OBJECTIVE. The purpose of this article is to compare the clinical effectiveness and complications. the treatment of benign urethral strictures.

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1 Vascular and Interventional Radiology Original Research Na et al. Stent Placement for Benign Urethral Strictures Vascular and Interventional Radiology Original Research Han Kyu Na 1 Ho-Young Song 1 Hyun Jin Yeo 1 Jung-Hoon Park 1 Jin Hyoung Kim 1 Hyungkeun Park 2 Choung-soo Kim 2 Na HK, Song HY, Yeo HJ, et al. Keywords: externally covered stent, urethral stent placement, urethral stricture DOI: /AJR Received February 23, 2011; accepted after revision May 18, This study was supported by the Korea Healthcare Technology R&D Project, Ministry of Health and Welfare, Republic of Korea (grant A102065). 1 Department of Radiology and Research Institute of Radiology, 388-1, Poongnap 2-dong, Songpa-gu, Seoul , Republic of Korea. Address correspondence to Ho-Young Song (hysong@amc.seoul.kr). 2 Department of Urology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Republic of Korea. WEB This is a Web exclusive article. AJR 2012; 198:W55 W X/12/1981 W55 American Roentgen Ray Society Retrospective Comparison of Internally and Externally Covered Retrievable Stent Placement for Patients With Benign Urethral Strictures Caused by Traumatic Injury OBJECTIVE. The purpose of this article is to compare the clinical effectiveness and complications of externally and internally covered stents for the treatment of benign urethral strictures. MATERIALS AND METHODS. From July 2002 to June 2010, 59 retrievable self-expandable polytetrafluoroethylene-covered nitinol stents were placed in 33 men with benign urethral strictures. These included 34 internally covered stents placed in 18 patients (group I) and 25 externally covered stents in 15 patients (group E). Stents were routinely removed 4 months after placement from patients who experienced no complications. Complications, duration of stent placement, removal techniques, and maintained patency rates were compared in the two groups. RESULTS. Twenty-eight complications (82.4%) occurred in 15 patients in group I, whereas 12 complications (48%) occurred in six patients in group E (p = 0.005). Three (16.7%) patients in group I and nine (60%) in group E had their stents electively removed without complications (p = 0.010). Stent migration was more frequent in group I (47.1%) than in group E (24.0%). No tissue ingrowth was detected in group E, whereas six cases of tissue ingrowth occurred in group I (p = 0.034). The standard technique was used more frequently in group E (70.8%). The median stent indwelling period was significantly shorter in group I than in group E (51.5 vs 114 days; p = 0.34). CONCLUSION. Despite their relatively high complication rates, externally covered stents are more effective with fewer complications than internally covered stents in the treatment of benign urethral strictures. S ince 1988, insertion of uncovered urethral stents as a treatment for urethral strictures has shown overall success rates of % within 18 months [1 4]. Over the long term, however, uncovered stents have shown reduced patency. For example, at 10 years, only two of 15 patients (13%) continued to show stent patency [5]. These disappointing results were primarily due to tissue ingrowth through the wire mesh of uncovered stents. To provide firmer resistance against tissue ingrowth, polyurethane-covered stents were introduced, which showed better preservation of luminal patency [6, 7]. However, degradation of polyurethane membrane began to take place. In a previous study [8], four of 22 (18%) polyurethanecovered stents showed membrane degradation or separation due to tissue ingrowth. This led to the development of stents covered internally or externally with polytetrafluoroethylene (PTFE) [8 12]. Although mem- brane degradation was not observed, three of 32 patients (9%) experienced separation between the wire mesh and PTFE layer of internally covered stents [13]. To overcome both membrane degradation and separation, stents externally covered with PTFE were applied in clinical use. To our knowledge, few studies have compared the effectiveness of internally and externally covered stents [14, 15]. We hypothesized that stents externally covered with PTFE could effectively prevent tissue ingrowth. The purpose of this study was to compare the clinical effectiveness and complications of externally and internally covered stents in the treatment of benign urethral strictures. Materials and Methods Patients and Study Design We retrospectively reviewed the collected records and images of patients who underwent stent placement for a benign urethral stricture refractory to endoscopic urethrostomy. The study protocol was W55

