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J Thorac Cardiovasc Surg 1997;114:527-534
© 1997 Mosby, Inc.
GENERAL THORACIC SURGERY |
Received for publication Dec. 27, 1996 revisions requested Feb. 14, 1997; revisions received March 18, 1997 accepted for publication March 19, 1997. Address for reprints: Klaus Wassermann, MD, Klinik III für Innere Medizin der Universität Köln, Building D/0, Josef-Stelzmann-Straße 9, 50924 Köln, Germany.
Abstract
Background: Although widely established in the management of malignant airway lesions, currently available tracheobronchial prostheses made of silicone have their drawbacks resulting from rigidity and wall thickness. Therefore we present clinical follow-up data obtained with a novel thin-walled expandable prototype silicone airway stent (Polyflex stent, Willy Rüsch AG, Kernen, Germany) in 19 patients. Methods: Seventeen of 19 patients had tracheobronchial complications of infiltrating cancer: five had respiratory-digestive fistulas, 14 had mixed-type obstructions (mucosal infiltration plus extrinsic compression), and two had diffuse tracheal hemorrhages from the tumor surface (three patients had more than one complication). Two of 19 patients had benign postintubation stricture and malacia. Overall, 33 stents were implanted either simultaneously or in a consecutive manner. Scanning electron microscopy was performed both on prototype stents and on other available silicone stents for comparison. Results: The treatment improved the patients' clinical condition substantially. The mechanical properties of the new prosthesis were excellent. Important stent-associated side effects were early mucus retention (n = 7), infolding of the inner silicone layer (n = 2), and stent dislodgment (n = 2). As of February 1997, 10 patients have died of causes unrelated to stent placement. Seven patients with malignant airway disease are still alive from 2 weeks up to 7 months after initial treatment. Scanning electron microscopy of explanted and unused prototypes suggested that an extremely ragged luminal microstructure may contribute to the firm adhesion of secretory material and that technical smoothing of the surface avoids such complications. Conclusions: The novel self-expandable silicone airway stent may be a promising addition to commonly used stent types. Short-term and medium-term management of fistulas, tumor surface bleeding, and strictures (malignant and benign) is satisfactory. Scanning electron microscopy of stents provides information on peculiar features of microstructure and material that may be of use in clinical research and technical innovation.
Malignant and benign stenoses of the central airways are a continuing challenge to the practicing broncholaryngologist. Newly developed cylindrical and bifurcated endoluminal silicone prostheses have passed their clinical tests. They are currently being used to restore and maintain airway patency, with negligible side effects. Long-term results are promising.
1-3 However, because of their unfavorably low internal/external diameter ratio and their lack of flexibility, 4 they may impose an unwanted resistance to air flow. In addition, tracheoesophageal fistulas cannot be sealed and diffuse mucosal bleeding unresponsive to laser treatment cannot be appropriately stopped by those devices. Bacterial adhesion to the stents sometimes poses serious long-term problems. 1 We present preliminary clinical follow-up data obtained with a novel thin-walled (0.45 mm) self-expanding cylindrical stent made of a texture of tightly woven polyester (TREVIRA) filaments. The meshes are covered with a silicone layer. The prosthesis has an elastic recoil that enables it to regain the original shape after short-lived deforming events such as cough, forced expiration, and stent insertion itself, and to adapt smoothly to wall irregularities. Because the prosthesis is completely coated with silicone, tumor and granulation tissue should not grow through the meshes. The stent should seal the bronchial wall by intimate contact with the mucosa over its entire length (Fig. 1).
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Patients.
From September 1995 to February 1997, 19 patients have been treated. Seventeen of 19 patients had tracheobronchial complications of infiltrating cancer: five had respiratory-enteric fistulas (primary, n = 2; secondary, n = 3), 14 had mixed-type obstructions, 1 and two had diffuse blood loss from the tumor surface. Three patients had more than one complicating condition. Two of 19 patients had benign postintubation disease: malacia below the cricoid in one and considerable tracheal restenosis caused by granulation tissue as a result of the insertion of a nitinol mesh wire stent in the other (Table I).
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Operative techniques.
