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Covered, Retrievable, Expandable Urethral Nitinol Stent: Feasibility Study in Dogs¹

¹ From the Departments of Radiology (G.Y.K., G.C.K., T.S.S., H.Y.S.) and Biomedical Engineering (T.H.K., J.O.L., J.H.L.), Asan Medical Center, University of Ulsan College of Medicine, 388-1, Poongnap-Dong, Songpa-Ku, Seoul 138-736, Korea. From the 2000 RSNA scientific assembly. Received April 5, 2001; revision requested May 11; revision received August 10; accepted September 7. Supported by grant HMP-98-G-2-043 from the Highly Advanced National Project, Ministry of Health and Welfare, Republic of Korea. Address correspondence to H.Y.S. (e-mail: hysong@www.amc.seoul.kr).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To evaluate the feasibility of using a retrievable urethral stent and to evaluate whether granulation tissue resolves after stent removal in a canine urethra.

MATERIALS AND METHODS: Polyurethane-covered retrievable 0.10-mm-thick (n = 11) or 0.15-mm-thick (n = 10) nitinol wire stents were placed in the urethras of 20 dogs. In one dog, a second stent was placed in the urethra because of complete migration of the first stent into the urinary bladder. The stents were removed with retrieval hook wires 4 weeks (n = 10) and 8 weeks (n = 10) after placement. Fourteen dogs were sacrificed just after stent removal, and the other six dogs were sacrificed 2 weeks after stent removal. Information concerning procedure success, stent migration, and tissue response was obtained.

RESULTS: Stent placement was technically successful in all dogs. Follow-up urethrograms showed partial (n = 4) or complete (n = 1) stent migration. Stent removal failed in two dogs due to partial or complete migration. Granulation tissue was observed at both ends of the stent in 17 dogs. Urethrograms and urethral specimens obtained 2 weeks after stent removal showed diminished granulation tissue and decreased thickness of the papillary projections of the epithelium compared with results obtained immediately after stent removal.

CONCLUSION: Although some design modifications are necessary to reduce current complications, the polyurethane-covered retrievable nitinol stent seems feasible for use in the urethra. Stent-induced granulation tissue formation improved after stent removal.

© RSNA, 2002

Index terms: Animals • Interventional procedures, experimental studies, 841.1299 • Stents and prostheses, 841.1299 • Urethra, interventional procedures, 841.1299

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Self-expanding metallic urethral stents have been used with much success since the late 1980s for alleviating obstructive symptoms caused by urethral stricture (1–6). However, formation of granulation tissue is inevitable after stent placement, and this formation contributes to further complications, such as restenosis or blockage (3,5–10). In addition, complications, such as stent migration, fracture of the metallic mesh, and tumor ingrowth, occurred in some cases during and after the procedure (5,7–10). Repeated interventions, such as balloon dilation, stent replacement, stent removal, and resection of granulation tissue, were necessary for maintenance of stent patency in these cases.

Similar complications also occurred in the esophagus and trachea after stent placement (11,12). Some of these complications, such as delayed migration, fracture of the stent, and formation of granulation tissue, could be avoided by using a retrievable stent in cases of esophageal and tracheal strictures (13,14). However, only a few types of retrievable urethral stents have been introduced, and the resolution of granulation tissue after stent removal has not been sufficiently documented in the literature.

The purpose of our study was to determine the feasibility of using a retrievable urethral stent and to evaluate whether granulation tissue resolves after stent removal in the canine urethra.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
All experimental procedures were performed in accordance with the National Institutes of Health guidelines for humane handling of animals and were approved by the Committee of Animal Research at our institution.

Stent Construction
Constructed in our research laboratory, the stent was woven 16 times from a single thread of 0.10- or 0.15-mm-thick nitinol wire filament into a tubular configuration (Fig 1). The stent has 16 bent points on the upper and lower end portions. The wire filament is crossed spirally in an unengaged manner. One segment of the wire filament runs alternately on and under other segments, which are arranged in the direction opposite to that of the segment, so that a number of meshes (openings) are formed by the crossing points of the segments. To prevent mucosal hyperplasia through the stent wires, the stent was covered by being dipped in 12% polyurethane solution (ChronoFlex AR; Cardiotech International, Woburn, Mass).