2 Na et al. approved by our institutional review board, and all patients provided written informed consent for urethral stent placement at enrollment. Between July 2002 and June 2010, 33 consecutive men, ranging in age from 24 to 77 years (mean age, 49.7 years), underwent urethral stent placement for benign urethral strictures (Table 1). All strictures were caused by trauma, including traffic accidents in seven patients, falls in 10, indwelling Foley catheters in 10, postsurgical trauma in five, and unknown trauma in one. Patients were divided into two groups. In group I, 34 retrievable self-expandable nitinol stents internally covered with PTFE were placed in 18 patients between July 2002 and October In group E, 25 retrievable self-expandable nitinol stents externally covered with PTFE were placed in 15 patients between December 2005 and June The timing of stent placement in the two groups differed because externally covered stents were introduced later to overcome the disadvantages of internally covered stents. We compared baseline characteristics in the two groups, including patient age, stricture site, and stricture length. Patients were considered for stent placement if they had benign urethral strictures successfully treated at least twice with endoscopic optimal urethrotomies, and partial or complete urethral occlusion resulting in dysuria. We excluded patients who were treated with both types of stents (externally and internally covered), had urethral strictures due to carcinoma regardless of complete recovery, and were treated with stents covered by materials other than PTFE. A Stent Construction Two types of stents were used in this study. Internally covered stents were placed in group I, and externally covered stents were used in group E. With the exception of the location of the covering material and their suturing method, the stents used in groups I and E were essentially the same (Fig. 1A). Stents and their retrieval sets have been described in detail elsewhere [7]. Briefly, each stent was woven from 0.1-mm-thick nitinol wires into a tubular configuration and was 10 mm in diameter and mm in length when fully expanded. To prevent tissue ingrowth, the stent was covered internally or externally with an expanded PTFE membrane. To make the stent retrievable, 2-mmdiameter nylon loops were hooked inside each bend in the distal end of the stent and another site, with nylon thread passing through each nylon loop to form larger loops (drawstrings), filling the circumference inside the distal stent. All stents were constructed as described by the manufacturer (Niti- S Urethral Stent, Taewoong). Methods used for adherence of the membrane and the wire mesh were different in both types of stents. A polyurethane solution was used for adherence of the lining inside the skeleton of each internally covered stent, whereas the linking of the externally covered stent was sutured at both ends [16]. These two stent types differed in conformability; because of the use of polyurethane solution as an adhesive, internally covered stents were reluctant to maintain in a bent state (Fig. 1B). Stent Placement and Removal Stent placement and removal were performed by one of two interventional radiologists (with 25 and 5 years experience in interventional radiology, respectively). Detailed procedures for stent placement and removal have been described elsewhere [10]. Before stent placement, retrograde urethrography was performed to evaluate the site, severity, and length of each stricture. Patients underwent oral prophylactic antibiotic therapy 1 day before the procedure and for at least 1 week afterward. After disinfection of the external urethral orifice with 0.05% chlorhexidine, the urethra was only topically anesthetized with 10 ml of lubricating jelly containing 2% lidocaine, without additional sedation. Retrograde urethrography was performed under fluoroscopic guidance, and the stricture sites were marked on the skin of the patient. A inch guidewire (Terumo