Induction of tracheal stenosis in a rabbit model-endoscopic versus open technique

Establishment of a survival rabbit model for laryngotracheal stenosis: A prospective randomized study

Objective

To develop a reproducible survival rabbit model for laryngotracheal stenosis (LTS).

Methods

Seventy New Zealand white (NZW) rabbits were randomly divided into experimental groups (n = 30) and a control group (n = 40). In experimental groups, a nylon brush was inserted retrograde from the tracheotomy through the subglottis and rotated until a full layer circumferential mucosal injury to cartilage exposure, assisted by fiberoptic laryngoscopy (FOL) visualization. Experimental group 1 (n = 10), rotated 10 times; group 2 (n = 20), rotated 20 times. The control group underwent tracheotomy only without nylon brush scraping. The rabbits underwent FOL at 1st, 4th, 8th, and 12th week postinjury respectively to observe the formation of LTS. They were euthanized and the larynxes and tracheas were subjected to gross and histopathological examination at 12 weeks postinjury.

Results

The control group all survived, while five cases in experimental groups died from LTS and/or mucous plug. Histological observation showed that the control group had intact laryngotracheal mucosal epithelium without any stenosis; the experimental groups showed proliferation of fibroblasts and thickening of collagen fibers. The mean stenosis in control group was 9.31 ± 0.98%, while that in experimental group 1 was 32.78 ± 7.07% and 58.25 ± 8.96% in experimental group 2. The difference between the three groups was statistically significant (χ 2 = 47.98, p < .05).

Conclusions

We successfully developed a reproducible survival rabbit model for LTS using a nylon brush through FOL visualization combined with tracheostomy. This model can provide a mature and stable animal model for the exploration of wound-healing pathophysiology and the effect of interventions.

Level of evidence

NA.

The Wound Healing of Autologous Regenerative Factor on Recurrent Benign Airway Stenosis: A Canine Experimental and Pilot Study

INTRODUCTION

Benign airway stenosis (BAS) is a severe pathologic condition. Complex stenosis has a high recurrence rate and requires repeated bronchoscopic interventions for achieving optimal control, leading to recurrent BAS (RBAS) due to intraluminal granulation.

METHODS

This study explored the potential of autologous regenerative factor (ARF) for treating RBAS using a post-intubation tracheal stenosis canine model. Bronchoscopic follow-ups were conducted, and RNA-seq analysis of airway tissue was performed. A clinical study was also initiated involving 17 patients with recurrent airway stenosis.

RESULTS

In the animal model, ARF demonstrated significant effectiveness in preventing further collapse of the injured airway, maintaining airway patency and promoting tissue regeneration. RNA-seq results showed differential gene expression, signifying alterations in cellular components and signaling pathways. The clinical study found that ARF treatment was well-tolerated by patients with no severe adverse events requiring hospitalization. ARF treatment yielded a high response rate, especially for post-intubation tracheal stenosis and idiopathic tracheal stenosis patients.

CONCLUSION

The study concludes that ARF presents a promising, effective, and less-invasive method for treating RBAS. ARF has shown potential in prolonging the intermittent period and reducing treatment failure in patients with recurrent tracheal stenosis by facilitating tracheal mucosal wound repair and ameliorating tracheal fibrosis. This novel approach could significantly impact future clinical applications.

Comparative evaluation of mechanical injury methods for establishing stable tracheal stenosis animal models

The study aimed to assess the stability of various mechanical injury techniques in creating tracheal stenosis animal models using endoscopic assistance and investigate the viability of tracheal stoma in this process. Twenty-six healthy adult New Zealand white rabbits were randomly assigned to an experimental and control group. The experimental group underwent tracheal incision followed by steel brush scraping with endoscopic assistance, while the control group received nylon brush scraping. Within the control group, two subgroups were formed: Group A underwent scraping without tracheal stoma, and Group B underwent scraping followed by tracheal stoma. Additionally, a sham operation was performed on a separate group without subsequent scratching, resulting in no stenosis formation. Endoscopic observations were conducted at 7, 14, and 21 days post-scraping, followed by histological examinations of euthanized rabbits on the 21st day. Notably, all rabbits in the non-stoma group survived without complications, whereas Group B rabbits faced mortality post-operation. Histological assessments revealed inflammatory cell infiltration, fibroblast proliferation, and collagen fiber deposition in narrowed tracheal specimens. Steel brush scraping with endoscopic assistance proved more effective in inducing stable tracheal stenosis compared to nylon brush scraping. However, the survival challenges of rabbits with tracheal fistula require further investigation.