View larger version (128K): [in this window] [in a new window] [Download PPT slide]   Figure 1. At top, polyurethane-covered retrievable urethral stent. In the middle, 9-F braided sheath. At bottom, hook wire.

Stent Introducer Set and Retrieval Set
A urethral stent introducer set includes a 9-F braided sheath (Cook, Bloomington, Ind), a dilator, and a pusher catheter. The pusher catheter was made of a 9-F dilator (Cook), of which the distal tapered segment was cut off. The stent retrieval set included a 9-F braided sheath, a hook wire, and a 0.035-inch guide wire (Terumo, Tokyo, Japan). The hook wire was constructed in our research laboratory with a nitinol wire. The end of the hook wire was constructed in a question mark configuration to facilitate hooking of the drawstring of the stent (Fig 1). The distal 20-mm section of the question mark portion was positioned at an angle of about 300° to the axis. An additional bend was made in this section with pliers so that the hook would not catch the end of the sheath when the hook was withdrawn.

Stent Placement
Stent placement and removal were performed by three individuals (G.Y.K., T.S.S., H.Y.S.). A 0.10-mm-thick (n = 11) or 0.15-mm-thick (n = 10) nitinol wire stent was placed in the membranous urethra in 20 male mongrel dogs (Yangsung, Yong-In, Korea) with a body weight of 16–25 kg (Fig 2). In one dog, a second stent was placed in the membranous urethra 2 weeks after the first stent placement because of complete migration of the first stent into the urinary bladder. Anesthesia was induced in each dog with intramuscularly administered ketamine hydrochloride and was maintained with intravenously administered pentobarbital sodium. After disinfection of the external urethral orifice with 0.05% chlorhexidin hydrochloride, the urethra was lubricated with 2% lidocaine jelly, and a retrograde urethrogram was obtained to determine the position of the external urethral sphincter.

View larger version (91K): [in this window] [in a new window] [Download PPT slide]   Figure 2a. Images show the technical steps in placement of urethral stent. (a) Initial right anterior oblique retrograde urethrogram. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a stent introducer set (arrowheads) into the urethra over the guide wire. (d) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the sheath over the pusher catheter. (e) Right anterior oblique image obtained with fluoroscopic guidance shows expansion of the stent (arrowheads) in the urethra.

View larger version (95K): [in this window] [in a new window] [Download PPT slide]   Figure 2b. Images show the technical steps in placement of urethral stent. (a) Initial right anterior oblique retrograde urethrogram. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a stent introducer set (arrowheads) into the urethra over the guide wire. (d) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the sheath over the pusher catheter. (e) Right anterior oblique image obtained with fluoroscopic guidance shows expansion of the stent (arrowheads) in the urethra.

View larger version (98K): [in this window] [in a new window] [Download PPT slide]   Figure 2c. Images show the technical steps in placement of urethral stent. (a) Initial right anterior oblique retrograde urethrogram. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a stent introducer set (arrowheads) into the urethra over the guide wire. (d) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the sheath over the pusher catheter. (e) Right anterior oblique image obtained with fluoroscopic guidance shows expansion of the stent (arrowheads) in the urethra.

View larger version (93K): [in this window] [in a new window] [Download PPT slide]   Figure 2d. Images show the technical steps in placement of urethral stent. (a) Initial right anterior oblique retrograde urethrogram. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a stent introducer set (arrowheads) into the urethra over the guide wire. (d) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the sheath over the pusher catheter. (e) Right anterior oblique image obtained with fluoroscopic guidance shows expansion of the stent (arrowheads) in the urethra.

View larger version (90K): [in this window] [in a new window] [Download PPT slide]   Figure 2e. Images show the technical steps in placement of urethral stent. (a) Initial right anterior oblique retrograde urethrogram. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a stent introducer set (arrowheads) into the urethra over the guide wire. (d) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the sheath over the pusher catheter. (e) Right anterior oblique image obtained with fluoroscopic guidance shows expansion of the stent (arrowheads) in the urethra.

