The concept of inserting a stent to open a blocked viscus is intuitively attractive to any minimally invasive surgeon. Urologists have been far ahead in applying this idea in clinical practice. The first reported use of a ureteric stent by Gustav Simon  dates back to the 19th century. Albarrano made a purpose-built ureteric stent over 100 years ago but the pigtail stent as it is currently known was designed by Finney and Hepperlen in 1978.
As the insertion of a pigtail stent became an established technique, the range of indications widened. Not surprisingly, the undesirable side-effects of JJ stents began to surface. Irritative bladder symptoms, haematuria, pain, infections and reflux have been well documented. More troublesome are migration in either direction, fragmentation or encrustation. Obstruction caused by the dampening of peristalsis has been well researched.
Attempts to make a better stent have not ceased. Metallic stents were initially used for coronary arteries and biliary ducts. Milroy et al. introduced their application in urology; urethral and subsequently prostatic stents made from stainless steel were inserted with some success. The concept of stenting only the obstructed part of a ureter was first introduced by Lugmayr and Pauer . The wallstent made from meshed steel wire is inserted through a special device into the ureter. Although it succeeds in proximal decompression, there can be recurrence of obstruction from the ingrowth of tumour tissue. Removing these stents is extremely difficult.
Nickel-titanium alloy has a unique thermal ‘shape-memory’; the Memokath-051® stent (Engineers & Doctors A/S, Hornbaek, Denmark) made from this alloy softens at temperatures of < 10°C but regains its original shape when re-warmed to 55°C. This device was a natural successor to the wallstent.
We first reported the use of the Memokath-051 in 1999 . This funnel shaped stent is deployed in its un-expanded form through a tailor-made delivery system after dilatation of the stricture. Sterile water at 55°C is injected when the stent is placed in the correct position, to induce expansion. The thermal characteristics allow removal of this stent by cooling if necessary. A similar stent, the Memotherm® does not have the property of softening, because of metallurgical differences, and hence cannot be removed.
The Memokath-051 stent has undergone many modifications since the original version that was 10 cm long and 9.5 F shaft diameter. The current version is available in 3, 6, 10, 15 and 20 cm lengths; it has a shaft diameter of 10.5 F and its wide end expands to 22 F. The dilator assembly and the delivery system have been modified for smoother insertion. They achieve durable decompression causing minimal complications. During the past 6 years we have encountered a few migrations; encrustation is rare. Most patients with malignant ureteric obstruction outlive the stent. They do not need re-admissions for stent changes, which offers a significant improvement in their quality of life (Fig. 1). It is also cost-effective; our evaluation suggests that two admissions for the change of a JJ stent cost as much as one Memokath stent insertion.
Refractory benign strictures which have recurred after repeated open or endourological procedures have been treated successfully (Fig. 2). Selected uretero-ileal strictures have been treated with a modified design of this stent.
Is this therefore a panacea for all ureteric strictures? It is not easy to answer this question at present. The lack of randomized studies and the few patients involved (our series being the largest, with 55 insertions in 43 patients) make interpretation difficult. Insertion needs a fully equipped endourological theatre and some training. Encrustation has been reported in patients with poor renal function, hypercalciuria, and in stents protruding into the bladder. Migration can occur in benign strictures, especially in PUJ obstruction and retroperitoneal fibrosis. Prolonged dilatation probably loosens the stent. Modified designs are currently under evaluation for these conditions.
This stent is best suited for patients with malignant ureteric obstruction with a reasonable life-expectancy. It should also be offered to patients with a short lifespan if quality of life is significantly altered by the side-effects of a JJ stent. Refractory benign strictures need to be considered individually, as they often are the most difficult to manage. Innovations in the materials and design of ureteric stents will continue. Patients will demand and indeed benefit from this truly minimally invasive therapeutic option.