A number of benign and malignant disorders of the upper airways can cause tracheobronchial narrowing, stricture, compression, or collapse (i.e., tracheobronchial malacia), ultimately leading to symptomatic and potentially life-threatening dyspnea. These tracheobronchial compromises can be managed with endobronchial dilation in addition to placement of endotracheal, bronchial, or tracheobronchial stents. Generally, stent placement can be accomplished safely and provides immediate relief of symptoms in the acute setting. Over the long term, stent placement has been shown to improve the patient’s quality of life. The use of endobronchial stents has accelerated recently as a result of the proliferation of new biocompatible materials, novel stent designs, and easier techniques for deployment.

Although stents have been described in reports dating back to the 1800s, the concept of using stents to relieve acute tracheobronchial obstruction was not reported until the mid-1950s.¹ Dumon² designed a dedicated endoluminal upper airway stent in the late 1990s, and it remains today one of the most commonly used silicone stents. The self-expanding metal stents manufactured from biocompatible metal alloys also were pioneered in the 1990s using technology initially developed for vascular and coronary stents.^3,4 The ideal tracheobronchial stent has yet to be perfected, and there are potentially life-threatening risks associated with all stents currently on the market. Making the correct stent selection therefore is critical for the well-being of the patient.

The indications for deployment of airway stents include (1) extrinsic compression of the central airways with or without intraluminal components owing to malignant or benign disorders; (2) complex, inoperable tracheobronchial strictures; (3) tracheobronchial malacia; (4) palliation for recurrent intraluminal tumor growth; and (5) central airway fistulas (i.e., esophagus, mediastinum, or pleura).

Presenting signs and findings may include dyspnea, cough, hemoptysis, recurrent lung infections, wheezing, and stridor. On occasion, a patient may be referred for evaluation of findings made on a screening CT scan. Since many types of endobronchial therapies are available for patients with airway disorders, it is important to recognize that stenting is just one such modality and the patient may benefit from a combination of treatments.

Desirable Stent Characteristics

The ideal stent should be easy to insert and remove yet resistant to migration. It should be sufficiently strong to support the airway yet flexible enough to withstand (and collapse with) cough without fracturing, narrowing, or moving. The material from which the stent is made should be biologically inert to minimize the formation of granulation tissues. The stent should not change in size when collapsed, or a scar may form at its two ends. The stent should be available in a variety of lengths and sizes, and its walls should be as thin as possible for a maximal intraluminal diameter to prevent retention of secretions. The stent should permit movement of secretions across its surface to prevent inspissation of secretions that could obstruct the stent, yet prevent any accumulation of secretions within it or distal to it. Finally, the stent should perfectly appose the airway wall to cover defects and prevent the ingrowth of tissues that may obstruct its lumen without causing airway ischemia or injury.

Tracheobronchial stents are classified according to their material composition (i.e., plastic, metal, or mixed) (Table 56-1). Plastic tracheobronchial stents usually are made of silicone, which is inexpensive and inert. They have solid walls, which prevent luminal obstruction secondary to tissue ingrowth, and they are removed easily, although rigid bronchoscopy usually is required for their insertion and removal. As a consequence of their relative mobility, plastic stents have a higher rate of migration (~10%). They are also somewhat thicker than metallic stents, which limits the intraluminal diameter and increases the probability of having retained airway secretions, particularly with smaller caliber stents.

The large number of metallic stents are composed of substances such as stainless steel, alloys that incorporate cobalt and chromium, and Nitinol, a biologically inert titanium and nickel alloy. Some of these metals are calibrated to expand maximally at body temperature. The most commonly used are the self-expandable metallic stents (SEMS). These stents are stored in the collapsed state and revert to the fully expanded state on release in the correct airway location. Deployment mechanisms vary among SEMS. The walls of SEMS are thinner than plastic, yielding a larger intraluminal diameter, which permits deployment of smaller stents. The stents are quite strong and are designed to last a lifetime. Because the delivery systems are smaller, they can be inserted with a flexible bronchoscope, sparing the necessity of general anesthesia.

