Stent placement is indicated for patients with submucosal and extraluminal pathology or tracheobronchomalacia, as well as for patients with intraluminal pathology (Table 1 and Figure 2). Tracheobronchomalacia is a special entity denoting functional airway obstruction with destruction of the surrounding airway cartilage. In patients with malignancies, the most common indication for stent placement is bronchogenic carcinoma, which can present as extraluminal compression with or without an intraluminal lesion. Tracheobronchial stenting is the only immediate treatment for unresectable extraluminal compression, promptly stabilizing a threatened airway while the primary tumor is treated with radiation or chemotherapy.

Open in New Tab   Full Size Figure   Download Figure

Figure 2 A 56-year-old woman with a left main bronchial obstruction caused by cervical cancer metastasis. A, B: Coronal reconstructed computed tomography (CT) scan (A) and 3D-reconstructed CT scan (B) showing complete obstruction (arrows) of the left main bronchus with left lung atelectasis; C: Radiograph taken 6 mo after left bronchial stent placement, showing good expansion of the stent (arrows) and good aeration of the left lung.

The only definitive contraindication for placement of an airway stent is in patients with external compression of the airway by a vessel. Stent placement in these patients was associated with unacceptably high rates of erosion, hemorrhage, and death[12].

At present, two types of stents are available, silicone and metallic. The advantages of metallic stents include their favorable ratios of wall to inner diameter and their ability to be placed using flexible bronchoscopy and/or fluoroscopy. Metallic stents can be classified as balloon-expandable and covered and uncovered self-expandable metallic stents. Silicone stents have been associated with high rates of migration, resulting in reocclusion, adherence of secretions due to the impairment of mucociliary clearance, and unfavorable wall to inner diameter ratios. Among the advantages of silicone stents are their ability to be repositioned or removed as many times as needed, which is especially important for benign tracheobronchial stenoses and for slowly growing, frequently recurring stenoses[12]. General anesthesia and the use of a rigid bronchoscope are mandatory for the placement of silicone stents. Among the advantages of balloon-expandable stents, including Strecker and Palmaz stents, and of uncovered self-expandable metallic stents, including Gianturco Z, Ultraflex, and Polyflex stents, are their lower rates of migration and interference with mucociliary clearance. Among their disadvantages are difficult removal and the growth of tumors or granulation tissue through the stent meshes. Because balloon-expandable metallic stents require an additional step, balloon dilation, and sometimes require rigid bronchoscopes, they are less commonly used. Uncovered and covered self-expandable metallic stents have become increasingly popular due to their relative ease of placement. These stents can be compressed into a small delivery device, and once deployed, are embedded into the surrounding tissue via a radial force. Whereas uncovered self-expandable metallic stents cannot be removed or exchanged easily, covered self-expandable metallic stents, such as Ultraflex, Wall, and Alveolus stents, can be removed or exchanged relatively easily. Therefore, temporarily placed retrievable, covered self-expandable metallic stents can be used to treat benign airway strictures as well as to treat malignant airway strictures in combination with radiation therapy and/or chemotherapy (Figure 3)[13,14]. In addition, covered self-expandable metallic stents can be used to seal esophagorespiratory fistulae. Recently introduced barbed retrievable covered metallic stents have shown particularly low migration rates[15].

Open in New Tab   Full Size Figure   Download Figure

Figure 3 A 67-year-old man with a left main bronchial stricture caused by non-small-cell lung cancer. A, B: Axial computed tomography (CT) scan (A) and anteroposterior view (B) of 3D surface-rendered reconstruction CT scan performed 3 d before stent placement, showing a severe left main bronchial stricture (arrows); C: Radiograph showing retrievable covered stent (arrows) 12 mm in diameter and 4 cm in length placed at the stricture. A sizing catheter (arrowhead) is also shown; D: Radiograph showing the collapse of the proximal end (arrowhead) of the stent while the retrievable hookwire (arrow) was withdrawn into the sheath; E, F: Axial CT scan (E) and anteroposterior view (F) of 3D surface-rendered reconstruction CT obtained 6 mo after stent removal, showing marked improvement in the stricture (arrows).

Expandable metallic stents can be inserted only under fluoroscopic guidance and only by radiologists with patients under topical anesthesia. Due to the importance of bronchoscopic evaluation immediately before and after stent placement, bronchoscopic assistance is valuable. Moreover, it is relatively easy to insert a guide wire across the stricture into the distal portion of the trachea or bronchus through the working channel of the bronchoscope.

