UvA - DARE ( Digital Academic Repository ) Endoscopic biliary drainage

T Background:Background: Clogging of biliary stents remains an important problem. In vitro studiess have shown less sludge formation in Teflon stents. Recently, clinical studies with Teflonn stents have produced contradictory results. The aim of this study was to investigatee whether the surface properties of the endoprostheses could explain the variationn observed in clinical studies. Methods:Methods: A total of 9 different types of unused roF endoprostheses were examined by scanningg electron microscopy {SEM}: polyethylene Amsterdam-type, polyurethane Amsterdam-type,, Teflon Amsterdam-type, Teflon Tannenbaum-type and a Tannenbaum-typee stent with a thin stainless steel mesh between inner and outer layers. Results:Results: All polyethylene stents had a relief with tiny lumps. All Teflon stents had multiplee shallow pits and ridges along the entire longitudinal axis. Both Tannenbaum-typee stents also had multiple particles protruding into the stent lumen withh adjacent holes in the wall of the stent. The polyurethane stent had an extremelyy smooth surface. Conclusion:Conclusion: SEM of Teflon made stents showed a markedly irregular inner surface, whichh may explain the controversial results of clinical studies. Our results indicate thatt the inner surface of a new stent should first be evaluated by SEM before clinical trialss are initiated. INTRODUCTION N Endoscopicc insertion of biliary stents is the standard method of palliation for pancreatobiliaryy malignancies. A major limitation is stent clogging. Different attempts to prolongg patency by changing stent size, stent design or altering bile composition havee been investigated, but stent clogging remains an unsolved clinical problem. In vitroo studies have shown that stents constructed of Teflon have a lower friction coefficientficient compared with stents made of other plastics and therefore have the maximumm potential for preventing stent blockage (1). In the in vivo studies it was shown thatt polyethylene stents with side holes accrued significandy more sludge than stentss of the same material without side holes (1). Soehendraa and colleagues designed the so-called Tannenbaum stent, a Teflon stent withoutt side holes, and they reported a substantially longer patency of these stentss (2,3). It is not known which factor, the Teflon material or the absence of side holes,, contributes most to this favorable result. Sung et al (4). showed in a randomizedd trial that polyethylene stents with and without side holes perform equally well. Recently,, we compared polyethylene and Teflon Amsterdam-type stents in a randomizedd trial and could not show any difference in patency (5). The results of these clinicall studies are contradictory. The combination of Teflon material and the absence of sidee holes results in superior patency rates, whereas omitting side holes in the designn of a polyethylene stent or the use of Teflon material in a conventional design doess not improve stent patency. In both studies, it was emphasized that the smooth1244 ENDOSCOPIC BILIARY DRAINAGE nesss of the stent surface is of great importance, either by omitting side holes or by usingg material with a low friction coefficient. We performed scanning electron microscopyy (SEM) of unused commercially available biliary endoprostheses, made off different materials and manufactured by different companies, to study the smoothnesss of the stent surface. In addition, we measured the inner and outer diameterss of the 10F stents. METHODS S AA total of 9 unused 10F endoprostheses were examined by SEM: polyethylene Amsterdam-typee (Olympus Optical Co., Ltd., Tokyo, Japan), (PBN Medicals, Stenloese,, Denmark), (Scandimed, Glostrup, Denmark), (Wilson Cook, WinstonSalem,, N.C.); polyurethane Amsterdam-type (Biosearch, Sommerville, N.J.); Teflon Amsterdam-typee (PBN Medicals); Teflon Tannenbaum-type (Wilson Cook) (Prof. Soehendra,, Hamburg, Germany); and a Tannenbaum-type stent (Olympus) with a thinn stainless steel mesh embedded between an inner Teflon coating and an outer polyamidee layer. SEMM of the inner surface of biliary stents was performed in the following manner. Stentt segments of about 2 cm in length were mounted on stubs, sputter coated with approximatelyy 15 nm of gold, and random areas were examined with a scanning electronn microscope (SEM 525; Philips, Eindhoven, The Netherlands). The elemental compositionn of the added x-ray contrast additives of the stents was determined with ann energy dispersive x-ray analysis (EDAX) 9900 system on a scanning electron microscopee (SEM 525; Philips). Thee scanning electron microscope was operated at 25 kV; however, in some stents imagingg of the surface was impossible at 25 kV because the x-ray contrast additives underr the stent surface were a much better source of secondary electrons than the stentt surface. To minimize this interference, imaging of the surface of these stents wass done at low accelerating voltage (5 kV); these low energy electrons do not reach thee additives because they are not able to penetrate into the stent material, contrary too the case with 25 kV electrons. Measurementt of internal and external stent diameter was performed by light microscopy. . RESULTS S Polyethylenee stents manufactured by Olympus, PBN