Is There Room for Improvement in Increasing the Stent Patency of Biliary Plastic Stents?

Endoscopic stent insertion is an effective method for biliary decompression that contributes to the regression of symptoms and improvement in quality of life for patients suffering from obstructive jaundice due to malignant biliary obstruction or benign stricture. Although there are two types of stents, i.e., plastic stent and self-expandable metal stents (SEMS), SEMS have largely replaced plastic stents for palliation of malignant biliary obstruction and are being increasingly used to manage benign strictures. At this rate, it seems conceivable that SEMS will practically substitute plastic stents, leaving no place for plastic stents to stand. However, this is a far-fetched speculation since SEMS itself is far from being perfect and poses its own limitations as follows. First, the degree of adverse events tends to be more severe with SEMS compared to plastic stents once they are present. Second, the stent patency of SEMS is still not satisfactory. There is no doubt that SEMS do have longer patency than plastic stents. However, clinical studies have not always been consistent in showing superiority of SEMS over plastic stents both in malignant and benign biliary strictures, except for lowering revision or reintervention rate. Third, uncovered SEMS are almost impossible to remove once they are deployed, whereas plastic stents are easy to remove. With much effort being put into developing and promoting diverse therapeutic modalities in order to increase survival of patients suffering from inoperable cancers causing distal malignant biliary obstruction, ease of stent revision would be an important feature in these patients. This is especially true for those undergoing local therapy with photodynamic therapy or radiofrequency ablation after which plastic stents are generally placed. Thus, with plastic stents still in demand, is there any room for improvement in increasing the stent patency of biliary plastic stents so as to regain its rightful place? In order to answer these questions, it would be necessary first to briefly go over the mechanisms underlying plastic stent occlusion. Since the introduction of plastic stent in 1979, the mechanisms of stent occlusion have been extensively studied and can eventually be summed down to two factors: bacteria and reflux. Many studies have shown that the initial event leading to stent occlusion is the adhesion of bacteria to the internal plastic stent surface. Once adherent, bacteria multiply within the glycocalix matrix to subsequently form endoluminal microcolonies. Therefore, attempts to reduce stent clogging have focused on altering stent design and reducing microbial colonization. However, many strategies including increasing stent diameter, changing stent composition and shape, modifying stents to have no side holes, placing stent above the papilla, administering long-term prophylactic antibiotics, and impregnating stent with antibiotics have failed to adequately prolong stent patency. Even when some experiments showed some glimpse of hope, they did not meet our expectations in clinical studies. There have been few clinical randomized studies showing promising results, but selection bias and intention-to-treat principle for analysis render results of these trials less convincing. Nevertheless, efforts put into plastic stent were not all in vain since plastic stents with antireflux valve prolonged stent patency by 1.5 times, albeit far from being on a par with SEMS. Hydrophilic coating method is another method for preventing plastic stent occlusion that showed promising result in in vitro studies. This method was employed based the fact that bacterial adhesion to the plastic surface is closely related to the surface hydrophobic-