Interact CardioVasc Thorac Surg 2009;8:225-229. doi:10.1510/icvts.2008.186528
© 2009 European Association of Cardio-Thoracic Surgery
Institutional report - Experimental |
Comparison of bioabsorbable materials for use in artificial tracheal grafts
Hisashi Tsukadaa,b,*,
Shojiro Matsudac,
Hajime Inoueb,d,
Yoshito Ikadae and
Hiroaki Osadaa,b
a Department of Surgery, Division of Chest Surgery, St. Marianna University School of Medicine, 2-16-1 Sugao Miyamae, Kawasaki 2168511, Japan
b St. Marianna University Graduate School of Medicine, Institute of Advanced Medical Science, 2-16-1 Sugao Miyamae, Kawasaki 2168511, Japan
c Gunze Ltd, Research and Development Center, One Ishiburo Inokura-shin-machi Ayabe, Kyoto 6238512, Japan
d Department of Plastic Surgery, St. Marianna University School of Medicine, 2-16-1 Sugao Miyamae, Kawasaki 2168511, Japan
e Department of Indoor Environmental Medicine, Nara Medical University, 840 Shijo-cho, Kashihara, Nara 6348521, Japan
Received 18 June 2008;
received in revised form 6 October 2008;
accepted 9 October 2008
 Financial support: Magnifying brochoscopy with infrared light observation system was supported by a Grant-in-Aid for Scientific Research (B) 15390424 from the Japanese Ministry of Education, Culture, Sports, Science and Technology.
*Corresponding author. Chest Disease Center, Beth Israel Deaconess Medical Center, Harvard Medical School, 185 Pilgrim Road, Deaconess Suite 207, Boston, MA 02215, USA. Tel.: +1-617-632-9976; fax: +1-617-632-8253.
E-mail address: htsukada{at}bidmc.harvard.edu (H. Tsukada).
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Abstract
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Limited information exists regarding the usefulness of bioabsorbable materials in the design of tracheal grafts. The aim of this study was to evaluate the feasibility of three bioabsorbable materials for use as artificial trachea. Three sets of grafts were prepared: Group 1 (n=6), knitted polyglactin 910 mesh; Group 2 (n=3), copolymer of L-lactide and  -caprolactone sponge reinforced with polyglycoride fibers; and Group 3 (n=8), copolymer of L-lactide and -caprolactone sponge covered with knitted poly-L-lactide mesh. All grafts were internally reinforced with a titanium stent. A 10-cartilage-ring-length of canine mediastinal trachea was resected and replaced by a bioabsorbable prosthesis with the aid of an omental flap. In Groups 1 and 2, the patency rates decreased below 50% within two months after surgery. In Group 3, six of eight dogs maintained patency rates above 50% from 10 months to 2 years after surgery. Grafts prepared with a copolymer of L-lactide and -caprolactone sponge covered with knitted poly-L-lactide mesh (Group 3) can function for up to two years after surgery. These results provide evidence toward the feasibility of utilizing bioabsorbable materials as a tracheal prosthesis.
Key Words: Trachea; Tracheal surgery; Prosthesis; Experimental surgery; Bronchoscopy
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1. Introduction
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Extensive tracheal reconstruction remains challenging for thoracic surgeons. Grillo warns in his review to not revisit methods that have yielded unacceptable results [1]. Despite advances in material science, an optimized long-term tracheal replacement has yet to be developed [2]. Our laboratory had previously developed a porous type of artificial trachea using Dacron material in which we had observed functioning for more than two years [3]. However, these animals developed gradual intraluminal stenosis as a result of intraluminal fibrous tissue growth. The gradual stenosis was likely a result of continuous foreign body reaction against Dacron. Both bioabsorbable and non-absorbable materials have been used in designing tracheal grafts [1–4]. We hypothesize that bioabsorbable materials will limit foreign body reaction in long-term use as tracheal grafts, thus extending the functional longevity of the grafts. The aim of this study was to evaluate the efficacy of three bioabsorbable materials as an artificial trachea.
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2. Materials and methods
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2.1. Bioabsorbable materials
(1) knitted polyglactin 910 mesh (Ethicon, Somerville, NJ) (Fig. 1a); (2) copolymer of L-lactide and -caprolactone (P(CL/LA)) (Gunze Ltd, Kyoto, Japan) sponge reinforced with polyglycoride fibers (Fig. 1b); and (3) knitted poly-L-lactide (PLLA) mesh (Gunze Ltd, Kyoto, Japan) (Fig. 1c).
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Fig. 1. Scanning electron photomicrograph of bioabsorbable materials. (a) Knitted polyglactin 910 mesh. (b) A sponge made of copolymer of L-lactide and -caprolactone. (c) Knitted poly-L-lactide mesh.
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2.2. Prosthesis design
Each prosthesis consisted of bioabsorbable material(s) and a titanium stent. The spiral stent was made of 0.4 mm wire with an outer diameter of 16 mm and a length of 50 mm. Stents were also reinforced by three vertical props to prevent stent contraction.
