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Doi:10.1016/j.urology.2004.03.005Ureteral stents have become an integral part of tulas. Straight silicone stents provided good
contemporary urologic practice. They are typ- internal drainage and developed less encrustation ically placed to prevent or relieve ureteral obstruc- than other compounds, but had no distal or prox- tion due to a variety of intrinsic or extrinsic etiol- imal features to prevent migration. Minor im- ogies. These include obstructing ureteral calculi, provements followed, with closing the proximal ureteral strictures, congenital anomalies such as end of the stent to facilitate passage and using a ureteropelvic junction obstruction, retroperito- “pusher” to hold the stent in place during wire neal tumor or fibrosis, trauma, and iatrogenic in- jury. Stents are also placed to provide urinary di- modifications to prevent stent migration, includ- version or postoperative drainage or to help ing a distal flange to prevent proximal migration identify and prevent inadvertent injury to the ure- and sharply pointed barbs to prevent downward migration and expulsion. In 1974, the Gibbonsstent became the first commercially available HISTORY OF STENTS
“modern” internal ureteral stent. Following the The concept of stenting the urinary system began Gibbons stent, single-pigtail stents were designed as an adjunct to open surgery to facilitate upper that could be straightened and placed over a wire tract drainage or to align the ureter. Gustav Simon cystoscopically to prevent distal, but not proximal, performed the first reported case during the 19th century by placing a tube in the ureter during open solved with the use of double-J stents, which con- cystostomy, and Joaquin Albarrano created the first catheter intended for use in the ureter in the The word “stent” is commonly used in genitouri- nary reconstructive surgery. Two related uses of was first reported in 1839, early catheters were the stent are given in most medical dictionaries.
First, a stent is a device used to maintain a bodily development of plastics, such as polyethylene and orifice, cavity, or contour, and second, it is a cath- polyvinyl, allowed for stents to become more rigid eter, rod, or tube within a tubular structure to and easier to place; however, problems such as maintain luminal patency or protect an anastomo- bladder irritation, infection, encrustation, and mi- sis or Etymologically, the word “stent” is an eponym related to three English dentists named using polyethylene tubes to help repair ureters and Stent who contributed to improving a substance In 1967, the era of the modern long-term in- dental impression compound into a urologic tool is dwelling ureteral stent began when Zimskind and attributed to of Holland during World War I, as the name became associated with a support for cone tubing inserted endoscopically to bypass ma- oral skin grafts and later adopted in plastic surgery.
lignant ureteral obstruction or ureterovaginal fis- Urologic stenting first appeared in print beginningin the 1970s. wrote a brief commentaryin 1972 titled, “Splint, Stent, Stint,” concluding: From the Department of Urology, University of California, David “Urologists are always talking about putting a tube Geffen School of Medicine, Los Angeles, Los Angeles, California; in a ureter or urethra. When they do this, it is not a and Department of Urology, New York-Presbyterian Hospital,Columbia University College of Physicians and Surgeons, New splint. It may be a stent. It probably is never a stint.
Perhaps the process is most properly described as Reprint requests: Mantu Gupta, M.D., Department of Urology, leaving a tube or stent in an organ.” The word Columbia University College of Physicians and Surgeons, 161 “stent” became forged into the urologic literature Fort Washington Avenue, Irving Pavilion, 11th Floor, New York,NY 10032 when Montie et explicitly defined the term: Submitted: December 29, 2003, accepted: March 1, 2004 “When referring to an intraluminal device to main- UROLOGY 64: 9 –15, 2004
tain patency until healing has taken place, the stent teral replacement using glass, tantalum, or vital-lium were first described before World War In PROPERTIES OF STENT MATERIALS
contemporary practice, devices constructed from The modern self-retained internal ureteral stent synthetic polymeric compounds are the most use- is a synthetic polymeric biomaterial device that can ful materials implanted in the urinary tract. Cur- be placed by a variety of endourologic techniques.
