Double-wire woven nitinol stent for treating dogs with refractory tracheal collapse: A case series

This retrospective case series describes the signalments, clinical signs, diagnostic test results, and postoperative complications obtained from the medical records of 10 client-owned dogs that underwent treatment for grade IV tracheal collapse using double-wire woven nitinol stents between October 2017 and September 2021. Respiratory signs resolved in all dogs immediately after tracheal stent placement. Mild to moderate stent fractures were identified in five dogs, of which two showed concurrent respiratory distress necessitating re-stenting after several months. Minor complications, such as stent migration, were absent. The double-wire woven nitinol stent optimised for the canine trachea showed favourable outcomes and minimal complications.

Tracheal collapse is a progressive disorder in which the tracheal cartilage weakens and the tracheal lumen cannot be maintained and collapses, resulting in death (Della Maggiore 2020).Dogs with end-stage tracheal collapse develop clinical signs, such as a goose-honking cough, open-mouth breathing, paradoxical abdominal movement, cyanosis, and dyspnoea, which may be fatal if they lead to respiratory distress (Durant et al. 2012;MacPhail 2018).Recently, intraluminal stents have been widely used instead of surgical methods, including extraluminal stent placement, because of the simple procedure, shorter anaesthesia time, and immediate recovery and discharge after the procedure (Durant et al. 2012).
While intraluminal stent placement has several advantages, complications have been documented, including stent fracture (19-50%) or migration (8-37%), excessive granulation tissue formation, aspiration pneumonia, and tracheitis (21-58%) (Sura and Krahwinkel 2008;Durant et al. 2012;Weisse et al. 2019).To minimise such complications, the selection of an appropriately sized stent with suitable mechanical properties, the use of high-quality surgical techniques, and the treatment of underlying diseases are essential.
Based on the mechanical properties and clinical results from previous studies on self-expanding metal stents (Kim et al. 2022a;Kim et al. 2023), it was determined that a D-type stent with a wire https://doi.org/10.17221/61/2023-VETMEDdiameter of 0.006 (D6) would be most suitable for the treatment of canine tracheal collapse.
This case series illustrates some of the major complications and clinical outcomes obtained through long-term follow-up after the application of canine-optimised stents in dogs diagnosed with end-stage, refractory grade IV tracheal collapse.

Case description
All included dogs were admitted to a veterinary teaching hospital between October 2017 and September 2021.The research was approved by the Institutional Animal Care and Use Committee.The signalment and diagnosis-related information of the 10 dogs obtained before surgery are presented in Table 1.The stent size, complications, number of stent placements, survival time, and cause of death are listed in Table 2.
Stent placement was indicated for dogs with no improvement in clinical signs after more than 4 weeks of medical treatment for tracheal collapse.Tracheal and principal bronchial movements during respiration were dynamically evaluated and the grade of tracheal collapse was assessed using fluoroscopy.Stent size was selected using static thoracic radiography under the animal's natural respiration.The distance from 1 cm posterior to the cricoid cartilage, to 1 cm anterior to the carina was defined as the stent length.A total of 110-120% of the maximum diameter of the corresponding trachea was selected as the stent diameter.
An intraluminal double-wire woven nitinol stent [Niti-S Biliary Uncovered Stent (D-type); Taewoong Medical] was manufactured and used in the study.
Stent placement was performed using methods described in previous studies (Yoon et al. 2017;Kim et al. 2022b).The average stenting duration in all dogs was 12 min (range, 5-15 min), and the average anaesthesia time was 25 min (range, 10-40 min).
Immediately after stent placement, signs of dyspnoea and goose-honk coughing resolved in all dogs.
Reassessments were conducted after discharge, and the degree of fracture was radiographically evaluated using the stent fracture grading system as follows (Allie et al. 2004;Scheinert et al. 2005): 0 = normal, no stent fracture; 1 = mild fracture, single strut fracture only; 2 = moderate fracture, multiple single-stent fractures; 3 = severe fracture, complete separation of the stent segment (Scheinert et al. 2005).
Dogs 3 and 8 experienced stent fraying, an indication of a minor stent strut fracture, but this was not associated with clinical signs (Sura and Durant 2012) (Figure 1A).Five dogs with tracheal kinking showed mild to moderate stent fracture and a 3-50% increase in body weight.
Re-stenting was performed in two dogs, on day 330 after initial stenting in Dog 8 with a grade 1 fracture (Figure 1B) and on day 533 in Dog 10 with  a grade 2 fracture (Figure 1C).A total of four stents were placed in Dog 8; the re-stenting periods were 330, 209, and 80 days and the survival period was 659 days.Two dogs, Dogs 6 and 8, died from respiratory distress.The causes of death for the other dogs were congestive heart failure, pancreatitis, and cardiac arrest due to worsening underlying diseases.

