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METALLIC TRACHEOBRONCHIAL STENTS
PRASHANT N CHHAJED, MONIQUE A MALOUF,ALLAN R GLANVILLE
Lung Transplant Unit, St. Vincent’s Hospital, Sydney, NSW 2010, Australia.
Significant airway stenosis is usually managed by dilatation and/or stent insertion. Rigid bronchoscopy with bougie dilatation for airways stenoses has largely been replaced by brochoscopic balloon dilatation. Tracheobronchial stents have proved valuable in the management of both benign and malignant airway stenoses. Certain metals (stainless steel, alloys incorporating cobalt and chromium, nitinol) are inherently inert and acceptable when placed in the living tissue. A variety of metallic stents have been used in the last two decades that include the Palmaz, Strecker, Gianturco, Wallstent and Ultraflex stent. The common problems associated with different stent types include migration, granulation tissue formation and mucus plugging. The advantage of the Ultraflex stent made of a nickel titanium alloy, is its high biocompatibility, excellent tissue tolerance and shape memory effect. This article discusses the use of metallic stents in the management of benign and malignant lesions in the tracheobronchial tree.
INTRODUCTION
Significant airway stenosis is usually managed by dilatation and/or stent insertion. [1-4] Rigid bronchoscopy with bougie dilatation for airway stenosis has largely been replaced by bronchoscopic balloon dilatation.[4,5] The word ‘stent’ connotes to an artificial support that maintains patency of a hollow tubular structure. Tracheobronchial stents have proved very valuable in the management of both benign and malignant airway stenoses.
Inoperable central airway stenosis due to a malignant tumour is a relatively common condition and may be life threatening. Patients with malignant lung disease often face a limited prognosis. Thus, interventional procedures are primarily of palliative character needed to maintain airway patency. Over the last two decades a variety of stents (silicone and metal) have been introduced in the pursuit of an ideal stent. The Dumon stent is inserted using a rigid bronchoscope and has probably been the most commonly used silicone stent.6 The advantage of this stent has been that it can be easily removed and that it provides a solid barrierto prevent encroachment of tumour. The disadvantages of this stent type include its tendency to migration, relatively unfavourable wall-to-lumen ratio, mucus retention and the possibility to stimulate formation of granulation tissue. [7] Also insertion of this stent necessitates training in rigid bronchoscopy.
Experiments have identified certain metals (stainless steel, tantalum, alloys incorporating cobalt and chromium, nitinol) that are inherently inert and acceptable when placed in the living tissue. A variety of metallic stents have been used in the last two decades that include the Palmaz, Strecker, Gianturco, Wallstent and Ultraflex stent. [2-8,11] Our experience with the use of the metallic stents has been with the latter three. The common problems associated with different stent types include migration, granulation tissue formation and mucus plugging. [2] The advantage of the Ultraflex stent made of a nickel titanium alloy, is its high biocompatibility, excellent tissue tolerance and shape memory effect.[12-14] This article discusses the use of metallic stents in the management of benign and malignant lesions in the tracheobronchial tree.
Mechanical characteristics
Characteristics of the stent depend on the construction design, amount of metal within the stent and the metals properties regarding the elasticity and plasticity. Stress is the measure of force per unit area, and strain the measure of fractional deformation. Elastic deformation is reversible and bears a linear correlation between stress and strain. Plastic deformation, on the other hand, is an irreversible process leading to permanent change resulting from reorganisation in the atomic structure of the metallic stent. The point at which the change occurs is called the yield point and represents the stress at which the permanent change occurs.
The Palmaz stent (stainless steel) is the only device, which exhibits plasticity.[8] The Gianturco stent has a metallic structure that is a broad lattice and increases the pressure per unit surface area, leading to possible penetration into the mediastinum.[15] The Wallstent seems to exhibit its expansile strength based on contact with the wall of the lesion. Eccentric compression on such a stent tends to produce a concentric compression over a segment of the stent, so that when inserted the stent may lose wall contact and fail to mount adequate resistance to precisely the point of constriction. Under this situation, balloon dilatation of the stent apposes it and restores the stent-wall contact, allowing the stent to offer resistance to the compression. [8] The radial expansion forces exerted by the Ultraflex stent are higher as compared to the Wallstent for the same constrained diameter.