Radifocus Guide Wire M, Terumo) was introduced through the urethral orifice across the stricture and into the urinary bladder. A sizing catheter (Song-Lim Biliary Catheter, S&G Biotech) was introduced over the guidewire into the proximal part of the stricture to measure the length of the stricture. After removing the sizing catheter, a 9-French sheath with a dilator was passed over the guidewire and was advanced until the proximal tip of the sheath reached 1 cm beyond the stricture. After removing the guidewire and the dilator, a PTFE-covered retrievable nitinol stent (diameter, 10 mm) was loaded into the sheath and was advanced to the stricture site. By slowly withdrawing the sheath with the pusher catheter held in Fig. 1 Stents internally and externally covered with polytetrafluoroethylene (PTFE). A, Retrievable self-expandable nitinol stents internally (top) and externally (bottom) covered with PTFE. PTFE membrane was sutured at both ends of externally covered stent (arrows), whereas polyurethane solution was used as adhesive in internally covered stent. B, Internally (top) and externally (bottom) covered stents 1 minute after bending into arched form. Internally covered stent returned immediately to its original shape, whereas externally covered stent showed better conformability to its bent state. C, End view of internally (top) and externally (bottom) covered stents with drawstrings attached to distal end. B C W56

3 Stent Placement for Benign Urethral Strictures place, the stent was freed and allowed to self-expand within the stricture. After stent placement, retrograde urethrography was performed to verify its position and patency. When the retrograde urethrography showed that the stent expanded less than a half of the preset expanded diameter of the stent, dilation of the placed stent was performed with the use of an 8-mm balloon catheter (Synergy ballooon catheter, Boston Scientific). Three techniques (standard, distal mesh, and eversion) were used for stent removal [6]. In the standard technique, the drawstring was grasped into the sheath, and the entire stent was collapsed into the sheath by withdrawing the hookwire. When the standard technique failed, the stent was removed in its expanded state, by either grasping the distal third of the stent mesh (distal mesh technique) or everting the stent (eversion technique). Stents were routinely removed 4 months after placement from patients who experienced no complications. Stents were prematurely removed from patients with complications, including migration, tissue ingrowth, and severe pain. Definition and Interpretation of the Results Clinical effectiveness was compared in groups I and E by evaluating technical success rate, clinical success rate, complications, reinterventions, stent indwelling period, removal techniques, and overall cumulative improvement rate after last stent removal. Technical success was defined as successful insertion of the stent in the proper position and good passage of contrast media through the stricture site. Clinical success was defined as urination without residual urine in the bladder 1 3 days after stent placement. Complications included migration, tissue ingrowth, formation of granulation tissue, and severe pain. A major complication was defined as any event that resulted in additional treatment, including an increased level of care; hospital stay beyond observation status, including readmission after initial discharge; or permanent adverse sequelae, including substantial morbidity or disability. All other complications were classified as minor [17]. Complete migration was defined as stent migration completely out of the stricture, whereas partial migration was defined as the stent remaining partially within the stricture. Tissue ingrowth was defined as luminal narrowing leading to urethral obstruction or detachment of the covering material from the nitinol wire mesh by tissue squeezing itself into the interstices of the metallic skeleton. Formation of granulation tissue was defined as a filling defect accompanied by luminal narrowing at the proximal or distal end of the inserted stent. Maintained patency was defined as the absence of recurrence of clinical dysuria after last stent removal and was evaluated by the Kaplan- Meier method [18]. When assessing the duration of the stent placement and removal techniques, we excluded one patient with an