With the patients under general anesthesia, orolaryngeal intubation was performed with the use of an operative laryngoscope attached to a chest support (Karl Storz, Tuttlingen, Germany). The tip of the laryngoscope had an internal diameter of 20 mm. It was positioned to expose the entire circumference of the vocal cords. Ventilation was sustained by a low-frequency oxygen jet through a nozzle fixed at the proximal orifice of the instrument. The laryngoscope thus served as an access for necessary manipulations within the tracheobronchial tree, and at the same time it allowed continuous administration of oxygen. Before stent placement in patients with central obstruction, a set of semirigid plastic applicator tubes of increasing diameter were used to dilate the stenosis. In five of them, mechanical debulking with a rigid forceps preceded the bougie maneuver. During dilation, the jet was attached to the tube so that efficient ventilation could be maintained. The distal end of the applicator tube was extended just beyond the aboral edge of the stenosis. For verification of the tube's correct position, which has to be marked at the outside, bronchoscopic examination through the tube is indispensable. For insertion of the stent into the applicator, an open-ended flexible basket attached to the opposite tip of the pusher entraps the prosthesis, compressing it on its way into the narrow sheath (Fig. 2, A). After the stent has been inserted into the distal end of the tube, the basket is torn off with the stent in place. The applicator set plus indwelling pusher and stent is then guided to its premarked intratracheal or intrabronchial position. After that, the tube is drawn back over the pusher, releasing the stent into the airway (Fig. 2, B). Bronchoscopic examination then confirms the correct position of the prosthesis. In cases of improper positioning, the stent can be repositioned with the use of a rigid optical forceps. If necessary, atraumatic extraction through the relaxed vocal cords is performed with ease.
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Results
Clinical results.
To date we have implanted 33 prototype stents of variable diameters and lengths into 19 patients (see 
Table I). The predominant location was the trachea. Two patients were provided with two serial overlapping Polyflex stents each during the first intervention to seal a large bleeding tumor area or to reopen an unexpectedly long stenotic segment of the right main-stem and intermediate bronchi. In another two patients, indwelling Dumon prostheses were replaced by Polyflex stents. In one the stent was used to decrease air flow resistance by a more favorable inner/outer diameter ratio; in the other it was used to seal a tracheoesophageal fistula underneath the Dumon stent and at the same time bridge an obstruction located at the interface with an adjacent distal Dynamic stent. A rigid tracheal restenosis produced by granulation tissue that developed as a reaction to a nitinol mesh stent, inserted because of postintubation stricture, was dilated cautiously, and a Polyflex stent was inserted to entirely cover the nitinol prosthesis (Fig. 3, A and B).
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Primary endoscopic treatment improved the patients' clinical condition substantially. Tumor bleeding and hemoptysis were suppressed, fistulas were efficiently sealed for a certain time (14 days, 1, 1, 3, and 6 months), and satisfying tracheobronchial patency could be achieved in all patients with benign disease and in all but one patients with malignant obstruction (see Table I).
Supplemental overlapping stents were needed in four patients who had tumor overgrowth or extension of fistula after 2 weeks (n = 2) and 1 month (n = 2). In three patients, two of whom had additional indwelling esophageal stents, secondary fistulas developed either spontaneously or after radiotherapy of infiltrating esophageal tumor underneath a tracheal prosthesis 4 and 4.5 months after the initial intervention. Overgrowing tumor necessitated stent exchange in another patient after 2 months. Continuing tumor growth and compression beyond the tracheal carina plus ostium of the main-stem bronchi in one patient required substitution of tracheal Polyflex stents by a bifurcated prosthesis (Dynamic stent) 1,2 after 10 months. In two patients, the inner silicone layer of the prosthesis became detached from the filament meshwork and folded inside 1 and 3 months after initial treatment. These stents were replaced by new ones to prevent further luminal narrowing. Stent dislodgment occurred only in the benign cases and was the reason for three exchanges within 1 day to 1 month from previous treatment (Table II).
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SEM studies.
Low-magnification SEM studies of the outside wall suggest that the typical weaving texture of the polyester fibers makes the stents anchor on the mucosal surface (Fig. 4, A). Thin strata (4 to 10 µm) of the otherwise large silicone layer (450 µm) span the outer vertex of the filaments, thus fixing the entire layer to the frame. Interestingly, it is through these strata that rupture lines of torn and infolded membranes run (Fig. 4, B). The inner surface of the early prototypes at low magnification (100 to 500x) and high magnification (1000x to 2000x) has an extremely ragged appearance (Fig. 4, C). A dense agglomeration of irregular lines, frequent jagged spikes about 50 µm high, and sudden sharp brinks can be seen, as well as quite a few pores with orifices of up to 20 µm in diameter. Cells and secretory material are occasionally found submerged in these pores. The respective features can be observed both in explanted and in hitherto unused stents. The improved latest prototypes show a flat, smooth inner surface (Fig. 4, D). Dumon and Dynamic stents 1 were scanned for comparison. At low and high magnifications their luminal surfaces appear smooth and evenly structured by a delicate pattern of wavelike folds distributed in regular intervals over the whole area. No crude irregularities or micropores can be observed.
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Although biomechanical studies are still unpublished, endoscopic evidence suggests that the currently presented prototype is as resistant to extrinsic stress as are conventional silicone stents. The occurrence of membrane infolding does not argue against these properties, because the supporting fiber frame remains stable. Kinking resulting from excessive compliance or even stent fracture was never observed.
