Thermoplastic Stents: A New Concept for Endoluminal Prosthesis

• Introduction
• Materials and Methods
• The Stent
• The Implantation
• The Animal Experiment
• Results
• Trachea
• Esophagus
• Discussion
• References

Background and Study Aims: Intraluminal stenting of organs with stenoses or fistulae in anatomically difficult locations (for instance cardia, pylorus, large bowel), with a tendency to kinking or increased motility, still carries a high risk of stent dislocation. In the search for a solution, we report on the use of a new thermoplastic stent in animal experiments.

Material and Methods: The new stent consists of a plastic-coated wire mesh which can be heated electrically. Once it is warmed up to 55 °C, its size and shape can be changed. After being expanded by a dilatation balloon across the stenosed area, the stent can be fitted onto the inner organ surface. This guarantees a low dislocation risk and high stability. In an animal experiment, stents were endoscopically placed in the trachea and the surgically stenosed esophagus of two dogs. The animals were observed for 3 months.

Results: The thermostents were implanted easily and without complications. It was possible to mold the thermostent evenly onto the intraluminal wall. No stent dislocation, bleeding or perforation was observed. Upon histologic evaluation, granulation tissue was found to be growing through the wire mesh of the stent.

Conclusion: It was shown that the stent described here can be implanted without major problems. The greater effort of the implantation procedure, in comparison with self-expanding stents, is compensated by the special mechanical characteristics of the stent. These characteristics may permit implantation in anatomically difficult locations where up to now stenting has been impossible or inadequate.

Introduction

Stenoses and fistulae are increasingly being treated successfully by implanting endoprostheses. The latest developments in endoscopic stents are promising, approaching high specificity for each indication and adjustment for the individual patient. However, medical problems without acceptable solutions for stent implantation remain [1] [2] [3] [4] . In the following study a newly designed thermoplastic stent is presented.

Thermoplastic materials change their plasticity according to temperature. This property is already applied in medicine, for instance in external bracing of cervical spine injuries or of extremities [5] [6] . Why not use it for stabilizing internal structures? Although the idea has been considered in the past, the use of thermoplastic endoprostheses has not been attempted in clinical settings. One of the major problems is uniform heating of the thermoplastic material [7] [8] .

Materials and Methods

The Stent

Similarly to most expandable stents, this stent is an endoluminal prosthesis which consists of a wire mesh pattern (Figure [1]) permitting dilation of the stent diameter and shortening of length simultaneously. The thermoplastic material softens at temperatures above 50 °C and hardens below 40 °C. The stent incorporates a constant heating wire with a diameter of 0.05 mm, which runs through the entire stent and heats uniformly when low voltage is applied at both wire ends. After the stent is inserted at the appropriate location, it is softened and its diameter can be increased up to three times. The stent can then be molded evenly onto the luminal surface, with a wall thickness of 0.7 mm and a maximum distance between the mesh wires of 4 mm. The endoprostheses can be manufactured in variable lengths and diameters and are not translucent at radiography.

The Implantation

The nonexpanded stent is placed over a dilatation balloon and kept in place by small increments of balloon inflation. In the present experiments a flexible mitral valvuplasty balloon with low compliance (PTMC 30; Danimed) and an expansion capacity of 30 mm was used. The desired site of stent placement is located endoscopically and is marked externally on the radiograph film. Using a flexible endoscope, a guide wire is introduced across the stenosed area. Both stent electrodes are connected to the extracorporeal power supply. The power-supplying electrical wires are isolated extrudable hooks which permit an external connection and disconnection of the stent to the contact electrodes (Figure [2]). The stent, previously fitted on the dilatation balloon, is then placed over the guide wire into the lumen and advanced to the marked position under fluoroscopy.

