The mechanical characteristics and performance evaluation of a newly developed silicone airway stent (GINA stent)

GINA stent, a radiopaque silicone airway stent that was recently developed by the authors, was designed to minimize migration, mucostasis, and granulation tissue formation. The mechanical tests performed in the current study confirmed that the GINA stent has a lower possibility of migration and granulation tissue formation, compared to the Dumon stent. Moreover, the results of the performance evaluation using porcine models suggest that the performance of the GINA stent is not inferior to that of the Dumon stent.

The conditions regarding an ideal airway stent are cost-effectiveness, ease of insertion and removal, devoid of migration or granulation tissue formation, not excessive but adequate expansion force against airway stenosis, adequate flexibility to preserve the airway physiology, and without any impairment of the mucociliary clearance7. However, no stent is capable of fulfilling all of the aforementioned conditions; if one characteristic is superior, another tends to be inferior. For instance, if the stent has low expansion force and the likelihood of granulation tissue formation is reduced, the airway fixation of the stent declines, thereby increasing the risk of stent migration. Accordingly, metal stents have a low migration rate, but a high rate of granulation tissue formation, whereas silicone stents have a high migration rate, but a low rate of granulation tissue formation1. In terms of the inhibition of mucostasis in the stent, it is desirable to maintain the mucociliary clearance, and necessary to be flexible so that the stent inner diameter is sufficiently reduced during exhalation. The uncovered metal stent could best preserve the mucociliary clearance. However, removal of the stent is difficult, owing to epithelialization, in addition to the problem of tissue ingrowth within the stent. Consequently, an uncovered metal stent is not recommended for the management of benign airway stenosis; the use is restricted to the palliation of malignant airway stenosis, but tumor ingrowth should be a concern8. After all, improving the flexibility of the stent (enabling the reduction of stent cross-sectional area) is the most rational method of resolving mucostasis, thereby facilitating the removal of airway secretions through the enhanced expiratory flow.

In the present study, the GINA stent showed a lower expansion force and higher flexibility, compared to the Dumon stent. Although the current study did not observe any substantial difference between the two types of stents with regard to the formation of granulation tissue in the porcine models, it could be attributed to the short observation period. Granulation tissue formation is a common complication of silicone airway stents, although less than in metal stents1,4,9. Excessive expansion force and low flexibility are the predisposing factors associated with granulation tissue formation9,10,11,12. The low expansion force and high flexibility of the GINA stent implies that less force is required to expand and bend the stent, resulting in less pressure on the airway, leading to diminished mucosal inflammation and granulation tissue formation.

Despite the low expansion force, the GINA stent displayed a higher anti-migration force, compared to the Dumon stent, which was further confirmed by the animal experiments. The aforesaid superiority can be attributed to the creative surface design of the GINA stent, which comprises a right-angled triangle-shaped outer ring pertaining to the cartilaginous trachea and a raised, three-line arrangement pertaining to the membranous trachea. During the formulation of the design of the GINA stent, we reviewed several previously developed airway stents and the most inspiring were the Freitag stent and the Natural stent13,14,15. The two aforementioned stents have a common outer ring for cartilaginous trachea, which plays a role in the inhibition of migration. The current design improved on this feature (to maximize the anti-migratory friction) by transforming the outer ring into a right-angled triangle shape, and including a raised, three-line arrangement for the membranous trachea. The right-angled triangle-shaped outer ring is specifically designed to further suppress the proximal migration of the stent, which is more dangerous, compared to distal migration (proximal migration can lead to glottic obstruction or complete stent breakaway, resulting in suffocation)16,17. The present study confirmed this through the results of the mechanical test, which showed an improved anti-migration force in relation to the GINA stent backward direction. Moreover, the in-vivo effectiveness was recognized to a certain extent through the evaluation of short-term performance. Migration is a common complication associated with airway stents, especially silicone stent18,19, and several attempts have been made to inhibit the same. The Montgomery T tube was fabricated with a side arm that passes through a tracheostomy, which provides the stent with a fixation to the trachea20. Recently, an external fixation method was introduced, which resolved the cosmetic problem associated with the Montgomery T tube21. However, these methods can only be employed for the management of upper tracheal stenosis. In case of lower tracheal or bronchial stenosis, a bifurcation stent might facilitate the prevention of migration. However, the stent insertion is a challenging endeavor, owing to the size of the stent, which is excessively large, compared to the segment of stenosis22,23. Nevertheless, in order to prevent the migration of the stent, it is necessary to improve the friction (i.e., anti-migration force) using stents of suitable dimensions [diameter larger than the stenosis, but slightly smaller (80–90%), compared to the airway diameter around the stenosis]4,24 as well as by improving the stent surface design (like studs, spikes, or protruding arcs)5,16,21,25,26.

Another important feature of the GINA stent is the flexible, dynamic structure, which enables the reduction of the stent cross-sectional area. The GINA stent has a flat part, similar to the membranous portion of the actual tracheobronchial tree, which makes the stent more contractible and facilitates the removal of airway secretions through an enhanced expiratory flow. Freitag and Kim have already demonstrated that flattening a part of the stent improves the mucostasis13,14,15,27. The current performance study did not observe any substantial difference between the two types of stents with regard to the mucostasis, which might be ascribed to the short observational period.

Lastly, the GINA stent is radiopaque, which makes stent-tracking easier. The radiolucency of silicone stents (such as Dumon stents) has been considered to be a major drawback and efforts have been made to improve the same. Recently, a radiopaque version of the Dumon stent was developed.

Despite the success associated with the development of the GINA stent, the current study does not preclude limitations. First, the sample size of the animals that were involved in the experimental evaluation of the performance of the stents was small and the duration of observation was short. The current study did not observe any substantial difference between the two types of stents with regard to the mucus retention and granulation tissue formation, which is presumed to be due to the short duration of the experiment. The current study observed a difference between the two types of stents with reference to migration. However, it was not statistically significant, on account of the small sample size. Second, the current study did not compare the histology of the tissues at the sites of stent insertion. However, a follow-up study to ascertain the difference between the two types of stents with regard to the degree of injury at the site of stent insertion will be planned in the future. Third, the current study performed the mechanical tests on the basis of the advice provided by the stent manufacturing company (S&G Biotech, Gyeonggi-do, Korea) and previous studies5,28,29. Due to the lack of a validated method, existing studies have used simple or complex methods according to the nature and requirements of the respective studies. Consequently, the authors were compelled to conduct the experiments in a selective manner, in accordance with the laboratory conditions. For instance, it is more desirable to assess the anti-migration force using ex-vivo tracheal tissue or the materials that mimic the same, but we could not. Therefore, the current results should be interpreted with due consideration of the limitations.

In conclusion, the authors have developed a new stent through strategic design, which has reduced migration, despite the low expansion force and increased flexibility, in order to reduce the likelihood of granulation tissue formation. The scenario warrants future clinical trials to demonstrate the efficacy and safety of GINA stents in humans.