indwelling stent in the urethra at the time of evaluation. Follow-Up All patients underwent retrograde urethrography 1 3 days after stent placement to verify stent patency and location. Patients were thereafter advised to visit the outpatient clinic every 1 or 2 months until the stent was removed. Retrograde urethrography was performed 1, 3, and 12 months after stent removal to assess stricture recurrence. All patients were followed up by telephone every 6 months to obtain information regarding hematuria, incontinence, dribbling, pain, and infection until the time of this writing. Reinterventions such as additional stent placement or removal were performed when complications occurred. We also evaluated the stent indwelling period in the urethra and the maintained patency rate. Statistical Analysis The Mann-Whitney U test was used to compare pairs of independent continuous variables, whereas the Fisher exact test and the chi-square test were used to compare categoric variables. Kaplan-Meier analysis was used to evaluate the maintained patency rates after last stent removal in both groups. A two-sided p value less than 0.05 was considered statistically significant for all tests. All statistical analyses were performed using SPSS version 18.0 (SPSS). Results Patient Characteristics There were no significant differences between groups I and E in terms of mean patient age, stricture length, stricture location, or causes of benign strictures groups (Table 1). Stent Placement and Removal All stents were successfully placed without technical failure and were well tolerated in all patients. All patients were able to void immediately after stent placement, resulting in a clinical success of 100%. A total of 58 stents were removed from 33 patients, all showing technical success. In group I, 18 patients underwent placement of 34 stents, with all being successfully removed. In group E, 15 patients underwent placement of 25 stents, with 24 being removed. Twenty-two stents, 10 in group I and 12 in group E, were removed 4 months after stent placement. There was hyperplastic tissue between the PTFE membrane and the wire mesh in five patients in group I. The tissue pushed the membrane into the stent lumen, which made the stent removal difficult. They experienced 1 6 days of mild hematuria after stent removal, but bleeding stopped spontaneously without requiring blood transfusion. The techniques used for stent removal are summarized in Table 2. Although the overall rate of each technique did not differ significantly between the two groups, the standard technique was used more frequently in group E (70.8%). TABLE 1: Characteristics of the Patient Population Before Stent Placement Demographic Characteristic Group I Group E p No. of patients Age (y), mean ± SD 49.1 ± ± a Cause of trauma (no. of patients) b Traffic accident 4 3 Fall 8 2 Indwelling Foley catheter 4 6 Post surgery 1 4 Unknown 1 0 Stricture length (mm), mean ± SD 28.4 ± ± a Stricture location (no. of patients) b Membranous 5 5 Bulbous 10 9 Cavernous 3 1 Note All subjects were male. Patients in group I had internally covered stents, and patients in group E had externally covered stents. a Mann-Whitney U test. b Fisher exact test. W57

4 Na et al. TABLE 2: Techniques Used for Stent Removal Technique Group I (n = 34) Group E (n = 24) p Standard 20 (58.8) 17 (70.8) a Distal mesh 10 (29.4) 4 (16.7) a Eversion 3 (8.8) 3 (12.5) b Cystostomy site 1 (2.9) 0 (0.0) > b Note Except for p values, data are no. (%) of stents removed. Patients in group I had internally covered stents, and patients in group E had externally covered stents. One stent in group E was not removed at the time of evaluation. a Chi-square test. b Fisher exact test. TABLE 3: Complications and Reinterventions Complications and Reinterventions Complications during follow-up and their reinterventions are detailed in Table 3. Twenty-eight complications (82.4%) occurred in 15 patients in group I, whereas 12 complications (48%) occurred in six patients in group E (p = 0.005). Three patients in group I (16.7%) and nine in group E (60%) underwent elective stent removal (p = 0.010). Stent migration was more frequent in group I (n = 16 [47.1%]) than in group E (n = 6 [24.0%]), although the difference did not reach statistical significance (p = 0.070). In group I, 15 stents in six patients showed partial distal or proximal migration, whereas one