Flexible Ureteroscopy Combined with Novel Ultra-Flexible Ureteral Access Sheaths for Treating Allium Stent Encrustation: Initial Experience with 15 Cases

Introduction

The Allium ureteral stent is a self-expanding nickel-titanium alloy scaffold coated with a polymer sleeve. Its efficacy in treating ureteral strictures has been well documented. ^1,2 Allium ureteral stents need to be left in place long-term for the treatment of ureteral strictures and as a long-term indwelling device. ^2,3 In particular, patients who have undergone radiotherapy and chemotherapy for pelvic malignancies often develop malignant ureteral strictures. ⁴ These patients frequently require lifelong stent placement to maintain ureteral patency, with the Allium stent often being the stent of choice. ⁵ Additionally, the Allium stent is commonly used to treat ureteral strictures after kidney transplantation. ^6,7 Despite the use of polymer membrane technology in Allium stents, encrustation or stone formation on the stent surface over time is inevitable. Some studies have indicated that encrustation can occur on the stent surface within 6 weeks of placement. ⁸ Stent encrustation is one of the primary factors affecting the efficacy and longevity of stent treatment. ⁹ Encrustation stents typically require removal and replacement with new ones, and given that the cost of an Allium stent exceeds that of a silicone Double-J stent, the replacement expenses are expensive. ³ Currently, there are virtually no established methods for treating encrustation on Allium ureteral stents. Previous reports have mentioned the use of laser lithotripsy; however, the effectiveness of this approach is limited, and it can damage the Allium polymer membrane, significantly impacting the stent’s lifespan and functionality. ^10,11 Therefore, it is crucial to explore a method for treating encrustation on Allium stents, which may extend their usable life.

This study investigates the use of flexible ureteroscopy (f-URS) combined with a novel ultra-flexible ureteral access sheath (UF-UAS) to safely and effectively manage encrustation on Allium ureteral stents. The UF-UAS features a silicone-based lumen approximately 10 cm long at the distal end without wire reinforcement. ¹² We provide a detailed description of the procedure for using the UF-UAS to clean encrusted Allium stents and analyze the preliminary treatment outcomes.

Patients and Methods

All patients undergoing UF-UAS treatment for Allium ureteral stent encrustations from January 2022 to January 2024 were included. The present study was approved by the Ethics Committee of the First Affiliated Hospital of Nanchang University (2022037). Inclusion criteria: patients aged 18–80 years with encrustation on Allium stents. Exclusion criteria: kidney transplanted, ureteral reimplantation, Allium stent lumens too small to accommodate a 10F UF-UAS, uncontrolled urinary tract infections, and patients unable to undergo general anesthesia. Antibiotic therapy was administered for individuals with positive urinary cultures, with repeat assessment before surgery to confirm adequate control of urinary tract infection.

All patients underwent preoperative and first postoperative day evaluations, including urinary ultrasonography, kidney–ureter–bladder (KUB) plain film, and noncontrast computed tomography (NCCT), to assess the length, dimensions, displacement, and damage to the Allium stents, the presence of residual stone fragments, and hydronephrosis. The Allium stent damage was defined as visible deformation of the nickel-titanium structure on NCCT. Stone-free status was defined as the absence of renal stone or encrustation fragments ≥2 mm on NCCT. Follow-up evaluations, including urinary ultrasonography and KUB, were conducted 3 months postoperatively to assess for increased hydronephrosis and displacement of the Allium stent.

Surgical techniques

The UF-UAS (Woek, Nanchang, China) refers to a 10 cm section on the distal end of the ureteral access sheath with increased flexibility, allowing for passive deflection exceeding 270° along the f-URS. This region is unreinforced with wire, comprised entirely of a silicone composite engineered for favorable pliability (Supplementary Video SV1 and Fig. 1).

OPEN IN VIEWER

FIG. 1.

The ultra-flexible ureteral access sheath friction transmitted shear forces to detach encrustation from the Allium ureteral stent surface. (A) Flexible ureteroscopy, (B) encrustations, and (C) ultra-flexible ureteral access sheath.

