Compared with the ureteral obstruction caused by extrinsic compression of malignancies, non-malignant refractory ureterostenosis is rarer. It is challenging for urologists not only because of deteriorating renal function but also difficulty in choosing the appropriate treatment plan for the individual. Traditional silicone or plastic ureteral stents are most commonly used and cost-effective. Despite their advantages, they have shown a relatively high failure rate when used to treat some chronic ureteral stenosis, especially in patients with retroperitoneal fibrosis or iatrogenic injury. In our study, we reviewed patients treated by the Resonance or the Allium metallic stents for ureterostenosis caused by non-malignant reasons (primarily retroperitoneal fibrosis). Both two types of stents showed good efficacy to protect renal function from deterioration. The Resonance stents caused more irritative complications while the Allium stent had higher frequency of displacement and re-obstruction.
The majority of our cases were retroperitoneal fibrosis, including 18 IRPF and 11 SRPF. IRPF is related to autoimmune diseases and characterized by chronic non-specific inflammation of the tissues around the retroperitoneal aorta with progressive hyperplasia of fibrous tissue. The factors of SRPF include drug side effects, infection, trauma, dissection, radiation therapy, malignant tumors [9,10,11,12]. Fibrous tissue surrounds and compresses its adjacent structure with the ureters involved especially. In our study, IRPF cases were diagnosed by rheumatologists, and SRPF cases were caused by radiotherapy. Although SRPF patients had history of cancer, most of them underwent 18F-FDG-Positron Emission Tomography to rule out malignant ureteral obstruction, reinforcing the accuracy of SRPF diagnosis . Although percutaneous nephrostomy or ureterolysis can drain the upper urinary tract and relieve the stenosis, it reduces the patients’ quality of life or confronts great operative difficulty. Metallic stents become promising ways to treat RPF in some refractory cases.
The Resonance metal stent has a special coil structure and alloy composition, making it resistant to external compression and the ingrowth of proliferative tissue. The usage of Resonance metal stents has been reported in patients with benign ureteral stenosis primarily due to ureteropelvic junction obstruction or benign stricture, showing good drainage and good tolerance in most of the patients [14,15,16,17]. Benson et al. reported no stent failure in 15 patients with benign obstruction after a median follow-up time of 14 months. López-Huertas et al.  reported that the Resonance stents succeeded in 92% of the patients with benign obstruction. Both of these studies included 1 case of IRPF without stent failure. The Allium stent is a self-expanding, large-caliber ureteral stent to treat benign or malignant ureteral stenosis. It displayed good efficacy in some benign conditions like post ureteroscope lithotripsy stricture. Moskovitz used Allium stents to treat benign urinary stricture after endoscopic lithotripsy, urinary diversion and renal transplant. All stents were patent except from 1 case of endoscopic ureteral lithotripsy after 11 months . The data regarding to RPF was still lacking. Our study compared the efficacy of the Resonance and the Allium stents in benign cases, primarily RPF. The result was favorable in protecting the renal function and reliving the hydronephrosis. This was expectable as both the two stents had greater tensile strength than traditional stents. Although the creatinine level and GFR did not increased significantly, both stents could improve the renal function or protect renal function from deteriorating. However, besides the successful cases in our study, we also experience 2 intraoperative failure when placing the Allium stents for two SRPF patients. The stent distorted and failed to dilate the stricture after self-expansion. This had not happened for the Resonance stent yet, suggesting the Resonance had more intensity against foreign force and were more suitable in severe ischemic fibrosis stenosis conditions secondary to radiation.
During follow-up, the Allium group showed 4 cases of displacement, accounting for 26% of the group cases. Among them, 2 cases were SRPF, 1 case was IRPF, and 1 case was post-lithotripsy stenosis. 2 of 4 patients received replacement of the thicker Allium stents; 1 patient received the Resonance stent replacement; 1 patient received observation. The rate was higher than previous literature, where 7 out of 49 (14.2%) stents migrated . Meanwhile, only 1 patient with the Resonance stent had migration, but not necessary for intervention.
Stent occlusion was not rare for both the two types of stents. Moskovitz el at reported 1 occlusion case out of 49 Allium stent placements. Lopez et al. showed a high success rate of the Resonance stent in treating benign ureteral obstruction, with only 1 failed case because of the extremely tortuous ureter. Wang et al. reported a higher re-obstruction rate (22.7%) of the Resonance stent, but the failure mainly happened in the malignant cases with radiotherapy history. In our study, the Allium group had significant higher rate of postoperative re-obstruction, mostly in SRPF cases. Thus, close follow-up is necessary to discover the position changes or occlusion especially after the Allium stent placement. And the Resonance stents might be a safer choice for SRPF secondary to radiotherapy. During follow-up of RPF cases, images like plain X-ray help to understand whether the ureters and the stents are shifted to the midline and to decide whether to replace or remove the stents.
Irritable lower urinary tract symptoms are common complications of indwelling catheters. Likewise, moderate to severe OAB happened frequently in the Resonance group but not the Allium group. Some patients even requested to remove the Resonance stent due to disturbing OAB. Also, patients with the Resonance stents had significantly higher chance of recurrent urinary infection. On the contrary, the Allium stents were well tolerated. Those patients who had the Resonance stent withdrawn and replaced by the Allium stent were also satisfied with the relief of irritable symptoms. The Resonance stent is a full ureteral stent which is connected from the renal pelvis to the bladder, which may impair the peristaltic function of the ureter and leaves a “foreign body” inside the bladder. The Allium stent is segmental and only expands the narrowed ureter without affecting the peristalsis of other segments of the ureter. These reasons might contribute to the higher frequency of irritation and infection in the Resonance group. Although more patients with the Resonance stents had encrustation formed on the bladder and lower ureteral stents, the blood creatinine, GFR and CT images showed no signs of aggravated obstruction.
Some researcher proposed that metal stents are not suitable for benign ureteral obstruction, especially when ureteral stricture was caused by proliferation into the ureteral lumen. Careful selection of suitable cases before surgery and close follow-up after surgery are critical to the success of metal stent implantation . In our study, there were 4 patients with refractory stenosis after ureterolithiasis surgery. The problem was completely solved by inserting Allium stent because the structure of the stent is a metal stent with double layers of polymer attached to the inside and outside of the metal mesh stent. The hydrophilic walls prevent the problem of the proliferation into the ureteral lumen. Also, according to our early experience, we placed tandem ureteral stents (TUS) in 3 patients with RPF but all stents failed at 3-month follow-up. Although studies showed TUS had good efficacy in benign conditions like ureteral stricture or severe stone disease , their efficacy in treating refractory cases with RPF needs further study.
In summary, both the Resonance and Allium stents could effectively protect the renal function. The Resonance stents showed higher frequency of irritable symptoms, while the Allium stents had more possibility of re-obstruction. The main limitation of the study is it small samples, mostly due to the rarity of non-malignant ureterostenosis like RPF. To evaluate the quality of life with ureteral stents, Ureteral Stent Symptom Questionnaire should be the most proper method. But we did not apply this questionnaire as it was not valid in the Chinese language at the early study period. Also, because the Resonance stents were introduced much earlier in our hospital, the follow-up time of the Resonance group was significantly longer than the Allium group. This bought bias to our study as the long-time efficacy and safety of the Allium stents were not possible to evaluate. For the same reason, more patients with IRPF were treated with the Resonance stents especially at the early stage of the study period. This bought bias to the disease consistency at baseline. The long-time efficacy and safely of the Allium stents to treat non-malignant ureterostenosis needs further study.