rently, self-retained internal ureteral stents are It is designed to retain its position within the ureter manufactured from “high polymers.” A polymeric and drain urine between the renal pelvis and blad- chain has tens of thousands of chemical “mono- der. The basic properties of a ureteral stent have mers,” which are linked by “polymerization.” been reviewed by Mardis et “Memory” pro- These chains are organized in various configura- duces the ability to maintain the position within tions resulting in complex and versatile macromol- the ureter and is a consequence of polymeric cross- linking, which results from physical or chemical Polyethylene was the first synthetic polymer bonding between macromolecular chains. “Du- used to fashion ureteral stents. It consists of poly- rometer” refers to the strength of the memory, olefin and is flexible, odorless, translucent, and which can be varied within the same material and nonreactive in the body. The stiffness of polyeth- depends on the type of cross-linking used. As ylene made it useful for the management of ure- cross-linking increases, the durometer changesfrom soft to hard. Elasticity allows the shape of the teral strictures, but it was found to promote protein stent to be manipulated. Variations in tensile deposits, leading to an increased likelihood of crys- strength reflect the mechanism of crystallization and cross-linking in the biomaterials. Greater ten- Furthermore, long-term exposure of polyethylene sile strength allows for the creation of a higher internal/external diameter ratio and a greater num- composed of alternating silicone and oxygen at- It is nonirritating and resistant to encrusta- elongation capacity is the percentage of elongation tion. However, silicone stents migrate easily, have at stent breakage. Thermoplastic elastomeres, such poor mechanical strength, and have a high coeffi- as polyurethanes and a variety of other proprietary cient of friction, making it difficult to use when copolymers, generally have more elongation ca- negotiating strictures or tortuous ureters. Polyure- pacity than thermoset elastomeres, such as sili- thane, a common generic class of condensation Biodurability refers to the ability to exist polymers derived from polyisocynate and a polyol within the body without degradation of stent struc- has been used in an effort to combine the flexibility ture and function. The urinary tract presents an of silicone with the stiffness of polyethylene. Poly- unstable and hostile environment to stent func- urethane is highly versatile and inexpensive, but tion, replete with free radicals, oxidizers, enzymes, has been shown to induce more epithelial ulcer- and supersaturation of urinary mucoids and crys- talloids. Stent degradation within the urinary tract ited durability and demonstrates slow in vivo bio- has been reported for polyethylene, polyurethane, and silicone internal ureteral Biocompat- Proprietary modifications to silicone led to the ibility is the utopian state at which the material development of C-Flex (Consolidated Polymer present within the ureter has no significant effect Technologies, Clearwater, Fla), a silicone-modi- on the interface between itself and the urothelium,such as the ability to resist encrustation and infec- fied styrene/ethylene/butylene block thermoplastic tion. The biocompatibility of a stent may be en- copolymer. The partial silicone composition of C- hanced with use of hydrophilic polymers that have Flex makes these stents softer than polyurethane low protein absorption and low bacterial adher- and theoretically less likely to develop encrusta- The coefficient of friction describes how tion. Silitek (ACMI, Southborough, Mass) is a pro- easily a stent is passed or exchanged. Surface fric- prietary, second-generation, polyester copolymer tion is generally dependent on the durometer and a that followed silicone and polyurethane. Percuflex greater durometer lowers the coefficient of friction.
(Boston Scientific, Natick, Mass) is a proprietary Applying a hydrophilic coating reduces the surface olefinic block copolymer developed by Boston Sci- coefficient of friction. Radiopacity refers to the ease entific/Microvasive that becomes soft and flexible of stent visualization during fluoroscopy. All stents at body temperature. This material also has excel- are radiopaque and some contain fillers that fur- lent memory and strength. Tecoflex (Thermedics, Woburn, Mass) is a proprietary aliphatic thermo-plastic polymer that is not silicone based. Stents STENT BIOMATERIALS AND COATINGS
made with this material have a smooth surface and The use of biomaterials within the urinary tract a comparatively large inside diameter. This mate- dates back to ancient Egypt when lead and papyrus rial allows for ink labeling instead of laser labeling, UROLOGY 64 (1), 2004
which may theoretically decrease the likelihood of tients with strictures of various etiologies and lo- encrustation from the rough surface created by the laser. Sof-Flex (Cook Urological, Spencer, Ind) is a be placed either alone or in combination with con- proprietary compound from Cook Urological.
ventional indwelling ureteral stents in patients Other stent materials from Cook include Endo-Sof with long strictures. These stents gradually be- and Ultrathane. Pellethane (C.R. Bard, Covington, come covered with urothelium after implantation, Ga) is a proprietary copolymer from Bard used in thus preventing encrustation and infection. The their Inlay stents. Flexima is a newer proprietary problems associated with metal stents include col- material from Boston Scientific/Microvasive de- lagenous ingrowth, hyperplastic epithelium, distal signed to resist buckling during insertion. Vertex ureteral narrowing, intense fibrosis, and subse- (Applied Medical, Rancho Santa Margarita, Calif) is a proprietary material from Applied Medical.
