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Ann Thorac Surg 2000;69:1030-1034
© 2000 The Society of Thoracic Surgeons

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ORIGINAL ARTICLES: GENERAL THORACIC

Long-term safety and tolerance of silicone and self-expandable airway stents: an experimental study

^a Departments of Surgery and General Thoracic Surgery, University of Perugia Medical School, Perugia, Italy
^b Department of Pathology, University of Perugia Medical School, Perugia, Italy
^c Institute of Veterinary Surgery, University of Perugia Medical School, Perugia, Italy

Address reprint requests to Dr Puma, Clinica Chirurgica Generale e Toracica, Ospedale Civile, 05100 Terni, Italy
e-mail: italylink{at}mclink.it

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. A variety of respiratory stents are currently available, but the ideal airway prosthesis seems far from being recognized. The objective of this study was to verify safety and long-term effect on the bronchial wall of three different types of airway stents.

Methods. Twelve healthy adult sheep were divided in three groups, scheduled to receive: (1) bare self-expandable metallic stents (Gianturco); (2) silicone stents (Dumon); and (3) covered self-expandable synthetic stents (Polyflex). Insertions were performed through a rigid bronchoscope under general anesthesia. Chest roentgenogram was performed 1 and 6 months after surgery, and flexible bronchoscopy after 6 months. Twelve months postoperatively, the animals were killed and a postmortem examination was carried out.

Results. All Polyflex stents migrated during the observation period; one late migration was observed in the Dumon group. Microscopic study showed: (1) Gianturco stents: full-thickness perforation of the bronchial wall covered by a thick layer of a chronic inflammatory infiltrate. Infection by Candida at the bottom of some ulcerations; (2) Dumon stents: mild bronchial inflammation (squamous metaplasia, submucosal inflammatory infiltrates; granuloma-like infiltrates). In case of displacement, no significant changes of the previously stented bronchus occurred; and (3) Polyflex stents: no changes of the previously stented bronchi.

Conclusions. Gianturco stents proved unsafe in the long term, owing to the risk of severe airway wall damage. The Polyflex stent is well tolerated but presents a high migration rate. Silicone stents show several limitations but appear to be well tolerated by the host mucosa.

	Introduction

Top Abstract Introduction Material and methods Results Comment References

 
Tracheobronchial stenting is an accepted treatment for both benign and malignant stenoses not amenable to surgical resection. A variety of stents are currently available and new prototypes are periodically introduced, proving that, to date, there is not an ideal respiratory stent for all conditions.

Basically, these prostheses can be divided into silicone tubes and expandable stents. The latter are anchored to the tracheobronchial wall, owing to their radial expansion force, while silicone tubes have a predetermined diameter and migration is prevented by the stent’s shape (Dumon, Hood-stent, bifurcated tubes, Montgomery T tube).

Both silicone and expandable stents present favorable properties as well as a variety of limitations (significant complications have been reported, some of these being life-threatening or lethal) [1–3]. Stenting is indicated for an already damaged tracheobronchial wall, and the airway changes directly attributable to the stent are unknown. In fact, the risk of damage to the airway wall by the different stents has not been completely evaluated in the long term because most of the experimental studies available are characterized by a relatively short observa-tion period [4–6] or by questionable methods [7]. Because airway stenting is also needed in patients with benign diseases and long life expectancy, from an ethical point of view, the risk of iatrogenic lesions must be considered.

The aim of this experimental study was to evaluate safety and long-term effect on the bronchial wall of three different types of respiratory stents.

	Material and methods

Top Abstract Introduction Material and methods Results Comment References

 
Thirteen healthy Comisane sheep, aged 2 to 6 years, were included in the study. All animals have received human care in compliance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH publication 85-23, revised 1985). We chose the adult sheep for the following reasons: species approved as laboratory animal by our national law; easy stalling; relatively favorable anatomy; and not expensive.

The first animal was killed to enable a bench evaluation in order to assess the most suitable bronchus to be stented. It was immediately clear that for any position the stent would have crossed a bronchial orifice with subsequent obstruction by silicone and covered expandable stents. The right caudal lobe was chosen because of its diameter (roughly 12 to 13 mm at the proximal end) and for the excellent collateral ventilation.

The remaining 12 animals were evenly divided into three groups scheduled to receive: (1) bare self-expandable metallic stent 15/25 mm (outside diameter/length) (Gianturco; Cook Inc, Bloomington, IN); (2) silicone stent 12/30 mm (Dumon-type; Hood Laboratories, Pembroke, MA); or (3) self-expandable synthetic covered stent 15/30 mm (Polyflex; Rusch, Kernen, Germany).

The Gianturco stent is one of the most widely used metallic respiratory prosthesis, even though it was initially designed as a vascular stent [8]. It is a stainless steel zigzag wire capable of spontaneous expansion when pushed out from its cartridge. The Dumon stent is a widely used silicone tube whose external surface is covered with multiple studs designed to prevent stent migration. Polyflex is a new prototype of airway stent that presents some promising features: it is a polyester autoexpandable prosthesis with an internal silicone layer to prevent tumoral ingrowth through the mesh.