We defined the technical success of stent placement as adequate placement of a stent in the membranous urethra with good passage of contrast media through the stent lumen.

Stent Removal Technique
The stents were removed with a retrieval hook wire at 4 weeks (n = 10) and 8 weeks (n = 10) after placement (Fig 3). After the same anesthetic was administered as during stent placement, a guide wire was introduced through the urethra and then across the stent into the urinary bladder. A sheath with a dilator was passed down over the guide wire into the distal stent lumen. After the dilator was removed from the sheath, a hook wire was introduced into the sheath and was advanced until the hook wire was passed through the sheath into the stent lumen. Then the sheath with the hook wire was pulled out through the stent so that the hook wire grasped the drawstring. When this was accomplished, the hook wire was withdrawn through the sheath to collapse the distal stent when it reached the sheath tip. The sheath, the hook wire, and the stent were then pulled out of the urethra.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. Images show the technical steps in removal of the urethral stent. (a) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a sheath (arrowheads) with a dilator. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a hook wire (arrow) into the sheath. (d) Right anterior oblique image obtained with fluoroscopic guidance shows grasping of the drawstring with the hook wire. (e) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the hook wire and the stent through the sheath.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. Images show the technical steps in removal of the urethral stent. (a) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a sheath (arrowheads) with a dilator. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a hook wire (arrow) into the sheath. (d) Right anterior oblique image obtained with fluoroscopic guidance shows grasping of the drawstring with the hook wire. (e) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the hook wire and the stent through the sheath.

View larger version (96K): [in this window] [in a new window] [Download PPT slide]   Figure 3c. Images show the technical steps in removal of the urethral stent. (a) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a sheath (arrowheads) with a dilator. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a hook wire (arrow) into the sheath. (d) Right anterior oblique image obtained with fluoroscopic guidance shows grasping of the drawstring with the hook wire. (e) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the hook wire and the stent through the sheath.

View larger version (103K): [in this window] [in a new window] [Download PPT slide]   Figure 3d. Images show the technical steps in removal of the urethral stent. (a) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a sheath (arrowheads) with a dilator. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a hook wire (arrow) into the sheath. (d) Right anterior oblique image obtained with fluoroscopic guidance shows grasping of the drawstring with the hook wire. (e) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the hook wire and the stent through the sheath.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 3e. Images show the technical steps in removal of the urethral stent. (a) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a guide wire (arrowheads) into the urethra. (b) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a sheath (arrowheads) with a dilator. (c) Right anterior oblique image obtained with fluoroscopic guidance shows insertion of a hook wire (arrow) into the sheath. (d) Right anterior oblique image obtained with fluoroscopic guidance shows grasping of the drawstring with the hook wire. (e) Right anterior oblique image obtained with fluoroscopic guidance shows withdrawal of the hook wire and the stent through the sheath.

Follow-up Examination
Retrograde urethrography was performed immediately after stent placement as well as before and after stent removal to verify the position of the stent and the patency of the urethra. In the first four dogs, urethroscopy was performed by a urologist 2 weeks after stent placement. Fourteen dogs were sacrificed just after stent removal. The other six dogs (three with 0.10-mm-thick stents and three with 0.15-mm-thick stents) were sacrificed 2 weeks after stent removal to determine whether the granulation tissue that formed as a result of placement of the stent would resolve after stent removal. A urethrogram was also obtained just before each of the dogs was sacrificed.

Information concerning urinary outlet obstruction and gross hematuria was obtained by means of daily examination of dogs after urination. The stent diameter was measured on the follow-up retrograde urethrogram and compared with its diameter immediately after placement. Measurement of the stent diameter was based on the consensus of two observers (G.C.K., T.H.K.). On the urethrogram, grading of irregular filling defects caused by granulation tissue formation was based on the consensus of three observers (G.Y.K., G.C.K., T.H.K.). Grade 0 was identified when the filling defect was within normal range. Grade 1 was identified when the filling defect was mild urethral wall irregularity. Grade 2 was identified when the filling defect was smaller than half of the stent diameter. Grade 3 was identified when the filling defect was larger than half of the stent diameter.