The development of tissue-engineered stents is Bioabsorbable polymeric materials are designed in its infancy, but this area has the potential to to retain their tissue-supporting properties for de- revolutionize biomaterial science and the clinical fined periods. After placement, they are gradually applications of biomaterials. The use of autologous biodegraded into tissue-compatible compounds tissue would be advantageous because of its inher- that are absorbed and replaced by healing tissue.
ent biocompatibility. Cartilaginous stents have These stents will be most useful for clinical situa- been successfully created in vitro and in vivo using tions in which temporary upper urinary tract drainage is desired. Bioabsorbable materials used technology might one day become clinically useful in urologic stents are high-molecular-weight poly- in the treatment of urethral or ureteral strictures.
mers of polylactic and polyglycolic Poly-L- The goals of coating ureteral stents are to facili- lactide-co-glycolide bioabsorbable ureteral stents tate passage over a guidewire beyond an obstruc- have been shown to be similar to standard ureteral tion and to reduce or eliminate biofilm formation stents with respect to fluid flow results and post- and encrustation. Hydrophilic coatings consist of operative appearance on retrograde pyelography in nondissolvable polymers that swell on contact a porcine model after cutting balloon device endo- with water and retain water within their polyan- ionic structure that layers on its surface. The sur- have less favorable biocompatibility than standard face water not only reduces the coefficient of fric- stents because of inferior healing of the incised tion, improving ease of stent insertion, but also ureteral musculature. Temporary ureteral drainage contributes to biocompatibility by reducing fric- stents (Boston Scientific/Microvasive) have been tional irritation and cell adhesion at the biomateri- al-urothelial interface. LSe is Cook Urological’s ion more, dissolution of these stents can be monitored implantation process in which specific ions are im- by radiographic examination during a period of 2 planted into the chemical structure. LSe is de- to 7 days owing to the inclusion of a radiopaque signed to reduce the coefficient of friction without filling in the bulk biomaterial. In a multicenter presoaking in water and lower the surface energy Phase II clinical trial, 90% of the stents dissolved as of the AQ is a hydrophilic coating made by expected with a median elimination time of 8 days, Cook Urological that can be applied to lower the and 89% of the patients were satisfied with their coefficient of friction. Other hydrophilic coatings include SL-6 by Applied Medical and Hydro Plus pand at body temperature to hold their position by Boston Scientific/Microvasive. A new hydro- philic coating, polyvinylpyrrolidone, has been stents do not communicate with the bladder or shown in vitro to decrease both hydroxyapatite en- kidney and would avoid vesicoureteral reflux and crustation and adherence of a hydrophobic Entero- possibly would have a lower infection rate. An al- coccus faecalis Another coating that has ternative strategy is o control degradation by mod- potential for reducing encrustation is the phospho- ulating the urinary pH to an alkaline level when the rylcholine group of compounds. Phosphorylcho- line-coated stents have been shown to be less vul- Metallic, superalloy titanium and nickel/tita- nerable to encrustation and colonization by nium are materials currently used for permanently bacterial biofilm than the uncoated stents at 12 implanted stents, such as the Wallstent (Medin- rent, Lausanne, Switzerland) or the Memokath 051 studies, heparin-like polysaccharides appear to (Engineers and Doctors of Copenhagen, Copenha- have great potential to reduce biomaterial encrus- gen, Denmark), to relieve conditions such as ma- tation. Heparin-coated stents have been shown not to have any biofilm formation or encrustation after Varying success rates using these devices have been reported, ranging from 14% to 100%, in pa- rial-related urinary tract encrustation involves UROLOGY 64 (1), 2004
coating biomaterials with oxalate-degrading en- The Towers peripheral stent is a grooved stent zymes derived from Oxalobacter formigenesA re- manufactured by Cook Urological. The outer di- duction in the amount of encrustation on oxalyl ameter ranges from 6 to 8 F and the length from 22 coenzyme A decarboxylase-coated and formyl co- to 32 cm. The LithoStent by ACMI features three enzyme A transferase-coated disks versus control grooves spiraling down the exterior of stent-like disks was demonstrated in an in vivo rabbit blad- gutters designed to prevent ureteral strictures and der implantation model after a 30-day implanta- A recent study has shown that silver nitrate Spiral stents are polyurethane stents supported and ofloxacin-blended caprolactam-L-lactide co- by a built-in metal spiral wire intended to keep the polymer-coated self-reinforced poly-L-lactic acid stent patent and improve drainage. In a study of 14 Urospiral urethral stents have good biocompatibil- patients with spiral stents placed for chronic ure- ity and caused less tissue reaction than stents with teral obstruction, only one stent change occurred a pure copolymer coating in a rabbit model. Fur- thermore, the silver nitrate coating was effective in spiral-ridged stents were compared with smooth- preventing biofilm formation and stent encrusta- walled double-J stents in an in vitro mechanical ureteral model, extraluminal flow was greatestwith the 7F spiral-ridged STENT DESIGNS
A mesh stent is a lightweight, self-expanding stent designed to preserve drainage while minimiz- A double-J stent has an open hook configuration ing irritative symptoms. In a porcine model, al- at either end of the ureteral stent, and a double- though not statistically significant, the mesh stent pigtail stent has a full retentive coil at either end.