Establishment of two canine models of benign airway stenosis and the effect of mitomycin C on airway stenosis

OBJECTIVES

Cuffed endotracheal intubation and stent implantation were employed to simulate two types of benign airway stenosis and further to analysis the different features between them from trachecscopic characteristics, gross anatomy to histopathological changes. In addition, our study explored the therapeutic effect of mitomycin C at different concentrations on granulation tissue caused by stent implantation in order to provide a new therapeutic strategy for clinical treatment of benign airway stenosis.

METHODS

Twelve beagle dogs were randomly divided into four groups, with three dogs in each group. Group A: Three beagle dogs were intubated through oral trachea after general anesthesia and cuff pressure maintained at 200 mmHg for 24 h. Group B, Group C and Group D: endotracheal coated self-expanding metal stents were placed after general anesthesia under the guidance of bronchoscope. On the Day7 after stent implantation, Group B, as control group, was injected phosphate buffer solution of 1 ml into granulation tissue at the end of stent; Group C was injected mitomycin C of 1 ml at 0.4 mg/ml and Group D was injected mitomycin C of 1 ml at 0.8 mg/ml into granulation tissue at the end of metal airway stent respectively, the same method as Group B. Bronchoscopy was used to observe tracheal lumen on the seventh day, fourteenth day and twenty-first day after modeling and pathological changes were examined on twenty-first day.

RESULTS

Two models of benign airway stenosis can be established by cuffed endotracheal intubation and stent implantation. There was tracheal rupture in the trachea cartiage ring in the cuffed endotracheal intubation group, but was't in stent implantation group. Histopathological characteristics were different between cuffed endotracheal intubation and stent implantation groups. In stent placement groups, we found that the stenosis degree of mitomycin C at 0.4 mg/ml was approximately 19%-32%, mitomycin C at 0.8 mg/ml was approximately 16%-21% and the control group was approximately 36%-47%.

CONCLUSION

The two models of canine benign tracheal stenosis induced by cuffed endotracheal intubation and stent implantation are relatively simple, reliable and reproducible and have different characteristics. Mitomycin C could inhibit proliferation of granulation tissue and attenuate the degree of airway stenosis caused by stent implantation.

3D bioprinted silk fibroin hydrogels for tissue engineering

The development of biocompatible and precisely printable bioink addresses the growing demand for three-dimensional (3D) bioprinting applications in the field of tissue engineering. We developed a methacrylated photocurable silk fibroin (SF) bioink for digital light processing 3D bioprinting to generate structures with high mechanical stability and biocompatibility for tissue engineering applications. Procedure 1 describes the synthesis of photocurable methacrylated SF bioink, which takes 2 weeks to complete. Digital light processing is used to fabricate 3D hydrogels using the bioink (1.5 h), which are characterized in terms of methacrylation, printability, mechanical and rheological properties, and biocompatibility. The physicochemical properties of the bioink can be modulated by varying photopolymerization conditions such as the degree of methacrylation, light intensity, and concentration of the photoinitiator and bioink. The versatile bioink can be used broadly in a range of applications, including nerve tissue engineering through co-polymerization of the bioink with graphene oxide, and for wound healing as a sealant. Procedure 2 outlines how to apply 3D-printed SF hydrogels embedded with chondrocytes and turbinate-derived mesenchymal stem cells in one specific in vivo application, trachea tissue engineering, which takes 2-9 weeks.

A modified rabbit model of tracheal stenosis and a household endoscope. More simplicity and accessibility

Purpose:

To develop a simpler animal model for benign tracheal stenosis and introduce a low-cost household endoscope for postmodeling endotracheal evaluation.

Methods:

Twenty rabbits were randomly divided into a model group (15 rabbits, subjected to transoral nylon brush scraping of the trachea) and a mock group (5 rabbits, merely exempted from scraping), a household endoscope was then introduced for weekly endoscopic examination. Meanwhile, other 15 rabbits (modeling like the model group) underwent batch tracheal resection at different postintervention times for pathological analysis.

