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Original Research |
1 Department of Radiology, Asan Medical Center, University of Ulsan College of
Medicine, 388-1, Poongnap-dong, Songpa-gu, Seoul 138-736, South Korea.
2 Department of Radiology, Seoul National University Bundang Hospital, Seoul,
South Korea.
Received July 6, 2006;
accepted after revision October 11, 2006.
Address correspondence to J. H. Shin
(jhshin{at}amc.seoul.kr).
Abstract
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MATERIALS AND METHODS. Polyurethaneor polytetrafluoroethylene (PTFE)-covered retrievable expandable nitinol stents were placed fluoroscopically in 24 patients with benign tracheobronchial strictures. Improvement in respiratory status and complications were evaluated. Maintained patency of airway after temporary stenting was calculated and compared between the 2- and 6-month stenting groups. Factors for maintained patency after temporary stenting were evaluated.
RESULTS. A total of 30 stents were successfully placed and well tolerated in 24 patients. Tissue hyperplasia, stent migration, and bronchial obstruction of the left upper lobe occurred in 36.7%, 13.3%, and 3.3% of patients, respectively. All stents were successfully removed electively either 2 (n = 12) or 6 (n = 12) months after placement or when complications occurred (n = 6). During the follow-up period (mean, 24 months), dyspnea recurred in 15 of the 24 patients. The 6-month stenting group showed a lower recurrence rate (41.7% vs 83.3%, p = 0.045) and a better mean maintained patency (39.7 ± 7.8 vs 9.4 ± 5.4 months, p = 0.001) than the 2-month stenting group. Multivariate analysis showed that duration of stent placement (p = 0.002) and the occurrence of tissue hyperplasia (p = 0.026) were associated with maintained patency after temporary stenting.
CONCLUSION. Temporary placement of a covered, retrievable, expandible nitinol stent may be a safe and effective treatment for benign tracheobronchial strictures during the period the stent is in place. A high symptomatic recurrence rate of 62.5% was found after stent removal. Shortterm placement of the stent and tissue hyperplasia were associated with decreased airway patency.
Keywords: airway • fluoroscopy • interventional radiology • stents • tracheobronchial strictures
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To facilitate the removal of metallic stents, Song et al. [1] designed the covered retrievable nitinol stent and reported that it was associated with little ingrowth of hyperplastic tissue and was easily removed in case of complications or when placed only temporarily. Although some investigators have analyzed the use of covered retrievable expandable nitinol stents in patients with malignant tracheobronchial strictures [9], studies examining the use of such stents for benign tracheobronchial strictures have involved only small numbers of patients and short follow-up times [1, 8]. Our study retrospectively evaluated long-term results and factors affecting airway patency after the temporary placement of covered retrievable nitinol stents for benign tracheobronchial strictures that were refractory to balloon dilation.
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The causes of the benign tracheobronchial strictures were endotracheobronchial tuberculosis (n = 13), stricture after tracheostomy (n = 6), anastomotic stricture after tracheal resection (n =3), and stricture after trauma (n = 2). The diagnoses of endotracheobronchial tuberculosis were established by bronchoscopic biopsy, and those of other benign strictures were made on the basis of the individual clinical situations. The stricture sites were at the trachea (n = 7), left main bronchus (n = 15), and right main bronchus (n = 2). All patients presented with dyspnea, noisy respiration, and coughing. Ten (59%) of the 17 patients with left or right main bronchial strictures showed unilateral complete lung collapse. The duration of symptoms before stent placement ranged from 3 to 480 weeks (mean, 42.3 weeks).
Stent Placement and Removal
Stents (Song airway stent, Taewoong) were woven from a single thread of
0.2-mm-diameter nitinol wire in a tubular configuration and covered with a 12%
polyurethane solution or polytetrafluoroethylene (PTFE) to prevent tissue
hyperplasia through the stent wires. PTFE has been used as a stent covering
material at our institution since October 2003 because it is considered more
durable and less degradable than polyurethane. The present study involved 24
polyurethane- and six PTFE-covered stents (six patients had two stents).
