 70 years ( age range : 47 - 85 years ) . the study protocol was approved by the institutional review board of our university and also by the ethics committee of the institute for medical science . the procedure was explained in detail to all the prospective patients , and a written consent was obtained from all the subjects before the procedure . the study inclusion criteria were ( i ) the presence of malignant obstruction of the common bile duct below the hilar confluence , and this was caused by unresectable malignant tumor or reocclusion of a previously inserted self - expandable uncovered biliary stent , and ( ii ) obtaining an informed consent . the exclusion criteria included surgical resectability of the tumor , hilar obstruction or postoperative anastomotic site obstruction due to malignant recurrence , or the presence of a generally fatal patient condition such as sepsis or disseminated intracoagulopathy . thirty - seven patients had percutaneous biliary drainage performed , of which 34 were treated with a covered stent as the first stent , and three were treated for failure of a standard uncovered stent . six patients underwent sphincterotomy via endoscopic guidance before stent insertion . in our study , 14 patients had biopsy - confirmed adenocarcinomas and 23 patients lacked a confirming biopsy , but these 23 patients had been diagnosed by the findings on abdominal spiral computed tomography ( ct ) , magnetic resonance ( mr ) imaging , percutaneous cholangiography and endoscopic retrograde cholangiopancreatography ( ercp ) . thirteen patients had common bile duct carcinoma , eight patients had pancreatic head carcinoma , four patients had gallbladder carcinoma , six patients had periampullary carcinoma and six patients had gastric carcinoma . the site of obstruction was the proximal common bile duct ( cbd ) in three patients , the middle cbd in six patients and at the distal cbd in 28 patients . the stents used in this study were self - expanding biliary ptfe covered niti - s stents ( taewoong medical corporation , seoul , korea ) , as are described in figure 1 . the niti - s stent we used was made of 0.007-inch single nitinol wire , which has excellent biocompatibility and expansile force , and the stent was also self - expanding without any sharp spurs at either end ( low trauma ends ) . to prevent stent migration , a ptfe membrane covered the stent on the inside of the metal wire . the stents were 5 - 8 cm long when fully expanded to a maximum diameter of 10 mm , and they were positioned using a 9-fr introducer system . the stent was held , compressed and elongated on the delivery catheter by a cylindrical rolling membrane ( the outer sheath ) . the delivery catheter had three markers : one at a first - proximal portion ( the proximal marker ) , the second at the actual stent length position ( the stent length marker ) , and the third at the distal position of the loaded stent ( the stent marker ) . the devised niti - s stent became shortened by 51% after deployment , which is similar to that of wallstents ( 50% ) ; the nitis - s stent had tungsten markers on its proximal and distal ends that helped with manipulation during deployment ( 13 ) . stent placement was accomplished within two weeks after performing percutaneous transhepatic biliary drainage ( ptbd ) . stent placement was accomplished in three patients during the follow - up tubogram after protracted ptbd or stent placement . a right intercostal and/or left epigastric approach was chosen depending on the extent of tumor . stent insertion was performed over an extra stiff amplatz guidewire ( 180 cm , 0.035 inch , curved tip ; cook inc . , care was taken to precisely locate the covered mid - portion of the stent exactly at the stricture . balloon dilation after stent insertion was not performed in any of our patients ; instead , an 8.5-fr or 10.2-fr biliary drainage catheter was left in place for seven days . this catheter was used only for flushing and maintaining access to the biliary tree , and not for drainage . thereafter , imaging was repeated via the drainage catheter to check the stent 's patency , function and position . the catheter was removed if free bile flowing into the duodenum was documented or if the bilirubin level was normalized or markedly reduced . all the patients received 200 mg of prophylactic antibiotics that were administered intravenously over a period of 12 hours ( flomoxef sodium ; flumarin , ildong , ansung , korea ; or fosfomycin sodium ; fonofos , chong keun dang , chunan , korea ) before and usually 48 - 72 hours after the procedure . coagulation tests were performed before the procedure , and if a patient had disseminated intravascular coagulopathy , then stent placement was not performed . sonography of the upper abdomen was performed , and the bilirubin levels were checked at each examination . the patients were also followed by conducting telephone interviews with the referring physicians or with the patients when the patients failed to appear at the follow - up examinations . if stent occlusion was suspected , then the stent was revised percutaneously using a 10-fr drainage catheter . finally , the cause of death was ascertained for all the patients who died within the observation period . successful decompression of the biliary system was defined by a decrease of the serum bilirubin level of more than 30% versus the baseline value within a week of stent insertion ( 10 ) . we measured the distance from the bifurcated line between the right and left intrahepatic ducts to the end of the biliary stent . proximal stent migration was defined as a stent that migrated into the common hepatic duct , and distal stent migration was defined as a stent that migrated into the duodenum or out of the patient 's body . the extents of these migrations were established by comparing the stent ends with