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Ann Thorac Surg 2004;78:1438-1440
© 2004 The Society of Thoracic Surgeons
a Department of Thoracic and Cardiovascular Surgery, Kansai Medical University, Moriguchi, Igaki Medical Planning, Kyoto, and Shiga Medical Center for Adults, Shiga, Japan
Accepted for publication July 3, 2003.
* Address reprint requests to Dr Saito, Department of Thoracic and Cardiovascular Surgery, Kansai Medical University, 10-15 Fumizonocho, Moriguchi, Osaka 570-8507, Japan
saitoy{at}takii.kmu.ac.jp
Abstract
PURPOSE: The aim of this study was to determine whether it is possible to deliver and deploy a new device, a poly-L-lactic acid (PLLA) tubular knitted airway stent, under bronchoscopic guidance in a dog model.
DESCRIPTION: The delivery system consisted of a flexible balloon catheter (controlled radial expansion balloon dilator, M00558440, Boston Scientific Corporation, MA, USA) preloaded with a stent. A delivery catheter preloaded with a stent was advanced to a target point in the trachea under bronchoscopic guidance. Once the stent was positioned, the balloon was inflated for sixty seconds. The stent was in full contact with the tracheal wall upon deflation of the balloon.
EVALUATION: The stents were successfully delivered into the tracheal lumen and successfully deployed in all dogs.
CONCLUSIONS: This is the first study to prove the feasibility of delivering and deploying the PLLA stents in a dog model, using a balloon expansion technique. Further investigation with large numbers of subjects and long-term follow-up will be necessary to assess the utility of the bioabsorbable knitted tubular stent before clinical applications begin.
Treatment of tracheobronchial stenosis is problematic. Airway stenoses can be caused by tracheobronchomalacia, extrinsic compression, postintubation tracheal injuries, sequelae after tracheostomy, or malignant or benign tumors. Conservative methods include stenting the stenotic area, but an ideal stent has not yet been developed. The aim of this study is to determine whether it possible to deliver and deploy the poly-L-lactic acid (PLLA) tubular knitted airway stent under bronchoscopic guidance for clinical application in the future.
The most common types of stents in clinical use are made of metallic wire or silicone. An ideal stent should possess several characteristics [1]: it should be easy to insert and remove, if necessary [2]; it should be available in different sizes to match the obstruction [3]; once placed, it should maintain its position without migration [4]; it should be firm enough to resist compressive forces, yet have sufficient elasticity to conform to the airway contours [5]; it should be made of inert material, so as not to irritate the airway, precipitate infection, or promote granulation tissue [6]; it should exhibit the same characteristics of the normal airway so that mobilization of secretions is not impaired [1].
The Dumon [2] stent, a silicone tube, is well established, especially for treatment of stenoses with intraluminal tumor growth or granulation tissue. Silicone stents have several problems, such as disturbance of physiologic mucociliary function of tracheobronchial epithelium, which can result in accumulation of secretions inside the stent and obstruction of its lumen. Because silicone stents are relatively thick, the internal diameter of the stent is thus smaller than the normal airway lumen.
A metallic nitinol self-expandable airway stent is often used in patients with airway stenosis, due to extrinsic tumoral compression [3]. Metallic stents have the following advantages [1]: simplicity of insertion and fixation [2], better clearance of secretions [3], accommodation to varying tracheal dimensions [4], and a high internal-to-external diameter ratio. However, metallic stents are not without problems. They can be removed by rigid bronchoscopy, but once they have been covered by epithelium, it is not easy to remove them using a conventional bronchoscope alone. Open surgery may be required.
The use of many of these stents has been limited to the pediatric population. In a pediatric patient who is growing year by year, it may be necessary to exchange existing stents for larger ones. In fact, it may be impossible to safely remove an expandable metal stent from the airway.
Bioabsorbable airway stents offer benefits: the device need not be extracted, and the airway preserves its normal function after the stent has been resorbed. Poly-L-lactic acid stents begin to degrade in 5 to 6 months, the total resorption (degradation of the particles) time of PLLA is more than one year. However, we can change the time of resorption if the properties of polymer, such as molecular weight, size of wire, and shape, are changed.