Stents at least 10 mm longer than the stricture were selected for placement so that the proximal and distal regions would rest on the upper and lower margins of the stricture, respectively. In adults, stents of 16- or 20-mm diameter were used for the trachea and of 10- or 12-mm diameter for the bronchus. A 10-mm-diameter stent was used in the trachea of the child patient. To enable stent removal, nylon loops were hooked inside each bend of the upper end of the stent, and two other nylon threads were passed through each of these nylon loops to form a larger loop (drawstring) to fill the inside circumference of the proximal end of the stent, as previously described [1, 9]. The end of the retrieval hookwire was shaped like a question mark to hook the stent drawstring, as previously described [16]. The stent introducer set and stent retrieval set were similar to those described elsewhere [1, 9]. To assist in deciding stent diameter and length, the severity and length of strictures were evaluated using conventional radiography or fluoroscopy, CT including 3D reconstructions, and bronchoscopy.
The details of stent placement are provided elsewhere [1]. In brief, topical anesthesia of the pharynx and larynx was routinely achieved with an aerosol spray before the procedure. Drugs for sedation were routinely used. With bronchoscopic guidance, a 0.035-inch exchange guidewire (Radiofocus Guide Wire M), Terumo) was inserted through the mouth across the stricture into the distal portion of the trachea or bronchus. A straight 5-French graduated sizing catheter (Royal Flush II angiographic catheter, Cook) was passed over the guidewire to the distal part of the stricture to measure the length of the stricture. After measuring the length of the stricture, balloon dilation was initially performed in all patients. After balloon dilation, the 0.035-inch exchange guidewire was then changed to a super-stiff J-tip guidewire (Amplatz Super Stiff Guidewires, Medi-tech/Boston Scientific), and the balloon was removed and the guidewire left in the trachea or bronchus. Then under fluoroscopic guidance, a 14-French sheath with a dilator was passed over the guidewire into the trachea or bronchus and was advanced until the distal tip of the sheath reached 1 cm beyond the stricture. The dilator and the guidewire were removed from the sheath, after which a stent was compressed and loaded into the sheath and then positioned in the trachea or bronchus using a pusher catheter.
The indications for stent removal included either elective removal or stent-related complications such as stent migration or marked tissue hyperplasia above or below the stent. Stents were electively removed either 2 or 6 months after placement. We initially removed the stent 2 months after placement; however, because of frequent restenosis, we later decided to remove stents 6 months after placement. The two removal techniques used were the standard and the eversion techniques, as detailed elsewhere [16] (Fig. 1A, 1B, 1C).
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Technical success was defined as successful placement of the stent without any major complications. Clinical success was defined as improvement in the respiratory status based on PFT results 1 month after stent placement. Recurrence was defined as the return of symptoms and identified airway restenosis on bronchoscopy after temporary stenting. Maintained patency was defined as no recurrence of clinical dyspnea during the follow-up period after temporary stenting. In terms of complications, sputum retention was considered when it was symptomatic and increased after stent placement, and tissue hyperplasia was confirmed using bronchoscopy and biopsy. Membrane degradation was defined as a membrane tear or opening in the polyurethane or PTFE covering.
Statistical Analysis
Statistical analysis was conducted using SPSS software (version 11.5;
SPSS), and p values less than 0.05 were considered to indicate a
significant difference. PFT results just before and 1 month after stent
placement were analyzed using the Wilcoxon's signed rank test. The Fisher's
exact test was used when there was a statistical relationship between tissue
hyperplasia or stent migration and the sites of stricture.
The study population was divided into two groups according to the duration of temporary stenting, either 2 months or 6 months. The Student's t test was used to compare continuous data, and the chi-square or Fisher's exact test was used to compare categoric values between the two groups. Maintained patency after temporary stent placement was calculated using the Kaplan-Meier method and was compared between the two groups using log-rank tests. Complications were compared between those with polyurethane-covered and those of PTFE-covered stents using the chi-square test or Fisher's exact test.