resulted in less inflammation along the urinary The double-J stent is the original, silicone, closed- tract at 1 week than a standard 7F double-pigtail tipped stent to which ACMI/Surgitek holds the through the mesh stent tended to be greater than double-pigtail stent, which have a variety of names, the flow through the standard stent at both 1 and 6 designs, and compositions. Standard double-pig- weeks. When the flow characteristics were com- tail stents must be sized correctly because those pared with other stents in a separate porcine ani- that are too long can cause bladder irritation, and mal study, the mesh stent had the greatest fl those that are too short can migrate up the ureter. A Tail stents have a standard pigtail and a 6F or 7F 24-cm ureteral stent is well suited for most adults, shaft at the proximal end, but the distal end tapers but should be individualized according to the ure- into an elongated 3F closed-tip tail to decrease teral length of the patient. Multiple-coil stents weredesigned to meet the “one size fits all” concept.
stent-related bladder irritability. Drainage is Multiple-coil stents leave redundant length in the achieved around the distal portion of tail stents, bladder coiled, so the trigone is not irritated. Some and, consequently, they are contraindicated when stents are manufactured with a nylon suture at- trauma to the distal ureter is suspected. Tail stents tached to the distal end, which can be left at the resist reflux because the distal third of the tail is urethral meatus, allowing for removal after short- occluded and approximately 3 to 5 cm of the tail’s term drainage without the need for repeat cystos- occluded portion lies within the intramural tunnel and the distal ureter. Boston Scientific/Microvasive Open-ended ureteral stents have no coils and makes a tail stent called the Percuflex Tail Plus.
thus are not suitable for long-term urinary drain- Tail stents have been shown to produce signifi- age. However, they can be secured externally to cantly fewer irritative voiding symptoms and result provide temporary drainage. Open-ended stents in significantly less postoperative flank pain after are useful in helping to direct and advance a guide- wire into a ureteral orifice. They can assist in col- Dual-durometer stents incorporate a smooth lecting upper urinary tract urine samples and per- transition from a firm biomaterial at the renal end mitting retrograde pyelography. Furthermore, to a soft biomaterial at the bladder end. Two such open-ended ureteral stents are commonly placed devices are the Sof-Curl (ACMI) and Polaris (Bos- intraoperatively to identify ureters to avoid inad- ton Scientific/Microvasive) stents, which have long vertent injury during abdominal or pelvic surgery.
tapered tips at the renal end and are coated with a Transilluminating ureteral stents have been used hydrophilic-bonded hydrogel that decreases their to prevent ureteral injuries during gynecologic ureteral stents, the Polaris stent was shown to have Grooved stents have grooves spiraling down the superior lubricity, which eases stent passage, and exterior of the stent to improve extraluminal flow lower flexural strength, which minimizes bladder aimed at postlithotripsy or holmium laser cases.
