Chronic pancreatitis is a result of unresolved and prolonged inflammation of the pancreas and eventually leads to pancreatic fibrosis and obstruction of the pancreatic and biliary ducts, the prevalence of which is 50/100 000 people. Common bile duct stenosis is a secondary outcome of chronic pancreatitis which occurs in 8-30% of patients. The obstruction of biliary duct is often associated with liver cirrhosis and previous studies have shown that the fibrosis of liver and pancreas can be diminished or the organs can regenerate when the ducts are opened (see references below). While I do not want to further discuss the disease itself, but rather focus on its treatment and prevention with use is metal-based stents (elaborated on below), here are two links to the websites which provide an overview of chronic pancreatitis and pancreatic fibrosis: https://pancreapedia.org/reviews/introduction-to-pancreatic-disease-chronic-pancreatitis, https://pancreapedia.org/reviews/pancreatic-fibrosis.
The treatment of stenosis of common bile duct (or pancreatic duct) is mainly done through drainage and opening of the ducts. This can be performed by surgical or endoscopic drainage and use of endoprosthesis such as stents. Several types of stents have been developed and used in the past, the most common types are plastic and metal stents. Metal stents often are covered and based on this are further subdivided into uncovered self-expandable metal stents (USEMS), partially covered self-expandable metal stents (PCSEMS), and fully covered self-expandable metal stents (FCSEMS).
Clinical trials generally agree that placement of PCSEMS and FCSEMS as opposed to USEMS and plastic stents results in better clinical outcomes. The commercially available metal stents are made of Nitinol (Ni-Ti alloy), Platinol (Pt-Ni-Ti), Elgiloy (Co-Cr-Ni alloy), or Stainless steel and their structures vary depending on whether they’re braided or laser-cut. The different alloys differ in terms of their expandability in the duct, elasticity, and thus ability to adjust to the shape of the bile duct. In general, Nitinol- based SEMS perform better as they are more elastic. Moreover, the patency of such stent was shown to be better in multiple studies, and the axial force (explained below) is generally lower than for steel- or Elgiloy-based stents which leads to lower dislocation rate of the Nitinol-based stent. Regardless of the materials from which they are made, the major issue with USEMS is that the bare metal directly interacts with the tissue, so hyperplasia and ingrowth of tissue between the mesh is often observed. Such a stent is then difficult to remove and very often fails as the overgrowth of cells again occlude the duct at the site of stent placement. Therefore, surgeons prefer to use PCSEMS and FCSEMS as opposed to USEMS.
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he stents can be covered with various biomaterials, including polyurethane, silicone, or polytetrafluoroethylene. Those materials are relatively resistant to hydrolysis and therefore the coatings are durable in the duct. It has been suggested that covering of metal stent with membrane of approximately 50 um was enough to minimize hyperplasia and ingrowth of tissue but maintain the biodegradable properties of the membranes. This results in the increase of patency of the stent and allows for its removal when the stent is no longer needed (figure on the left presents examples of PCEMS and FCSEMS). The drawback of such design is that the stents could potentially migrate and therefore anchoring fins, flaps, or have dilated, flared ends are included in the design which reduce dislocation of these stents. The definite benefit of the covering is that the stents could include drug-releasing systems with for example anti-inflammatory agents or other drugs beneficial in treatment of pancreatitis.
As for the mechanical properties of stents, two major values are the most important in defining the stent’s characteristics, the axial force (AF) and radial force (RF). AF is the force applied onto a bend of a stent which results in straightening of the stent, and RF is the force applied by the stent onto the surrounding tissue by the expansion of the stent. Ideally, the stents should have low AF (no occlusion) and medium RF (medium force exerted on the duct itself, full expansion of the stent). The tables below summarizes some of the commercially available stents, their coverings, as well as AF and RF, migrations rate, and structure. SEMS come in different sizes, however studies have shown that regardless of their diameter, the clinical outcomes are very similar.
It seems that SEMS with coverings are very tunable in terms of their mechanical properties (), durability, antimigratory design without causing hyperplasia and maintaining the ease of removal of the stent, and can be further modified to include drug-releasing coatings and improve the overall efficiency of the stents as treatments for biliary duct/ pancreatic duct occlusion, and ultimately prevent chronic pancreatitis and formation of fibrotic scarring of the pancreas and liver.
References:
Behm, B., Brock, A., Clarke, B. W., Ellen, K., Northup, P. G., Dumonceau, J. M., & Kahaleh, M. (2009). Partially covered self-expandable metallic stents for benign biliary strictures due to chronic pancreatitis. Endoscopy, 41(06), 547-551.
Hammel, P., Couvelard, A., O’toole, D., Ratouis, A., Sauvanet, A., Fléjou, J.F., Degott, C., Belghiti, J., Bernades, P., Valla, D. & Ruszniewski, P., 2001. Regression of liver fibrosis after biliary drainage in patients with chronic pancreatitis and stenosis of the common bile duct. New England Journal of Medicine, 344(6), pp.418-423.
Isayama, H., Nakai, Y., Kawakubo, K., Kogure, H., Togawa, O., Hamada, T., Ito, Y., Sasaki, T., Yamamoto, N., Sasahira, N. & Hirano, K., 2011. Covered metallic stenting for malignant distal biliary obstruction: clinical results according to stent type. Journal of Hepato‐Biliary‐Pancreatic Sciences, 18(5), pp.673-677.
Isayama, H., Nakai, Y., Toyokawa, Y., Togawa, O., Gon, C., Ito, Y., Yashima, Y., Yagioka, H., Kogure, H., Sasaki, T. & Arizumi, T., 2009. Measurement of radial and axial forces of biliary self-expandable metallic stents. Gastrointestinal Endoscopy, 70(1), pp.37-44.
Shamah, S., Waxman, I., Chapman, C. G., Haider, H., & Siddiqui, U. D. (2018). Sa1293 Largers US experience with partially covered self expandable stents (SEMS) for malignant biliary obstruction: size does not matter. Gastrointestinal Endoscopy, 87(6), AB200.