ERBD is effective in the prevention and treatment of cholangitis and enables physiologic drainage of the bile, making it a suitable palliative therapy for patients with unresectable malignant biliary obstruction.2
The indications of ERBD include malignant biliary obstruction due to pancreatic cancer or cholangiocarcinoma, suppurative cholangitis, common bile duct stone impaction, bile duct stricture after surgery, and obstructive jaundice concomitant with chronic pancreatitis. In malignant biliary obstruction, the placement of biliary plastic stent rapidly improves the jaundice and allows semi-permanent biliary drainage in unresectable cases. However, the plastic stent is not permanent and requires replacement or additional stent insertion in several months when the occlusion occurs. Efforts have been made to reduce the stent occlusion, such as by decreasing bacterial adhesion inside the stent or by changing the material or structure-size and shape, that is-of the stent,14
without an apparent success yet.
Plastic stent is not expensive and replaceable, but requires frequent replacement due to the stent migration or frequent occlusion leading to the recurrence of obstructive jaundice or cholangitis.8
Metal stent was developed in 1989 to overcome these limitations and has been in use since then.5
SEMS is associated with less occlusion and cholangitis and shorter duration of hospital stay compared with the plastic stent.7
The plastic stent induces occlusion mostly by the formation of biliary sludge. The metal stent, however causes occlusion mostly by the tumor ingrowth, where the tumor grows through the gaps of the stent mesh; by the tumor overgrowth, where the tumor grows to the upper and lower space of the stent; and by epithelial hyperplasia inside the stent.16
The rate of metal stent occlusion due to the tumor ingrowth or overgrowth after some time, has been reported variously from 7% to 42%.5
According to a prospective trial by Davids et al.,7
the early efficacy of drainage by the plastic stent and the metal stent are not significant different at 95% and 96%, respectively. This study also revealed no difference of the early efficacy by the materials of the stents, with the polyethylene plastic stent and the Hanaro covered SEMS respectively showing 91.3% (21/23) and 90.5% (19/21) of functional success rates. Given the fact that a stent of 7 Fr or more is physiologically sufficient to induce biliary drainage,21
this lack of difference in the early efficacy of drainage between the plastic stent group and the Hanaro covered SEMS group in our study may come from using 10 Fr plastic stents. The time to biliary stent occlusion of a 10 Fr plastic stent has been reported average 134 to 164 days.22
Davids et al.7
reported stent patency of 273 days for metal stent (Wallstent; Schneider, Minneapolis, MN, USA) and 126 days for polyethylene plastic stent in distal malignant biliary obstruction. Our findings also showed 233.6 days and 94.6 days of mean stent patency for the Hanaro covered SEMS group and the plastic group, respectively.
The advantage of the SEMS is that the stent diameter can be increased remarkably to allow early sufficient drainage, the duration of patency is longer than the plastic stent, and the frequency of migration is low.7
But several disadvantages of the SEMS have been also pointed out, including the difficulty of insertion technique compared with that of the plastic stent,24
the possibility of injuries such as bleeding or perforation in the duodenum and the papilla,25
high cost, inability to remove once placed, and the possibility of stent occlusion due to the tumor ingrowth or overgrowth.26
Despite the development of covered SEMS, using a thin layer of polyurethane, silicon, polytetrafluoroethylene or Gore-Tex to cover SEMS (30 Fr), to prevent functional failure due to the tumor ingrowth or overgrowth, tumor ingrowth is still reported to occur with the membrane covered metal stent.27
Suh et al.31
reported, in a study of 59 patients who were diagnosed with extrahepatic biliary stricture due to the malignant tumor, that the tumor ingrowth was the cause of the stent functional failure in 6.6% (2/29) of the covered SEMS group compared to 24% (7/30) of the uncovered SEMS group, suggesting that the covered SEMS may prevent the tumor ingrowth efficiently. The functional failure of the Hanaro covered SEMS in our study was due to stent occlusion in 9 patients and of migration in 1 patient (late stent migration); among these 9 patients of occlusion, 4 patients (44.4%) were associated with the tumor ingrowth, 1 patient with the tumor overgrowth, and the remaining 4 patients with the biliary sludge, reflecting that the covering membrane of the SEMS cannot prevent the tumor ingrowth completely. Tumor ingrowth in the covered SEMS may occur when the membrane was damaged during the stent insertion, by the bile or biliary stone, or by bacterial infection, but little data are available regarding the exact cause of tumor ingrowth.
Early complications occurring during the stent insertion or within 2 weeks after the insertion include cholangitis, cholecystitis, perforation, bleeding, and acute pancreatitis, at 6.4% to 26.3% of incidence, among which cholangitis occurring most frequently.22
Davids et al.7
reported that 11% of the plastic stent group and 12% of the metal stent group were associated with the early (within a week after the stent insertion) complications, which is similar to our results of 12.5% (3/23) for the plastic stent group and 14.3% (3/21) for the Hanaro covered SEMS group. Isayama et al.32
reported that the membrane attached to a stent often blocks the orifice of the cystic duct or the pancreatic duct causing cholecystitis or pancreatitis. In this study, there were 3 cases of early complications in the both groups, repectively. In the Hanaro covered SEMS group, there were pancreatitis in 1 patient, cholangitis in 1 patient and cholecystitis in 1 patient, all of them were recovered with conservative treatment. In the plastic stent group, pancreatitis occurred in 1 patient and cholangitis in 2 patients; the former was recovered with conservative treatment, while each of the patients with cholangitis required SEMS insertion and PTBD, respectively.
There is still a controversy in what type of stent should be used for ERBD in terms of cost-effectiveness. Prat et al.33
reported 3.2 months of survival for ≥30 mm of tumor diameter and 6.6 months of survival for ≤30 mm of tumor diameter in 101 patients with unresectable malignant biliary obstruction. Therefore, they insisted that the metal stent may be more effective than the plastic stent when the tumor diameter is 30 mm or less in unresectable malignant biliary obstruction.33
Schmassmann et al.17
also reported that the metal stent was more effective than the plastic stent in patients with more than 6 to 9 months of life expectancy.
Cahen et al.34
emphasized that modification of the SEMS design was necessary for convenient removal and repositioning of the stent. The lasso attached on the covered SEMS in our study allowed functional success in 1 case of technical failure during the insertion by enabling the stent repositioning at ease.
In conclusion, the placement of the covered SEMS with longer stent patency seemed to be more effective than that of the plastic stent in patients with unresectable malignant biliary obstruction who their life expectancy are estimated to be longer than 6 to 9 months based on their general condition, the presence of comorbidity and biliary stricture diameter. The lasso of the covered SEMS seemed to be helpful in repositioning of the stent to achieve functional success, although this issue needs to be further studied in prospective studies with more cases.