A Self-Expanding Thermolabile Nitinol Stent as a Minimally Invasive Treatment Alternative for Ureteral Strictures in Renal Transplant Patients

Abstract

Background and Purpose:

Ureteral obstruction in renal transplant allografts secondary to strictures can pose a challenging problem. Its incidence is reported between 0.5% and 4.7%. Usually, open surgical repair is performed. We present a series of patients in whom a metal Memokath™ 051 stent has been used as a minimally invasive treatment alternative.

Methods:

We analyzed our data on the use of thermo-expandable metallic Memokath 051 stents for ureteral strictures in renal transplant patients.

Results:

Between 2003 and 2010, eight male kidney recipients with a mean age of 49 years and obstructed ureters after kidney transplantation were treated with ureteral Memokath insertion. In six patients, the obstruction was at the level of the anastomosis, and in two, at the pelviureteral junction. After a mean follow-up of 4 years, half of the stents are in situ providing a good graft function. The average indwelling time is 4 years. Spontaneous resolution of the stricture without the need for further stent insertion was seen in three patients after a mean indwelling time of 7.3 months. There was one treatment failure in a patient with an obstructed, dilated, and convoluted ureter that was unable to withhold the stent in situ. No perioperative complications were recorded in this series. The overall success rate was 87%.

Conclusion:

Ureteral stent placement with the Memokath 051 is a safe minimally invasive treatment alternative for ureteral strictures in renal transplant recipients.

Introduction

Ureteral stenosis develops in 0.5% to 4.7% of patients after kidney transplantation.¹ A main contributing factor is the impaired ureteral blood supply depending solely on the vessels originating from the renal pelvis and running in the periureteral adventitia distally. This may explain why 75% of strictures are located in the distal ureter near the ureterovesical anastomosis.² Surgical correction with ureteroneocystostomy or pyeloureterostomy using the native ureter has been the treatment of choice. However, this approach is invasive and can be associated with significant patient morbidity, especially in high-risk patients. Furthermore, this kind of reconstructive surgery can be quite challenging, especially after previous surgical interventions. Alternative minimally invasive treatment options are warranted.

Various minimally invasive techniques such as balloon dilation, laser or electric-wire-balloon ureterotomy, and long-term stent placement with a Double-J (JJ), or as used by us with double pigtail stents (called JJ or Double-J stent in the following). and self-expanding metal stents have been reported, each with its own set of advantages and disadvantages.

Ureteral stent insertion with a Double-J stent or urinary diversion with a nephrostomy are considered as “last resort” options.¹ There remains a need to exchange both regularly, which can be technically challenging because of the distorted anatomy in renal transplants.³ Encrustation, blockage, dislocation, and infection are frequent. In particular, urinary tract infections are of concern in the immune-suppressed transplant patients because they can lead to renal scarring and affect graft survival.⁴

In our department, we have a 9-year experience with the use of the Memokath™ (MK) 051 stent (PNN Medical A/S, Kvistgaard, Denmark) in the management of benign and malignant ureteral strictures. It has been proven a minimally invasive, safe, and effective alternative to open surgical repair or long-term Double-J stent placement on long-term clinical follow-up.⁵ There are only a few cases reported on its use in renal transplant ureters, however. We would like to share our experience with a small series of eight patients with ureteral stenosis after kidney transplantation who were treated with the insertion of a ureteral MK 051.

Methods

The stent

The MK 051 ureteral stent is a thermoexpandable, nickel-titanium alloy (nitinol) ureteral stent with a shaft diameter of 10.5F. Its fluted proximal end expands to 20F after flushing with 55°C hot saline. Available lengths are 30, 60, 100, 150, and 200 mm. The MK has a tight spiral structure that prevents urothelial ingrowth between the coils and allows adaption to the natural curves of the ureter. This results in a decrease of outward pressure, lessens the risk of secondary ischemic injury to the ureteral wall, and helps preserve peristalsis. The stent's outer titanium coating resists corrosion and encrustation in the urinary tract.^6,7 These characteristics allow easy insertion and removal of the MK.⁵

Patients

Over a 7-year period (2003–2010), eight renal transplant patients were referred to us with ureteral stenosis. All patients were male, with a mean age of 49 years (range 29–77 years). For all of them, a minimally invasive therapy was the preferred first-line option because of high anesthetic risk and/or anticipated surgical difficulties after previous interventions. In five patients, the obstruction was at the level of the vesicoureteral anastomosis, in one at the anastomosis of a Boari flap and ureter, all being <1 cm in length.

One of those patients presented with a previously inserted, now completely encrusted ureteral metal mesh stent. The problem with these previous generation mesh stents is that they overgrow with urothelium. Thus, once there is a complication, it is almost impossible to remove them from the ureter. We solved this problem by inserting a MK through the mesh stent once we had cleared it from encrustations in a combined anterograde-retrograde endourologic procedure.