At the appropriate location, a direct voltage of around 10 V (DC) is applied. The voltage used varies with the size and electrical resistance of the stent and has to be determined for each type individually. Before introducing the stent, it is advisable to connect the contact electrodes to the power supply, usually an external transformer. When the stent has been correctly positioned (Figure [3]), electrical power can be applied. The electrical current heats the constant wire and with it the thermoplastic coating. After the necessary temperature for sufficient stent softening is reached (5 - 8 seconds taken to reach 55 ° C), the dilatation balloon is inflated. The flexible stent expands with the balloon and is molded anatomically onto the luminal surface. When the appropriate size and shape are achieved, the power supply is terminated. After an interval of 5 - 8 seconds the stent hardens and maintains its new shape. The balloon is deflated and extracted with the guide wire; the stent remains in position.

Before the power contacts are removed, the stent location is verified endoscopically. If necessary, its position and shape can be corrected with endoscopically introduced and maneuvered forceps and balloon, by reheating the thermostent. Even after longer periods the stent can be reheated and replaced. This requires repeated endoscopic connection of the external contact electrodes to the stent. This also applies to the stent removal procedure.

The Animal Experiment

In order to test the mechanical and biological behavior of the stent in vivo, the effects of stent implantation in the trachea and stenosed esophagus of two healthy Labrador dogs were studied in a pilot experiment.

Under general anesthesia a tracheal stent (45 mm in length, 22 mm maximum diameter) was introduced using the method described above, placed 4 cm proximal to the tracheal bifurcation and expanded to 1.8 cm (Figure [4]).

The clinical status of the dogs was unremarkable 2 weeks later and the esophageal stent implantation was begun. Under general anesthesia the middle third of the esophagus was surgically exposed and ligated to a lumen of 10 mm. The wound was closed and in the same session the stent was implanted endoscopically over the stenosed area using the method described above.

The animals were followed up for 3 months. The clinical status was monitored daily by assessing respiration rate, coughing, eating habits, weight and general appearance. We performed conventional radiography at weekly intervals, upper gastrointestinal series with water-soluble contrast medium every 4 weeks, and endoscopic check-ups every 6 weeks. After completion of the experiment macroscopic and histologic evaluations of the trachea and esophagus were conducted.

Results

The thermostent implantation procedure at the designated location occurred without complications. The appropriate temperature for sufficient stent softening was reached after a few seconds. After balloon inflation the stent in the tracheal region showed a smooth positioning along the inside of the trachea with a slight stretching of the elastic posterior wall. Across the esophageal stricture the stent resembled the hour-glass form of the stenosed esophagus (Figure [5]). During the entire observation period neither the tracheal nor the esophageal stent site showed signs of bleeding, perforation or dislocation.

Trachea

In the first week following the implantation of the tracheal stent both animals exhibited intermittent coughing attacks which always ended spontaneously. At no point were respiratory complications observed. Endoscopic and histologic evaluation at 6 weeks showed increased granulation tissue growth through the wire mesh of the stent. Further analysis confirmed the presence of typical granulation tissue with scar formation, possibly arising from thermal lesions. The clinical appearance of the dogs was unremarkable up until completion of the experiment.

Esophagus

Endoscopic evaluation and radiographic studies of the upper gastrointestinal area revealed the presence of granulation-like tissue with a similar growth tendency through the wire mesh as was found in the tracheal stent. In contrast, the clinical appearance of the dogs was different. Although an increase in body weight was measured initially, a slowly increasing dysphagia was observed over an 8-week period. The histologic evaluation showed signs of a sustained burn injury.

Discussion

In comparison with other stent models, the major advantage of the thermostent described here is the fact that its size and shape can be molded to fit virtually any wall surface, and then maintained (Table [1]). With self-expanding and dilatable stents, dislocation rates up to 37 % have been reported, especially in the esophageal region [3] [9] . The described thermostent allows a firm local anchorage and guarantees a low dislocation risk.

A well known problem in intraluminal stenting of soft tissue organs is kinking of the organ at the end of the rigid tube or stent with consequent luminal obstruction [10]. The mechanical properties of the thermostent take the anatomical kinking of areas like the cardia or the pylorus into account and lessen the possibility of obstruction.