patient showed a complete migration into the bladder. This completely migrated stent was removed through the suprapubic cystostomy site by introducing a snare. In group E, six stents in three patients partially migrated, with none showing complete migration. All stent migrations occurred within 2 months (mean, 23 days), except for one stent in group I. One patient in group E underwent a total of five stent placements because of continual migration. The first three stents, which were 10 mm in diameter, migrated within 6, 23, and 30 days, respectively. To prevent migration, a 12-mm-diameter stent was introduced, but it migrated within 14 days. Finally, a 14-mm-diameter stent was introduced; this stent has remained in his urethra for 142 days. Tissue ingrowth rates of the two groups differed significantly (p = 0.034), with none detected in group E and six (17.6%), three major and three minor, occurring in group I. Three of these stents showed extensive detachment of the PTFE layer from the nitinol wire mesh because of severe tissue ingrowth, resulting in occlusion (Fig. 2). Bleeding occurred during removal of all three stents. The three stents with minor tissue ingrowth showed luminal narrowing and voiding difficulty, but the patients maintained these stents for 4 months, resulting in routine removal. Five cases of granulation tissue formation occurred in each group (p = 0.729). Obstructions due to granulation tissue formation were managed by stent removal and Complications and Reinterventions Group I (n = 34) Group E (n = 25) p Complications Migration 16 (47.1) 6 (24.0) a Tissue ingrowth 6 (17.6) 0 (0.0) b Granulation tissue 5 (14.7) 5 (20.0) b Severe pain 1 (2.9) 1 (4.0) > b Total no. of complications 28 (82.4) 12 (48.0) a Reinterventions Stent removal 20 (58.8) 8 (32.0) a Additional stent placement 4 (11.8) 4 (16.0) b Total no. of reinterventions 24 (70.6) 12 (48.0) a Note Except for p values, data are no. (%) of stents. Patients in group I had internally covered stents, and patients in group E had externally covered stents. a Chi-square test. b Fisher exact test. insertion of an additional stent. One patient showed minor granulation tissue formation at the distal end of the stent but had no difficulty in voiding and showed spontaneous improvement within 1 month after stent removal (Fig. 3). All patients experienced mild pain for 1 3 days after the procedure, with one in each group complaining of severe pain. Despite 20 days of analgesic medication, the stents were removed for pain relief, with pain spontaneously resolving within 1 week. Follow-Up The mean duration of stent placement is summarized in Table 4. The median stent indwelling period was 51.5 days in group I and 114 days in group E (p = 0.34). Patients were followed up for a mean of 52 months (range, 3 99 months) after last stent removal. Nine of 33 patients, seven in group I and two in group E, experienced recurrences 1 5 months after removal of the last stent. The remaining 24 patients, 11 in group I and 13 in group E, no longer suffered from dysuria. Maintained patency rates (Fig. 4) at 1, 4, and 12 months were 83%, 61%, and 61%, respectively, in group I, and 93%, 93%, and 87%, respectively, in group E (p = 0.607, p = 0.046, and p = 0.134, respectively). The Kaplan-Meier plot appeared to level off 6 months after last stent removal, because no further recurrence occurred since that time. The overall cumulative improvement rate after last stent removal was higher in group E (87%) than in group I (61%). Discussion The results of this study substantiated the superiority of externally covered stents over internally covered stents for the treatment of urethral strictures. Externally covered stents showed greater resistance to tissue ingrowth, longer duration of stent placement, relatively simple stent removal, and lower stent migration rates than internally covered stents. Tissue ingrowth through the interstices between the wire filaments of uncovered stents has been found to result in stent occlusion or luminal narrowing [1 5]. Polyurethane-covered stents were developed to prevent tissue ingrowth and seemed to show better outcomes [7 9, 16]. Luminal narrowing or occlusion, however, was observed even when using covered stents [10, 11, 19 21], because the covering material either degraded or detached from the wire filaments [6, 22]. Better outcomes after 6 months were reported with un- W58

