The word ‘stent’ derives from the name of a British dentist Charles Thomas Stent (1807–1885). He used metallic scaffolds to immobilize tissues in his dental practice. His family name became the name for tubular scaffolding devices developed for holding open vascular obstructions by a device preventing the vessel wall from apposing. In today's nomenclature ‘stenting’ means insertion of a hollow device to create a pathway, and also support a structure, in tubular organs obstructed by benign or malignant causes. This also describes what urethral or ureteral catheters do and they are not called ‘stents’ but ‘catheters’. Some of the devices we call ‘ureteral stents’ in reality do not scaffold anything; they just connect two cavities (the renal pelvis to the bladder) by creating a pathway through an obstructed segment of the ureter. As such, what we call double-J stent, pigtail stent, Silhouette stent or Resonance stent are not stents but simple pathway opening devices.
Those urologists working in their speciality before the 1970s know that a ureteral catheter (UC) was inserted to relieve an obstructed ureter or, was left indwelling after a difficult ureterolithotomy procedure, to maintain urinary drainage as the ureter healed.
At that time UCs were not left in-situ for longer than 48–72 hours because of the risk of an ascending upper urinary tract infection. In 1967, PD Zimskind reported his experience with endoscopic long term implantation of a ‘silicone rubber ureteral splint’ to connect the renal pelvis to the bladder. Without an anchoring mechanism this ureteral splint occasionally migrated downward. To prevent such migration RP Gibbons developed a barbed silicone tube with flanges at its lower end for directly draining the kidney into the bladder.1 This was inserted retrogradely, however a combination of the additional caliber caused by the barbs (7 Fr catheter + the barbs = 11 Fr) and the high friction of the silicone made it somewhat difficult to insert this device. In1978 Roy P Finney developed a smooth ureteral tube fixed by adding J-shaped curves at its two ends, making the Gibbons device obsolete. It was Finney who for the first time added the word ‘stent’ to this tubular device and named it a ‘double-J ureteral catheter stent’.2 Finney's stent came in two calibers – 7 and 8.5 Fr, tapered with both ends closed. Drainage holes were placed along the shaft at 1 cm increments. The stents came in lengths of 16, 26 and 28 cm. These were the first double-J stents and the direct precursors of the pig-tail ureteral catheters, these carried forward the name ‘stent,’ which has now been in use for almost 35 years.4
Material used for ureteral catheter stent manufacturing evolved with the development of polymers. Initially silicone elastomer, because of its resistance to early encrustation in the presence of sterile urine, was used for Zimskind's, Gibbons's and Finney's devices. Increased use of silicone double-J stents showed that they encrusted easily in the presence of bacteria in the urine. Additional problems were the high friction coefficient of the silicone, which made stent insertion difficult and the wall thickness of the stent needed to maintain its small lumen. The availability of polyurethane and other thermoplastic elastomers allowed the design of double-Js of varying stiffness with a better wall to lumen ratio. The availability of newer polymers allowed for the design of composite stents with a softer tip at the sensitive, bladder end, thermosensitive polymers allowed for materials which are stiff during insertion and later softer at body temperature, all resulting in better patient comfort.
Several new stent coatings have been used to try to prevent bacterial adhesion, and encrustation; or to prevent stent-related symptoms. To give some examples; diamond-like amorphous carbon coating can reduce the friction of the stent but not the encrustation. Heparin coating can prevent encrustation. Various pharmacological agents have been put into stent coatings; triclosan eluting stents (polychloro-phenoxy-phenol is an antibacterial and antifungal agent) gave promising results in the prevention of stent induced infections but failed to receive FDA approval due to concerns about bacterial resistance. Ketorolac eluting stents (a non-steroidal anti-inflammatory drug used as an analgesic) did not significantly decrease stent related discomfort symptoms as was hoped. To date, none of these coatings can prevent bacterial adhesions, encrustations or stent related symptoms in a satisfactory manner.3 The ideal ureteral stent coating has therefore still to be found.
Coiled ureteral double-J stents
The full metal Resonance stent was first launched in 2007 with the idea to create a non-crushing device to be used in malignancies. It has a wire core connecting both ends of the device which keeps the coils touching each other and maintains the J-shape of the stent's ends. Although the coils along its straight body form an almost watertight tube, at its J-segments the coils are less tight, allowing the urine to enter from its kidney end and come out through its bladder end.