The implantation of balloon-expandable stents involves an active dilation of the organ. The relatively high rigidity of the thermostent in its unheated state stabilizes the achieved dilation and does not permit any external compression. This might be an advantage over “shape memory” stents which have similar mechanical properties, but which are much softer when expanded [8].

The disadvantages of conventionally used rigid endoluminal tubes are not likely to occur with the stent described here, since placement according to the individual anatomical site is possible, including placement exclusively in the short stenotic segment. Compared with conventional rigid tubes, the good fixation of the thermostent within the stenoses prevents stent parts overlapping healthy tissue which may lead to mechanical complications.

A major advance is the possibility of shape and position correction of the stent even after its implantation. The same holds true for stent extraction which is difficult and risky with most commonly used stents [11] [12] .

The effort of implantation, compared with modern self-expanding stents, is much greater, mainly due to the necessity for a balloon catheter and the heating technique. Acquiring this technique is more demanding and requires experience.

Compared with other stents, the one presented here has a widely meshed wire frame. It is assumed that tumor or granulation tissue advances more readily through a wide wire mesh than through tighter-meshed stents [13]. The development of thermostents with a smaller wire diameter and tighter mesh frame may be necessary. This could also enable the implantation of stents into smaller vessel structures, such as the biliary tree, or blood vessels. Another solution may be the coating of the entire stent wall, which could also prevent tissue growth through the wire mesh. This would extend the indication for stent implantation in the treatment of fistulas.

The granulation tissue growth observed in our experiment has also been confirmed in other stent applications [14] [15] . This is not only a foreign body response but also a reaction to thermal injury. For prevention of the latter, the method of softening the thermoplastic material by electrical heating needs to be optimized.

Due to the large wall diameter of the stent presented here, we suggest application in large luminas of the bronchial and gastrointestinal system, and especially in difficult anatomical locations such as the cardia opening of the stomach or the pylorus region, where stent kinking or high mechanical stress can be anticipated [16].

The described properties of the thermostent may also be useful in treating stenoses of the large bowel, especially the rectum, where an optimal solution for stenting has not yet been found [17] [18] . A further indication could be the treatment for tracheomalacia, where conventional stents have shown a high tendency to dislocation [19].

Improvement in the handling of stent implantation and control of the heating temperature, and covering the stent to avoid the growth of granulation tissue through the wire mesh, are necessary. In conclusion, we propose further clinical investigations for the application of the new generation of thermostents.