Results:

The model group presented a low mortality and few complications. The endoscope could obtain adequate images for stenosis assessment, which showed that the models presented homogeneous injury after scraping and developed a mature scar stricture at 28 days postoperatively with a mean stenosis degree of 65.9%, and 71.4% (10/14) above Myer–Cotton’s grade II. The pathological findings were consistent with the clinicopathological process of human. No stenosis was found in mock group.

Conclusion:

The modified model is simpler, minimally invasive and reliable, while the household endoscope is competent for model’s follow-up, providing easily accessible and useful tools for facilitating more extensive studies of benign tracheal stenosis.

Rabbit model of consistently survivable subglottic stenosis using a modified brush technique

INTRODUCTION

Several animal models of subglottic stenosis (SGS) have been described in the literature, however many result in severe stenosis that requires early intervention and carry a high mortality rate. This limits the application of the model and may require the use of additional animals to achieve desired results due to procedural complications. A novel endoscopic method of inducing SGS in a rabbit model was developed as part of a larger investigation on the treatment of this condition. The objective of this study was to develop an animal model for survivable subglottic stenosis.

METHODS

12 New Zealand white rabbits underwent 2 trials of prolonged intubation that were not successful in inducing SGS. A partially sheathed nylon brush injury technique was then designed and implemented. Airway assessment consisted of rigid bronchoscopy 6 weeks and 8 months after injury.

RESULTS

12 rabbits undergoing subglottic brush injury had focal posteriorly based subglottic stenosis on bronchoscopy at 6 weeks and 8 months post-injury. One rabbit was euthanized after the brush induced subglottic injury but prior to 6 week bronchoscopy due to an unrelated orthopedic injury. This animal was therefore excluded from analysis and replaced. No rabbits required early airway intervention or sacrifice. All survived a period of 8 months.

CONCLUSION

Inducing subglottic injury with a partially-sheathed nylon brush safely and reliably creates a controlled SGS with zero procedure-related mortality over 8 months. This model could be the basis for a longer-term evaluation of subglottic scar evolution and intervention.

Subglottic Stenosis: Development of a Clinically Relevant Endoscopic Animal Model

OBJECTIVE

Develop a clinically relevant and reproducible endoscopic animal model for subglottic stenosis amenable to testing of minimally invasive therapeutic modalities.

STUDY DESIGN

Cohort study.

SETTING

Division of Laboratory Animals Research, University of Pittsburgh.

SUBJECTS AND METHODS

Subglottic stenosis was induced endoscopically via microsuspension laryngoscopy in 26 New Zealand white rabbits. A trimmed polypropylene brush connected to a novel electronic stenosis induction apparatus was used to create circumferential trauma to the subglottis. By using open source image analysis software, the cross-sectional areas of the stenotic and native airways were compared to calculate the percentage of stenosis and the Myer-Cotton classification grade.

RESULTS

Of the 26 rabbits, 24 (92%) exhibited stenosis after the first attempt. The mean percentage of airway stenosis was 57% (range, 34%-85%; SD, 15%). Five rabbits (19.2%) died on the day of stenosis induction from procedural complications. Of the 21 rabbits, 2 demonstrated no stenosis 7 days after initial injury and so underwent reinduction of airway injury, upon which they developed stenosis. Overall, 14 of the 21 rabbits (67%) exhibited moderate to severe stenosis (grade 2 or 3).

CONCLUSION

The stenosis induction apparatus reliably induced stenosis with a low mortality rate as compared with that of other methods in the literature. The device could be improved to generate a predetermined potentially reproducible grade of stenosis as desired by the operator. This method sets the stage for a clinically relevant and reproducible subglottic stenosis disease model that is amenable to testing of minimally invasive treatment modalities.

Permanent Implantable Medical Devices in Exotic Pet Medicine

Medical devices are defined as implantable if they are intended to remain in the body after the procedure. In veterinary medicine, use of such devices is marginal but may find some indications. Use in exotic pet medicine is even more challenging due to size restriction and the limited data available. This review focuses on the esophageal and tracheal stent in the case of stricture, ureteral stent and subcutaneous ureteral bypass in the case of ureteral obstruction, permanent urinary diversion in the case of bladder atony, and pacemaker in the case of severe arrythmias. Comparative aspects are developed.