5 Stent Placement for Benign Urethral Strictures A D covered Wallstent devices (Boston Scientific) than with polyurethane-covered stents because of breaches in the polyurethane membranes [20]. Two types of stent were developed to prevent membrane degradation, those covered internally [8, 12, 13, 23] or externally [9, 16] with PTFE. However, tissue ingrowth due to detachment of the PTFE layer from the wire filaments was reported in the internally covered stents, although the PTFE membrane remained intact [13]. Indeed, we observed tissue ingrowth due to detachment of the PTFE layer from six internally covered stents but no instance of tissue ingrowth in any of the externally covered stents. Because the latter stents contained PTFE membranes externally wound onto the nitinol skeleton, these stents effectively prevented both membrane degradation and detachment. For ideal treatment, the stent should remain in the stricture until complete recovery but should be removed before invasion of granulation tissue at the end of the stent. We found that most externally covered stents could remain in the urethra for nearly 4 months, the optimal indwelling period [7, 8, 12], before complications occurred. In contrast, the median indwelling period of internally covered stents was 51.5 days, with many stents removed before 4 months because of complications. Premature removal before complete recovery has been associated with higher rates of recurrence [12]. Indeed, we found that the overall cumulative improvement rate (Fig. 4) was higher for externally than for internally covered stents (87% vs 61%). Difficulties in stent removal have been associated with tissue ingrowth [6, 8, 22, 24], resulting in more complicated removal procedures and longer operation times. We found that it was easier to remove externally covered stents than internally covered ones, with the former having fewer complications. The standard stent removal technique is safer than other methods because the former minimizes bleeding and irritation of the urethral wall. However, the standard technique was not feasible for most cases of tissue ingrowth, because of B Fig year-old man (group I, internally covered stent) with tissue ingrowth. A, Retrograde urethrogram shows urethral stricture just before stent placement. B, Radiograph 2 weeks after stent placement shows good contrast passage. C, Radiograph 6 weeks after stent placement shows severe tissue ingrowth (arrows) in middle of stent. D, Radiograph shows irregular urethral wall immediately after stent removal. E, Photograph of removed stent shows detachment of polytetrafluoroethylene membrane from wire mesh (arrowheads) at site of tissue ingrowth. disconnection of the hookwire or drawstring. Paradoxically, although the rate of tissue ingrowth was much higher in group I, the rate of use of the standard removal technique was not significantly different (p = 0.248) between group I (58.8%) and group E (70.8%). We surmise that this tendency resulted from removal difficulty due to granulation tissue formation. Untying of drawstrings also occurred because of the difficulty of detaching the stent from the urethral wall, which was caused by tissue hyperplasia proximal and distal to the stent [8]. Because neither stent type had incorporated any countermeasures against granulation tissue formation, granulation tissue occurred at similar rates in groups I (14.7%) and E (20.0%) (p = 0.729). Formation of granulation tissue, as well as tissue ingrowth, therefore reduced the rate of use of the standard removal technique. Further research is needed to verify the relationship between tissue ingrowth and difficulty in stent removal. Contrary to our expectation, we found that the migration rate of externally covered stents C E W59