References

• 1 Lambert R. Esophageal cancer: Which stent, who places it, and where?.  Endoscopy. 1995;  27 509-511
  PubMedSearch in Google Scholar
• 2 Rey J F, Romanczky T, Greff M. Metal stents for palliation of rectal carcinoma: preliminary report on 12 patients.  Endoscopy. 1995;  27 501-504
  PubMedSearch in Google Scholar
• 3 May A, Hahn E G, Ell C. Self-expanding metal stents for palliation of malignant obstruction in the upper gastrointestinal tract. Comparative assessment of three stent types implemented in 96 implantations.  J Clin Gastroenterol. 1996;  22 261-266
  CrossrefPubMedSearch in Google Scholar
• 4 Richard A, Kozarek M D. Expandable endoprostheses for gastrointestinal stenoses.  Gastroenterol Endosc Clin N Am. 1994;  4 279-295
  PubMedSearch in Google Scholar
• 5 Gaskill S J, Marlin A E. Custom fitted thermoplastic Minerva jackets in the treatment of cervical spine instability in pre-school age children.  Pediatr Neurosurg. 1990 - 1991;  16 35-39
  PubMedSearch in Google Scholar
• 6 Breger-Lee D E, Buford W L. Update in splinting materials and methods.  Hand Clin. 1991;  7 569-585
  PubMedSearch in Google Scholar
• 7 Beck A. A new balloon-expandable plastic endoprosthesis. Initial report of experience with the malleable thermostent.  Radiologie. 1990;  30 347-350
  PubMedSearch in Google Scholar
• 8 Tsgawa C, Nishijima E, Muraji T. A shape memory airway stent for tracheobronchomalacia in children: an experimental and clinical study.  J Pediatr Surg. 1997;  32 50-53
  CrossrefPubMedSearch in Google Scholar
• 9 Valek V, Hrobar P, Mrazova J, Spurny V. Metal stents in patients with malignant and benign esophageal stenoses.  Rozhl Chir. 1997;  76 319-324
  PubMedSearch in Google Scholar
• 10 Sen S, Balaratnam N, Wood L A, Allison M C. Buckling of redundant expansible stent distal to an oesophageal cancer: endoscopic management.  Endoscopy. 1998;  30 422-424
  PubMedSearch in Google Scholar
• 11 Grund K E, Storek D, Becker H D. Highly flexible, self-expanding knitted metal mesh stents: innovative palliative therapy of malignant dysphagia.  Endoscopy. 1995;  27 286-294
  PubMedSearch in Google Scholar
• 12 Cotton B. Metallic mesh stents: is the expanse worth the expense?.  Endoscopy. 1990;  24 421-423
  PubMedSearch in Google Scholar
• 13 Kozarek M, Ball T J, Patterson D J. Metallic self-expanding stent application in the gastrointestinal tract: caveats and concerns.  Gastrointest Endosc. 1992;  38 1-6
  PubMedSearch in Google Scholar
• 14 Mitsuoka M, Hayashi A, Takamori S, et al. Experimental study of the histocompatibility of covered expandable metallic stents in the trachea.  Chest. 1998;  114 110-114
  PubMedSearch in Google Scholar
• 15 Fraga J C, Filler R M, Forte V, et al. Experimental trial of balloon-expandable, metallic Palmaz stent in the trachea.  Arch Otolaryngol Head Neck Surg. 1997;  123 522-528
  CrossrefPubMedSearch in Google Scholar
• 16 Pinto I T. Malignant gastric and duodenal stenoses: palliation by peroral implantation of a self-expanding metallic stent.  Cardiovasc Intervent Radiol. 1997;  20 431-434
  CrossrefPubMedSearch in Google Scholar
• 17 Saida Y, Sumiyama Y, Nagao J, Takase M. Stent endoprosthesis for obstruction colorectal cancers.  Dis Colon Rectum. 1996;  39 552-555
  PubMedSearch in Google Scholar
• 18 Dohmoto M, Hunnerbein M, Schlag P M. Application of rectal stent for palliation of obstructing rectosigmoid cancer.  Surg Endosc. 1997;  11 758-761
  CrossrefPubMedSearch in Google Scholar
• 19 Filler R M, Forte V, Chait P. Tracheobronchial stenting for the treatment of airway obstruction.  J Pediatr Surg. 1998;  33 304-311
  Search in Google Scholar

