Keywords

1 Introduction

Even though metal might be the first material used for unblocking urinary tract, first widely used stents in the ureter were polymeric. And polymers do have their problems with longevity, compression, encrustation, irritation etc. which has led researchers to try other materials—amongst them metal alloys. First metallic stents used in ureter were made from stainless steel (Wallstent, Palmaz-Schatz) and afterwards focus was mainly on nitinol (nickel titanium oxide) as well as other alloys (tantalum, platinum, niobium, cobalt, etc.) with or without PTFE (polytetrafluoroethylene) or polymer coating. In shape/structure, they can replicate typical JJ design [1] (Passage—nitinol JJ stent, Resonance—nickel-chromium-cobalt-molybdenium JJ stent) or have coil (Memokath—nitinol coils, Allium—loose nitinol coils with polymer coating) or mesh (Uventa—nitinol mesh with PTFE coating) structure. By mechanism of deployment we can recognise baloon-expandable, self-expandable, thermo-expandable and non-expandable metallic ureteral stents.

First documented metallic stent used in ureter was vascular permanent stent (Wallstent) placed in two patients with malignant obstruction by Lugmayer in 1992 [2]. Afterwards many vascular stents were tried but high rates of complications and inability for easy removal and replacement led to their discontinuation and development of purpose-based urological metallic stents which could be more easily removed and replaced [3].

2 Indications for Metallic Ureteral Stents

For kidneys to function properly, urine produced should flow freely through ureters in order to reach bladder or substitute reservoir. Should drainage become impaired excessive intrarenal pressure will develop and can subsequently lead to kidney damage and eventually to loss of function. This blockage can come from within the ureter (internal) or outside of it (external) and by nature of cause described as malignant, benign or post radiotherapy. Two main ways of unblocking an obstructed renal unit can be considered; either internally via ureteral stent or externally by means of nephrostomy and both ways should provide uninterrupted urinary drainage.

Internal unblocking of renal unit using stent is minimally invasive and should offer long enough indwelling time with the ideal stent being easy to insert and remove, made of biocompatible and MRI-compatible material and causing no adverse host reaction (inflammation, urothelial hyperplasia, tumour ingrowth etc.) and being resistant to incrustation. Unfortunately, such stent still does not exist but some materials and designs cover many of requirements.

Even though both polymeric and metallic stents can be considered in all of benign, malignant and post-radiotherapy settings but we will usually opt for metallic stents in situations in which longer indwelling times are projected with benign conditions (resistant post inflammatory strictures), malignant obstruction (due to internal occlusion or external compression) or post-radiotherapy strictures [4].

3 Complications and Side Effects of Metallic Ureteral Stents

Even with careful and proper usage complications will inevitably arise and same is with metallic ureteral stents [5,6,7]. Some complications are inherent with stent design and others come from material used or applied coating. Many case reports and review papers have summarised either single stent experience or problems with specific patient population and none of them have yet discussed complications on a sufficiently large number of patients so workgroup within COST Action 16217—ENIUS (European Network of multidisciplinary research to Improve the Urinary Stents) has led literature search in order to identify, catalogue and review in a systematic way all published complications and patency rates for metallic ureteral stents used for ureteric obstructions [data prepared for publication].

In our systematic review 319 publications were identified and 111 acceptable full text papers were thoroughly examined leading to 88 being included in final analysis. That translates to database of 1749 patients with 2194 ureter units receiving 2394 stents with 1188 complications documented. It is worth noting that some of complications are due to disease itself (especially malignant) others correspond to stent type and shape or material and cation used. Even though some patients did not experience any complications or side effects, others have had multiple stent related complications but in total this translates to 68% per patient and 50% per stent risk of complication. Only 3.4% of papers (3/88) have used verified system for reporting complications (modified Clavien-Dindo classification [8]) which also poses problem in real-world data acquisition. Complication reporting in general and specifically using standardised approach is obviously not at the highest standards among academic urologic community and further actions are needed in order for that to be changed in future.

Complications related to stent placement (regardless of stent type) were low in our dataset and only 22 failures and 4 significant difficulties were documented in attempt to place 2394 stents which comes to less than 1.1% in total.