A semirigid ureteroscopy (9.8F, Karl Storz, Germany) was performed to assess ureteral and Allium stent conditions and facilitate guidewire (VALENCA, Hunan, China) placement. The UF-UAS (10/12F or 11/13F, Woek, Nanchang, China) was inserted under guidewire guidance. Under f-URS (7.5F, PUSEN, Zhuhai, China) visualization, the UF-UAS was positioned just proximal to the Allium stent. The proximal stones on the Allium stent were pulverized using a holmium laser (200 μm; energy 0.6–0.8 J; frequency 10–20 Hz, Moses Laser, Yokneam, Israel). To prevent damage, the laser was not directed at the Allium stent itself. The Allium stent encrustations were managed using the following methods. The f-URS was withdrawn into the UF-UAS, approximately 3–4 mm from the sheath orifice. Deflecting the f-URS served to maneuver the UF-UAS tip into direct contact with the Allium encrustations. Repeated UF-UAS movements created friction between the UF-UAS tip and stent incrustations to divestiture encrustations. Repeated friction facilitated removal of additional encrustation, plaque, and biofilm from the Allium surface. By observing the extent of deformation at the UF-UAS distal tip, the clinician can infer the level of frictional forces. That can then be utilized to optimize the deflection angle of the F-URS. Throughout the procedure, care was taken to avoid direct laser activation on the Allium stent polymer. Encrustations were primarily removed through physical abrasion using the silicone portion at the distal end of UF-UAS. Continuous irrigation (peristaltic pump: 60–100 mL/min) and continuous negative pressure suctioning must be maintained during lithotripsy.

The UF-UAS was connected to a vacuum device, with preset negative pressures of 1–5 kPa. The surgeon titrated the actual intraoperative negative pressure in real time as needed through the pressure modulation port. Dislodged encrustation fragments from the Allium stent may be large debris. Laser firing at the UF-UAS orifice can further fragment these pieces into smaller particles that can then be flushed out through the UF-UAS lumen by the irrigation flow. Any dropped pieces could undergo additional fragmentation and retrieval after concurrent advancement of the UF-UAS and f-URS into the renal pelvis. Final inspection of the renal collecting system and entire Allium length was conducted to prevent residuals. Depending on the intraoperative situation, some patients had a Double-J stent placed, which was removed one month postoperatively.

Results

A total of 15 consecutive patients (10 women, 5 men) underwent the procedure (Table 1). Mean patient age was 63.3 (range: 56–79 years) years. Mean preoperative Allium stent dwell time was 19.9 (range: 6–38 months) months. The cohort included 3, 9, and 3 patients with Allium lengths of 6, 8, and 10 cm, respectively. Twelve patients (80.0%) presented with hydronephrosis resulting from Allium-associated proximal ureteral stones, with stent encrustation discovered intraoperatively. Two patients (13.3%) had hydronephrosis because of proximal or distal Allium-related ureteral polyps, with stent encrustation discovered intraoperatively. One patient (6.7%) suffered recurrent urinary tract infections, with cystoscopy revealing encrustations. Preoperative urine cultures were positive in seven (72.7%) patients.

Mean operative time was 33.8 (range: 27–47 minutes) minutes (Table 2). Three patients (20.0%) had one Double-J stent each postoperatively. One patient (6.7%) had 1.5 cm Allium migration postoperatively. The preoperative mean serum creatinine level was 94.5 μmol/L (range: 67.4–146.1 μmol/L), whereas the postoperative mean serum creatinine level decreased to 85.7 μmol/L (range: 61.5–127.7 μmol/L). Postoperatively, 13 patients with no stone, Grade A = 86.7%; 1 patient with ≦2mm fragments, Grade B = 6.7%; and 1 patient with 2.1–4 mm fragments, Grade C = 6.7%. No patients with >4mm fragments, not stone-free = 0. The average postoperative hospital stay was 1.1 days. NCCT imaging revealed no significant damage to the nitinol structure of the Allium stents in any patient. No patients exhibited fever or other complications. During the 3-month follow-up with KUB and renal ultrasonography, no additional migrations of the Allium stent or increased hydronephrosis were observed.

Discussion

Ureteral strictures can arise from various etiologies including benign ureteral obstruction and malignant ureteral obstruction (MUO). ^2,7,13,14 MUO often necessitate long-term or permanent stenting. Allium ureteral stents are frequently utilized for MUO. Prolonged Allium stents dwell times predispose toward encrustation and infectious biofilm accumulation which can result in luminal occlusion and severe infections. ¹⁵ In vitro studies demonstrate encrustation onset within 6 weeks of Allium stents placement. ⁸ Reported follow-up data indicate Allium stents encrustation obstruction incidence ranges widely from 2.5% to 29%, with potential for complete luminal blocking ultimately necessitating nephrostomy. ^3,4 The encrustation pattern of Allium stents differs structurally from that observed in silicone Double-J stents or metallic Resonance® stents. ¹⁶ The Allium stent features a larger internal lumen with polymer coating, where encrustation occurs within the internal cavity, whereas other stents have smaller luminal diameters with encrustation primarily occurring on the peripheral surface. Although extracorporeal shock wave lithotripsy (ESWL) represents a primary treatment modality for stent encrustation, its application differs between stent types. ¹⁷ For Double-J stents or Resonance® stents, ESWL can effectively remove encrustations without stent removal. ¹⁷ However, the hollow structure of Allium stents, combined with encrustation throughout the entire lumen, presents significant challenges for precise ESWL targeting. Currently, there are no published reports documenting the use of ESWL for treating Allium stent encrustations. Limited reports describe laser ablation for encrustations but this risks damage to the Allium stents polymer coating, necessitating replacement. ¹⁰ Encrustation or damage of the polymer coating on the Allium stent severely impacts its functional lifetime. ¹⁵ Although removing the Allium stent is relatively straightforward, the Allium stent’s high costs necessitate extending its usable lifespan when possible. ⁴ Currently, there is an absence of clearly defined and effective methods for the removal of encrustations and other deposits from Allium ureteral stents.