Animals were sacrificed with an overdose of pentobarbital sodium. The urethras were excised, incised longitudinally, and sectioned to allow both gross and microscopic examinations. At histologic examination, the number of epithelial layers, the thickness of the submucosal fibrosis, the thickness of the papillary projection of the uroepithelium, and the degree of inflammatory cell infiltration were evaluated by a pathologist.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Stent Placement and Removal
Stent placement was technically successful and well tolerated in all dogs; no difficulties were experienced during the insertions.

Stent removal from the urethra was technically successful in all dogs 4 weeks after placement and in nine of 10 dogs 8 weeks after placement. In one dog, stent removal failed because the stent was pointing nearly perpendicularly into the wall of the urethra at the curved area of the bulbous urethra owing to its partial downward migration. Stent removal was technically difficult but successful in two dogs in which the hook wires broke at the bending point when they were being pulled out through the stents. The stents were then removed with another hook wire.

Thirteen of 19 removed stents were stained with blood, but only one dog bled after stent removal. Gross hematuria occurred in this dog after stent removal; however, it ceased spontaneously the next day. The remaining six removed stents showed no grossly abnormal findings.

Findings during Follow-up
No urinary outlet obstruction or gross hematuria occurred during the follow-up period. Urethroscopy performed 2 weeks after stent placement in four dogs showed the lumen with the stent remaining patent in three of four dogs. In one dog, complete migration of the stent into the urinary bladder occurred. Stent removal from the urinary bladder failed because the drawstring could not be grasped with the retrieval hook wire. The dog was sacrificed 4 weeks after placement of the second stent. At autopsy, the urinary bladder and the stent showed no clinically important abnormal findings.

The urethrographic findings after stent placement are summarized in Table 1. The stents reached 63% (mean, 6.3 mm ± 0.4 [SD]) of their fully expanded diameter immediately after placement and 90% (9.0 mm ± 0.3) of their fully expanded diameter before stent removal. Follow-up urethrograms showed partial (<10 mm) downward or upward stent migration in four dogs. Irregular filling defects induced by tissue response at the proximal or distal end of the stent were observed in 17 of 20 dogs (Fig 4). Focal disruption of the polyurethane membrane was detected at the removed stent in one dog.

View larger version (107K): [in this window] [in a new window] [Download PPT slide]   Figure 4a. Urethrographic findings after stent placement and removal. (a) Right anterior oblique urethrogram obtained just after stent placement shows an expanded stent (arrowheads) and the normal urethra (arrows). (b) Right anterior oblique urethrogram obtained 8 weeks after stent placement shows grade 2 filling defects (arrowheads) at the proximal and distal ends of the stent. (c) Right anterior oblique urethrogram obtained just after stent removal shows the remaining filling defects (arrowheads). (d) Right anterior oblique urethrogram obtained 2 weeks after stent removal shows a slightly widened segment (arrowheads) of the urethra in which the stent was placed. Note that the filling defects disappeared.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 4b. Urethrographic findings after stent placement and removal. (a) Right anterior oblique urethrogram obtained just after stent placement shows an expanded stent (arrowheads) and the normal urethra (arrows). (b) Right anterior oblique urethrogram obtained 8 weeks after stent placement shows grade 2 filling defects (arrowheads) at the proximal and distal ends of the stent. (c) Right anterior oblique urethrogram obtained just after stent removal shows the remaining filling defects (arrowheads). (d) Right anterior oblique urethrogram obtained 2 weeks after stent removal shows a slightly widened segment (arrowheads) of the urethra in which the stent was placed. Note that the filling defects disappeared.

