A new thermo-expandable shape-memory nickel-titanium alloy stent for the management of ureteric strictures


Mr Kulkarni Department of Urology, Ashford and St Peter’s Hospital, Ashford TW15 3AA, UK.



To assess the ease on insertion, patient tolerance, undesirable side-effects, degree of encrustation and duration of upper tract decompression with a new thermo-expandable shape memory alloy ureteric stent.

Patients and methods

From November 1996 to October 1998, 15 patients with ureteric strictures were treated with a new nickel-titanium shape-memory alloy stent, the Memokath 051 (Engineers & Doctors A/S, Hornbaek, Denmark). A total of 22 insertions were carried out. Ureteric obstruction was caused by recurrent colorectal carcinoma in four patients; two patients each with transitional cell carcinoma of the bladder, iatrogenic injury or ischaemia at the uretero-ileal anastomosis; and one patient each with metastatic lymph nodes from prostatic carcinoma, radiation-induced fibrosis, pelvi-ureteric junction obstruction, metastatic carcinoma of the vagina and extra-luminal endometriosis. The stent has a shaft diameter of 9 F and its proximal end expands to 17 F. The first three patients were treated with the original version, which expanded to 14 F. The unexpanded stent is inserted into the ureter after initial dilatation of the stricture to 12 F. The stent is expanded by injection with sterile water preheated to 50 °C. The procedures were carried out under a general anaesthetic and patients were allowed home the next day. The follow-up protocol included initial intravenous urography (IVU) at 6 weeks, with assessment of a mid-stream urine sample and renal function tests. These were repeated at 3-monthly intervals. Isotopic renography was performed when indicated.


The mean (range) follow-up was 10.6 (2–21) months; there was complete relief of upper tract obstruction in all patients. No stent-related symptoms, e.g. pain, sepsis, haematuria or frequency, were noted and no encrustation has occurred so far. The stent migrated in the first three patients with the original smaller diameter of stent but decompression of the upper tracts was maintained. None of the modified wider stents have migrated. The return of peristalsis in the proximal ureter was detected during IVU. There was no apparent endothelial growth through the stent material and no re-admissions for stent-related complications.


Early experience with this new stent is very encouraging. All patients have maintained satisfactory decompression of their upper tracts with no need for repeated hospitalization for stent changes. There have been no untoward side-effects so far. This stent appears to have a valuable place in the long-term management of ureteric strictures; it is probably most suited for malignant ureteric obstruction. It should be considered in the management of selected benign strictures that require long-term JJ stenting.


Ureteric obstruction is conventionally managed with JJ stents; unavoidably, this can cause irritation of the bladder epithelium, bleeding, pain, encrustation and reflux. These adverse effects necessitate regular replacement or ultimate removal, requiring open surgery, to maintain renal function. Greater understanding of the interaction between stent material and urine, with improvements in biomedical engineering, has reduced some of the undesirable features associated with these stents. An ideal stent should be inert, cause no pain or reflux and remain free from encrustation. It should also be suitable for long-term use in the ureter when the aetiology of obstruction is untreatable. None of the JJ stents currently available fulfils these criteria. We present our initial results using a new metal ureteric stent, the Memokath 051 (Engineers & Doctors A/S, Hornbaek, Denmark) which seems to be free of most of the side-effects related to conventional JJ stents.

Patients and methods

The Memokath 051 is made from an alloy of nickel and titanium that has a unique ‘shape-memory’; it softens at temperatures of < 10 °C but regains the original shape when re-warmed to 50 °C. The stent has a shaft diameter of 9 F and its wider fluted proximal end expands to 17 F (Fig. 1). It is available in lengths of 30, 60 and 100 mm. A new longer and wider version became available in late 1998; this has a shaft diameter of 10.5 F and its wide proximal end expands to 20 F. Versions are also available in 150 and 200 mm lengths. The stent in its unexpanded form is mounted over a hollow catheter, with its wide end held with plastic lugs to prevent movement during insertion. This entire assembly is inserted into the ureter through a 12 F sheath which is radio-opaque. The sheath is coupled with a stepped dilator; its inner core can be removed after dilatation. The working end of stent assembly has a port for injecting hot water to induce stent expansion.

Figure 1.

The Memokath 051 ureteric stent.


A retrograde ureterogram is taken to delineate the precise length and location of the stricture (Fig. 2). The image is stored in the memory of an image-intensifier if possible and the upper and lower limits of the stricture are also marked on the abdominal wall using radio-opaque markers for correct positioning of the stent after dilatation.

Figure 2.

A retrograde pyelogram to assess the length of the stricture.