Expandable stents were chosen slightly wider in diameter than silicone stents (15 vs 12 mm) to ensure optimal anchoring to the wall, paying attention to avoid excessive disproportion in diameter between the stent and the bronchial lumen.

All surgical and bronchoscopic procedures were carried out under general anesthesia. Preoperative antibiotic prophylaxis was performed with 1 g of Cephalexin intramuscularly. The animals were premedicated with acepromazine maleate (0.08 mg/kg intramuscularly). Anesthesia was induced and maintained with Propofol (0.4 mg/kg intravenously). Tracheal intubation was carried out for oxygen administration (4 L/min). Noninvasive blood pressure, electrocardiogram, and pulse oximetry were continuously monitored.

Because of sheep tracheal length, a temporary low tracheotomy was necessary to allow the use of a human ventilating rigid bronchoscope (8.5 mm internal diameter; Karl Storz, Tuttlingen, Germany). Before the insertion of the rigid scope through the tracheotomy, 10 mL of 2% lidocaine was instilled into the tracheal lumen. Oxygen was then delivered through the ventilating rigid bronchoscope.

Bronchial diameter was carefully measured before stent insertion with an open endoscopic forceps. Each stent was inserted with its special introducer system, and correct positioning was endoscopically assessed. At the end of the procedure, the tracheal stoma was closed with synthetic absorbable interrupted sutures.

All animals underwent clinical control every week in the first 3 months and twice a month subsequently. After 1 and 6 months, a chest roentgenogram was performed; in the Gianturco group, a chest roentgenogram was also performed immediately after surgery. Six months after stenting, a flexible bronchoscopy was carried out for intraluminal control and cultural and cytological sampling. Twelve months postoperatively, the animals were sacrificed by intravenous administration of barbiturates and curariform drugs. Larynx, trachea, lungs, and esophagus were en bloc removed, and pathological examinations were performed.

	Results

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Stent placement was absolutely easy and uneventful in all animals. Mean operative time was 25 minutes. No animals died after the procedure. In the early postoperative period, an irritative cough was observed in the Gianturco and Dumon groups, mainly under stress condition (capture, clinical examination). Chest roentgenogram showed no evidence of pulmonary infiltrates; progressive widening of the Gianturco stents was observed.

Fiberoptic control revealed: (1) Gianturco group: good airway patency with stable positioning of all the stents, only minimally covered by respiratory mucosa. Definite overexpansion in comparison with the initial diameter. Mucosal inflammation, minor granulomas, and necrotic debris were also evident at both ends in three out of four animals. The crossed bronchial orifice was always patent. (2) Dumon group: good airway patency, stable positioning of all the stents, minimal mucosal inflammation, minor granuloma at the proximal end in one animal. There were secretions adhering to the inner surface of the stents. (3) Polyflex group: two stents migrated and were subsequently expelled with the cough. In the two animals with the stent in place, good airway patency, minimal mucosal inflammation, and no granulomas were noted. Secretions adhered to the inner surface. In case of migration, no pathological findings were detected on the bronchial wall at the level of the previous stent deployment.

Cultural studies of the bronchial secretions endoscopically collected showed only saprophytic bacteria in all animals. Cytological examination revealed normal bronchial cells, neutrophils, and macrophages, particularly in the Gianturco group. Six-month endoscopic data are summarized in Table 1.

View larger version (150K): [in this window] [in a new window]   Fig 1. Gianturco group: microscopic features of the stented bronchus. Full-thickness ulceration of the bronchial wall surrounded by a thick layer of chronic inflammatory infiltrate (Hematoxylin & Eosin; x100).

View larger version (142K): [in this window] [in a new window]   Fig 2. Gianturco group: mycotic spores, likely due to Candida infection, at the bottom of an ulceration (PAS; x400).

View larger version (136K): [in this window] [in a new window]   Fig 3. Dumon group: borderline separating normal columnar epithelium from squamous metaplasia (Hematoxylin & Eosin; x400).

View larger version (134K): [in this window] [in a new window]   Fig 4. Dumon group: chronic inflammatory infiltrate bulging towards the bronchial lumen, mainly composed by histiocytes and lymphocytes (granuloma) (Hematoxylin & Eosin; x100).

Twelve-month postmortem data are summarized in Table 2.

	Comment

Top Abstract Introduction Material and methods Results Comment References

The "optimal" airway stent should fulfill the following requirements. It should be: atraumatic for the airway wall; well anchored to the wall; thin with a wide internal lumen; easy to insert; removable without tracheo-bronchial damage; available in different diameters and lengths, better if customizable; easy to manage if complications occur; and not expensive. It should feature transversal and longitudinal elasticity. It should allow: insertion under direct endoscopic control; ventilation during insertion; repositioning; and ventilation of a crossed patent bronchus. It should avoid retention of secretions and tumor ingrowth. It should not interfere with mucociliary function or entail tracheostomy.