Multivariate Cox proportional hazard regression analysis with forward stepwise selection was used to evaluate the multivariate factors affecting maintained patency after temporary stenting. The variables used in this analysis included age, sex, symptom duration, cause, site, length and diameter of strictures, stent placement duration, stent covering material, the presence of tissue hyperplasia, and stent migration.
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A total of seven complications (23.3%, 7/30) occurred after stent placement. The complications comprised symptomatic tissue hyperplasia (n = 2), stent migration (n =4), and bronchial obstruction of the left upper lobe (n = 1). Tissue hyperplasia was mild or asymptomatic in 81.8% (9/11) of patients and was symptomatic in 18.2% (2/11). Stent migration occurred more frequently in the trachea, and no relationship was seen between tissue hyperplasia and the stricture site (Table 1). To relocate the stent obstructing the left upper lobar bronchus, the drawstring was grasped using biopsy forceps and pulled upward until the distal end of the stent was just proximal to the orifice of the left upper lobar bronchus. Six stents were removed to overcome six complications (stent migration [n = 4] and symptomatic tissue hyperplasia [n = 2]) in six patients. In these six patients, a second stent was inserted. Symptomatic sputum retention did not occur in any patient.
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Stents were electively removed from 12 patients 2 months after placement and from an additional 12 patients 6 months after placement. All stents were successfully and safely removed using standard (n = 23) or eversion (n = 1) techniques. Balloon dilation before removal was necessary for three patients in whom tissue hyperplasia resulted in a narrowing at the proximal end of the stent. Removed stents were examined to determine membrane degradation. We found that membrane degradation occurred in five (20.8%) of the 24 polyurethane stents and in none of the six PTFE stents (p = 0.298) (Table 2).
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Follow-Up and Statistical Analysis
During the mean 24-month follow-up period (range, 12-64 months) after
elective stent removal, dyspnea recurred in 15 patients (62.5%, 15/24). The 2-
and 6-month temporary stenting groups were compared in terms of clinical
characteristics (Table 3). The
2-month group was found to have a higher recurrence rate (83.3% [10/12] vs
41.7% [5/12], p = 0.045). The two groups did not differ significantly
in terms of a range of age; sex; duration of symptoms; cause; site, length,
and diameter of stricture; or presence of tissue hyperplasia.
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The cumulative maintained patency rates after stent removal are shown in Figure 2. Overall, the mean maintained patency time after stent removal was 25.2 ± 5.9 months. The maintained patency rate was 50% at 1 year and appeared to plateau at 2 years (32%) after stent removal. The 6-month temporary stenting group showed a better mean maintained patency than the 2-month group (39.7 ± 7.8 vs 9.4 ± 5.4 months, p = 0.001) (Fig. 3).
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Problems associated with stent placement in benign tracheobronchial strictures were identified in a recent study showing that airway stents in benign disease increased the length of the inflamed lumen, caused esophagorespiratory fistulas, promoted subglottic stenoses, and ultimately made surgery impossible in 30% of patients judged surgically correctable before stent placement [18]. However, technical improvements in stent design, including retrievable, absorbable, and bifurcated stents, have resulted in airway stent placement expanding in its indications in benign diseases [5].
Retrievable airway stents have a definite advantage in patients with benign strictures, in whom it is assumed the stent will no longer be needed after healing of the underlying stricture or that a sufficient duration of stent support will prevent restricture after stent removal. We believe that benign airway strictures should be exclusively treated using temporary rather than permanent stents to avoid serious stent-related complications.
There are few reports regarding temporary stent placement for benign tracheobronchial strictures. One study of 21 patients reported that the Dumon silicone stent was successful in 17 patients, with only four patients (19%) showing recurrence 18 months after temporary stenting [19]. Although those authors suggested 18 months was the optimal time for temporary stenting with Dumon silicone stents for benign airway disease, the followup period of that study was relatively short (mean, 8.6 months).
Our study found that restricture occurred in 15 (62.5%) of the 24 patients over a mean 24-month follow-up period (range, 12-64 months) after temporary stenting for either 2 or 6 months and that the mean maintained patency time after stent removal was 25.2 ± 5.9 months. Comparing the 2- and 6-month temporary stenting groups, the 6-month group showed a lower recurrence rate (41.7% vs 83.3%) and better mean maintained patency (39.7 ± 7.8 vs 9.4 ± 5.4 months) than the 2-month group.