Two patients had ureteral strictures at the pelviureteric junction, one of them from a massively distorted, secondarily obstructed ureter. The MK were inserted under general anesthesia as a day-case procedure with prophylactic gentamycin cover following a standardized technique. All patients received additional oral broad-band antibiotics for 1 week postoperatively. Seven stents were inserted in a retrograde fashion, one in an anterograde fashion. Stent placement was successful in all cases.

Patient details, stent data, stricture characteristics, and follow-up details were collected. In addition to their regular follow-up with the nephrologist, all patients were followed up in our own specialized endourology outpatient clinic at 6 weeks, 3 months, and every 6 months thereafter with clinical examination, serum urea, creatinine, and electrolytes levels, radiography of the kidneys, ureters, and bladder to confirm the position of the MK, and ultrasonography of the kidneys. If inconclusive, intravenous urography and/or renal isotope studies were used to exclude malfunction through obstruction.

Results

After a mean follow-up of 55 months by March 2013, four (50%) MK were in situ, patent, and functioning well with the patients' kidney function being normal. Average indwelling time of these stents is about 4 years.

In two (25%) patients, spontaneous resolution of the stricture without the need for further stent placement was seen after 2 and 8 month, respectively. The stricture between the transplant ureter and Boari flap resolved spontaneously, and the stent was expelled after 2 months. In another patient, the MK was expulsed into the bladder after 8 months. Both patients remain in our follow-up and a recurrent stricture did not develop. A third patient underwent endoscopic examination of the MK for suspected partial blockage. On removal of the MK after 13 months, the ureter was found unobstructed and remained so on follow-up.

Only in one patient did MK placement fail. The graft had been transplanted in an extraanatomical upside-down fashion, which led to a massively distorted and obstructed ureter. The ureter was bridged by a full-length MK but could not hold the stent, which migrated after 1 month and was consecutively replaced with a conventional JJ stent. Including the three spontaneously resolved strictures, the overall long-term success rate of managing ureteral strictures in renal transplant patients with MK was 87%.

No perioperative specific complications and morbidities were recorded in this series, and all patients tolerated the stents well without bothersome lower urinary tract symptoms (LUTS).

Discussion

In our series of eight patients, after a mean follow-up of 50 months, we have four (50%) patients in whom the MK is in situ and patent. The mean indwelling time is 47 months. Another case series with four patients showed a patency rate of 75% after 20 months.⁸ In this series, one stent needed to be exchanged because of recurrent infections after 14 months, a complication that we did not encounter.⁸ Further reports on the use of MK in transplant kidneys are rare, and if part of a larger cohort, those patients are usually not explored in detail.^5,9
–11 Unfortunately, there are no real data that could be used in comparison.

The use of another nitinol metallic stent (Sinuflex,^® Optimed, Ettlingen, Germany) has been reported in 16 patients, and a patency rate of 75% after a mean follow-up of 51 months was shown.¹² In contrast to the MK stent with its tight spiral structure preventing urothelial overgrowth and making removal and exchange easy, however, this stent is a mesh wire stent. Inherent to this kind of device is the urothelial overgrowth making removal impossible. Therefore, such mesh stents cannot be considered as a good treatment option for benign ureteral strictures.^13
–15

As described above, we had one patient who had a previously placed now completely blocked mesh stent. This was deblocked and and MK stent was inserted. This case has been previously reported in detail.¹⁵

Of our eight patients, three (28%) had a spontaneously resolved stricture after 2, 8, and 13 months indwelling time, respectively. In two patients, the stent migrated into the bladder. Stent migration or expulsion is, therefore, not always bad news. In our total series, there is a 14% spontaneous resolution rate usually diagnosed on retrograde studies when the stent is removed for planned exchange.⁵ This has also been observed by others.⁸

Stent migration is a known complication of MK stents and is known to happen especially in patients with soft, no-fibrotic (eg, functional) stenosis.¹⁰ Accordingly, one patient with a long and massively distorted ureter after extra-anatomically implanted kidney expulsed the stent as well and was treated with Double-J stent placement afterward.

On the other hand, for fibrotic strictures, it is recommended to avoid balloon dilation because this may widen the ureter too much, causing the stent to slip in the early postoperative phase.⁹

None of our patients reported bothersome LUTS; these are usually seen in patients with JJ stents in whom they can significantly affect quality of life. This finding of good tolerability of the MK is in keeping with a previous study comparing JJ stents and MK in terms of stent-related symptoms and quality of life.¹⁶

Our success rate (87%), long-term patency rate (50%), and complication rate are comparable to data reported from other case series with nontransplant patients. The patency rates reported range from 26%¹⁷ to 62%.⁵

Conclusion

The thermoexpandable, nickel-titanium alloy MK 051 stent is a safe, minimally invasive treatment option for carefully selected patients with ureteral strictures post-kidney transplantation with good long-term results.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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