Preclinical evaluation of a pediatric airway stent for tracheobronchomalacia

Abstract

Objectives:We sought to demonstrate in an animal model that helical NiTi stents designed for malacic airways could be delivered and removed without significant trauma while minimally impeding mucus clearance during the period of implantation.Methods: Stents were delivered and removed from the tracheas of healthy 20 kg swine (n=5) using tools designed to minimize trauma. In 4-week experiments, the stents were implanted on day 0, removed after 3 weeks and euthanasia was performed after 4 weeks. Weekly bronchoscopies, Xrays and mucus clearance examinations were performed in vivo. Hematoxylin and eosin (H&E) staining and scanning electron microscopy (SEM) imaging were used to evaluate foreign body response, tracheal tissue reaction and damage and to measure non-ciliated regions.Results: In all in vivo experiments, the stent was implanted and removed atraumatically. Mucus clearance was maintained throughout the experiment period. H&E stained slides showed that foreign body response and tracheal tissue damage were localized to the stented subsections. Tracheal tissue reaction and damage was further restricted to the epithelium and submucosal layers. SEM imaging revealed that the cilia was absent only over the contact area between the trachea and the wire forming the helical stent.Conclusions: Helical NiTi stents designed to provide radial support for malacic airways were well tolerated in a porcine model, providing for mucus clearance while also enabling atraumatic removal.

In Vivo Molding of Airway Stents

“…This outer diameter is about three times the diameter of the NiTi wire used in refs. [ 2,22 ] but is small enough that it does not impede airflow and mucus transport as verified by our in vivo trial described later in this report. This larger diameter also reduces the stress in the regions of contact between the stent and the tissue further reducing the risk of tissue erosion.…”

“…Furthermore, helical airway stents made from superelastic nitinol (NiTi) metal have previously been demonstrated to possess a number of desirable properties including resistance to migration, minimal disruption to mucus flow, and the ability to maintain patency of malacic airways while not eroding through tracheal tissue. [ 12,22,23 ] Moreover, if a helical stent becomes endothelialized, it can be removed with minimal damage to the airways using an unscrewing motion similar to how a corkscrew is removed from a cork. [ 12 ]…”

“…While the use of polymers enables the design of stents with a broad range of mechanical stiffnesses, they are also much more compliant than metals. Since helical NiTi stents have been demonstrated to provide sufficient radial support and resistance to migration while not eroding through tracheal tissue, [ 12,22,23 ] our goal was to demonstrate that we could fabricate in vivo molded stents of a comparable stiffness. We selected tubing with an inner diameter of 1.14 mm and an outer diameter of 1.57 mm.…”

“…A stent pitch of 18.6 mm was tested to match prior published results. [ 12,22 ] Since stents are preloaded against the airway, we collected data around operating points corresponding to diametral compressions of 0.5–3.0 mm. To capture hysteresis around each operating point, stent diameters were cyclically compressed around each operating point by ±0.2 mm to produce Figure 3e.…”

“…Our goal in selecting the stent tubing cross section had been to produce an in vivo molded stent with approximately the same stiffness as the 18.6 mm pitch NiTi stents reported in ref. [ 22 ] . As seen in Figure 3e, the in vivo molded stents produce approximately the same force as the NiTi stents for displacements up to 1 mm and produce higher forces for larger displacements.…”