We surmise that the high recurrence rate in our study can be attributed to the short-term duration of stent placement (2 months) in half of the study patients. It may be that recurrence rates would have been lower if the duration of stent placement had been longer than 6 months. However, long-term placement of a metallic stent may increase the chance of complications and can make stent removal difficult or even impossible [16, 18, 20]. We propose that 6 months may be the optimal period of temporary stenting for benign airway disease using a covered metallic stent.
The characteristics of metallic and silicone stents are described in detail elsewhere [3, 13, 14, 21]. In 1990, Dumon [22] designed a silicone stent that became one of the most commonly used tracheobronchial stents in the world, and some believe it is the gold standard with which future stents will be compared [23]. Many authors have reported that the major advantage of the Dumon stent is that it is easier to remove or relocate than the expandable metallic stent [3, 14, 19]. Therefore, the Dumon stent was believed to be more appropriate in treating benign tracheobronchial stricture than expandable metallic stents [19]. However, removal or relocation of a Dumon stent requires rigid bronchoscopy with general anesthesia. Furthermore, repeated rigid bronchoscopy and stent repositioning have been reported to be necessary more frequently for Dumon stents than for expandable metallic stents [24]. On the other hand, the removal of covered retrievable nitinol stents does not require bronchoscopy or general anesthesia and is safe and effective [16].
Excessive tissue hyperplasia is a common drawback of metallic stents under most circumstances [25, 26]. The constant friction of stents and the highly localized pressures they exert against the mucosa possibly promote the development of tissue hyperplasia [13]. Although covered metallic stents can decrease the ingrowth of hyperplastic tissue through the wire mesh, tissue may still grow over either end of the stents. Most tissue hyperplasia may be mild or asymptomatic, as found in our study. However, some authors have proposed that the growth of tissue in the tracheobronchial tree eventually causes symptom recurrence, necessitating further treatment [1].
Our study found that tissue hyperplasia was one of the risk factors for decreased airway patency after stent removal. Although hyperplastic tissue can recede after stent removal [1, 16], we believe that formation of such new tissue may prevent healing and remodeling in tracheobronchial strictures. A correctly sized stent is critical in preventing tissue hyperplasia because an undersized stent results in excessive friction of metal against the airway wall, and an oversized stent results in excessive radial pressure against the tracheobronchial mucosa and impairs microcirculation. Topical application of drugs such as mitomycin-C or intraluminal brachytherapy using irridium 192 (192Ir) may have the potential for decreasing tissue hyperplasia [27, 28].
Polyurethane is a common covering for stents used in nonvascular luminal organs because it can be prepared in solution and easily forms a membrane [1, 8, 9, 16, 29]. However, polyurethane membrane defects have been reported in several clinical studies dealing with covered stent placement and removal in the airway and in the biliary tract, in transjugular portosystemic shunt (TIPS), and in the upper gastrointestinal tract [16, 30-32]. These findings suggest that polyurethane degrades over time because it may not be biostable. Development of a nondegradable covering material is critical because such degradation results in hyperplastic tissue ingrowth and makes stent removal difficult or even impossible.
PTFE is a strongly chemical-resistant material and has been used as a covering in biliary stents, in TIPS, and in esophageal and gastroduodenal stents [31, 33-35]. There have been no reports of membrane degradation or disruption in clinical studies using PTFE stents. In our study, PTFE- and polyurethane-covered stents did not significantly differ in terms of the overall incidence of complications. However, after stent removal, the membrane itself was intact in all six PTFE stents, whereas five (20.8%) of the 24 polyurethane stents were perforated.
In conclusion, temporary placement of a covered, retrievable, expandible nitinol stent may be a safe and effective treatment for benign tracheobronchial strictures during the period that the stent is in place. A high symptomatic recurrence rate of 62.5% was found after stent removal. Factors decreasing airway patency after temporary stenting were short-term (2 months) placement of the stent and tissue hyperplasia.
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