Recent advances in metallic stents used in the stent‐in‐stent method for hilar malignant biliary obstruction

The optimal treatment strategy for unresectable hilar malignant biliary obstruction (HMBO) should be determined based on both patient- and stent-related factors.1 Patient-related factors include HMBO-causing diseases, anatomical considerations, and advances in chemotherapy. Stent-related factors include type of stent (metallic stent [MS] or plastic stent [PS]), drainage area (unilateral or bilateral), placement of the lower end of the stent (above the papilla or not), and method for multiple stent placement (stent-in-stent [SIS] or side-by-side [SBS]). Many studies have compared SIS and SBS methods for HMBO; however, conclusions vary from report to report regarding procedure success, clinical success, and complication rates for initial drainage.2-4 Notably, the SIS method has been proven to be technically more difficult than the SBS method. A successful SIS method depends on the second stent, which needs to be inserted beyond the first stent mesh into the drainage area; however, the small mesh area hinders the passage of the stent. Therefore, dedicated uncovered self-expandable MS (SEMS) for the SIS method has been investigated. The Niti-S Y-type stent (Taewoong Medical, Gyeonggi-do, South Korea) is an uncovered SEMS developed as a hybrid of spiral and Z stents. The mesh of the central region of this stent was made larger by omitting the Z component (forming a central area of a relatively wide-open mesh), resulting in a central open-weave portion precisely 10 mm long. Kim et al.5 reported that the SIS method performed using the Niti-S Y-type and Zilver (Cook Endoscopy, Winston-Salem, NC, USA) stents achieved a technical success rate of 85% (29/34) and a clinical success rate of 100.0% (29/29). The BONASTENT M-Hilar (Standard Sci. Tech Inc., Seoul, South Korea) is an uncovered SEMS developed specifically for the SIS method, has an independent central cross-wire structure, and is suitable for second stent insertion. Yamao et al. reported that the SIS method performed using the BONASTENT M-Hilar stent achieved a technical success rate of 93.5% (43/46) and a clinical success rate of 93.0% (40/43).6 Half of the cases required additional dilation for second stent insertion. This may have occurred because the cell size of BONASTENT M-Hilar is as small as 1.6 mm; therefore, the tip of the delivery sheath for the second stent was caught in the cell and could not be inserted. The Hilzo Biliary Moving Cell Stent (BCM Co., Ltd, Seoul, South Korea) is a “moving cell” uncovered SEMS developed for the SIS method. This stent allows the cell size to expand from 4 to 10 mm depending on the thickness of the stent sheath during the second device insertion.7 Kawai et al.8 reported that the SIS method performed using the Hilzo Biliary Moving Cell Stent achieved a success rate of second stent insertion without initial mesh expansion of 85.2%, a median time to recurrence of biliary obstruction of 271 days, and a stent dysfunction rate of 44.4%. The Niti-S Large Cell D-type (LCD) biliary stent (Taewoong Medical) is an uncovered SEMS designed with a cell diameter as large as 6 mm to facilitate a second stent insertion using the SIS method. A comparative study of the effect of cell size on the SIS method performed using BONASTENT M-Hilar as a small cell size stent and Niti-S LCD biliary stent as a large cell size stent reported no significant differences in success rates (small cell size stent vs. large cell size stent: 100% vs. 100%; early complication rates: 38.1% vs. 18.2%; late complication rates: 14.3% vs. 22.7%; and stent occlusion rate: 42.9% vs. 45.5%) between both groups.9 However, the delivery system of the Niti-S LCD biliary stent had a thickness of 8.5F, reducing its ability to be inserted beyond the hilar biliary stenosis and second stent insertion in the SIS procedure. Niti-S Large Cell SR Slim (LC slim delivery; Taewoong Medical) is an uncovered SEMS developed for the SIS procedure. It has a delivery tip thinner (6F) and more flexible than conventional tips, improving mesh passage even under strong bile duct flexion conditions.10 In this issue, Ishigaki et al.11 compared clinical outcomes of LC slim delivery and conventional LCD for unresectable HMBO. The study included 83 patients with HMBO; 31 received LC slim delivery, whereas 52 received LCD. The overall technical and clinical success rates, recurrent biliary obstruction rates, and time to recurrence of biliary obstruction were similar for both methods. However, multiple regression analysis showed that LC slim delivery was associated with a shorter stent placement time of 18 min, compared with 23 min in the LCD group. In addition, the early adverse event (AE) rate was reported to be 10% in the LC slim delivery group without cholangitis or cholecystitis compared with 23% in the LCD group. This comparative study is interesting because it focused on the evolution of the delivery tip rather than the stent itself. The authors also noted that using thin, appropriately bent delivery tips significantly reduced stenting time. Considering that the LC slim delivery and LCD stents are almost identical in shape, focusing on the delivery tip was a significant clinical improvement. The authors particularly emphasized that LC slim delivery was associated with fewer AEs than LCD (10% and 23%, respectively), and the reason for the low AE rate in the LC slim delivery group was that shortened procedure time prevented development of postprocedure cholangitis and liver abscesses. Shortening stenting time and reducing technical difficulties may prevent excessive cholangiography and inappropriate guidewire manipulation. However, it should be emphasized that excessive cholangiography and abusive guidewire manipulation can still cause AEs even with LC slim delivery. The authors also reported that slim delivery facilitated the placement of multiple SIS, with 32% of the LC slim delivery group able to insert three to four stents. Although multiple stent placements naturally lead to longer examination times, slim delivery also contributed to shorter examination times. Notably, endoscopic sphincterotomy (EST) was performed more frequently in the LC slim delivery group than in the LCD group (76% vs. 27%, P < 0.01). The need for EST may decrease because of the smaller diameter of the delivery sheath. However, a contrasting result was observed in the present study. Because EST increases the risk of developing retrograde cholangitis above the papilla, SIS without EST using LC slim delivery is expected to be clinically useful, and further case accumulation and verification are necessary. The authors also noted that advances in chemotherapy have greatly improved the prognosis of patients with HMBO so that initial drainage treatment of HMBO almost always leads to stent dysfunction, requiring reintervention. Tumor ingrowth and sludge accumulation are causes of stent dysfunction, and a stent that is easy to insert despite these situations is desired for reintervention. For reintervention of the SIS method, it is sometimes difficult to perform drainage on the side of the first stent because it is necessary to break through the cells of the two overlapping stents; therefore, LC slim delivery is expected to be useful for reintervention. In the future, a prospective comparative study of whether PS or MS is better for reintervention of the SIS method, including the indication of endoscopic ultrasound-guided biliary drainage and percutaneous transhepatic biliary drainage, is required. Here, we described recent advances in MS in the SIS method for HMBO. Because the SIS method for HMBO is fairly anatomically and technically complicated, this editorial aimed to aid in organizing and comprehending available knowledge about the MS for the SIS method and contribute to as many cases of HMBO as possible. Author M.T. is an Associate Editor of Digestive Endoscopy. The other authors declare no conflict of interest for this article. None.