DISCUSSION
The indications of stent placement are listed in Table 1. [8] Despite the reported side effects, cumulative evidence suggests beneficial effects of stent placement in selected patients. An ideal stent should have the following properties: (1) be easy to insert and remove; (2) should not migrate; (3) be made of a biocompatible material; (4) should not cause obstruction of segmental openings; (5) should not impair mucociliary clearance and (6) adapt to the varying dimensions of the airways during respiration and coughing. Such a stent still remains to be developed. The advantages of the metallic stents include its ease of placement, dynamic expandability, visibility on an ordinary radiograph and preservation of ventilation when placed across lobar orifices. [ 16 ]
TABLE 1
Indications for stent placement in the tracheobronchial treeBenign Structure
Post infectious : tuberculosis, histoplasmosis (fibrosing mediastinitis)
Post lung transplantation : anastomotic, segmental
Post inflammatory: Wegener’s granulomatosis, sarcoidosis, radiation
Post intubation/tracheostomy
Post laser resectionTracheobronchomalacia
Post lung transplantation
Post tracheostomy
Relapsing polychondritis
TracheobronchomegalyTumours
Papillomatosis
AmyloidosisMiscellaneous
Tracheal narrowing following muscle flap operation
for tracheoesophageal fistulaMalignant Extrinsic compression
Submucosal involvement
Following laser resection, electrocautery, photodynamic therapy or cryotherapy to main airway patency
Tracheo-bronchial oesophageal fistula (in conjunction with oesophageal stent)
Fig1. Double gianturco stent.
TABLE 2
Characteristics of various metallic stentsType of stent Description Size Comment Gianturco Stainless stell zigzag wire, proximal and distal ends have 0.010-inch wire hooks. 15-30mm in diameter
Single and double post expansion lengths of 25 or 50 mm
High pressures exerted over the airway mucosa may lead to ischaemia and perforation Wallstent Cobalt based superalloy filaments each 100 micron in diameter braided in a cylindrical mesh. Covered stent has a layer of polyurethane. 7-24 mm in diameter
20-70 mm in lengthMetal filaments and end of stent are free and sharp Ultraflex Nitinol-a nickel titanium alloy braided ina cylindrical mesh. Covered stent has a layer of polyurethane. 8 to 20 mm in diameter
40-80 mm in length‘Shape memory effect’
Smooth proximal and distal ends
Fig2 ---- Ultraflex stent.
Dasgupta et al, have reported a 45% improvement in FEV1 following stent placement for tracheobronchomalacia.[17] In our institute the mean improvement in FEV1 post stent placement for pure bronchomalacia was 673 ml or 81% and with combined dilatation and stent placement for benign bronchostenosis was 625 ml or 52%.18 In a series of 40 patients with inoperable tracheobronchial cancer, the average karnofsky Performance Index improved from 40 to 70 after deployment of the Wallstent which denotes a change from dependence needing assistance to a subnormal/normal activity.[15] The three month survival in the same study was 32.5%.
The common problems associated with different stent types include migration, granulation tissue formation and mucus plugging.[2] Routine surveillance bronchoscopy within 2 to 3 months after silicone stent insertion did not detect a high incidence of stent related complications among patients without new respiratory symptoms.[19] However, there is some argument that instead of leaving surveillance bronchoscopy till 2 to 3 months after silicone stent insertion, the duration of surveillance bronchoscopy should be brought forward.[20] Stent fracture and perforation of the airway wall have also been reported with certain stent types. Although relevant in patients with malignant airway disease these are seen more often in patients with benign lesions because of their longer survival. Granulation tissue formation, a common problem noted with metallic stents results from an excessive reaction to the stent and resembles a foreign body reaction. This may lead to formation of webs and recurrent stenosis.[21] Also metallic stents are difficult to remove once the process of neoepithelialisation is initiated.
Fig 3 Endobronchial mass in the right main bronchus
Fig 3A ---- Covered Wallstent inserted in the right main bronchus after debulking with ND-YAG laser.
Fig4 ----- Uncovered Ultraflex stent inserted in the right main bronchus of a patient having combined stenosis and bronchomalacia following lung transplantation.
Fig5 ----- Endobronchial mass blocking the left lower lobe bronchus.
The main advantage described with the Gianturco stent is its metal lattice that becomes overgrown with respiratory mucosa in a few weeks so that muco-ciliary clearance is not impaired.[22,23] Complications have included migration, restenosis, stent frature, formation of false passage and perforation and fatal massive haemoptysis due to erosion of the pulmonary artery.[8] The complications observed with the Gianturco stent in our experience include granulation tissue formation and stenosis (60%), migration (20%), stent fracture (25%) and formation of false passage (12.5%) including fatal rupture of the left pulmonary artery in one patient.[2]
Fig 5A ------ Covered Wallstent inserted in the left lower lobe bronchus.