Development of a survival animal model for subglottic stenosis

OBJECTIVE

To develop a reproducible survival animal model for subglottic stenosis.

STUDY DESIGN

Prospective study.

METHODS

We evaluated five methods of inducing airway injury in 30 New Zealand white rabbits to produce a subglottic stenosis model. Experimental groups comprised: group 1 (n = 5), which underwent 4-hour intubation; group 2 (n = 5), which underwent induced subglottic injury with a nylon brush; group 3 (n = 10), which underwent subglottic injury with a nylon brush, followed by 4-hour intubation; group 4 (n = 5), which underwent subglottic injury with Bugbee cautery in 50% of the subglottic circumference, followed by 4-hour intubation; and group 5 (n = 5), which underwent subglottic injury with Bugbee cautery in 75% of the subglottic circumference, followed by 4-hour intubation. Five animals were used as controls. Endoscopy of the airway and sacrifice of animals were planned at an interval of 14 days postinjury. Histologic measurements were analyzed.

RESULTS

No animals in groups 1 or 2 developed stenosis. In group 3, 50% of animals developed symptomatic grade 3 subglottic and tracheal stenosis, necessitating early endoscopy and sacrifice in three animals. Four animals in group 4 developed grade 1 subglottic stenosis, and four in group 5 developed grade 2 subglottic stenosis. Histologic measurements of lumen areas within each of these two groups were similar; all animals survived the follow-up period.

CONCLUSION

We successfully developed a reproducible survival model for induced subglottic stenosis using a combination of cautery-induced subglottic injury followed by 4-hour intubation. This model lays the foundation for future studies that evaluate endoscopic interventions for the management of subglottic stenosis.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:989-994, 2019.

A comparison of tracheal scaffold strategies for pediatric transplantation in a rabbit model

OBJECTIVES/HYPOTHESIS

Despite surgical advances, childhood tracheal stenosis is associated with high morbidity and mortality. Various tracheal scaffold strategies have been developed as the basis for bioengineered substitutes, but there is no consensus on which may be superior in vivo. We hypothesized that there would be no difference in morbidity and mortality between three competing scaffold strategies in rabbits.

STUDY DESIGN

Pilot preclinical study.

METHODS

Tracheal scaffolds were prepared by three methods that have been applied clinically and reported: preserved cadaveric ("Herberhold") allografts, detergent-enzymatically decellularized allografts, and synthetic scaffolds (nanocomposite polymer [polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU)]). Scaffolds were implanted into cervical trachea of New Zealand White rabbits (n = 4 per group) without cell seeding. Control animals (n = 4) received autotransplanted tracheal segments using the same technique. Animals underwent bronchoscopic monitoring of the grafts for 30 days. Macroscopic evaluation of tissue integration, graft stenosis, and collapsibility and histological examinations were performed on explants at termination.

RESULTS

All surgical controls survived to termination without airway compromise. Mild to moderate anastomotic stenosis from granulation tissue was detected, but there was evidence suggestive of vascular reconnection with minimal fibrous encapsulation. In contrast, three of the four animals in the Herberhold and POSS-PCU groups, and all animals receiving decellularized allografts, required early termination due to respiratory distress. Herberhold grafts showed intense inflammatory reactions, anastomotic stenoses, and mucus plugging. Synthetic graft integration and vascularization were poor, whereas decellularized grafts demonstrated malacia and collapse but had features suggestive of vascular connection or revascularization.

CONCLUSIONS

There are mirror-image benefits and drawbacks to nonrecellularized, decellularized, and synthetic grafts, such that none emerged as the preferred option. Results from prevascularized and/or cell-seeded grafts (as applied clinically) may elucidate clearer advantages of one scaffold type over another.

LEVEL OF EVIDENCE

NA. Laryngoscope, 127:E449-E457, 2017.

Diagnosis of subglottic stenosis in a rabbit model using long-range optical coherence tomography

OBJECTIVES/HYPOTHESIS

Current imaging modalities lack the necessary resolution to diagnose subglottic stenosis. The aim of this study was to use optical coherence tomography (OCT) to evaluate nascent subglottic mucosal injury and characterize mucosal thickness and structural changes using texture analysis in a simulated intubation rabbit model.

STUDY DESIGN

Prospective animal study in rabbits.