SEMS are available in two forms: covered and uncovered. The benefit of the uncovered variety is that within 3 to 4 months of placement, the stent becomes incorporated in the walls of the trachea or bronchus and lined with a new ciliated epithelial tissue layer, which facilitates the patient’s ability to clear secretions. The uncovered metal stent is, for all practical purposes, a permanent prosthesis that permits tissue ingrowth between the metallic components, making it extremely difficult to remove the stent if it fails. For this reason, uncovered SEMS often are not recommended for benign tracheobronchial processes or for patients who are expected to survive for a long time. Covered SEMS were developed specifically to deal with the complication of tumor ingrowth. These are usually coated with Silastic or polyurethane and are essentially identical to the uncovered variety.

Since neither the metallic nor plastic stents have all the desired characteristics of the ideal stent, a number of manufacturers recently have introduced combination stents. These are made partly of metal and partly of silicone or other plastic materials. Nitinol is a component of many of the combination stents. The metal in the stent provides strength and helps reduce migration, while the plastic material provides contiguous coverage and helps maintain its shape.

The two most commonly used balloon-expandable metallic stents are the Palmaz stent and the Strecker stent. The Palmaz stent (Johnson & Johnson, New Brunswick, NJ, and Interventional Systems, Warren, NJ) is reportedly the device used most commonly in children, in part, because of its small size. The stent consists of a 150-μm diameter slotted stainless steel mesh tube. It is available in lengths ranging from 10 to 40 mm. A balloon 6 to 10 mm in diameter fits inside the stent for manual expansion by as much as 6 to 12 mm. An appropriate size for expansion in children is 8 mm for the trachea and 6 mm for the bronchus. After balloon expansion, the stent ceases to exert outward pressure on the airway wall. The stent has been used in primary tracheomalacia or bronchomalacia, external compression of the trachea or bronchi, and collapse of the trachea or bronchi from previous surgery. Occasionally, it needs to be re-expanded, particularly after a violent coughing fit. The Strecker stent is made of a tantalum filament that is fashioned into a cylindrical wire mesh. The stent is flexible, whether compressed or expanded. When expanded, the stent does not change in length. The Strecker stent is 2 to 4 cm long and can be expanded by 8 to 11 mm. This stent has been used successfully in patients with tracheobronchial obstruction.

Specific examples of the SEMS include the Gianturco-Z (William Cook, Bjaeverskov, Denmark), the Wallstent (Boston Scientific Corporation, Natick, MA), and the Ultraflex stent (Boston Scientific Corporation, Natick, MA). The Gianturco-Z is composed of 460-μm stainless steel filaments that are arranged in a zigzag configuration. The diameter of the stent when expanded is 15 to 40 mm.⁵ The stent is available in 2- and 2.5-cm lengths. In its original design, it has metal hooks to prevent migration. The Gianturco-Z stent has been used to expand the tracheobronchial region in benign disease (e.g., posterior anastomotic strictures, tracheal stenosis, and tracheobronchomalacia). The stent exerts adequate radial force and does not shorten when deployed. It does have a tendency to spring forward if released too quickly. Complications are sometimes reported, including breakdown or unraveling of the stent and fatal hemoptysis after erosion into the pulmonary artery.

The Wallstent (originally the Schneider stent) is a stainless steel device composed of approximately 15 to 20 braided (100-μm diameter) filaments (Fig. 56-1). The filaments are arranged in a crisscross fashion to form a cylindrical mesh. Stent diameters range from 6 to 25 mm; lengths range from 2 to 7 cm. The stent exerts adequate radial force and is flexible. However, it can shorten to 20% to 40% of its original size on deployment. An important advantage of using the Wallstent is the ability one has to cut small openings into the mesh when the stent traverses bronchial openings. A disadvantage is that it changes length whenever it is compressed, potentially causing scars and stenosis at its edges.

Stents made of Nitinol (e.g., Nitinol, InStent, and Ultraflex stent) are thermally triggered and change shape in response to temperature changes (Marmen effect).⁶ The Nitinol wire is heated, made into a helical shape, and then cooled for deployment. With release into the target site, the high body temperature causes the stent to coil back into its original helical shape. Alternatively, a current of 1.5 to 3 A or 3 to 5 V can be applied to the stent for 1 to 2 seconds until it reaches a temperature of 40°C, causing it to convert to the fully expanded state. Ultraflex stents are configured such that the wire backbone is perpendicular to the airway wall, which prevents substantial shortening or lengthening with changes in airway width and lends considerable stability to the stent (Fig. 56-2).

Silicone Stents