What is the ideal shape of a self-expandable metal stent with no migration or obstruction?

An ideal biliary stent is one that is permanently patent and physically gentle on the bile duct wall. To achieve this, many endoscopists and company researchers have been making efforts, achieving various ingenuities and concepts. Stent patency is primarily damaged by food impaction, sludge formation in the stent, tumor ingrowth/overgrowth, and stent dislocation/migration. Many measures have been applied to overcome these issues, including stenting above the papilla (inside stent), use of a mechanistic valve to prevent duodenal juice/food reflex, use of special materials such as polytetrafluoroethylene and silicon coating for inner smoothing, application of a membrane that covers a metal stent to prevent ingrowth, flared or tapered structure of a metal stent end, and use of multihole membrane covering a metal stent to prevent migration.¹

Biliary stents are first divided into “plastic” and “metal” stents. According to several meta-analyses of stent patency in malignant distal biliary obstruction, metal stents are superior to plastic stents; however, they are more commonly associated with postendoscopic retrograde cholangiopancreatography pancreatitis, an important postoperative complication, due to the large diameter. Metal stents are also classified as covered (CSEMS) and uncovered self-expandable metal stents (USEMS). Each type of metal stent has both advantages and disadvantages: the former tends to dislocate/migrate due to covered surface smoothness preventing tumor ingrowth, whereas the latter tends to be occluded by tumor ingrowth through the mesh gap preventing migration. Although the patencies of CSEMS and USEMS are controversial, the drawbacks of both stents as described should be addressed. CSEMS can be further subclassified into fully covered SEMS (FCSEMS) and partially covered SEMS (PCSEMS); the former was developed later to overcome the drawbacks of PCSEMS, such as tumor ingrowth/overgrowth and difficult removability. Although PCSEMS are generally considered best choice due to the issues associated with FCSEMS and USEMS, in their current form, PCSEMS still have issues, despite its longer patency than FCSEMS, as indicated in a recent meta-analysis (369 days vs. 238 days, d-value = 0.116).²

Several countermeasures have been tested to prevent dislocation/migration of CSEMS. First, the flared structure of both ends of CSEMS showed no stent migration, lower tumor ingrowth, and a longer patency than USEMS with the same flared ends.³ However, the rates of tumor overgrowth and sludge formation, both of which were significant issues, did not significantly differ between the two SEMS groups. Second, CSEMS with a tapered-and-flared structure in the distal end showed no dislocation/migration and sustained biliary decompression with no stent trouble until pancreatoduodenectomy.⁴ However, long-term outcomes of this novel stent are unclear and need to be clarified in a large multicenter cohort study. Third, a very unique CSEMS with a multihole membrane (MHSEMS) has recently been developed. The largest retrospective study under a multicenter setting published by Takahashi et al. in this issue of Digestive Endoscopy, showed a very long patency of 446 days, which is longer than that of PCSEMS (219.3 ± 159.1 days) reported by Kitano et al.,^{1, 3} as well as good removability of 88.9%. The multihole system on the membrane of FCSEMS is a novel concept that uses mild protrusions of the tumor and normal mucosa of the bile duct through the holes as anchors, which can be a double-edged sword, as the protrusions can also introduce tumor ingrowth. However, MHSEMS unexpectedly revealed the surprising long patency due to the low migration rate (symptomatic migration rate = 1.9%), probably as migration may be more likely to affect and shorten the time to recurrent biliary obstruction than ingrowth according to the authors' discussion, which could be a new finding.

Unfortunately, MHSEMS migration was not completely prevented by the multihole system, which is based on the dilemma between ensuring a lack of migration and easy removability. Regarding CSEMS removability, one prior study revealed that clinical outcomes following reintervention for CSEMS with recurrent biliary obstruction was improved by CSEMS exchange with a new stent compared with balloon cleaning of an in situ previous stent.⁵ Meanwhile, easy stent removability is also important for the patients whose survival time is longer than before due to improved chemotherapy with various anticancer drugs and immune checkpoint inhibitors. However, researchers should also aim to produce a stent with no occlusion and no migration for an ideal stent with permanent patency and no exchange.

There are several physical or mechanical properties probably associated with the stent patency and migration that characterize SEMS, including axial force (AF), defined as the force to recover to a straight position after bending; radial force (RF), defined as chronic outward force; and resistance force to migration (RFM), which represents antimigration potential and is measured by the maximum value of force during the stent retraction at a speed of 1 mm/s until the distal end of the stent is dislocated from the silicone wall.^{6, 7}

Thus far, no reports have clearly revealed a correlation between patency and AF/RF; however, one study by Minaga et al.⁷ investigated the relationship between RFM and RF in five braided CSEMS. These researchers indicated no correlation between these factors, but instead that RFM after full stent expansion (diameter 10 mm) correlated strongly with all three stent flare structure variables: outer diameter of the flare (coefficient 0.952), height of the flare (coefficient 0.943), and taper angle of the flare (coefficient 0.906). Therefore, the multihole system should also be examined for RFM in various settings, including differing outer diameters of the compressed stent and an animal model where mucosal protrusion can chronologically develop or change through the multiple holes for most appropriate RFM.

To develop a gentle stent for the bile duct, a slim CSEMS with a 6 mm diameter would be better than a CSEMS with an 8 mm or larger diameter because adverse events such as bleeding and pseudoaneurysms due to SEMS compression of the bile duct have been reported.⁸ However, slim CSEMS are associated with more frequent stent migration, with a rate of 15.6% compared with the previous rate (FCSEMS, 9.8%; PCSEMS, 4.3%), whereas that of MHSEMS with a 10 mm diameter is much lower.^{9, 10} Therefore, 6 mm MHSEMS may be better to reduce bile duct damage in addition to application to hilar biliary obstruction, as the authors referred to.

It is also known that the hole size of a new stent can be adjusted according to the size of the “cell” formed by the wire spacing. However, since the size of the cell affects the tumor ingrowth and the mechanical properties of the stent, the larger the cell size, the more disadvantages the stent has. Overall, the optimal hole size, number, and array to prevent migration while effectively maintaining patency should further be examined in the future, balancing these factors based on the biliary obstruction location.

Authors declare no conflict of interest for this article.

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