Long-standing ureteric obstructions significantly compromise renal function. Alleviating the obstruction is essential for pain relief, reduction of infection risk, and restoration of adequate renal function. Surgical urinary diversions including open nephrostomy, enteral conduits, cutaneous ureterostomy, and enteric substitution are not always indicated because of underlying disease and/or co-morbidities. They are also often technically difficult and can cause many complications [3]. Conventionally, polymeric stent or PCN were employed, but numerous complications and/or negative impact on patient quality of life distressed many patients. Ureteric metallic stents were introduced as a less invasive and more long-lasting treatment without frequent stent changes for ureteric obstructions and would be suitable for patients who are not fit for further oncologic treatment or frequent anesthesia. Patients who wish to avoid PCN or frequent stent change would also be good candidates for metallic stent. Various types of ureteric metallic stents have been developed, and each has benefits and limitations [14, 15].

The Urexel™ stent is a recently developed self-expanding segmental metallic stent. There are some previous reports have shown that Urexel™ provided favorable results in MUO [11] and ureteric stricture following kidney transplantation [12] when placed in an antegrade manner. Kim et al. [11] enrolled 19 MUO patients for prospective comparison of Urexel™ (10/19) and double-J ureteral stent (9/19). The Urexel™ group showed higher success rate at three months (90% vs. 35%) and at six months (57% vs. 21%), with a higher overall success rate (p = 0.041). Stent migration occurred in one patient in the Urexel™ group at one month after stent placement. Recently, a retrospective study was reported comparing ureter unit (54/84) in Urexel™ and (30/84) ureter unit in double-J ureteral stent for MUO in advanced gastric cancer, and the cumulative success rates at 1, 3, 6, and 12 months were 77%, 74%, 70%, and 70%, in the Urexel™ group and 72%, 60%, 53%, and 26%, in the double-J group, reporting that Urexel™ group is better in maintaining patency [16]. Tsauo et al. reported a retrospective study with 175 ureteral units treated with Urexel™ in an antegrade manner for MUO, and stent malfunction was found to be 21.4% (37 ureteral units) during the follow-up period. Through this, it was shown that Urexel™ placement was effective [17]. Cao et al. [12] reported the outcomes of temporarily placed Urexel™ in eight patients with refractory ureteral stricture following kidney transplantation. Stent migration occurred in two patients (28.6%), and the overall success rate after stent removal was 71.4% (5/7) during a mean 22.6 months of follow-up.

In addition to Urexel™, research on other ureteral metal stents cannot be overlooked. In the case of Resonance®, the success rate at 6 months is reported to be 77.3% and at 12 months is 70.3% [18], and for UROSOFT tumor stent, the success rate at 5 months is reported to be 52% [5]. In this study, the 6-month success rate is reported as 83.3% and the 12-month success rate is reported as 70%, and the results are comparable to the previous studies. However, the ureteral stents mentioned above are stents that require periodic replacement, so their use concept is different from Urexel™.

Retrograde ureteral stent placement has several advantages over antegrade insertion. It does not require nephrostomy, so there is no potential risk related with percutaneous access such as pleural complication, internal solid organ injury, and significant bleeding. Retrograde placement enables an endoscopic procedure, so concomitant procedures including diagnostic endoscopy, biopsy of abnormal tissue, and stent removal are possible. Further, when stent location is not favorable, the position of the distal stent is easily adjustable with retrograde procedures. Above all, it is more familiar to a urologist. There are no data demonstrating the clinical outcomes of retrograde Urexel™ stent placement; the current study aims to address this lack.

Stent migration is one of the most hindersome complications in metallic stent placement [3]. Soft strictures, insufficient anchorage, and propulsion by antegrade peristalsis were considered as contributing factors for migration [19]. In the early days of metallic mesh stent use, tissue ingrowth was a major cause of failure [20]. To overcome this, a covered metallic mesh stent has been used, although it did not address the issue of migration [21]. Thereafter, the Uventa™ stent was introduced and showed favorable results in terms of migration [15, 22, 23]. However, the mesh exposed to the outside of the PTFE cover adhered to the ureteral walls, and stent removal is difficult [24].

A metallic ureteral stent mainly is used to avoid frequent stent change to achieve long-term or permanent indwelling. However, in cases of complications including stent malfunction, migration, recurrent urinary tract infection, and intractable stent-induced pain, the metallic ureteral stent should be removed. This indicates the need for feasibility of stent removal or exchange. Most of the currently available metallic stents have their own mechanism to facilitate stent removal [25,26,27]. In consequence, although migration and removability are opposing requirements, there is need for a stent that offers both. Urexel™ has a 1-cm-long additional segment with external mesh that is not covered by silicone. We expect this distinct design to contribute to preventing stent migration. In this study, stent migration was observed in 23.1% (3/13) of cases and was the major cause of overall failure. This is similar to cases with other metallic stents such as Allium® URS (14%) [25] and Memokath™-051 (20%) [15, 20, 28, 29]. Stent migration was observed in three of the initial six cases and not at all in the later six cases. The surgeon’s experience with Urexel™ stent is one explanation for this finding, but the sample size of the study was not large enough to make any conclusions.

The metallic mesh of Urexel™ is not exposed except for 1 cm at both ends and is surrounded by a silicon membrane to prevent adhesion to ureteral walls and reduce stone formation around the stent. When Urexel™ is pulled from the end, the lumen narrows. These features prevent attachment to the ureteral wall and aid in easy removal. Stent removal was necessary in six patients with Urexel™ placement, and there was no adverse event during stent removal. Complications did not exceed Grade 3b, and no serious complication such as fistula formation was observed. The possibility of stent removal via either antegrade or retrograde method was also demonstrated in a previous report about temporary Urexel™ indwelling in eight kidney transplanted patients [12]. Two characteristics of the Urexel™ foster easy endoscopic removal. The nitinol mesh shrinks and becomes more flexible when cold saline is applied. The silicone membrane prevents ingrowth of tissue into the lumen of the stents.

The economic aspect should also be taken into consideration. Several studies have already reported the cost-effectiveness of the less repeat procedure of metal stents compared to repeated replacement of polymer ureteral stents [30, 31]. Urexel is a ureteral stent that is intended for permanent placement and is expected to provide greater economic value compared to existing metal stents. In addition, expectations are even higher in that it can minimize patient discomfort caused by ureteral stent replacement.

Our study has some limitations including its retrospective nature. The sample size was small and the follow-up period was insufficient. We could not evaluate statistical difference but could offer a descriptive report. However, we think our study showed acceptable initial outcomes with a new metallic ureteric stent for MUO. The Urexel™ stent could have a role in symptom relief and maintenance of renal function for MUO patients.