DISCUSSION
This case series report describes the outcome of canine-optimised tracheal stent placement in dogs with grade IV tracheal collapse refractory to medical therapy.
In dogs with severe tracheal collapse, a stent is used to support the tracheal wall against excessive mechanical stress, as the cartilage is weakened and can no longer support the tracheal wall as in healthy dogs.The more severe the cough and chronic tracheitis, the more clinical signs and damage to the tracheal wall and mucosa, leading to a reduced ability of the tracheal cartilage to support the lumen.In particular, intense pressure in the thoracic cavity occurs while coughing.In dogs with end-stage tracheal collapse, in which the physical properties of the trachea are poor, the trachea loses its straightened shape because it cannot withstand the pressure acting on the lumen during coughing, resulting in a phenomenon called tracheal kinking in severe cases.In this study, five of the 10 dogs showed tracheal kinking on fluoroscopy, and the stent fracture rate was high.
Being overweight in dogs with tracheal collapse significantly increases the risk of relapsing respiratory signs.In the present study, Dog 10 showed a 50% body weight increase after the first procedure and the highest grade of stent fracture (grade 2).As weight increases, adipose tissue accumulates in the dorsal muscles surrounding the trachea, causing tracheal and bronchial collapse to worsen (de Oliveira Lemos et al. 2020;Song et al. 2022).Five dogs in this study had a postoperative weight gain of 3-50% and showed mild to moderate stent fractures.Weight gain worsens the pathological conditions in dogs with tracheal collapse and influences the tracheal stents.Theoretically, when the tracheal radial force is increased, stent fatigue is increased by placing more significant pressure on the stent supporting the lumen of the trachea.This can eventually increase the possibility of fractures; thus, weight control is crucial to reducing stent fractures in all stented dogs.
Stent fracture is defined as the complete or partial separation of the embedded stent that maintains the luminal patency of the organ.In human medicine, several studies have classified stent fractures for vascular stents and evaluated the degree of stent occlusion, restenosis, and fracture, and found that there is no correlation with mortality (Scheinert et al. 2005;Kan et al. 2016).In veterinary medicine, there is no indication for predicting factors related to clinical signs or mortality rate according to stent fracture type to date.In the present study, the degree of fractures of canine tracheal stents was compared, referring to the morphological changes of stents in human medicine.Recurrence of respiratory signs to the extent requiring re-stenting was confirmed in dogs with mild or moderate stent fractures.Further studies related to prognosis are needed on methods to evaluate stent fracture grade in patients with canine tracheal stents.
In summary, a canine-optimised tracheal stent was applied in dogs with grade IV tracheal collapse in this study, with a low major complication rate and favourable clinical outcomes.Dogs with tracheal kinking confirmed by fluoroscopy and those with weight gain after stent placement were vulnerable to stent fracture and thus had a poor prognosis.A grading system for stent fractures could be helpful for evaluating the changes in morphologic details in a follow-up procedure.
Figure 1.Lateral thoracic radiograph view showing types of stent fracture during inspiration (A) Mild stent fraying shown on day 251 in Dog 3 (yellow arrow).(B) Type 1 stent fracture identified on day 220 in Dog 8.A segmental stent was placed from the cervical trachea to the thoracic inlet.(C) Type 2 stent fracture identified on day 533 in Dog 10

Table 2 .
Ten dogs grouped into mild and severe types according to the presence of tracheal kinking