the stent positions seen on the initial images . early migration was defined as migration that occurred within one week of deployment , and delayed migration was defined as migration that occurred from one week to one month after stent placement . for the cases of recurrent jaundice , any stent occlusion was confirmed by sonography or ct , which showed dilation of the intrahepatic duct and the presence of soft tissue density inside the stent . recurrent episodes of inexplicable fever were defined as cholangitis . at the time of death , a stent was assumed to be patent if the patient had normal or only mildly elevated serum bilirubin levels ( < 3 mg / dl ) . if the patients had obvious jaundice or if they had a higher bilirubin level than that before stent placement , then the stent was assumed to be obstructed . echogenic materials in the stents , as were determined by us , were considered to represent sludge , and floating material in the stent , as determined by percutaneous transhepatic cholangiography , was also considered to represent sludge formation . the bilirubin and amylase / lipase levels before and during follow - up were statistically analyzed by using the wilcoxon signed - rank test . the stent patency rate and the patient survival rate were calculated by performing kaplan - meier survival ( life - table ) analysis . the duration of primary patency was defined as the interval between stent placement and the recurrence of obstructive jaundice . if obstruction was not evident during a patient 's life , then the patency period was considered to be equal to the survival duration , but this data was censored . stent deployment was successful in all 37 patients , and this was performed via transpapillary placement of the covered stent ( 37/37 , 100% ) ( fig . two patients showed that the stent had migrated completely out of the biliary duct during the 7-week and 27-week follow - up period , respectively . the distance between the bifurcated line and the end of the stent immediately after stent placement was 6 - 35 mm ( mean : 15 mm ) . one week later , follow - up tubography showed no change of this distance ( range : 6 - 35 mm : mean : 15 mm ) . the follow - up plain films showed no changes in any of the stents in the right upper quadrant areas except for two patients who experienced delayed stent migration . the mean bilirubin level decreased from 11.1 mg / dl 6.86 standard deviation ( sd ) before the procedure to 5.12 mg / dl 4.6 sd at the first follow - up , and this difference was statistically significant ( wilcoxon test : p < 0.01 ) . significant changes in the bilirubin levels were observed between the baseline and the most recent follow - up after stent placement ( p < 0.05 ) . the amylase and lipase levels were 110.5 iu / l 150.6 and 133.4 iu / l 187.3 , respectively , before the procedure ; they were 94.0 iu / l 90.6 and 135.2 iu / l 185.0 , respectively , one week after the procedure , and they were 48.6 iu / l 38.1 and 50.3 iu / l 54.2 , respectively , at the last follow - up ( fig . 3 ) . the amylase and lipase levels did not increase at one week and at the last follow - up day after the insertion ( p > 0.01 ) . the amylase and lipase levels were found to be severely elevated one day after stent insertion in 11/37 patients ( 29% ) ; that is , the amylase / lipase level was 505 iu / l ( range : 107 - 1,455)/520 iu / l ( range : 77 - 1,019 ) . despite endoscopic sphincterotomy that was done in six patients , four patients revealed severe elevations of their amylase and lipase levels one day after stent insertion . these 11 patients had pancreatitis symptoms such as pain ; however , their amylase and lipase levels stabilized within seven days . the 30-day mortality rate was 8% ( 3/37 ) , which reflected fatal septic conditions in two patients and death from an unknown cause in one patient . three patients showed good patency of their biliary stent , according to one - week follow - up ultrasound sonography . however , the bilirubin level of two patients before stent insertion was from 11.0 mg / dl to 11.5 mg / dl and 13.5 mg / dl to 14.4 mg / dl , respectively , on the one - week follow - up , and one other patient showed a bilirubin level from 24.7 mg / dl to 22.0 mg / dl . the discrepancies between the image findings and lab findings were related to each patient 's conditions . the lesion was completely covered by the stent in four patients who received a 5-cm - long stent , in 12 patients who received a 6-cm - long stent , in 11 patients who received a 7-cm - long stent , in nine patients who received an 8-cm - long stent and in one patient who received a 9-cm - long stent . no case of biliary side - branch obstruction occurred , except for at the cystic duct . seventy - six percent ( 28/37 ) of the study patients experienced adequate palliative drainage for the remainder of their lives . three patients displayed stent sludge occlusion , and ptbd showed some filling defect in these stents ( fig . however , good stent patency was achieved via irrigation for two patients who both underwent ptbd two times during the follow - up . two patients had ptbd performed : one patient due to tumor ingrowth ( 34 weeks ) and another due to stent migration ( 7 weeks ) . the tubogram finding after ptbd showed stone formation , but we did not find the previously inserted ptfe stent in the abdomen ( fig . one patient underwent percutaneous transhepatic gallbladder drainage at 19 weeks due to acute cholecystitis , and that patient also underwent ptbd at 24 weeks due to overgrowth that was noted at 24 weeks . the duration of the follow - up study for all the patients was 2 - 81 weeks ( mean : 27.9 weeks ) . twenty - eight patients died during 2 - 81 weeks and nine patients lived until 43 - 80 weeks . according to the kaplan - meier survival analysis , the primary stent patency rates were 