A 0.038 inch guidewire is inserted up to the renal pelvis and the stricture dilated to 12 F with the dilator provided. The sheath must be passed well above the proximal limit of the stricture; balloon dilatation may be necessary, particularly in benign strictures. The dilator is removed, leaving the sheath in the ureter, and the guidewire is removed. A retrograde study can be performed through the sheath to confirm complete dilatation. The Memokath 051 stent assembly is inserted into the sheath. The metal stent can be imaged through the sheath and the sheath then withdrawn proximally to expose the stent. The entire assembly can be moved for final positioning of the stent. The proximal end is expanded by injecting 50 mL of sterile water, preheated to 50 °C, through the injection port. The retaining lugs are removed and the supporting catheter withdrawn. A retrograde pyelogram is taken through the sheath to confirm correct positioning of the stent and adequate decompression of the upper tract. The distal end of the stent can be visualized with a ureteroscope to confirm satisfactory positioning.

Between November 1996 and October 1998, 22 stents were inserted in 15 patients (five men and 10 women, mean age 59.4 years, range 31–86). The causes and locations of the strictures are listed in Table 1. Two patients had bilateral insertion of Memokath 051 stents. One patient had his stent changed twice because the disease progressed, leading to obstruction proximal of the stent. Stents were replaced in two patients because they migrated and in two because they were incorrectly positioned at first insertion in the early part of the series. JJ stents were inserted in 11 patients 4–6 weeks before inserting the Memokath 051 stent. Four patients had the Memokath 051 stent inserted as a primary procedure. One of these patients had an initial nephrostomy inserted 1 day before definitive stenting; in this patient stent placement was facilitated by a ‘rendezvous’ manipulation of a guidewire through the stricture and the stent was placed antegradely. Most of the strictures were in the lower third of the ureter ( Table 1). The mean (range) length of the stricture was 3.8 (1–7) cm.

Table 1.  Aetiology and location of ureteric obstruction. Thumbnail image of

All procedures were performed under general anaesthesia. A urethral catheter was left in the bladder and was removed the following morning. All patients received prophylactic antibiotics (intravenous gentamicin at anaesthetic induction, followed by norfloxacin for 5 days). A plain film of the kidneys, ureter and bladder was taken the next day (Fig. 3). The mean length of stay was 1.5 days; there was no operative mortality.

Figure 3.

A Memokath 051 stent inserted across the stricture.

All patients were followed using a protocol including IVU, and an assessment of MSU samples and renal function after 6 weeks. These were repeated at 3-monthly intervals or earlier if indicated. DTPA renography was performed when indicated. Ureteroscopy was carried out in two patients to assess encrustation a year after insertion. The mean (range) follow-up was 10.6 (2–21) months.


The stents migrated in the first three patients after treatment of their underlying malignancy (radiotherapy in two and hormone manipulation in the third). Although the patency of the strictured segment of the ureter was maintained, the lower end of the migrated stents (the original long version with a smaller funnel diameter) caused bladder irritation. These were replaced with short Memokath stents with wider proximal ends in two of these patients. The third patient remains asymptomatic. The remaining 12 patients experienced no adverse effects; in all, the ureteric obstruction was relieved (Fig. 4). There was no stent-related pain or sepsis in any of these patients. In the two patients who underwent ureteroscopy a year after stent insertion, no encrustation, calculi or obstruction from endothelial ingrowth was apparent. Two patients have had the stents indwelling for > 19 months with no untoward side-effects. The Memokath 051 stent can be removed easily with biopsy forceps, after cooling the stent to < 10 °C; the stent then unfurls and can be removed safely.

Figure 4.

An intravenous urogram taken after Memokath 051 insertion.


The first recorded ureteric stent insertion was by Gustav Simon in the 19th century (cited in [ 1]). A purpose-built ureteric catheter was made by Joaquin Albarrano in 1900; Gibbons devised a silicone stent in 1967. The first JJ stent was described by Finney and Hepperlen (cited in [ 1]) in 1978. Considerable efforts have since been made to improve stent design, with the aim of making insertion and retention easier, preventing migration and permitting maximum flow of urine into the bladder, but the ideal stent does not exist.

