In patients with central airway obstruction (CAO), expected results and adverse events after stent insertion are directly related to the biomechanical properties of the various airway stents (Table 1). A thorough understanding of these biomechanical properties, particularly retrievability, stent size, expansive radial force, fatigability, resistance to buckling, and hydrophilic properties, is critical for appropriate stent selection and for prevention of complications. In this editorial, we will address these properties as pertinent to the practice of bronchoscopists caring for patients requiring airway stents.
Operators contemplating stent insertion must consider stent retrievability, as airway stents may be required only for a short duration. This is the case in patients with malignant obstruction who respond to chemotherapy and/or radiation therapy or in patients with benign obstruction, such as postintubation tracheal stenosis, when occasionally stents may be successfully removed without stricture recurrence. More importantly, however, stents need to be removed when stent-related complications occur. In fact, stent fracture, excessive granulation tissue, and epithelialization as can be seen with metallic stents (uncovered, partially or fully covered) may result in embedding of the stent in the airway wall, which potentially precludes a safe stent removal. Published literature suggests that self-expandable metallic stents (SEMS) are “difficult” to remove after 3 months in vivo; however, there are case reports of SEMS removal 10 years after insertion. Complications of SEMS removal include stent fracture, inability to completely remove it (lose the airway), airway perforation with pneumomediastinum and pneumothorax, airway bleeding, laryngeal trauma/edema/spasm, retained stent pieces, reobstruction, respiratory failure, and death. In 1 study, the average number of procedures per patient required to remove the stents and address the immediate related complications was 2.6. Most indications for removal are reported in patients with benign obstruction, and most complications during removal occur in patients with uncovered SEMS and in patients with long-term stenting (after 3 to 4 mo)1,2
In general, the stent size is selected to achieve at least 50% patency in the obstructed airway, as suggested by flow dynamic studies.3 The ability to achieve airway patency is highly dependent on the expansive radial force of the stent material, which has been shown to vary by stent type.4,5 Expert opinion recommendations are to insert a stent 10% to 15% larger than the airway diameter after dilation. Oversizing a stent or choosing a stent with an excessive expansive radial force may impair the airway mucosal blood flow and promote granulation tissue formation. Indeed, significantly oversized stents are associated with airway necrosis due to obstruction of microcirculation. A mismatch between the shape of the stent and the shape of the airway, especially when devices are slightly oversized, can lead to critically high-localized pressures on the airway mucosa. In combination with impaired blood supply and hypoxia this can easily result in permanent tissue damage. Undersizing or choosing a stent with inadequate radial force will increase the risk for stent migration and inadequate airway patency. Migration is one of the most common complications associated with the use of stents, and is more common in benign CAO, likely because of the viscoelastic properties of the airway tissues. In patients with CAO due to cancer, stent migration can definitely occur in patients undergoing chemotherapy or radiation therapy. Stent undersizing and use of silicone stents are risk factors for migration. One study showed the following relationship between sent-to-airway diameter and migration rate: ratio 90% to 100%, migration 5.26%; ratio 80% to 90%, migration 6.06%; and ratio <80%, migration 15.38%.6
Fatigability and resistance to buckling are important properties that determine the risk for stent fracture and collapse.5,7 Fatigue life, defined as time to weakness and potential fracture, results from chronic compression of the stent both during tidal and especially during forced exhalation and coughing. Stent fracture is fortunately uncommon but may occur with metal stents, covered or uncovered. It may result in airway wall perforation and hemoptysis. This complication is related to the metal mesh fatigability. This property is particularly important in patients with benign CAO, especially tracheobronchomalacia, in which cycled compression of the stent with each exhalation may lead to stent fracture and its associated complications. Stent fracture is more common with metallic stents than with silicone stents and in patients with benign CAO where airway stents may be in place for many years. However, metallic stents are more likely to resist angulation, as may be seen when stenting is required in the naturally curved and tapered left main bronchus. In fact, 1 study has shown that patients with malignant CAO are more likely to receive a covered metallic stent in the left main bronchus, whereas silicone stents are more commonly used in the straighter airways such as the trachea and right main bronchus.8
Hydrophilic properties impact the risk of mucous plugging and may potentially lower respiratory tract infection rate following airway stenting. The hydrophilic inner coating in covered metallic stents and silicone stents is specifically designed to help prevent mucous build-up. Studies in patients with malignant CAO have shown that silicone and covered metal stents are associated with a higher risk for infection and silicone stents are associated with a higher risk for mucous impaction as well as stent migration.9
Stent-tissue interactions directly determine the various complications that may occur and the expected results with regard to maintaining airway patency. Many of these stent properties are currently considered proprietary and are not available to clinicians to guide stent selection in individual patients. The overall clinical behavior of a stent is, however, complex and can only be partially explained by the factors discussed in this article. A fair comparison between different stent designs in the same patient population is impossible at this time as there are no comparative studies. Pending future clinical outcome research and novel stent developments (biodegradable, drug-eluting, customized 3D printed stents), bronchoscopists should take into account the biomechanical properties of the currently available stents in their approach to patients suffering from CAO.
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9. Ost DE, Shah AM, Lei X, et al.. Respiratory infections increase the risk of granulation tissue formation following airway stenting in patients with malignant airway obstruction. Chest. 2012;141:1473–1481.