We have reported the biocompatibility and mechanical strength of PLLA stent in a rabbit model [4]. The total resorption time of PLLA wire (340 µm in size) used in this study is predicted 14 months.
Material and Methods
We assessed the feasibility of delivering and deploying PLLA stents in a dog model.
Stent Delivery System
The stents were made of single-stranded 340 µm wire, with an outer diameter of 20 mm and a length of 40 mm (Fig 1). The animals used in our study were six beagle dogs weighing approximately 10 kg.
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The delivery system consisted of a flexible balloon catheter (controlled radial expansion balloon dilator, M00558440, Boston Scientific Corporation, MA, USA) preloaded with a stent. The stent was wrapped around the catheter's balloon expansion site. The distal and proximal edges of the stent were covered and secured by small silicone sleeves to prevent displacement resulting from friction during passage through the airway (Fig 2).
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All stents were successfully delivered and deployed into the tracheal lumen with no technical problems. One month after stent placement, the PLLA stents were covered with tracheal mucosa (Fig 4). In one dog, marked granulation tissue developed at the distal end of the stent. This animal was sacrificed two months after stent placement. Autopsy of this animal revealed an incomplete laceration of the membranous portion of the trachea.
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The ideal stent should be simple to insert, fix, and remove; be biocompatible, not obstruct the airway; allow clearance of secretions, and accommodate to varying tracheal dimensions and shapes [5, 6]. In the pediatric patient who is growing, this stent could be preferentially utilized. Because it may be necessary to exchange existing stents for larger ones, the metallic stent is unfavorable for stenting in these patients. Bioabsorbable airway stents, on the other hand, are beneficial; extraction of the device is unnecessary.
The introduction of coronary angioplasty by Grunzig [7] stimulated the rapid technological growth of interventional cardiology. The concept of balloon dilation of stenotic lesions reported by Grunzig has been widely applied, not only in cardiology, but also in the treatment of patients with airway stenosis [8]. After Grunzig, Strecker and colleagues [9] designed a balloon-expandable mesh endoprosthesis made of a tantalum wire for use in peripheral arteries. Poly-L-lactic acid stent can be easily delivered into the trachea using the modified delivery system originally designed by Strecker. Insertion of it into the trachea requires no special training or equipment. The procedure is very similar to balloon dilation and can be performed easily by a skilled pulmonologist. The self-expanding force of this stent is less than that of a metallic stent such as the Ultraflex stent (Boston Scientific Corp., Boston, MA, USA). However, PLLA wire has shape memory, meaning it plastically deforms at lower temperatures, but reverts to its original manufactured shape when heated to high temperatures. We made use of this characteristic in our delivery and deployment technique. The knit design of this stent is highly resistant to extrinsic compression. Even at an external 50% or more diameter compression, the stent does not collapse. Under extreme torquing maneuvers, a constant inner lumen diameter is maintained. This stent can be in full contact with the tracheal wall, because its wire is knitted into a series of loosely interwoven loops, providing longitudinal and radial flexibility.
A series of placement in six dogs suggests that successful deployment is possible. Although almost all of the stents were successfully implanted, we overlooked a tracheal injury at the time of balloon expansion and stent deployment in one dog. Our experience of incomplete laceration of tracheal membranous portion suggests that attention should be paid while inflating a balloon to deploy the stent. Also, an expanding balloon suitable for the size of trachea should be chosen.
In conclusion, this is the first study to prove the feasibility of a balloon expansion technique for the deployment of tubular bioabsorbable knitted airway stents, made of PLLA, in a normal dog model. Further investigation with large numbers of subjects and long-term follow-up will be necessary to assess the utility of the bioabsorbable knitted tubular stent before clinical applications begin.
Disclosures and Freedom of Investigation
The PLLA stent was provided to us from Igaki Medical Planning (Kyoto, Japan). We have no further financial relationship with Igaki Medical Planning. The flexible balloon catheters (Boston Scientific Corporation, MA) were purchased by our Department in Kansai Medical University. The authors have performed a free and independent evaluation of this technology. The authors had full control of the design of the study, methods used, outcome parameters, analysis of data, and production of the written report.
DisclaimerThe Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
References
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