Mechanisms of Occlusion of Uncoated Self-Expanding Metal Biliary Stents (SEMS): Final Results of a Multi-Center Comparative Study

Abstract

Background: A large three-armed multi-center RCT was designed to compare patency, mechanism of stent failure and other placement features of two types of uncoated SEMS. The study included two diameters of SEMS (Wallstent (W) 10 mm, Boston Scientific; Zilver (Z) 10 mm and 6 mm, Cook Endoscopy) for the treatment of biliary strictures due to unresectable malignancy occurring at least 2 cm below the bifurcation. These SEMS are of different expansion design (foreshortening-W and non-foreshortening-Z) and are made of different metals (stainless steel-W and nitinol-Z). We now report final patency data and mechanisms of occlusion. Patients and Methods: 241 patients (pts) were randomized by remote telephone (Rapid System, Med Institute, West Lafayette, Indiana) at 9 participating centers into the three arms and followed to 95% study end. Increased number of occlusions in the 6 mm arm was noted at interim analysis which was then closed to randomization and reported (GIE 2006;63:298AB). Results: Symptomatic occlusion occurred in 25/64 (39%) of the 6 mm SEMS, but in only 39/177 (22%) in the 10 mm arms (p = 0.008). The occlusion rate of the two 10 mm SEMS were equal (19/89 (21%) for Wallstent 10 mm vs. 20/88 (23%) for Zilver 10 mm, p = 0.83). There was no difference in mortality in any of the three arms. Details of the 64 occlusions were characterized by protocol to be principally due to ingrowth (n = 41), overgrowth (n = 7), debris (n = 15), or blood clot (n = 1). Multiple forceps biopsies were collected on 15/41 (37%) of ingrowth cases and were positive for cancer in 7/15 (47%) but negative for cancer in 8/15 (53%). Conclusions: The studied biliary uncoated self-expanding metal stents placed for malignant obstruction below the bifurcation demonstrated lower patency at a 6 mm diameter compared to 10 mm. The 10 mm Wallstent and 10 mm Zilver arms had equal numbers of symptomatic occlusions in this study. The documented mechanisms of occlusion were due to benign tissue ingrowth in >50% of those sampled and ingrowth accounted for only 64% of the total. These findings may have important implications for future stent design especially with the possibility of adding drug-eluting technology to biliary metal stents.