3.1 Off Label Use of Bare Metal Stents (BMS) Designed for Vascular or Gastroenterological Use in Ureteric Obstruction

First papers reporting experience with off-label use of metallic mesh stents (developed for cardiovascular use) in ureters started to emerge in 1991 with promising results initially, but as soon as 1993 reports on poor outcomes started to emerge. Review and vast personal experience published by Liatsikos et al. in 2009 started era of review papers but no comprehensive set of data reporting on complications was published as yet. Majority of data in our dataset come from experience using Wallstent™ (Schneider, Zürich, Switzerland later Boston Scientific/Microvasive, MA, USA) and other data come from use of other stents mainly Strecker (Boston Scientific, MA, USA), AccuFlex (Boston Scientific, MA, USA), Protege (Endovascular Inc., MN, USA), Luminexx (Bard GmbH, Angiomed, Karlsruhe, Germany), Sinus-Flex (Optimed, Ettingen, Germany) and Palmaz-Schatz (Johnson and Johnson, Warren, USA).

A total of 29 papers have reported on use of (mostly vascular or biliar) BMS in 345 patients (258 with malignant and 87 benign conditions) with 359 stents implanted in malignant and 98 in benign ureter units with a total of 277 complications reported which translates to 80% of patients at risk of complication or 60% per stent used. Among complications most prevalent were obstruction or occlusion in 71 (26%), tumour overgrowth or ingrowth in 59 (21%), flank or abdominal pain in 39 (14%), urothelial hyperplasia in 33 (12%). Also, four serious complications needing surgery (including two nephrectomies due to chronic pyelonephritis and two laser surgeries to remove stents) were also reported. Reported patency rates ranged from 0% to 100% with most report around 30–80% (Figs. 1, 2, and 3).

Fig. 1
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Ureteroscopic assessment. Metallic stent encrustation. (Dr. F. Soria. JUMISC. Spain)

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Fig. 2
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Ureteroscopic assessment. Obstructive urothelial hyperplasia. (Dr. F. Soria. JUMISC. Spain)

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Fig. 3
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Fluoroscopic view. Ureteral metallic stent migration. (Dr. F. Soria. JUMISC. Spain)

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From these results we can conclude that early cardiovascular and biliary stents placed (off-label) in ureters had promising initial results but with follow up approaching 1 year they mostly suffered obstructive complications (occlusion, compression, tumour overgrowth or reactive hyperplasia) which were responsible for roughly 60% of incapacitated stents.

3.2 Off Label Use of Covered Metal Stents (CMS) Designed for Vascular or Gastroenterological Use in Ureteral Obstruction

Research advancements in cardiology has led to introduction of covered metal stents which were also tried in ureters and resulted in no benefit compared to off-label use of vascular/gastroenterological BMS with regards to complication rates with migrations and UTI’s being most common.

Only five studies in our dataset had some data on covered metal stents including two on Passager stent (Boston Scientific Corporation, Oakland, NJ, USA), one on polyurethane tube with metal wire (Mannheim hospital, Heidelberg University, Germany), one on Dacron covered nitinol mesh stent (Stanford, Nanture, France) and one on ePTFE covered nitinol stent (Hemobahn Endoprosthesis, W. L. Gore and Associates, Inc., Flagstaff, Arizona, USA). In total they report 72 patients (49 malignant and 23 benign obstructions) with 86 ureteral units (56 and 30 respectively) with 69 complications reported namely migration/dislocation in 20 (29%), urinary tract infections (UTI) in 11 (16%), vesicoureteral reflux in 9 (13%) and reactive hyperplasia in 7 (10%) being most common. One nephrectomy was carried out due to recurrent UTI’s. Patency rates reported ranged from 18.75% to 100%.

3.3 Covered Metal Stents Designed for Use in Urinary Tract

Purpose built covered metallic stents designed for use in urinary tract (Allium™ and Uventa™) could be considered as next generation of covered stents. Allium™ URS is segmental nitinol mesh stent fully covered with polymeric coating with high radial force in mid part and low radial force in outer parts. Uventa™ is segmental ureteral self-expanding metallic mesh stent with triple-layered structure consisting of nitinol mesh on outer and inner side and PTFE membrane in middle (Fig. 4).