85% and 78% at 20 and 50 weeks , respectively ; the survival rates were 49% and 27% at 20 and 50 weeks , respectively ; and the mean survival and patency durations were 33 weeks and 64 weeks , respectively . no procedure - related patient deaths were identified , and all the deaths were attributed to the patients ' natural course of disease . the covered stents showed patent lumens in 28 patients ; this was confirmed by such laboratory values as the bilirubin levels at the final follow - up . the reported follow - up durations and patency rates of covered stents in the medical literature have varied ( 10 - 19 )   hausegger et al . ( 10 ) have reported that the patency rates after 1 , 3 , 6 and 12 months were 96% , 69% , 47% and 31% , respectively . rossi et al . ( 16 ) have reported primary patency rates after 3 , 6 and 9 months of 72% , 46% and 46% , respectively . both of these investigations ( 10 , 16 ) did n't report good patency rates because their stent occlusion rates were somewhat high ( 33 - 37% ) . ( 12 ) reported that the mean patency period of their stents was 37.5 weeks . ( 19 ) , in a large series of covered stents , reported the primary patency rates at 3 , 6 and 12 months to be 90% , 76% and 76% , respectively , and we previously reported stent patency rates of 71% and 48% at 20 and 50 weeks , respectively ( 20 ) . in this present study , the primary and stent patency rates were 85% , and 78% at 20 and 50 weeks , respectively . ( 14 ) have reported good patency rates that were the result of lower stent occlusion rates . however , making comparison with those studies is difficult because each of their patients was treated at a different stage of disease , and the tumors differed for their histologic type , cellular growth and biological behavior ( 12 ) . it is resistant to bacterial growth , and this reduces the risk of occlusion caused by bile incrustation ( 18 , 22 ) . moreover , ptfe resists chemicals ( acids and alkalies ) and heat , and ptfe is self - lubricating and not prone to demolding . moreover , although ptfe has a low coefficient of friction and therefore good patency , it has poor adhesion to other materials ( 23 ) . in the present study , first , a 9-fr introducer system was used for implanting the ptfe - covered stents , which is a much smaller system than that required for the schoder et al . second , in order to prevent stent migration , the end portion of the stent and the outside of the stent are not coated with ptfe . third , the stent is designed to be covered with ptfe on the inner aspect of the stent . the stent wire surrounds the ptfe , and so the stent 's outer surface is similar to that of an uncovered stent . in our study , the disadvantage of the eptfe / fep - covered stent is that the fabric covering of the expanded stent may cause blockage of the cystic or pancreatic duct , and this can result in possible cholecystitis or pancreatitis ( 18 , 19 ) . in our study , three patients ( 3/37 , 8% ) underwent percutaneous transhepatic gallbladder drainage due to acute cholecystitis . the most obvious disadvantage of the ptfe stent was that stent insertion was followed by marked elevation in the amylase and lipase levels one day after insertion ; these levels subsequently decreased and then fell to normalized levels within seven days . the other disadvantage of the stent is the occurrence of stent obstruction due to sludge formation . in our opinion , this elevation of the amylase and lipase levels was caused by pancreatic sphincter hypertension because there was no space between the stent and the sphincter of oddi , as seen on post - endoscopic retrograde cholangiopancreatography ( ercp ) . the level of the lesion was the proximal cbd in one patient , the mid cbd in two patients and the distal cbd in eight patients . the cause of disease was cbd carcinoma in three patients , gastric carcinoma in three patients , ampulla of vater carcinoma in two patients and pancreatic head carcinoma in three patients . the second disadvantage of stenting is the possibility of stent occlusion due to food reflux from the duodenum into the proximal common bile duct . there are many studies that have made comparisons between covered and uncovered stents ( 24 - 26 ) . reported have that covered stents successfully prevented tumor ingrowth and they were significantly superior to uncovered stents for treating those patients suffering with distal malignant biliary obstruction ( 24 ) . stent occlusion occurred in covered stents for eight patients ( 14% ) after a mean of 304 days ( 24 ) . yoon et al . reported that the stent patency rates were 83% , 78% , 67% and 54% at 100 , 200 , 300 and 400 days , respectively , for covered stents and 83% , 66% , 54% and 36% , respectively , for uncovered stents , which was not significantly different ( 25 ) . park et al . have reported that the mean stent patency was 148.9 days for covered stents and 143.5 days for uncovered stents . migration of stent occurred in 0.6% of the patients and sludge formation in stents occurred in 20% of the patients in the above study . in our study , the covered stent showed similar patency rates , a high incidence of mild pancreatitis and a low incidence of sludge formation and stent migration . one patient had tumor ingrowth in the mid portion of their stent ; however , the image finding was not confirmed by choledochoscopy . tumor ingrowth may not occur in ptfe materials because the ptfe itself is a strong barrier . if it does occurs , it would be due to damage or natural degradation of ptfe . in conclusion , the ptfe - covered self - expandable nitinol stent is safe with acceptable complication rates . this study is similar to previous studies in regard to comparing between the patency rates and the survival rates . using the ptfe - covered self - expandable nitinol stent for treatingmalignant biliary obstruction