|}
Today, elastomeric materials, such as silicones, polyurethanes andhydrogel-coated polyolefins are used, with no clear winner, which canwithstand the urinary environment (Tunney et al., 1996). * Althoughsilicone has better long-term stability than other stent materials, itsextreme flexibility makes it difficult to pass over guidewires and throughnarrow or tortuous ureters. * Polyethylene is stiffer and easier to use forpatients with strictures; however, it has been known to become brittle withtime leading to breakage and is no longer commercially available.* Polyurethane has properties that fall in between polyethylene and silicone;however, stent fracture also has been an issue with polyurethanes. Attempts have been made to develop polymers with a combination of thebest of all properties. The key players are C-Flex (Concept PolymerTechnologies), Silitek and Percuflex (Boston Scientific) (Roemer, 2000).* C-Flex is proprietary silicone oil and mineral oil interpenetrated into astyrenelolefin block copolymer with the hope of reduced encrustation.* Silitek (Medical Engineering Corporation) is another silicone-basedcopolymer. * Percuflex is a proprietary olefinic block copolymer (Denstedt. et al., 1998). Metallic stents have been used recently to treat extrinsicureteric obstructions (Reinberg et al., 1994; Pauer et al., 1992).The effect of synthetic polymers on the urothelium of the urinarytract seems to be dependent on the bulk chemical composition of thepolymer, the chemical composition of its surface, coatings on the devicesurface, smoothness of the surface and coefficient of friction (Denstedt et.al., 1998).Typically, most ureteral stents are made of relatively smoothcatheters. [http://www.ncbi.nlm.nih.gov/pubmed/10772512 Koleski et al., (2000) ] tested a longitudinally grooved ureteralstent made by Circon in the pig ureter. The results indicated that thegrooved stent led to better drainage than a conventional stent. Theiropinion is that the ureter wall has a better chance of collapsing over asmooth surface than a grooved surface, especially when debris is present. Stoller (2000) had the same experience with the SpiraStent(Urosurge Corp.). This helical stent was superior at passing stones than a conventional smooth stent. There are a variety of ureteral stent configurations with different anchoring systems. Most stents today have a double [http://linkinghub.elsevier.com/retrieve/pii/S014067360002674X pigtail anchoring system]. (Tolley, 2000), Dunn et al, (2000) conducted a randomized, single-blind study comparing a Tail stent (proximal pigtail with a shaft which tapers to a lumenless straight tail) to a double pigtail stent. The Tail stent was found to be better tolerated than the double-pigtail concerninglower urinary tract irritative symptoms. A double-J ureteral stent and a flexible ureteropyeloscope are shown in the first diagram. The other two diagrams show a pigtail ureteral stent in place; the end of the pigtail is facing away fiom the ureteral opening in the second of these two diagrams. [[Image:Ureteral stent.jpg|thumb|right|400px]]