The advantages favouring the use of Wallstents have included: (1) migration of the stent is uncommon; (2) neoepithelialisation helps preserve mucociliary action and the stent exhibits a certain degree of dynamic compression with coughing which facilitates mucus clearance; (3) can be easily inserted in patients who are receiving mechanical ventilatory support where the use of rigid bronchoscopy might be contraindicated; and (4) if needed, patients can be easily intubated through the tracheal stent. In addition, the Wallstent is made of multiple metal filaments, which in theory may provide a stronger radial expansion force and a larger surface area is in contact with the wall of the airway, with a possibly lower risk of erosion.[11] A disadvantage of the Wallstent is the propensity to form granulation tissue at its proximal and distal ends due to the irritation from the uncovered end fibres.[8,24] In our experience with lung transplant recipients the complicationsobserved with the Wallstents have been granulation tissue formation and stenosis (27%), mucus plugging (27%) and migration (7%).[2] Mucus plugging noted in our group of patients with uncovered Wallstents could be explained by the partial neoepithelialization that may occur with this stent as opposed to the complete neoepithelialization that occurs with the Gianturco stent, which is designed as a broad metal lattice.
Vinograd et al, reported the use of nitinol stents in the trachea in a rabbit model.[12] Bronchoscopy revealed increased secretions in most animals in the first two weeks after implantation, but the secretions diminished significantly in ensuing weeks. There was no evidence of migration, gross inflammation or granuloma formation. If these results could be duplicated in humans then it could overcome the disadvantages associated with the Gianturco and Wallstents. Nitinol alloys appear to be biologically compatible with the human body.[12-14] The use of nitinol stents has been reported for the treatment of benign and malignant tracheobronchial stenoses[.9,21,25-27] The Ultraflex (nitinol) stent shares some common advantages with the Wallstent in terms of ease of deployment and its mesh characteristics. With the use of the Ultraflex stent in the management of benign tracheobronchomalacia and benign bronchostenosis, so far we have not noted mucus plugging or formation of granulation tissue at stent extremity.[9] The complications that we have observed include stent migration (two patients) and stricture formation through the stent in one patient with bilateral lung transplantation at the site of the anastomosis. In malignant lesions of the airways, the Ultraflex stent has been used in certain complicated situations, which include narrow stenosis, hourglass stenosis, curvilinear stenosis, bilateral mainstem bronchi stenoses, and long stenosis of varying diameters.[27] Miyazawa et al report that the uncovered Ultraflex stent can be used in all locations for malignant tracheobronchial lesions, even the subglottic region, without the risk of migration.[27]
Initially made without a covering, the limitation of the metallic stents became quickly evident, as tumoral regrowth occurred rapidly through the metallic meshes. Covered metallic stents were introduced to counter this problem of tumour ingrowth and the covered Wallstents have been widely used for this purpose. Due to its thin wall, auto-expandability, and flexibility, the Wallstent adapts easily to the local anatomic conditions and is effective in treating pure extrinsic compressions. For intraluminal neoplastic lesions, patency of the airways and relief of symptoms can be achieved by the complementary use of stent placement with laser, diathermy and brachytherapy. Monnier et al have reported partial obstruction of the prosthesis secondary to tumour recurrence at the extremities in nine out of 40 patients treated with Wallstents, but without aggravation of the dyspnoea index in six patients.[15] The main complications reported with the Ultraflex stent in malignant lesions at 2 month follow up period were tumour ingrowth (24%), tumour overgrowth (21%) and mucus plugging (9%).[27]
Stenting should be undertaken with caution and only after careful consideration in patients with elevated bleeding times or coagulopathies and a tumour stricture adjacent to a major vessel. Stent insertion is contraindicated in patients for whom bronchoscopic procedures are contraindicated. For benign lesions, in the presence of significant inflammation our practice is to dilate the lesion and defer stent placement if possible.
SUMMARY
Complications of stent fracture and perforation have lead to caution in the use of the Gianturco stents. Both the Wallstent and the Ultraflex stents can be inserted using the flexible bronchoscope. The Ultraflex stent shares some common advantages with the Wallstent in terms of ease of deployment and its mesh characteristics. In addition, nitinol alloys appear to be biologicallycompatible with the human body and appear to be associated with a low complication rate. Some may argue that the problem of the metallic stents with respect to benign lesions is that with time they cannot be removed without damaging the bronchial wall. The past decade has seen the adaptation of intravascular metal stents for use in the airways. New designs are being brought out in the quest for the perfect airway stent. Potential developments include metallic removable stents, biodegradable stents, and chemically and radioactive coated stents. Currently nitinol stents hold promise as a useful prosthesis for the management of benign and malignant airway lesions.
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