The fate of homograft tracheal transplants in sheep.

OBJECTIVE: An established method of tracheal substitution is not yet available, but homograft tracheal transplantation might provide a realistic tracheal replacement. With the objective of sequentially examining the healing of tracheal homografts, we have established a suitable large-animal model. METHODS: Five sheep received orthotopic tracheal transplantation of a 4-cm cervical tracheal homograft. The trachea was supported for 6 weeks with a self-expanding polyester stent. The plan was to euthanize the animals after 2, 4, 8, 12 and 16 weeks, or whenever complications occurred. RESULTS: The implantation itself was performed without complications. After 2 weeks the homograft was firmly encapsulated by connective tissue, without signs of necrosis or abscess. The original mucous membrane no longer existed; the cartilage rings were exposed. In all animals that were euthanized at the later dates, the homografts were completely absorbed and replaced by inflammatory scar tissue. This, in turn, was covered with a shiny cellular surface layer. CONCLUSIONS: The results from this animal experiment reveal-contrary to data published to date-that tracheal homografts are not incorporated but absorbed. They are replaced by scar/granulation tissue that cannot secure the stability of the trachea. Therefore, further experiments with respect to the biocompatability of homografts appear to be necessary.

On the use of unsealed polypropylene mesh as tracheal replacement.

The necessity of a cervical tracheal replacement arises with thyroid carcinoma, which occasionally infiltrates the trachea extensively, the rare primary tracheal tumors and, sporadically, benign stenoses. In the present study, we used an uncoated porous polypropylene prosthesis as cervical tracheal replacement in sheep. Specifically, we implanted a tracheal prosthesis of polypropylene mesh as a cervical tracheal replacement in five sheep, protecting the airways with self-expanding stents. Healing-in of the prostheses was checked bronchoscopically. The animals were killed after increasing survival times (7, 28, 64, 68, and >90 days), and incorporation of the prosthesis was examined macroscopically, microangiographically and histologically. Although medium-term survival was possible with a sufficiently wide airway, all animals were ultimately euthanized because of complications (airway stenosis, prolapse of prosthesis). Nevertheless, the results show that replacement of the cervical trachea with a polypropylene mesh can be successful under different experimental conditions.

The mechanical influence of tissue engineering techniques on tracheal strength: an experimental study on sheep trachea.

Tissue engineering is an attractive concept for facilitating the transplantation of different tissue types with a low immunogeneity and a well-preserved tissue structure. We examined the influence of treatment with trypsin/ethylenediamine tetraacetic acid (EDTA) on the mechanical properties of tracheae. Fresh sheep tracheae were stored in a trypsin/EDTA solution for various periods of time (24, 46, and 72 h) and their breaking strength was subsequently examined. The results were compared with native trachea. The treatment with trypsin/EDTA results in a considerable reduction of the mechanical stability under load, in this case compared with the tensile strength, that does not correlate with the results obtained from light-optical microscopy. The results from this study reveal that tissue preparation with trypsin/EDTA does not seem to be a suitable method of preparatory treatment of tracheae intended for transplantation.

A comparison of interrupted and continuous sutures for tracheal anastomoses in sheep.

OBJECTIVE: To study the tensile strength of tracheal anastomoses. DESIGN: Experimental study. SETTING: University medical school laboratory, Germany. ANIMALS: 15 sheep. INTERVENTIONS: Tracheal anastomoses with three different suturing techniques: a continuous suture and interrupted sutures with either a monofilament or a polyfilament material. Anastomoses were tested to breaking after being in place for 1, 2, 4, 8 or 24 weeks. RESULTS: After one week, with all three materials, the trachea broke at the anastomosis. In animals that survived longer, the trachea broke further away from the anastomosis. There was no significant difference between the mean values of the breaking force for continuous sutures and single interrupted sutures (p = 0.9). CONCLUSION: The suturing technique (continuous or interrupted) has less relevance for the tensile strength of the anastomosis than in vitro experiments suggest.

The mechanical stability under load of tracheal anastomoses after various phases in vivo.

OBJECTIVE: The objective of our study was to postoperatively examine the mechanical stability under load of tracheal anastomoses over different periods in time and to compare these with native tracheae. STUDY DESIGN: A randomized experimental study on animals. METHOD: We performed tracheal anastomoses on sheep with three different suturing techniques and different resection lengths (3, 6, and 9 cm): These anastomoses were subjected to a breaking test at different intervals in vivo (1, 2, 4, 8, and 24 wk) and compared with the tracheae of healthy sheep. RESULTS: After 1 week in vivo, the anastomosis itself tore off from the remaining trachea under tension, regardless of the suturing technique used and the length of resection. With all animals that survived for a longer period, the trachea broke a greater distance away from the anastomosis. The necessary breaking forces were only minimally lower than those required for breaking healthy tracheae and the difference is statistically insignificant. When all operated tracheae are combined and the forces compared with native tracheae, this reveals that the operated tracheae are significantly more stable (P =.015) and present a lower longitudinal elasticity (P =.004). CONCLUSION: During the first postoperative days, the stability under load of tracheal anastomoses is slightly lower than that of healthy trachea. This difference is, however, far from those values that can be measured intraoperatively on tracheal anastomoses. Thus, supplementary measures for the mechanical protection against suture line separation do not seem necessary.