Group 1: stents were simply wrapped with knitted polyglactin mesh. The mesh was either applied in two- or four-sheets thickness in order to control for graft pore size.
Group 2: stents were covered with a 0.7 mm thick layer of P(CL/LA) sponge reinforced with polyglycoride fibers (Fig. 2a).
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Fig. 2. (a) Group 2 graft; a titanium spiral stent reinforced with vertical props was covered by P(CLLA) sponge reinforced with polyglactin. (b) Group 3 graft; stent inserted into a P(CLLA) sponge conduit was inner and outer covered with a knitted PLLA.
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Group 3: utilizing the same design as in Group 2, the grafts were additionally lined internally and externally with a knitted PLLA mesh (Fig. 2b).
2.3. Implantation
A 10-cartilage-ring length of beagle dog (n=17, weighing 14–16 kg) mediastinal trachea was resected and a prosthesis was implanted with the aid of an omental flap. Details of the surgical procedures were described in our previous study [3].
2.4. Conventional bronchoscopy and histology
Bronchoscopy (model BF200, Olympus Optical Co Ltd, Tokyo, Japan) was performed at specified follow-up periods of two weeks, at one month and every month thereafter under intravenous pentobarbital sodium anesthesia using an endotracheal tube. Additional bronchoscopies were scheduled when dogs presented noticeable respiratory symptoms and at the time of animal sacrifice. The degrees of tissue in-growth, patency rate, and epithelialization on the internal surface of the in-grown tissue were examined by punch biopsy. Biopsy specimens were fixed in 10% phosphate-buffered formalin, embedded in paraffin, and sectioned. Sections were stained with hematoxylin and eosin (H&E). Patency rates were calculated by dividing the internal cut surface caliber of the graft portion by the native tracheal cut surface caliber on the bronchoscopic pictures.
2.5. Magnifying bronchoscopy
Magnifying bronchoscopy (XBF-200 HM, Olympus Optical Co Ltd, Tokyo, Japan) was performed at 6–12 months under intravenous pentobarbital sodium anesthesia using an endotracheal tube. Magnified pictures of in-growing vessels within the tissue on the inner surface of the grafts were observed in vivo under both normal white light and infrared light utilizing 25 mg of indocyanine green injection (ICG, Diagnogreen: Dai-ichi seiyaku Co Ltd, Tokyo, Japan).
2.6. Animal care
Animals were sacrificed with an overdose injection of sodium pentobarbital if intraluminal/anastomotic patency rates decreased to <50% or if surgical related complications occurred. This research study was approved for conduct by the Animal Care and Use Committee, based on the Guideline for Animal Experimentation of St Marianna University Graduate School of Medicine in Japan. All animals received humane care in compliance with the National Research Council's Guide for the Care and Use of Laboratory Animals, prepared by the Institute of Laboratory Animals and published by the National Institutes of Health (Publication No. 5377-3 1996).
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3. Results
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A summary of the results is displayed in Table 1. All dogs tolerated the surgical procedure well and perioperative mortality did not occur. In Group 1 (n=6), four dogs developed intraluminal graft stenosis due to excessive in-growth of tissue at 1–2 months after surgery and were euthanized as per protocol. One dog developed ileus at one week and another dog developed diaphragmatic herniation at six weeks after surgery. In Group 2 (n=3), all dogs developed intraluminal graft stenosis due to excessive tissue ingrowth at 3, 4 and 7 weeks after surgery, respectively.
In Group 3 (n=8), five dogs yielded intraluminal patency rates of <50% ranging from four months to two years after surgery. The graft harvested from dog #14 showed complete epithelialization with noticeable changes of epithelium along the length of the graft. Ciliated epithelium was observed within the first centimeter of anastomoses, followed by transitional epithelium, and lastly covered by cuboidal epithelium in the central area of the graft (Fig. 3). Biopsy specimens taken at six months from the central area of the graft in dog #10, showed ciliated epithelialization (Fig. 4). The remaining three dogs are presently being followed at postoperative times ranging from 10 months to 21 months after surgery and are in good condition (Fig. 5).
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Fig. 3. Complete graft epithelialization was confirmed by the graft harvested at four months after implantation from the third set. Ciliated epithelium cells lined on the graft at near the anastomosis. (H&E; original magnification, x200 from dog #14.)
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Fig. 4. A biopsy specimen taken from the central part of the graft at six months after surgery revealing ciliated cells. (H&E; original magnification, x400 from dog #10.)
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Fig. 5. Bronchoscopic findings of reconstructed trachea of the third set. Good luminal patency and graft incorporated into the native tissue were observed at 17 months after implantation. (From dog #10.)
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Magnifying bronchoscopy revealed angiogenesis originating from the native trachea in all animals that survived beyond six months (Fig. 6). Vessels were seen in the graft pores and apparently growing toward the graft surface. Native tracheal submucosal vessels and graft surface vessels adjacent to the native trachea were detected utilizing an infrared light immediately after intravenous injection of ICG. The central portion of the vessels, which were suspected of originating from the omental flap, was partially observed by infrared detection (Fig. 7).
