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).
The Dumon stent (Boston Medical Products, Westborough, MA) is a cylindrical silicone stent with external studs that are placed at regular intervals to prevent migration and limit contact with the airway wall, thereby reducing mucosal ischemia (Fig. 56-3). Several other varieties of silicone stents are currently available from Hood (Hood Laboratories, Pembroke, MA) (Figs. 56-4 and 56-5) in the United States and other manufacturers worldwide that have slightly different types of posts or rings to reduce migration. All these silicone stents are less expensive than metallic stents, are available in many sizes, and even can be custom manufactured within a few days. Y stents adjusted to cover the distal trachea and both mainstem bronchi are also available from most of these manufacturers and can be custom made with respect to sizes and angles. Once placed in the airway, the stent can be adjusted with a forceps and bronchoscope.
The Montgomery T-tube, which was introduced in the mid-1960s to support the trachea after laryngotracheoplasty (Fig. 56-6), also can be used for a number of stenting applications. In its original form, the device was an uncuffed silicone T-tube that was inserted with the long limb in the distal trachea, a short limb in the proximal trachea (or in some patients even through the vocal cords), and the T limb projecting through the tracheostomy stoma. Several modifications have been made in the original design to allow for a proximal cuff and other extras, such as a T-Y stent. The T-tube is supplied in sizes ranging from 4.5 to 16 mm (external diameter) and can be custom made as well. Smaller sizes (4.5–8 mm in external diameter) are available for pediatric use.
Other plastic and combination stents include the Reynder stent (Reynder's Medical Supply, Lennik, Belgium), a cylindrical silicone prosthesis that is more rigid than a regular silicone tube but requires a special introducer and a bronchoscope for placement. The Dynamic stent (Rusch AG, Duluth, GA) is a silicone Y stent with anterior and lateral walls that are reinforced with metal to simulate the tracheal wall. Special forceps are available for insertion within the rigid laryngoscope (Fig. 56-7). The Polyflex (Rusch AG, Duluth, GA) is a self-expandable stent made of polyester wire mesh within layers of silicone (Fig. 56-8). Essentially, this is a silicone stent that can be deployed with flexible bronchoscopy, but rigid bronchoscopy is required for removal. Novastent (Novadis, Saint-Victoret, France) is a thin silicone sheet that contains a small metallic hoop of Nitinol alloy. The silicone bands on the ends are designed to prevent migration.
The AERO™ stent (manufactured by MERIT ENDOTEK™, a division of Merit Medical Systems, Inc., South Jordan, Utah) is another recent addition to the family of composite stents (Fig. 56-9). It is a combination stent made of a Nitinol metal scaffold covered with a silicone-containing biocompatible membrane to minimize the possibility of tissue ingrowth and granuloma formation. The stent does not foreshorten on delivery, and the hydrophilic coating on the inner lumen of the stent minimizes the possibility of mucous adherence and accumulation.
AERO™ Tracheobronchial stent system. Manufactured by MERIT ENDOTEK™, a division of Merit Medical Systems, Inc., South Jordan, Utah.
Although we have attempted to review most of the stents that are or have been on the market, one should take account that additional stents are constantly invented and manufactured and the companies making them are bought and sold to larger device companies consistent with the observation that most do not solve all current clinical problems.