The techniques for providing topical anesthesia, introducing the guide wire and catheter into the tracheobronchial tree, and obtaining selective tracheobronchography are the same as for balloon dilation. Subsequently, the location of the narrowed lumen can be marked on the patient’s skin using radiopaque markers. With the patient in a supine position and with the neck fully extended, the delivery system, the proximal part of which is lubricated with jelly, is passed over the guide wire into the trachea and is advanced until the distal tip reaches beyond the stricture. When the stricture is severe (i.e. more than two-thirds of the lumen is narrowed), the stenotic portion is dilated with an angioplasty balloon catheter. The stent should be at least 10 mm longer than the stricture, so that the proximal and distal parts of the stent rest on the upper and lower margins of the stricture, respectively.

Following stent placement, its patency and location are evaluated by bronchoscopy. It is important to avoid inexact stent deployment that results in partial obstruction of a bronchial orifice or incomplete coverage of a tumor stenosis. If this occurs, the stent should be repositioned using bronchoscopic biopsy forceps or it should be removed and its placement reattempted.

Early investigators used forceps/rotation techniques to remove uncovered expandable metallic stents under general anesthesia[16,17]. In patients in whom the stent was tightly welded to the tracheobronchial wall, however, this removal procedure carried potential risks of mucosal bleeding and airway occlusion during the procedure. Our removal technique uses a hook-like device and has been reported to be safe and easy to perform because the stents (Song Airway Stent, S&G Biotech, Seongnam, Korea) are completely covered and designed for optimal removal[13-15,18]. To make the stent removable, a nylon loop is hooked inside each bend of its proximal end and two nylon threads are passed through each loop. To remove these stents, a hooked wire is introduced into the sheath and passed through it into the stent lumen. The sheath containing the hook is then pulled out of the stent so that the hook grasps the drawstring. When this occurs, the hook wire is withdrawn through the sheath, collapsing the proximal stent. The sheath, hook wire, and stent are then pulled out of the trachea.

In benign tracheobronchial stenosis, technical success rates of 100% and clinical success rates of 88-100% have been reported in 46 patients across three representative studies using uncovered self-expandable stents[19-21]. The most common causes of stenosis in these patients were post-lung transplantation strictures and tuberculosis. Stent fracture was the most commonly observed complication, occurring in seven patients (15%), followed by granulation tissue formation (7%) and stent migration (2%). Four patients (9%) required stent removal due to stent fracture or migration, but stent removal was difficult because the wire mesh was embedded in the airway walls. These findings indicate that the placement of permanent stents may not be ideal due to the formation of granulation tissue. We have placed 30 covered retrievable expandable metallic stents into 24 patients[13], resulting in technical and short-term clinical success rates of 100%. All stents were successfully removed electively, either 2 (n = 12) or 6 (n = 12) months after placement or when complications occurred (n = 6). The 6-mo 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 mo vs 9.4 ± 5.4 mo, P = 0.001) than the 2-mo stenting group. Although stent migration and tissue hyperplasia at either end of the stent was observed in 13% and 37% of these patients, respectively, stent removal was easy and safe.

Four representative studies of stent placement, two using uncovered[22,23] and two using covered[18,24] stents, in 133 patients with malignant tracheobronchial stenosis showed technical success rates of 98%-100% and clinical success rates of 82%-92%. Covered metallic stents were associated with much higher rates of stent migration (12%-17%) and sputum retention (20%-38%) than uncovered metallic stents (0% and 9%, respectively). In contrast, tumor ingrowth into the stent lumen occurred more often with uncovered (21%-23%) than with covered (0%) metallic stents because the former do not contain covering material between the wire mesh. In patients with malignant bronchial obstructions, involvement of the lower-lobe segmental bronchus has been associated with lower rates of radiologic and clinical improvement following stent placement[25].

In patients with benign disease, stent migration is more likely when there is no substantial extrinsic compression maintaining the stents in place and when short stents are placed in conical stenoses. In patients with malignancies, migration can be expected after tumor shrinkage from radiation or chemotherapy. Granulation tissue is more likely to form at the proximal and distal ends of the stent, and excessive granulation tissue can lead to obstruction of the airway. This occurs more commonly with metallic stents, especially uncovered stents (up to 7%), than with silicone stents because metal stents are more rigid and have multiple edges, therefore causing more irritation. The incidence of tumor overgrowth at the tip of the covered metallic stent depends on the follow-up period and extent of the malignancy at the time of stent placement and has been reported to occur in 6%-28% of patients[18,24]. Because airway obstruction by tumor ingrowth/overgrowth can be life-threatening, patients with new symptoms or radiographic findings should undergo further diagnostic evaluation. Sputum retention is more likely to occur after placement of silicone and covered expandable metallic stents than after placement of uncovered expandable metallic stents due to impaired mucociliary clearance in the former. Lack of mucociliary clearance can lead to obstruction and infection.