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Fig. 6. Magnified bronchoscopic view revealing angiogenesis through the porous graft suspected originating from the omental flap at 1 year after surgery. (From dog #12.)
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Fig. 7. Magnifying brochoscopic view with an infrared light observation system at one year after surgery. (a) Fine vessels were observed by the normal light. (b) The vessels observed in (a) were immediately detected by infrared light with intravenous ICG injection. (c) The other parts of fine vessels were observed by the normal light. (d) The vessels observed in (c) were not detected by infrared light with ICG injection. (From dog #15.)
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4. Discussion
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Tracheal resections in adults, of more than half of their original length, require some type of graft and technique to reconstruct the absent tissue [1]. Non-collapsibility and biocompatibility of the grafts are critical for the success of tracheal replacement. Cartilage plays the major role of supporting the tracheal lumen to prevent collapse. Martinod et al. recently reported the use of a fresh female sheep allogenic aortic graft to replace the cervical trachea of a male sheep [5]. Moreover, they confirmed the presence of regenerated tracheal cartilage in the graft recovered from the male recipient tissue. Although methods for tracheal graft reconstruction are trending towards tissue engineering, one major limitation is the ability to develop adequate long-term stiffness of the tissue-engineered tracheal cartilage [6]. We did not observe cartilaginous regeneration in any of our experimental animals. In our three modalities, the grafts worked as a scaffold, free of an exogenous cell source, in which the metallic stent served to maintain the internal lumen. Our grafts complement the airway stenting technique that is widely accepted in the clinical setting [7].
Bioabsorbable material is widely utilized as the scaffold for tissue regeneration today [6, 8]. Although the optimal scaffold material for tracheal tissue regeneration is debatable [9], bioabsorbable material could serve as the most biocompatible scaffold unless the in-growing tissue obstructs the internal lumen. Other literature have described the use of bioabsorbable materials as an external support scaffold; however, these materials are contraindicated for use in unsterile conditions [4]. The average strengthlessness period in vivo for Polyglactin 910, P (CL/LA), and PLLA are one month, three months and three years, respectively [8]. Material absorption typically depends on the response from the recipient tissue and is approximately twice the material strengthlessness period. Previous studies have reported delayed or rapid degradation of polyglactin in subjects with an acidic or alkaline environment [10, 11]. We had planned for eight canines to be assigned to each group in the original design of this study. However, all dogs in Groups 1 and 2, which utilized polyglactin, developed graft stenosis within two months after surgery. These polyglactin grafts were absorbed more quickly than we had anticipated and may possibly be related to their environmental conditions. Quick absorption had led to excessive tissue in-growth into the graft lumen. As a result of early observed graft stenosis, we suspended further tracheal replacements in Groups 1 and 2.
An allotransplantation study using cryopreserved trachea showed that donor epithelium had dissipated within three weeks of implantation, and the recipient epithelium had migrated into the graft from the anastomoses sites [12]. In addition, total re-epithelialization of a five-cartilage-ring length occurred 50 days after implantation. In our previous report and in this current study, our data confirmed the total epithelialization phenomenon along 5 cm grafts [3]. McDowell et al. also reported vigorous tracheal epithelium proliferation in vitro [13]. We regard the appropriate description of tracheal epithelialization in tracheal replacement studies are ‘migration’ and/or ‘bridging’ epithelium originating from the native trachea at anastomoses site rather than ‘regeneration’. This distinction is necessary to clarify any ambiguity within researchers as the concepts of ‘migration’ and ‘regeneration’ have been used interchangeably to describe tracheal epithelialization.
Olympus Optical Company Ltd. modified the conventional magnifying bronchoscopy to evaluate blood flow within the tracheal submucosal vascular network [3]. This scope made it possible to observe real-time blood flow of small sized vessels by monitoring intravenous ICG. Using this technology, we detected graft surface vessels adjacent to the native trachea but did not detect angiogenesis in the central area of the grafts under infrared light. Effective blood supply for the graft is essential for successful tracheal replacement. We believe further modifications such as topical addition of the vascular endothelial growth factor on the grafts [14], which would constitute improvements over the omental flap technique, may promote angiogenesis in the central area of the grafts. Currently, quantitative measurements are unachievable with this bronchoscopic technology, but future improvements may potentially allow for these measurements.
In conclusion, bioabsorbable materials may feasibly be used for tracheal prosthesis. Our data suggest that a stent covered with P (CL/LA) sponge and additionally lined internally and externally with a knitted PLLA sheet may function well for up to two years after surgery. These results provide evidence toward the feasibility of utilizing bioabsorbable materials as a tracheal replacement.
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Acknowledgements
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We thank Ethicon, Somerville, NJ for providing graft materials. We also thank Olympus Optical Co Ltd, Tokyo, Japan for technical support.
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References
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Abstract
1. Introduction
