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Pancreaticobiliary therapy (biliary): plastic stents, metal stents and … beyond
28 August 2019 (online)
When Nib Soehendra described in 1980 the technique of endoscopic placement of a stent to bypass a stricture of the common bile duct (CBD) , a new epoch of endotherapy was born. Soehendra’s stents were home-made from a Teflon tube, cut to the proper length and customized with hot water. Later, companies producing endoscopic accessories turned to polyethylene, a material with a smoother surface, and introduced commercially available products to the market, although Soehendra remained faithful to Teflon! The shape of the stents was also modified, from a single-pigtail configuration to a straight design, following the ideas of Kees Huibregtse in the Netherlands, and Peter Cotton and Joseph Leung who had recently moved from London and Hong Kong to the United States.
For several years, plastic stents of variable sizes (7, 8.5, 10, 11.5 Fr) represented the fundamental tools in endotherapy for both malignant and benign biliary strictures: plastic stents were used for preoperative and palliative drainage of biliopancreatic malignancies, and for dilation and calibration of postoperative biliary strictures or CBD compression from chronic pancreatitis. The main drawback of plastic stents was, and still is, their propensity to occlude. The process starts with the formation of a bacterial biofilm sticking to the inner surface of the stent as a result of bile contamination with intestinal bacteria. Then, the deposition of bilirubinate and palmitate salts over the biofilm, together with proteins and refluxed food residue, eventually leads to the occlusion of the stent lumen. This process occurs systematically and usually leads to recurrent symptoms of biliary obstruction within 3 – 4 months, depending on stent caliber, large bore stents being more durable than smaller sized stents. We learned that “the larger, the better.” However, plastic stent diameter was limited by the size of the operative channel of therapeutic duodenoscopes, to a maximum of 11.5 Fr. An alternative solution was definitely needed, especially for palliative purposes in malignant lesions.
To overcome the limitation of the operative channel size, the “next-generation” stent had to include a delivery catheter compatible with the operative channel (3.8 mm, later 4.2 mm), but at the same time the stent, once released over a guidewire, should expand as much as possible in order to reach a much larger diameter compared with the available plastic stents. The solution was to use braided metal wires, as already tested in vascular and urethral strictures. Several types of metal stents became available in the late 1980s and early 1990s. Some were self-expandable (i. e. the Gianturco Z-stent), other were expanded by balloon inflation (i. e. the Strecker tantalum stent). However, because of its ease of use, radial force, and precise delivery system, the front-runner immediately appeared to be the stainless steel, braided Wallstent, designed by Hans Wallstén in Switzerland  , which could reach a diameter of 10 mm or 30 Fr when fully expanded. In 1989, Huibregtse et al.  reported on the first series of 33 patients treated with endoscopically inserted Wallstents, heralding a new era of endoscopic stenting. Clinical success was reached in all but one patient in the series, in a short-term follow-up of only 4 weeks. This was followed by a much larger series heralding the combining of data from different endoscopy centers .
Since then, self-expandable metal stents (SEMS) have been refined in several ways. Nitinol, a more manageable material, has replaced stainless steel. Metal stents are now also available in the partially covered or fully covered (with various materials) versions. Thanks to the fully covered option, metal stents have also become potentially removable. These technological improvements have substantially broadened the indications of SEMS use. At the beginning of the SEMS era, several randomized controlled studies provided evidence that SEMS performed better than plastic stents in the palliative treatment of biliopancreatic malignancies involving the mid-lower third of the CBD . Then, multiple case series also showed the efficacy of uncovered single or multiple SEMS to palliate biliary obstruction occurring at the hepatic hilum. Today, covered SEMS are also used to manage non-neoplastic strictures, such as postoperative fibrotic strictures occurring after biliary surgery or orthotopic liver transplantation and strictures of the lower CBD caused by chronic pancreatitis . Randomized controlled trials are currently being conducted to compare removable fully covered SEMS with multiple plastic stents in these clinical settings. Preoperative drainage with SEMS in patients with resectable pancreatic cancer is also under investigation.
What’s next? The current frontier is to show the potential benefit of “active or therapeutic” devices: drug-eluting stents, light-activated photodynamic stents, and maybe radiofrequency- and radiotherapy-delivering stents. The development of such technological advances is possible and is limited only by human imagination and ingenuity.
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