Medicals, Scandimed and Wilsonn Cook were examined at operating voltages of 5 and 25 kV. At 5 kV all had a relativelyy smooth surface with tiny lumps protruding into the lumen (Fig. iA). Thee inset in Figure iA shows the stent image at 25 kV. At this voltage the image of thee stent surface is disturbed by the back scatter and secondary electrons generated byy the granular additives (contrast material) in the plastic material. Although most off the surface was relatively smooth, the polyethylene stents manufactured by SCANNINGG ELECTRON MICROSCOPY OF STENT MATERIALS 125 Scandimedd and PBN also had areas with barbs (Fig. iB). Small discrete particles protrudingg into the lumen and an occasional longitudinal shallow pit were seen in the polyethylenee stents manufactured by Olympus. Alll types of Teflon stents had multiple shallow pits and ridges along the entire longitudinall axis (Fig. iC and iD). Both Teflon Tannenbaum-type stents also had multiplee particles protruding into the stent lumen with adjacent holes in the wall of the stentt (Fig. iD). Thee stent with a stainless steel mesh and an inner Teflon coating had the same inner surfacee as the Teflon Tannenbaum stent with less explicit shallow pits and ridges alongg the longitudinal axis (Fig. iE). At low magnification a relief of the stainless steell mesh on the inner surface was visible (Fig. iF). The polyurethane stent had an extremelyy smooth surface, virtually free of irregularities (Fig. iG). Alll stents contained x-ray additives meant to augment visualization under fluoroscopy.. The nature of these additives was investigated by EDAX. In most cases it wass bismuth (4X), but barium (2x), tungsten and chromium (both ix) were also found.. The Wilson Cook Tannenbaum-type contained barium and bismuth. The naturee of the x-ray additives found in the different stents are listed in Table 1. Bismuthh and tungsten were mostly found in the form of fairly coarse particles. In contrast,, barium and chromium were found as much smaller particles homogeneouslyy distributed in the stent material. Depending on the roughness of the inner stentt surface and granule size, the bismuth granules in particular were sometimes foundd exposed at the lumen of the stent. Thee internal and external diameters of 10F biliary stents were determined via light microscopyy and are presented in Table 1. DISCUSSION N Duringg the past decade many randomized clinical trials have been performed with thee goal of optimization of stent design. Although the smoothness of the stent wall hass been considered to be an important factor for stent function (1,6,7) to date detailedd quality analysis of the commercially available stents used in the various trialss has not been performed. By using SEM we show in the present study that few stentss have surfaces with the desired perfect smoothness. Except one, all stents had variouss surface defects that are favored sites for microbial adherence. SEMM of stents made of Teflon demonstrated the most irregular inner surface of all stentss examined. It seems that the manufacturing process itself has an influence on thee surface smoothness, especially for Teflon stents. The shallow pits and ridges alongg the longitudinal axis are the result of the extrusion process used in manufacturingg these stents. The particles protruding into the stent lumen, which were only observedd in the Teflon Tannenbaum stent, were caused by the use of large granular additives.. Clearly, this significantly contributes to the roughness of the stent surface. Inn addition, it cannot be excluded that biliary components bind to the x-ray additives 1266 ENDOSCOPIC BILIARY DRAINAGE protrudingg in the lumen. Whether the release of these additives plays a role in cloggingg of the stents requires further investigation. The irregular surface could provide ann explanation for the discrepancies between in vitro performance and results of clinicall trials with Teflon stents. Only Binmoeller et al (2). reported superior performancee in an uncontrolled clinical trial of the Teflon Soehendra-type Tannenbaum stent.. Their results could, however, not be confirmed in a controlled clinical trial (8). Thee stent with a thin stainless steel mesh and an inner Teflon coating had an irregularr inner surface that was less explicit than the Teflon Tannenbaum stent. The differencee in surface smoothness between a Teflon stent and a Teflon coated stent may bee due to the fact that the latter stent is probably not manufactured by means of extrusion.. This stent is supposed to have a greater internal diameter compared with otherr polyethylene stents (9). This prompted us to determine the internal diameter off the stents investigated in this study. It turned out that this diameter varies; we couldd not confirm an increased internal diameter of the Teflon coated stent (Table 1). InIn conclusion, the surface smoothness of commercially available biliary endoprosthesess is highly variable. Additional damage to the stent surface may occur during thee mechanical process of placing the stent {10). Because the smoothness of the surfacee has been shown to influence stent performance considerably, we suggest evaluationn by SEM of stents that are to be evaluated in clinical trials. SCANNINGG ELECTRON MICROSCOPY OF STENT MATERIALS 127 Tablee l. Characteristics of 10F biliary stents.