Unlike the other silicone-based prostheses that are inflexible because of massive circular coating, the new prototype readily adapts to uneven bronchial contours such as conical or step-shaped strictures. Thus stenting of complex obstructive processes is possible without excess luminal decrease or subsequent dislodgment. In addition, overlapping of sequential prototypes to extend the supported tracheobronchial segment without spatial interruption elegantly avoids exchange maneuvers that may end in fatal hemorrhage, as was reported after previous brachytherapy. 1
Benign obstruction, in particular that resulting from malacia, certainly is difficult to stent with a cylindrical prosthesis. Dislodgment may occur either because of the stent's incomplete anchoring on a normal noninfiltrated mucosa or because of large differences in tracheal cross-sectional area during inspiration and expiration associated with a low grade of radial expandability of the stent itself. Some authors have, however, documented restoration of long-standing patency in benign stenoses with the help of mesh wire wall stents. 5
Airway stenting in tracheoesophageal or bronchoesophageal fistulas is not a common practice. However, given that esophageal stents fail to obliterate fistulas in 33% to 70% of cases owing to the absence of tumor-associated stenosis, 6,7 a different approach may be necessary. Indeed, airway prostheses of the currently presented type are capable of sealing connections to the digestive tract on their own, as was the case in three of five patients. The remaining two patients had additional esophageal stents. Sufficient, albeit transient, closure of tracheoesophageal or bronchoesophageal fistulas was achieved for a median of 1 month before recurrence of symptoms. This compares favorably with the 33.5 ± 25.8 days (mean ± standard deviation) during which lone esophageal stents sustain intact function in patients who have a fistula. 8 Presumably, neither treatment will prolong mean survival over mere supportive care. 6
For several reasons, the SEM technique was a valuable investigative tool to reveal biomaterial flaws underlying adverse clinical effects:
9 1,3 rare is indirect evidence in favor of our hypothesis. In addition, we did not observe problems with retained mucus with the latest improved stent types. 1 It was speculated that microbes might be able to damage and colonize the plastic substance, thus facilitating bacterial adhesion and subsequent chronic inflammation. 1 However, no signs suggestive of "biodeterioration" 10 could be found on SEM scrutiny, because the observed features were invariably present in explanted and unused stents alike. It is therefore possible that bacteria adhere to the devices, using the preformed irregular landscape. In vitro studies to quantify bacterial adherence on different stent types and materials are ongoing. In conclusion, the mechanical properties of the new self-expandable silicone stent make it a future candidate to restore and sustain patency in malignant central airway stenoses. The spectrum of indications for silicone stents may be enlarged to involve the transient seal of respiratory-enteric fistulas and diffuse mucosal bleeding.
However, drawbacks remain: Occasional infolding of the interior membrane may lead to secondary obstruction. Owing to its peculiar microstructure, the new prototype tends to retain mucus early after implantation. Bacterial adhesion is a property shared by the other commonly available silicone stents. Frequent dislodgment in benign stenosis as a result of malacia appears to disqualify the stent for this indication.
The SEM technique provides useful information on surface profiles, flaws, and potential changes of stent material before placement and after long-standing tissue exposure. SEM follow-up as a basis for eventual technical improvements should help to prevent untoward effects of indwelling prostheses. Before final recommendations can be given, long-term observations are warranted.
Footnotes
From the Third Department of Internal Medicinea and Department of Otolaryngology,b University of Cologne, Cologne, Germany. 
References
This article has been cited by other articles:
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  T. R. Gildea, S. C. Murthy, D. Sahoo, D. P. Mason, and A. C. Mehta Performance of a self-expanding silicone stent in palliation of benign airway conditions. Chest, November 1, 2006; 130(5): 1419 - 1423. [Abstract] [Full Text] [PDF]
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 S.-i. Watanabe, S. Shimokawa, G. Yotsumoto, and K.-i. Sakasegawa The use of a Dumon stent for the treatment of a bronchopleural fistula Ann. Thorac. Surg., July 1, 2001; 72(1): 276 - 278. [Abstract] [Full Text] [PDF]
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B. Schmidt, H. Olze, A. C. Borges, M. John, U. Liebers, O. Kaschke, K. Haake, and C. Witt Endotracheal balloon dilatation and stent implantation in benign stenoses Ann. Thorac. Surg., May 1, 2001; 71(5): 1630 - 1634. [Abstract] [Full Text] [PDF]
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 L. M. Seijo and D. H. Sterman Interventional Pulmonology N. Engl. J. Med., March 8, 2001; 344(10): 740 - 749. [Full Text] [PDF]
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K. Wassermann, F. Mathen, and H. E. Eckel Malignant laryngotracheal obstruction: a way to treat serial stenoses of the upper airways Ann. Thorac. Surg., October 1, 2000; 70(4): 1197 - 1201. [Abstract] [Full Text] [PDF]
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H. Hautmann, M. Bauer, K. J. Pfeifer, and R. M. Huber Flexible bronchoscopy: a safe method for metal stent implantation in bronchial disease Ann. Thorac. Surg., February 1, 2000; 69(2): 398 - 401. [Abstract] [Full Text] [PDF]
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