UROLOGY 64 (1), 2004
Magnetic material-tipped ureteral stents were tion, in whom indwelling ureteral stents have developed to eliminate the morbidity of cysto- failed. The proximal end of the stent is inserted scopic stent removal. The use of magnetic stents into the renal pelvis by way of a percutaneous ne- was initially reported in 1989, when Macaluso and phrostomy puncture. A subcutaneous tunnel is created from the flank to the bladder, and the distal tients. Retrieval of the Magnetip (ACMI/Surgitek) end of the stent is passed into the bladder through double-J stent was accomplished in 86% of cases using the Magnetriever (ACMI/Surgitek). Mag- Fistula stent sets specifically assist in the man- netic stents have also been used in an attempt to agement of ureteral fistulas. The stent has drainage eliminate the need for general anesthesia or cystos- ports solely in the pigtails to reduce fluid pressure copy in children. In 7 children, the stents were within the ureter. It is manufactured in several dif- removed with a magnetic-tip retriever without the need for cystoscopy, and only 1 patient requiredMore recently, an approach that uses GUIDEWIRES
a powerful rare-earth magnet on the retrieval cath- Accurate and atraumatic stent placement is eter and a stainless steel bead attached to the end of greatly enhanced by proper ureteral access using a the stent has been developed. The magnetic stent guidewire under fluoroscopic guidance. Guide- was removed without difficulty in 29 (97%) of 30 wires straighten the course to the ureter and secure patients, and no complications related to the place- the luminal path for passage of a stent, balloon dilator, or ureteroscope. Different types of guide- Injection stents are single-pigtail catheters that wires have been developed; each designed with convert into an indwelling double-pigtail ureteral unique structural properties to help accomplish stent, obviating a second cystoscopy. It is ideal for specific tasks. Guidewires are described by mate- patients undergoing shockwave lithotripsy who rial of composition, size (diameter and length), tip need a double-pigtail stent postoperatively, but si- design, surface coating, core (moveable or non- multaneously require contrast instillation during movable), and shaft rigidity. Guidewires may be the procedure for stone localization. Once the pro- composed of stainless steel or nitinol. Stainless cedure is completed, the injection apparatus is re- steel wires may be round or flat. Round wires have moved, leaving a double-pigtail stent.
a standard stiffness, and flat wires allow for a Urinary diversion stents have a single coil for the thicker, stiffer core without an increased diameter.
renal pelvis and a long straight end that is brought Nitinol-based wires are kink resistant and can be out for external drainage. The drainage holes are in designed with a stiffer core or a stripped core. Stan- the proximal end of the stent. They are commonly dard guidewires range in diameter from 0.018 to used to provide internal support to ureteral anas- 0.038 in., and available lengths vary from 145 to tomoses created during various types of urinary 180 cm. The distal 3 cm of guidewires are designed diversion. This allows for accurate monitoring of to be soft and flexible to minimize trauma to the urine output from each renal unit and facilitates ureteral wall during passage. Various tip designs removal of the stent after an adequate period of and shapes have been developed to facilitate pas- sage through tortuous ureteral anatomy or ureteral strictures. Tip configurations may be straight, an- used for temporary drainage from the ureteropel- gled, or J-shaped. Straight and angled guidewires vic junction to the bladder after incision of a stric- are also available with an 8-cm flexible tip. Shaft ture. These internal stents have a tapering diameter rigidity depends on the core materials. Guidewires with no sideports and are designed for postendo- may be coated with polytetrafluoroethylene pyelotomy procedures to prevent narrowing of the (PTFE, Teflon), which improves advancement of ureteral lumen while preventing ingrowth of the more rigid devices; a lubricious covering, used for ureteral wall to the stent. The larger caliber portion soft devices; or a hydrogel, which permits near fric- of the stent acts as a mold for the healing of the incised ureter. Applied Medical has a 7/10F endo-pyelotomy stent for use with the Acucise endopy- FUTURE DIRECTIONS
elotomy balloon catheter device. Cook Urologicalmakes a 6/10F and 7/14F stent, and Boston Scien- Advances in surface-coating technologies, along tific/Microvasive makes the RetroMax Plus in with the use of biodegradable materials will result in novel drug-delivery systems. By incorporating Subcutaneous urinary diversion (nephrovesical) various drugs and medications, such as growth fac- stents are used as an alternative to nephrostomy for tors, anti-inflammatory agents, and chemothera- urinary diversion in uremic cancer patients and peutic drugs, new therapeutic strategies for the patients with benign idiopathic ureteral obstruc- treatment of ureteral stricture disease and, possi- UROLOGY 64 (1), 2004
bly, genitourinary malignancy may be offered.
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VETERINARY RESEARCH INTERNATIONAL Journal homepage: www.jakraya.com/journal/vri Efficacy of Ivermectin – Clorsulon combination against mixed helminthic infection in goat – a case report Rajeev Ranjan Kumar1* and Stuti Vatsya2 Department of Veterinary Parasitology, College of Veterinary and Animal Sciences, G.B. Pant University of Agriculture and Technology, Pantnagar-263145 (Uttarakhand)