Minimally invasive techniques have virtually replaced the need for open surgery in ureteric obstruction [ 2]. An obstructed system can be immediately decompressed using a JJ stent or percutaneous nephrostomy, followed by the placement of a stent. The management of ureteric obstruction caused by recurrent benign disease or extrinsic compression by tumours can be difficult. Long-term JJ stenting is the only viable option if open diversion is to be avoided. Encrustation, pain, haematuria, sepsis and reflux [ 3] are inevitable complications of JJ stents; the stents may also migrate and even fragment [ 4]. A decrease in intraluminal flow and relative obstruction from diminished peristalsis are also well documented [ 2, 5, 6]. Stents must then be changed at regular intervals to maintain renal function. Improvements in stent design and materials have reduced the need for frequent changes but there is a clear need for a stent which can be left in situ for prolonged periods without the inevitable morbidity of a JJ stent. This is particularly relevant to patients with malignancy, where frequent hospitalization should be avoided.

Metal stents were initially used in the biliary tract. Gillams et al. [ 7] described woven stainless-steel stents of 9–14 F which were moderately successful and subsequently used in vascular surgery. Early experience in the urinary tract was in the prostatic urethra, with stents made from stainless-steel [ 8, 9]. Encrustation and urothelial hyperplasia develop rapidly around these devices [ 10]; incorporation of these stents into the urothelium makes removal difficult, a feature substantiated by in vitro studies [ 10]. The concept of metal stents in the ureter was an extension of their use in the lower urinary tract [ 11[12][13][14][15][16]–17]. Wallstents were first reported by Lugmayr and Pauer [ 11] in the management of malignant ureteric obstruction, with moderate success. They are clearly advantageous, as like the plastic stents they are not compressed by tumour tissue. However, endothelial hyperplasia has been reported to occlude these stents within weeks [ 14, 15] and tumour ingrowth through the mesh of the wallstent eventually results in recurrence of obstruction at the original site [ 11]; removal of these stents is virtually impossible.

Prostatic stents made from nickel-titanium alloy have been in use for over 6 years; they are easy to insert and have been reported to cause very little encrustation. Titanium has been reported to resist encrustation in the urinary tract [ 10]. This concept was extended to the ureter, with the Memokath series; we inserted the first Memokath 051 stent in November 1996 in a patient with extrinsic ureteric obstruction caused by metastatic lymph nodes from carcinoma of the prostate. The stent design was modified after the first three insertions because the stents migrated after treatment of the underlying malignancy. The proximal wide end of the original stent (14 F) allowed some movement between the stent and the ureter; this was changed to 17 F with no further problems. A reverse-mounted assembly for antegrade insertion is also available and the addition of wider and longer versions permits the clinician to select the most appropriate stent for a given patient.

No patient was re-admitted with stent-related sepsis, pain or other complication, and upper tract patency was maintained in all patients managed with this stent. There has been no radiological evidence of encrustation to date and no evidence of endothelial hyperplasia. One patient underwent cystectomy for recurrent TCC of the bladder. The Memokath 051 stent was inserted in the right ureter 14 months before cystectomy and the stent bearing ureter removed en-bloc with the bladder; there was no evidence of endothelial hyperplasia or encrustation in this patient.

Two patients with strictures at the uretero-ileal anastomosis were treated successfully with a Memokath 051 stent; both had been treated with repeated JJ stent insertions, because these stents frequently became encrusted and obstructed. One of these patients has a solitary kidney and thus renal replacement therapy was imminent; however, this has been avoided in both patients after successful decompression of the upper tracts by the Memokath 051 stent.

Previous JJ stenting is advantageous; the Memokath 051 can also be inserted antegradely through a nephrostomy after dilatation of the tract, as was carried out in one patient in the present study. Stricture ‘landmarks’ are important for accurately positioning the stent after dilatation. Benign strictures are often more difficult to dilate than malignant ones. Balloon dilatation may be necessary in such cases. The new wide version (shaft 10.5 F, wide fluted end expanding to 20 F) has been used in one patient so far. The 150 and 200 mm stents may be useful when strictures are long or multiple. The stent design is unique; its tight spiral structure probably prevents endothelial ingrowth. This feature is important in malignant ureteric obstruction if repeated stent change is to be avoided. The floppiness of the stent helps to retain ureteric mobility and recovery of ureteric peristalsis above and below the stenotic segment. The ‘thermal memory’ facilitates removal if necessary.

In conclusion, this new metallic stent seems to be remarkably free of side-effects usually associated with conventional JJ stents. Memokath 051 stents have remained in some of these patients with malignant ureteric obstruction for > 19 months with no morbidity. In no patients with the new Memokath 051 has it migrated. With no need for repeated stent changes, there has been a saving in resources and hospital beds. The stent improves the quality of life for patients with malignancy. This stent has a clear place in the long-term management of ureteric strictures; it seems most suitable for malignant ureteric obstruction and is valuable in selected patients with recurrent benign strictures.


We thank Mr Brian Ellis.