Such a stent is simply impossible to realize (some of the properties are mutually exclusive and suit specific requirements, ie, intraluminal tumor, benign stenoses). Metallic self-expandable prostheses do respond to many of the basic requirements, explaining their wide acceptance in clinical use. In particular, migration is nearly impossible, and occlusion by mucous accumulation is unlikely due to incorporation of the metallic mesh by the host mucosa. Nevertheless, severe complications are reported with the use of expanding wire stents (hemorrhage, perforation, dysphagia, granulomas, malposition, metallic mesh fracture, etc) [1, 2, 9].

The results of our study demonstrate that in the long term, Gianturco’s prosthesis is unsafe: the stent progressively expands to its maximum diameter, weakening the tracheobronchial wall, which undergoes erosion until perforation. Even though the animals in our study remained asymptomatic, the severity of bronchial wall damage observed (full thickness erosion surrounded by a chronic inflammatory infiltrate) make symptomatic perforation predictable in the next future. The study of Wallace and associates had reported similar results with a minor degree of severity, probably owing to the shorter observation period (4 to 10 weeks) [5]. Rauber and associates implanted self-constructed Gianturco stents in rabbit models and observed a marked widening of the stents within the first 2 to 3 weeks of follow-up. In our opinion, the methods of this study are questionable because of excessive disproportion between tracheal and stent diameters (10 vs 25 mm) [7].

We did not observe fragmentation of the metallic mesh; such a complication seems to be more frequent in tracheomalacia because of the very important variations in shape and diameter of the diseased trachea [2]. Furthermore, in the treatment of such condition, breakage of other metallic stents (Wallstent), made of a different alloy with a close network, has been reported [2].

In benign diseases, the use of bare autoexpandable metallic stents may be inadvisable because of their questionable removability, and moreover because of the long-term damage to the airway wall. In endoluminal neoplastic stenoses, these prostheses are again unsuitable due to tumor penetration into the mesh. Hauck and associates, with the use of different types of expandable metallic stents, reported 67% of tumoral penetration after a mean endoscopic follow-up of only 63 days [10]; similar results have been reported by other authors [11]. This problem has been resolved by covering the metallic stent with a plastic sheath. However, it seems evident that covered stents compared with bare metallic stents present new disadvantages (possible dislocation and obstruction) while maintaining some limitations (granulomas at the uncovered tips, difficult repositioning and removal) [12].

In our opinion, the use of metallic self-expandable stents is preferable only in the presence of hard extrinsic tracheo-bronchial compression in patients with a very poor prognosis [11] or in patients submitted to total laryngectomy, because of the possibility of stent incorporation into the host mucosa [13].

Polyflex is theoretically one of the best conceived airway stents. It is self-expandable and, due to its elasticity, it can fit even in distorted scoliotic segments; its synthetic mesh does not damage the airway wall; and the internal silicone layer avoids tumoral ingrowth. Furthermore, our results show a good tolerance of this stent. However, the use of Polyflex stent seems to be inadvisable because of the risk of migration. Even though the purpose of our study was not to evaluate the migration rate (consequently, the number of animals involved was small for a statistical analysis), the rate of Polyflex stent displacement observed is impressive (100% within 12 months). Such data were confirmed by our clinical experience: we observed four migrations within 6 months in five patients treated both for malignant and postintubation stenoses. After such preliminary results, we decided to discontinue the clinical experimentation of Polyflex stent.

Silicone stents show several disadvantages. They interfere with normal mucociliary function and can be obstructed by heavy secretions (particularly in patients with chronic obstructive pulmonary disease); their external-internal diameter ratio is generally not favorable; and ventilation of nonstenotic bronchi crossed by the stent is blocked. Furthermore, migration of a Dumon stent is possible (even though such a complication seems to be rarely life-threatening) [3, 14, 15]. Despite these limitations, silicone stents, far from being ideal, can be considered the first choice for most of benign and malignant unresectable airway stenoses because of their safety and excellent long-term tolerance. We believe that unless a standard treatment has been recognized, the first rule to apply must be "primum non nocere," and most silicone airway stents do fulfill such a requirement.

All silicone stents are removable, and our results demonstrate that even after a long stenting period, the underlying mucosa remains healthy. The inflammatory changes observed are reversible.

Dumon stent is an excellent prosthesis for both benign and malignant airway stenoses. When comparing with the Montgomery T tube (the other widely used silicone stent), it presents some advantages: tracheostomy is not required, and it is also effective at the bronchial level. In endoluminal neoplastic stenoses, Dumon stent migration rate can be kept low by choosing the proper stent diameter; tumor ingrowth is impossible and early recurrent obstruction can be avoided if the prosthesis is placed so as to cover all the intraluminal disease.

In benign high tracheal stenoses, Montgomery T tube still may be considered the safest stent because it does not entail problems of migration and obstruction; furthermore, it is removable anywhere. In florid postintubation tracheal stenoses, the T tube is the best bridge to tracheal resection and reconstruction, allowing time for the resolution of active inflammatory changes and weaning from steroids [16–19].

If carina is involved, T-Y or multiple stents are inadvisable because different effective bifurcated silicone stents are now available (Dumon-type, Hood, Dynamic stent).

	References

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