The Wallstent has remained the industry standard for biliary self-expanding metal stents (SEMSs). Recently, stents of differing designs, compositions, and diameters have been developed.

To compare the new nitinol 6-mm and 10-mm Zilver stents with the 10-mm stainless steel Wallstent and determine the mechanism of obstruction.

Randomized, prospective, controlled study.

Nine centers experienced in SEMS placement during ERCP.

A total of 241 patients presenting between September 2003 and December 2005 with unresectable malignant biliary strictures at least 2 cm distal to the bifurcation.

Stent occlusions requiring reintervention and death.

At interim analysis, a significant increase in occlusions was noted in the 6-mm Zilver group at the P = .04 level, resulting in arm closure but continued follow-up. Final study arms were 64, 88, and 89 patients receiving a 6-mm Zilver, 10-mm Zilver, and 10-mm Wallstent, respectively. Stent occlusions occurred in 25 (39.1%) of the patients in the 6-mm Zilver arm, 21 (23.9%) of the patients in the 10-mm Zilver arm, and 19 (21.4%) of the patients in the 10-mm Wallstent arm (P = .02). The mean number of days of stent patency were 142.9, 185.8, and 186.7, respectively (P = .057). No differences were noted in secondary endpoints, and the study was ended at the 95% censored study endpoints. Biopsy specimens of ingrowth occlusive tissue revealed that 56% were caused by benign epithelial hyperplasia.

SEMS occlusions were much more frequent with a 6-mm diameter SEMS and equivalent in the two 10-mm arms despite major differences in stent design, material, and expansion, suggesting that diameter is the critical feature. Malignant tumor ingrowth produced only a minority of the documented occlusions.