6 Na et al. 1.0 Group I (24.0%) was lower than that of internally covered stents (47.1%). We cast doubt on this result, because tissue ingrowth causes stents to become jammed in the urethra, preventing their migration [25]. Several reports have compared migration rates of internally and externally covered stents [13, 15, 19, 25, 26]. For example, rapid migration of externally covered urethral stents within 2 3 months was observed in 13 A C Fig year-old man (group E, externally covered stent) with improvement of urethral stricture. A, Retrograde urethrogram shows focal tight occlusion of urethra just before stent placement. B, Radiograph 1 week after stent placement shows good contrast passage. C, Radiograph 4 months after stent placement shows mild formation of granulation tissue (arrows) distal to stent. D, Radiograph 2 weeks after stent removal shows improvement in stricture and spontaneous alleviation of granulation tissue. TABLE 4: Stent Indwelling Period Stent Indwelling Period (d) Group I Group E Mean SD Median Note p = for difference between groups (Mann-Whitney U test). Patients in group I had internally covered stents, and patients in group E had externally covered stents. of 16 (81.3%) patients as a result of the nonanchorage of the stents, suggesting that stent migration may be prevented by using internally covered stents [15]. Moreover, a newly designed covered stent, consisting of a PTFE layer between two bare stents, was found to provide appropriate anchorage and to prevent wrinkling of the covering material in the ureter [19] and esophagus [13]. These studies reported B D Patency Rate Censored Case Group E Time (months) Fig. 4 Kaplan-Meier plot showing overall cumulative improvement rate of groups I (internally covered stent) and E (externally covered stent) after last stent removal. These rates appeared to level off 6 months after stent removal. All recurrences occurred within 5 months. that stents with wire mesh external to the covering material provide better anchorage because of the direct contact between the wire mesh and the mucosa. In contrast to these reports, internally covered stents showed higher migration rate in the present study. This finding may be due to differences in stent conformability (Fig. 1B). Internally covered stents conform less to a bent configuration, with the polyurethane solution used as adhesive tending to cause these stents to return to their original shape. Thus, the stents used in group I could not be properly anchored to the urethral wall because of the curved structure of the urethra, leading to frequent migration. Because most migrations occurred within 2 months (mean, 23 days) after stent placement, and because tissue ingrowth requires at least 2 months, stent migration in these patients could not have been prevented by tissue ingrowth. Further research, strictly controlling for other factors, is required to verify the nonmigration of externally covered stents. Despite their advantages, the externally covered stents used in the present study still have several drawbacks. First, they cannot prevent granulation tissue from forming. Granulation tissue has been shown to form at the proximal or distal end of the stent, as the stent itself acted as a chronic stimulus to the urethra [6]. Application of an antiproliferating agent, such as paclitaxel-eluting stent [27] or IN-1233 [28], may prevent the formation of granulation tissue. Second, the migration rate of externally covered stents was still too high (24%). Antimigration measures are essential for better clinical efficacy. For example, stents with anchorage pins have been associated with lower migration rates [16, 23]. W60

7 Stent Placement for Benign Urethral Strictures This study had several limitations. First, the mean follow-up period after last stent removal differed significantly between groups I and E (84 vs 15 months). However, most recurrences occurred within the first 5 months after last stent removal, with no further recurrences after that period, suggesting that the difference in follow-up period did not act as a serious bias. Second, stent placement techniques in group I and group E were performed consecutively. Improved outcomes in group E may result from greater experience with the procedure. In summary, retrievable self-expandable nitinol stents externally covered with PTFE membrane are clinically more effective than internally covered stents in the treatment of benign urethral strictures. Externally covered stents showed better patency with fewer complications, yet complication rates in both stent types were relatively high. References 1. Milroy EJ, Chapple CR, Cooper JE, et al. A new treatment for urethral strictures. Lancet 1988; 331: Milroy EJ, Chapple CR, Eldin A, Wallsten H. A new stent for the treatment of urethral strictures: preliminary report. Br J Urol 1989; 63: Yachia D, Beyar M. New treatment modality for penile urethral strictures using a self-expanding and self-retaining coil stent: UroCoil follow-up of 16 months