METHODS

Three-centimeter-long sections of endotracheal tubes (ETT) were endoscopically placed in the subglottis and proximal trachea of New Zealand White rabbits (n = 10) and secured via suture. OCT imaging and conventional endoscopic video was performed just prior to ETT segment placement (day 0), immediately after tube removal (day 7), and 1 week later (day 14). OCT images were analyzed for airway wall thickness and textural properties.

RESULTS

Endoscopy and histology of intubated rabbits showed a range of normal to edematous tissue, which correlated with OCT images. The mean airway mucosal wall thickness measured using OCT was 336.4 μm (day 0), 391.3 μm (day 7), and 420.4 μm (day 14), with significant differences between day 0 and day 14 (P = .002). Significance was found for correlation and homogeneity texture features across all time points (P < .05).

CONCLUSIONS

OCT is a minimally invasive endoscopic imaging modality capable of monitoring progression of subglottic mucosal injury. This study is the first to evaluate mucosal injury during simulated intubation using serial OCT imaging and texture analysis. OCT and texture analysis have the potential for early detection of subglottic mucosal injury, which could lead to better management of the neonatal airway and limit the progression to stenosis.

LEVEL OF EVIDENCE

NA Laryngoscope, 127:64-69, 2017.

Tracheobronchial brush cytology and bronchoalveolar lavage in dogs and cats with chronic cough: 45 cases (2012-2014)

Background

Animals with chronic cough can have normal bronchoalveolar lavage fluid cytology when small airway disease is absent. Cytology of a tracheobronchial brushing can detect inflammation in larger airways; however, evaluation of this technique has been limited in veterinary medicine.

Objective

To compare airway brush cytology to bronchoalveolar lavage fluid analysis in dogs and cats with chronic cough.

Animals

Forty dogs and five cats undergoing bronchoscopic investigation of chronic cough.

Methods

Prospective study. Bronchoscopy and bronchoalveolar lavage were performed followed by tracheobronchial brushing of central airways. Results of cytologic assessment of BAL fluid and brush cytology were compared for the presence or absence of inflammation and concordance of inflammatory cell type.

Results

Brush cytology detected central airway inflammation in 34 of 40 (85%) dogs with inflammatory BAL fluid. However, the type of inflammation reported differed in 23 of 34 dogs. In five cats with inflammation in BAL fluid, brush cytology detected inflammation in four; the type of inflammation was discordant in all cats.

Conclusions and clinical relevance

Brush cytology has good agreement with BAL regarding the presence of inflammation, although the type of inflammation detected with the different sampling techniques commonly varies. Brush cytology can provide supplementary information to BAL, and additional studies will provide further information on the role of tracheobronchial brush cytology in the diagnosis and management of respiratory conditions.

Rabbit model of tracheal stenosis induced by prolonged endotracheal intubation using a segmented tube

OBJECTIVE

Animal model of tracheal stenosis based on pathophysiology of prolonged endotracheal intubation has been rarely reported. We sought to verify the feasibility of inducing an animal model of tracheal stenosis by segmented endotracheal tube insertion in the New Zealand white rabbit model.

METHODS

Tracheal stenosis was induced by inserting a segmented endotracheal tube of 1.5cm length which was wrapped with a commercialized absorbable hemostat in 15 New Zealand white rabbits, while sham surgery controls (n=3) underwent tracheotomy and direct closure of tracheal exposure. The tube was removed transorally, 1 week after tube insertion. All rabbits were evaluated endoscopically at 1 week, 2 weeks and 3 weeks after the tube insertion. The rabbits were sacrificed 3 weeks after the surgery, and the excised tissue of trachea was processed along with the procedure of standard hematoxylin eosin staining and observed under a microscope.

RESULTS

Tracheal stenosis was induced in all rabbits (range 32-84% stenosis) with no death of rabbits during the study. The histological features of tracheal stenosis demonstrated thickening and fibrosis of lamina propria and submucosa with relatively intact cartilage framework.

CONCLUSIONS

We developed a rabbit model of tracheal stenosis induced by endotracheal intubation using a segmented tracheal tube. Since the model is based on the physiologic condition of prolonged endotracheal intubation, it may be used in variable studies related to tracheal stenosis.