This study presents a novel technique for managing Allium ureteral stent encrustations. In cases where ureteral obstruction is caused by calculi at the proximal end of Allium stents, this study employs holmium laser lithotripsy to restore ureteral patency, a method consistent with most studies focusing on stone fragmentation at the core. The novelty of this study lies in addressing encrustation and biofilm formation on the walls of Allium stents. For encrustations and biofilms on Allium stents, this research uses a physical scraping method under direct vision via f-URS and a UF-UAS to detach these deposits from the stent surface. Compared with direct laser application, this physical scraping technique minimizes damage to the stent. Using f-URS for scraping encrustations may also be a viable method for their removal. However, compared with using f-URS alone, the deformability of the silicone material at the front end of the UF-UAS allows for a broader contact area with the Allium stent.

During the surgery, it may be necessary to pay attention to certain details. We discovered that the connections between encrustations and the Allium stent were not robust, which enabled dislodgement with delicate friction. The blunt removal enabled by the UF-UAS silicone composition was unlikely to damage the Allium stent compared with laser lithotripsy. However, procedural skill is required to avoid excessive friction and potential stent migration. In this study, a peristaltic pump was used for continuous irrigation to maintain a clear surgical field. However, continuous irrigation requires vigilance regarding the risk of increased intrarenal pressure. ¹⁸ The UF-UAS can be connected to a vacuum suction device, which, in this study, was utilized to prevent elevated intrarenal pressure through continuous suction. It is crucial to select an appropriately sized UF-UAS to avoid excessive diameter. There must be sufficient clearance between the Allium and the UF-UAS, allowing the UF-UAS to actively maneuver and deflect within the lumen of the Allium stent.

It is undeniable that this surgical method may also result in certain complications. On the first postoperative day, one patient was found to have experienced a 1.5 cm displacement of the Allium stent. There is also the potential for residual stone fragments or encrustation debris to remain within the kidney, as evidenced by one patient in this study who had residual fragments larger than 2 mm detected by NCCT postoperatively. Additionally, during the procedure, the physician applied friction to facilitate the detachment of encrustations, but it proved challenging to control the amount of force exerted. The degree of contact with encrustations is controlled by adjusting the UF-UAS’ deflection angle, and the deflection angle of UF-UAS can be adjusted by the deflection of F-URS. The distal 10 cm of the UF-UAS is composed of silicone material without wire reinforcement, allowing for deformation upon contact with encrustations. We utilize this deformation as a visual indicator to indirectly assess the applied friction force. When excessive deformation is observed (indicating excessive pressure), we reduce the deflection angle of the F-URS to decrease the contact force.

Regarding stent-related damage, although no significant damage to the Allium stent’s polymer and nitinol alloy was observed under f-URS visualization or in the postoperative NCCT examination, the possibility of subtle, undetectable damage cannot be entirely excluded. Regarding potential damage to the stent’s biomembrane, whereas no visible trauma was detected under 20x magnification using f-URS, it is acknowledged that additional diagnostic methods may be necessary to identify microscopic damage. If such microscopic trauma exists, the question of whether it could increase the susceptibility of the Allium stent to re-encrustations requires further investigation. During the 3-month follow-up period, NCCT showed no evidence of stent re-scarring, displacement, or worsening hydronephrosis. Although the Allium stent may potentially develop re-encrustations complications, this would require a longer follow-up period for comprehensive evaluation.

This study reports a method for treating encrustation on Allium ureteral stents using UF-UAS. To our knowledge, this is the first study to explore the use of UF-UAS for addressing encrustations on Allium stents. Based on the analysis of our results and the 3-month follow-up, we believe this method is effective and may extend the lifespan of Allium ureteral stents, potentially reducing the need for their replacement. However, this study has certain limitations. The sample size is relatively small, which indicates the necessity for larger-scale studies. Additionally, this study is a single-center retrospective analysis and lacks controlled experiments. Therefore, prospective multicenter controlled trials are essential to further validate the reliability of this surgical method and to identify any potential limitations.

Conclusions

This study demonstrates the novel UF-UAS enables effective removal of stones, encrustation, and infectious biofilm from Allium ureteral stents. The use of silicone friction at the front end of the UF-UAS to detach encrustations and other deposits from the Allium stent avoids unnecessary damage to the stent. This safe technique shows potential to prolong Allium ureteral stents dwell times and decrease exchange requirements.