after removal of the stent. Eur Urol 1993; 24: Hussain M, Greenwell TJ, Shah J, Mundy A. Long-term results of a self-expanding Wallstent in the treatment of urethral stricture. BJU Int 2004; 94: De Vocht TF, van Venrooij GE, Boon TA. Selfexpanding stent insertion for urethral strictures: a 10-year follow-up. BJU Int 2003; 91: Ko GY, Kim GC, Seo TS, et al. Covered, retrievable, expandable urethral nitinol stent: feasibility study in dogs. Radiology 2002; 223: Song HY, Park H, Suh TS, et al. Recurrent traumatic urethral strictures near the external sphincter: treatment with a covered, retrievable, expandable nitinol stent initial results. Radiology 2003; 226: Shin JH, Song HY, Park H, et al. Removal of retrievable self-expandable urethral stents: experience in 58 stents. Eur Radiol 2006; 16: Ishii S, Sato M, Sonomura T, et al. Optimal covering material for stent-grafts placed in the portal vein in a canine model. Cardiovasc Intervent Radiol 2005; 28: Kim JH, Song HY, Shin JH, Jung HY, Kim SB, Park SI. Membrane degradation of covered stents in the upper gastrointestinal tract: frequency and clinical significance. J Vasc Interv Radiol 2008; 19: Dillon JG, Hughes MK. Degradation of five polyurethane gastric bubbles following in vivo use: SEC, ATR-IR and DSC studies. Biomaterials 1992; 13: Choi EK, Song HY, Shin JH, Lim JO, Park H, Kim CS. Management of recurrent urethral strictures with covered retrievable expandable nitinol stents: long-term results. AJR 2007; 189: Park JG, Jung GS, Oh KS, Park SJ. Double-layered PTFE-covered nitinol stents: experience in 32 patients with malignant esophageal strictures. Cardiovasc Intervent Radiol 2010; 33: Liatsikos EN, Siablis D, Kalogeropoulou C, et al. Coated v noncoated ureteral metal stents: an experimental model. J Endourol 2001; 15: Barbalias GA, Liatsikos EN, Kalogeropoulou C, et al. Externally coated ureteral metallic stents: an unfavorable clinical experience. Eur Urol 2002; 42: Yoon CJ, Song HY, Kim JH, et al. Temporary placement of a covered, retrievable, barbed stent for the treatment of hormone-induced benign prostatic hyperplasia: technical feasibility and histologic changes in canine prostates. J Vasc Interv Radiol 2010; 21: Martin LG, Rundback JH, Sacks D, et al. Quality improvement guidelines for angiography, angioplasty, and stent placement in the diagnosis and treatment of renal artery stenosis in adults. J Vasc Interv Radiol 2003; 14:S297 S Kim JH, Song HY, Choi EK, Kim KR, Shin JH, Lim JO. Temporary metallic stent placement in the treatment of refractory benign esophageal strictures: results and factors associated with outcome in 55 patients. Eur Radiol 2009; 19: Chung HH, Lee SH, Cho SB, et al. Comparison of a new polytetrafluoroethylene-covered metallic stent to a noncovered stent in canine ureters. Cardiovasc Intervent Radiol 2008; 31: Rossi P, Bezzi M, Salvatori FM, Panzetti C, Rossi M, Pavia G. Clinical experience with covered wallstents for biliary malignancies: 23-month follow-up. Cardiovasc Intervent Radiol 1997; 20: Wright KC, Dobben RL, Magal C, Ogawa K, Wallace S, Gianturco C. Occlusive effect of metallic stents on canine ureters. Cardiovasc Intervent Radiol 1993; 16: Kim JH, Shin JH, Shim TS, et al. Efficacy and safety of a retrieval hook for removal of retrievable expandable tracheobronchial stents. J Vasc Interv Radiol 2004; 15: Bezzi M, Zolovkins A, Cantisani V, et al. New eptfe/fep-covered stent in the palliative treatment of malignant biliary obstruction. J Vasc Interv Radiol 2002; 13: Yoon CJ, Shin JH, Song HY, Lim JO, Yoon HK, Sung KB. Removal of retrievable esophageal and gastrointestinal stents: experience in 113 patients. AJR 2004; 183: Song HY, Lee DH, Seo TS, et al. Retrievable covered nitinol stents: experiences in 108 patients with malignant esophageal strictures. J Vasc Interv Radiol 2002; 13: Trueba Arguiñarena FJ, Fernández del Busto E. Self-expanding polytetrafluoroethylene covered nitinol stents for the treatment of ureteral stenosis: preliminary report. J Urol 2004; 172: Shin JH, Song HY, Choi CG, et al. Tissue hyperplasia: influence of a paclitaxel-eluting covered stent preliminary study in a canine urethral model. Radiology 2005; 234: Kim JH, Song HY, Park JH, Yoon HJ, Park HG, Kim DK. IN-1233, an ALK-5 inhibitor: prevention of granulation tissue formation after bare metallic stent placement in a rat urethral model. Radiology 2010; 255:75 82 W61