Tubeless tracheotomy for survival airway surgery in the leporine model

OBJECTIVES/HYPOTHESIS

The ideal animal experimental tracheostomy technique is one that is 1) safe and easy to perform, 2) requires no tracheostomy tube, and 3) requires minimal cleaning or suctioning to maintain patency. The leporine model for airway injury has been well established and offers an inexpensive and practical animal model for experimental evaluation. However, previous research has demonstrated a high mortality rate with survival airway surgery in rabbits. This study demonstrates the feasibility of airway management in the leporine model using a simple maturing suture tracheostomy that avoids a tracheostomy tube.

STUDY DESIGN

Tracheostomy was performed in six New Zealand white rabbits in the setting of survival surgery over a 2-week study period.

METHODS

A vertical tracheal incision was made from the second to the sixth tracheal ring. The anterior portion of the tracheal rings was removed and the skin surrounding the stoma was sutured down to the tracheal wall. The lateral tracheal wall was then suspended to the soft tissue in the lateral neck.

RESULTS

All six rabbits survived the study period with minimal care and maintained stoma patency until sacrifice. Granulation tissue and edema were noted during the first week and largely resolved by the second week. An average of 5-mm-diameter stoma was measured 14 days after surgery without intraluminal stenosis or laryngeal edema.

CONCLUSIONS

This method meets the defined criteria for the ideal experimental tracheostomy, demonstrating potential benefit in a laryngotracheal stenosis model and a rabbit model of evoked phonation.

Subglottic injury: a clinically relevant animal model

OBJECTIVES/HYPOTHESIS

To develop a clinically aligned, reproducible model for subglottic injury.

STUDY DESIGN

Prospective randomized control pilot study.

METHODS

Juvenile (3-month-old) New Zealand White rabbits underwent intubation with a 3-cm length of an endotracheal tube that was chosen so that there would be no air leak below 20 cm of water. This tube was one or two sizes above the appropriate tube for the animal. It was held in situ with a suture placed at the trachea and secured over a button in the neck for a period of 1 week. Animals were sacrificed 1 week postextubation, and larynges were harvested. A range of histological techniques and gross morphology were utilized to examine the injury caused at the level of the subglottis. Unintubated animals constituted controlled specimens.

RESULTS

Intubated animals demonstrated considerable histopathology including evidence of ulceration, inflammation, granulation tissue, perichondritis, and chondritis when compared with control animals. Morphometric analysis demonstrated a significant increase in lamina propria thickness (P = .0013), mucosal thickness (P ≤ .0001), and in goblet cell areal density (P = .014). Analysis of mucin types found a significant decrease in acidic (P = .0001) mucin coinciding with a significant increase in mixed mucin types (P = .0013).

CONCLUSIONS

Our model provides a reliable and reproducible technique for acute/subacute injury to the subglottis secondary to intubation, which is consistent with previous histological findings of early changes associated with acquired subglottic stenosis (SGS). Future uses of this model could include the examination of current adjunctive therapies and their effects on limiting progression to SGS.

A model of canine tracheal stenosis induced by radiofrequency cauterization

OBJECTIVE

The current methods of management of tracheal stenosis have disadvantages and are controversial, therefore ideal experimental animal models for the further studies are required. The aim of this study was to establish a new model of canine tracheal stenosis by radiofrequency cauterization.

METHODS

The tracheal cartilage ring was injured by radiofrequency at the level of the sixth tracheal ring in 12 mongrel dogs. A fibrolaryngoscope was used to guide the procedure. The animals were observed after operation and examined after euthanasia. Endoscopic and histological examinations were undertaken to evaluate the progress of stenosis. The degree of stenosis was calculated using the formula: degree of stenosis=(initial lumen area-final lumen area)/initial lumen area × 100%.

RESULTS

Tracheal stenosis had developed in all dogs by the 21st day post operation. Costal retraction was observed in all dogs after the 18th day post operation. At the end of the study, gross and endoscopic examinations showed that stenosis had been induced to a satisfactory degree and without any complications. The median of the degree of stenosis was 92%, with a range of 84-94%. Histological examination showed that cartilage was damaged and that granulation tissue and collagen fibres had formed.

CONCLUSIONS

The model of canine tracheal stenosis induced by radiofrequency cauterization is a relatively simple, reliable, and reproducible animal model. This model may be useful in the development of new methods of treatment for tracheal stenosis.