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 4c. Urethrographic findings after stent placement and removal. (a) Right anterior oblique urethrogram obtained just after stent placement shows an expanded stent (arrowheads) and the normal urethra (arrows). (b) Right anterior oblique urethrogram obtained 8 weeks after stent placement shows grade 2 filling defects (arrowheads) at the proximal and distal ends of the stent. (c) Right anterior oblique urethrogram obtained just after stent removal shows the remaining filling defects (arrowheads). (d) Right anterior oblique urethrogram obtained 2 weeks after stent removal shows a slightly widened segment (arrowheads) of the urethra in which the stent was placed. Note that the filling defects disappeared.

View larger version (108K): [in this window] [in a new window] [Download PPT slide]   Figure 4d. Urethrographic findings after stent placement and removal. (a) Right anterior oblique urethrogram obtained just after stent placement shows an expanded stent (arrowheads) and the normal urethra (arrows). (b) Right anterior oblique urethrogram obtained 8 weeks after stent placement shows grade 2 filling defects (arrowheads) at the proximal and distal ends of the stent. (c) Right anterior oblique urethrogram obtained just after stent removal shows the remaining filling defects (arrowheads). (d) Right anterior oblique urethrogram obtained 2 weeks after stent removal shows a slightly widened segment (arrowheads) of the urethra in which the stent was placed. Note that the filling defects disappeared.

The histologic findings after stent removal are summarized in Table 2. The excised urethral specimens from most dogs showed a tissue response of varying degrees to chronic irritation at the proximal or distal end of the urethras in which stents were placed (Fig 5). In two dogs, erosion was also noted at the proximal and distal ends of the urethra in which a stent was placed. Excised urethral specimens showed the same histologic findings 2 weeks after stent removal as the excised urethras showed immediately after stent removal. However, histologic findings in the urethras examined 2 weeks after stent removal revealed decreased thickness of the papillary projections of the epithelium and diminished inflammatory cell infiltration compared with findings in urethras examined immediately after stent removal.

View larger version (104K): [in this window] [in a new window] [Download PPT slide]   Figure 5a. Gross and microscopic findings. (a) Gross specimen of the urethra incised longitudinally shows granulation tissue (arrowhead) at the proximal end of the urethra in which the stent was placed and some petechial hemorrhagic spots (arrows). (b) Photomicrograph of a specimen from the distal end of a urethra in which the stent was placed shows thickening (arrowheads) of the urethral epithelium. (Hematoxylin-eosin stain; original magnification, x100.)

View larger version (138K): [in this window] [in a new window] [Download PPT slide]   Figure 5b. Gross and microscopic findings. (a) Gross specimen of the urethra incised longitudinally shows granulation tissue (arrowhead) at the proximal end of the urethra in which the stent was placed and some petechial hemorrhagic spots (arrows). (b) Photomicrograph of a specimen from the distal end of a urethra in which the stent was placed shows thickening (arrowheads) of the urethral epithelium. (Hematoxylin-eosin stain; original magnification, x100.)

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

First, there is little chance of tissue proliferation into the stent lumen because a polyurethane membrane covers the stent. Second, fracture of the stent is unlikely because the stent is woven from a single thread of nitinol wire. Third, the risk of urethral injury from such a stent is much lower than it is with an uncovered stent, and the implantation is better tolerated and safer than it is with an uncovered stent, because the polyurethane-covered nitinol stent is inserted in its compressed state by using a delivery system with a smaller diameter. Fourth, the stent can be easily relocated if it is placed incorrectly, and the stent can be easily removed if a complication occurs. Last, migration of this stent appears to be less frequent because it exerts a radial force on the wall of the urethra.

Since Milroy et al (1) in 1988 reported their results in patients successfully treated with self-expanding metallic urethral stents, other stents have been designed and used (6,17–23). Most of these stents are metal, uncovered, permanently implanted, and intended to epithelialize and become completely incorporated into the urethra. However, the permanent stent remains in the urethral wall and keeps the lumen permanently open; consequently, uncontrolled dribbling occurs after voiding (1,3).