Fig. 4
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Fluoroscopic assessment. Ureteral Uventa™ metallic stent. (Taewoong Medical, Seoul, Korea)

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Our search has identified only one study reporting short term outcomes with use of three Allium urethral stents (Allium™ Medical, Caesarea, Israel) in two patients resulting in one obstruction.

Ten reports were included on use of Uventa™ (Taewoong Medical, Seoul, Korea) stent in 202 patients (158 with malignant and 44 benign disease) across 238 ureteral units with a total of 163 complications with 16 (10%) of which were serious (7 uretero-enteric fistula, 4 uretero-arterial fistula, 2 uretero-vaginal fistula and one pseudoaneurysm, ureteral perforation and sepsis each). Other more common complications include tumour overgrowth/ingrowth in 26 (16%), flank or abdominal pain in 21 (13%) and urothelial hyperplasia in 19 (12%) of cases. Reported patency rates ranged from 30% to 100% but mainly around 65–100%.

3.4 Memokath 051™ (PNN Medical A/S, Kvistgaard, Denmark)

Memokath 051™ is a thermo-expandable, spiral-shaped (coiled) memory nickel-titanium metallic alloy segmental stent and could be considered as next generation bare metal stent with reduced complications when compared to purpose built covered metallic stents. It was more often used in benign conditions than any other stent in our review.

Data from 21 paper on use of Memokath 051™ stent report on 423 patient (188 with malignant and 235 benign condition) with 469 ureter unit (214 and 255 respectively) and 230 complications with 48% of them (111) related to migration. Obstruction, occlusion or compression is reported in further 23% of cases (52). Only one serious complication (uretero-arterial fistula post radiotherapy for colon cancer) was reported. Patency was reported anywhere between 40% and 100% with figures around 70–80% being most common especially in larger series.

3.5 Resonance™ (Cook Medical, Bloomington, IN, USA)

Resonance™ is 6 Fr double pigtail full metal (nickel-chromium-cobalt-molybdenium alloy) tight spiral stent without internal lumen and is like Memokath 051™ also considered to be next generation purpose built BMS and was most widely used stent in our review.

Twenty-eight papers in our review reported use of 1085 Resonance™ stents implanted in 707 patients (with 462 due to malignancy and 245 for benign conditions) with 944 ureter units (621 and 323 respectively) with a total of 449 complications reported with UTI being the most common with 23% (103 cases) followed by compression in 20% (91 cases) and obstruction or occlusion in further 10% (43 cases). Among eight reported significant complications three were subcapsular haematomas (all in one series), three sepsis and two surgeries due to calcification (one cystolitholapaxy and another percutaneous nephrolithotomy). With a mean follow up of 1 year reported patency rates were between 10% and 100% with larger studies usually reporting patency rates around 70–90%.

4 Conclusion and Further Recommendations

Even though metallic ureteral stents in general exhibit better patency rates than polymeric stents in comparable patient populations and provide effective long term drainage they still have high rates of complications and side effects. Metallic ureteral stents (especially segmented ones) also tend to cause less stent-related symptoms than polymeric JJ stents.

As expected, purpose-built metallic ureteric stents outperform off-label vascular and biliary stents used in past but they still have nearly 50% complication chance with 2.6% of them graded as severe. Difference among stents in predominant type of complication arise from differences inherent in stent design or material used. Despite these negative issues, metallic ureteral stents still represent most appropriate salvage options for certain groups of patients with short life expectancy or those unwilling or unable to undergo surgery.

Choice which metallic ureteral stent should be preferred over others depend on local availability, stage and localisation of disease, patient characteristics and expectations, provider (urologist, interventional radiologist) preference and experience and cost and reimbursement policy [9].

In order to have better graded recommendations there is still unmet need for multi-institutional prospective randomised trial with adequate number of patients stratified to malignant, benign and post-radiotherapy group designed as head to head superiority trial of existing metallic ureteral stents with follow up period at least 12 months in order to obtain high quality data on their patency and complication rates.

In conclusion, due to high number of complications, stent failures, side effects and stent-related symptoms, stringent follow-up of these patients is necessary.