This was followed by the development of the Silhouette ureteral stent, which, when, compared to the non-reinforced polymer double-J stents has a thin coil-reinforced wall, with a 30% larger inner diameter to maximize intraluminal flow. It comes in calibers of 4–8 Fr. Both the Resonance and the Silhouette claim providing maximum compression and resistance to kinking.5,6
With the appearance of the Resonance and the Silhouette for use in malignant ureteral obstructions, mechanical testing was done to 5compare them with conventional polyurethane and silicone stents, compressing the examined stents between two metal surfaces.7 Such a simulation test is far from what happens in real life because no tumor is as hard as metal. The way even the hardest tumor develops is by cell division and this cannot create the compression resulting from the approximation of two metal surfaces. These studies were based on a false belief that conventional polyurethane stents can be crushed by tumors, causing occlusion of the stent.
Expandable metal and covered metal ureteric stents
The relief of ureteral obstructions caused by strictures or malignancies with regular double-J stents has high failure rates, because of the small intraluminal caliber and occlusion of the lumen by debris, and blood clots. Neither conventional double-J stents nor coiled stents can be compared with the self-expanding ureteral stents, which exert an outward radial force on the surrounding tissues. Based on stent usage in other tubular organs such as arteries, similar stents were tried in the ureter. The use of meshed bare metal stents was not widely accepted did not find wide acceptance mainly because a hyperplastic mucosal reaction developing into the stent lumen and the possibility of tumor ingrowth. To prevent this hyperplastic mucosal reaction again trying to follow the vascular experience, drug eluding metal stents are being tried in the ureter.
The first large caliber coiled full metal ureteral stent to enter urological use is the Memokath 051. This has a 10.5–12 Fr non-expandable body and a bell-shaped thermo-expandable end for anchoring it. The Memokath 051 is made of a polished nickel-titanium (nitinol) wire. To prevent the lumen occluding mucosal hyperplasia or tumor invasion, two self-expandable fully polymer covered metal stents were developed:
The URS (Allium ureteral stent) which comes in 24 and 30 Fr diameter and the TaeWoong Uventa ureteral stent which comes in 21, 24 and 30 Fr.
The Allium URS is made of an undulating nitinol wire fully covered with a thin membrane of ElastEon (polyurethane and silicone elastomer). It is indicated for obstructions of the lower ureter. The Allium URS can be removed by unraveling them to a ribbon-shaped strip.8
The TaeWoong Uventa has a two layered nitinol mesh tubular body with a polytetrafluoroethylene layer between them. The bare nitinol outer body allows some limited tissue ingrowth to anchor the stent in place.9 There is no information on how the Uventa can be removed after being left indwelling for a few months or on the development of encrustations on the inner nitinol structure. Since both the URS and the Uventa are relatively new stents, reports on both of them are limited thus far.6
References
- Gibbons RP, Mason JT, Correa RJ. Experience with indwelling silicone rubber ureteral catheters. J Urol 1974; 111:594–599.
- Finney RP. Experience with new ‘double-J ureteral catheter stent’. J Urol 1978; 120:678–681.
- Lange D, Chew BH. Update on ureteral stent technology. Ther Adv Urol 2009; 1:143–148.
- Liatsikos E, Kallidonis P, Kyriazis I, et al. Ureteral obstruction: Is the full metallic double pigtail the way to go? Eur Urol 2010; 57:480–486.
- Christman MS, L'Esperance JO, Choe CH, Stroup SP, Auge BK. Analysis of ureteric stent kinkling forces: The role of curvature in stent failure. BJU Int 2010; 105:866–869.
- Miyaoka R, Hendlin K, Monga M. Resistance to extrinsic compression and maintenance of intraluminal flow in coil-reinforced stents (Silhouette Scaffold Device): an in vitro study. J Endourol 2010; 24:595–598.
- Pedro RN, Hendlin K, Kriedenberg C, Monga M. Wire-based ureteral stents: impact on tensile strength and compression. Urology 2007; 70:1057–1059.
- Moskovitz B, Halachmi S, Nativ O. A new self-expanding, large caliber ureteral stent: Results of a multicenter experience. J Endurol 2012; 26:1523–1527.
- Kim JH, Song K, Jo MK, Park JW. Palliative care of malignant ureteral obstruction with polytetrafluoroethylene membrane-covered self-expandable metallic stents: initial experience. Korean J Urol 2012; 53:625–631.