This permanent stent also may induce mucosal hyperplasia through the openings between the wire filaments of the stent and may obstruct the lumen of the stent; as a result, repeat endoscopic resection, stent replacement, or even surgical removal may be required (5–9,24). To resolve these problems, several types of temporarily placed or bioabsorbable urethral stents were introduced (4,15,17,18,20,25). Some of them are removed after an appropriate period; however, some need not be removed because of their complete degradation within 12 months. However, use of such stents has not completely overcome these problems, and they require relatively large-sized introducer systems.

To analyze the differences in the formation of granulation tissue, depending on the thickness of the wires and periods of stent placement, we applied two types of wire thickness and two stent placement periods. There were varying degrees of tissue responses to chronic irritation at the proximal or distal end of the urethra in which the stent was placed. Similar alterations were confirmed in clinical series with other stents (2,5,8,23,26). Histologic changes after stent placement in the urethra were as follows in our study: increased number of epithelial layers; increased thickness of submucosal layer and development of submucosal fibrosis; papillary projection of urothelium; and inflammatory cell infiltration.

Although statistical analysis was not possible because of the small number of animals in each group, the histologic changes were the same in the 0.10- and 0.15-mm-thick wire groups and in the 4- and 8-week stent placement duration groups. Further investigation is needed to correlate wire thickness, duration of stent placement, and tissue response. In addition, acute angulation of the stent into the urethra in one dog induced a more severe tissue response. Thus, accurate placement of the stent is very important, since encroachment on the urethral wall results in a severe tissue response.

In our study, on urethrograms obtained 2 weeks after stent removal, most urethral luminal irregularities that were depicted immediately after stent removal returned to normal or improved. In addition, histologic findings obtained 2 weeks after stent removal revealed regression of the papillary projection and infiltration of inflammatory cells. Yachia and Beyar (4) documented that the urethral lining returned to normal at urethroscopy 3 months after stent removal.

We postulate that the urethrographic findings immediately after stent removal might be overestimated owing to acute inflammation, edema, and sludge. Thus, the filling defects were diminished after the inflammation and edema subsided during follow-up. In addition, we postulated that the tissue responses caused by chronic irritation returned to normal after stent removal, although 2 weeks might be considered too short a period for the underlying tissue changes to return to normal. However, there may be differences in human urethral strictures; our study was an animal study including normal urethras.

The polyurethane-covered, retrievable stent has important limitations because of its migration and disruption of the polyurethane membrane. In fact, in our study, five of 20 dogs showed partial or complete stent migration, and stent removal failed in two dogs because of stent migration into the bladder in one and because of stent migration to a position abutting the urethra in the other. Also, one removed stent showed focal disruption at the polyurethane membrane. Although we do not know the reason for disruption of the polyurethane membrane, we suspect that the polyurethane membrane may have been damaged while the stent was being pushed into the introducing sheath. Thus, further investigation is needed to overcome these limitations.

In conclusion, although a few complications were encountered in our study, the polyurethane-covered, retrievable, expandable nitinol stent seems feasible for use in the urethra. This stent can be removed with a retrieval hook without difficulty 4 or 8 weeks after placement in most cases. Moreover, granulation tissue resolved or decreased 2 weeks after stent removal.

Practical application: Although a few complications were encountered in our study, the polyurethane-covered, retrievable, expandable nitinol stent seems feasible for use in the urethra.

	FOOTNOTES

 
Author contributions: Guarantor of integrity of entire study, H.Y.S.; study concepts and design, H.Y.S., G.Y.K.; literature research, G.Y.K., G.C.K.; experimental studies, H.Y.S., G.Y.K., T.H.K., T.S.S., J.O.L.; data acquisition, G.Y.K., T.S.S.; data analysis/interpretation, G.Y.K., G.C.K., T.H.K.; statistical analysis, J.H.L., T.H.K.; manuscript preparation, G.Y.K., G.C.K.; manuscript definition of intellectual content, G.Y.K., H.Y.S.; manuscript editing, H.Y.S.; manuscript revision/review, J.H.L., G.C.K., T.H.K., T.S.S., J.O.L.; manuscript final version approval, H.Y.S.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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