M.D. S. Freudenberg

Chirurgische Klinik Mannheim Universitätsklinik der Universität Heidelberg

Theodor Kutzer Ufer 1 - 3

68135 Mannheim

Germany

Phone: +49-6221-480789

Email: sebastian.freudenberg@chir.ma.uni-heidelberg.de

References

• 1 Lambert R. Esophageal cancer: Which stent, who places it, and where?.  Endoscopy. 1995;  27 509-511
  PubMedSearch in Google Scholar
• 2 Rey J F, Romanczky T, Greff M. Metal stents for palliation of rectal carcinoma: preliminary report on 12 patients.  Endoscopy. 1995;  27 501-504
  PubMedSearch in Google Scholar
• 3 May A, Hahn E G, Ell C. Self-expanding metal stents for palliation of malignant obstruction in the upper gastrointestinal tract. Comparative assessment of three stent types implemented in 96 implantations.  J Clin Gastroenterol. 1996;  22 261-266
  CrossrefPubMedSearch in Google Scholar
• 4 Richard A, Kozarek M D. Expandable endoprostheses for gastrointestinal stenoses.  Gastroenterol Endosc Clin N Am. 1994;  4 279-295
  PubMedSearch in Google Scholar
• 5 Gaskill S J, Marlin A E. Custom fitted thermoplastic Minerva jackets in the treatment of cervical spine instability in pre-school age children.  Pediatr Neurosurg. 1990 - 1991;  16 35-39
  PubMedSearch in Google Scholar
• 6 Breger-Lee D E, Buford W L. Update in splinting materials and methods.  Hand Clin. 1991;  7 569-585
  PubMedSearch in Google Scholar
• 7 Beck A. A new balloon-expandable plastic endoprosthesis. Initial report of experience with the malleable thermostent.  Radiologie. 1990;  30 347-350
  PubMedSearch in Google Scholar
• 8 Tsgawa C, Nishijima E, Muraji T. A shape memory airway stent for tracheobronchomalacia in children: an experimental and clinical study.  J Pediatr Surg. 1997;  32 50-53
  CrossrefPubMedSearch in Google Scholar
• 9 Valek V, Hrobar P, Mrazova J, Spurny V. Metal stents in patients with malignant and benign esophageal stenoses.  Rozhl Chir. 1997;  76 319-324
  PubMedSearch in Google Scholar
• 10 Sen S, Balaratnam N, Wood L A, Allison M C. Buckling of redundant expansible stent distal to an oesophageal cancer: endoscopic management.  Endoscopy. 1998;  30 422-424
  PubMedSearch in Google Scholar
• 11 Grund K E, Storek D, Becker H D. Highly flexible, self-expanding knitted metal mesh stents: innovative palliative therapy of malignant dysphagia.  Endoscopy. 1995;  27 286-294
  PubMedSearch in Google Scholar
• 12 Cotton B. Metallic mesh stents: is the expanse worth the expense?.  Endoscopy. 1990;  24 421-423
  PubMedSearch in Google Scholar
• 13 Kozarek M, Ball T J, Patterson D J. Metallic self-expanding stent application in the gastrointestinal tract: caveats and concerns.  Gastrointest Endosc. 1992;  38 1-6
  PubMedSearch in Google Scholar
• 14 Mitsuoka M, Hayashi A, Takamori S, et al. Experimental study of the histocompatibility of covered expandable metallic stents in the trachea.  Chest. 1998;  114 110-114
  PubMedSearch in Google Scholar
• 15 Fraga J C, Filler R M, Forte V, et al. Experimental trial of balloon-expandable, metallic Palmaz stent in the trachea.  Arch Otolaryngol Head Neck Surg. 1997;  123 522-528
  CrossrefPubMedSearch in Google Scholar
• 16 Pinto I T. Malignant gastric and duodenal stenoses: palliation by peroral implantation of a self-expanding metallic stent.  Cardiovasc Intervent Radiol. 1997;  20 431-434
  CrossrefPubMedSearch in Google Scholar
• 17 Saida Y, Sumiyama Y, Nagao J, Takase M. Stent endoprosthesis for obstruction colorectal cancers.  Dis Colon Rectum. 1996;  39 552-555
  PubMedSearch in Google Scholar
• 18 Dohmoto M, Hunnerbein M, Schlag P M. Application of rectal stent for palliation of obstructing rectosigmoid cancer.  Surg Endosc. 1997;  11 758-761
  CrossrefPubMedSearch in Google Scholar
• 19 Filler R M, Forte V, Chait P. Tracheobronchial stenting for the treatment of airway obstruction.  J Pediatr Surg. 1998;  33 304-311
  Search in Google Scholar

M.D. S. Freudenberg

Chirurgische Klinik Mannheim Universitätsklinik der Universität Heidelberg

Theodor Kutzer Ufer 1 - 3

68135 Mannheim

Germany

Phone: +49-6221-480789

Email: sebastian.freudenberg@chir.ma.uni-heidelberg.de