Houben, Rudolph H.

ABSTRACTVascularized composite allotransplantation of bone is a possible alternative treatment for large osseous defects but requires life‐long immunosuppression. Surgical induction of autogenous neo‐angiogenic circulation maintains transplant viability without this requirement, providing encouraging results in small animal models [1–3]. A preliminary feasibility study in a swine tibia model demonstrated similar findings [4, 5]. This study in swine tibial allotransplantation tests its applicability in a pre‐clinical large animal model. Previously, we have demonstrated bone vascularized composite allotransplantation (VCA) survival was not the result of induction of tolerance nor an incompetent immune system [1]. Fourteen tibia vascularized bone allotransplants were microsurgically transplanted orthotopically to reconstruct size‐matched tibial defects in Yucatan miniature swine. Two weeks of immunosuppression was used to maintain allotransplant pedicle patency during angiogenesis from a simultaneously implanted autogenous arteriovenous bundle. The implanted arteriovenous bundle was patent in group 1 and ligated in group 2 (a neo‐angiogenesis control). At twenty weeks, we quantified the neo‐angiogenesis and correlated it with transplant viability, bone remodeling, and gene expression. All patent arteriovenous bundles maintained patency throughout the survival period. Micro‐angiographic, osteocyte cell count and bone remodeling parameters were significantly higher than controls due to the formation of a neo‐angiogenic autogenous circulation. Analysis of gene expression found maintained osteoblastic and osteoclastic activity as well as a significant increase in expression of endothelial growth factor‐like 6 (EGFL‐6) in the patent arteriovenous bundle group. Vascularized composite allotransplants of swine tibia maintained viability and actively remodeled over 20 weeks when short‐term immunosuppression is combined with simultaneous autogenous neo‐angiogenesis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:288‐296, 2020

Background Bone vascularized composite allotransplantation (VCA) is a possible alternative for the treatment of large bone defects. Clinical application of VCAs is limited by the need for life-long immunosuppression (IS). We report an alternative method to maintain bone allotransplant viability in a large animal model without the need for life-long IS by using autogenous vessel implantation. Methods Fourteen bone only VCAs were transplanted in a porcine tibia defect model with short-term IS. Two groups were used to evaluate the effect of the implantation of an autogenous arteriovenous (AV)-bundle, therefore the only difference between the groups was the patency of the AV-bundle. We radiographically evaluated bone healing and allogenic pedicle patency. AV-bundle patency and union were evaluated with micro-CT. Bone remodeling was assessed with histomorphometry and material properties were evaluated with axial compression testing and cyclic reference point indentation. Results Two subjects did not reach the final time point. Twelve tibiae healed proximally, and nine at the distal transplant–bone interface. Bone allotransplants showed their viability in the first 4 to 6 weeks by significant periosteal bridging arising from the transplant and maintained pedicle patency. Bone material properties were not affected by the implantation of an AV-bundle when compared with ligated AV-bundle controls, but diminished compared with normal bone. Significantly higher bone formation rates resulted from the implantation of a patent AV-bundle. Conclusion New periosteal bone formation and subsequent bone healing result from blood flow through the microsurgically repaired nutrient blood supply, demonstrated by maintained allogenic pedicle patency. The implantation of a patent autogenous AV-bundle has no adverse effect on material properties, but a positive effect on bone remodeling of endosteal surfaces despite thrombosis of the allogenic pedicle. Bone material properties change after transplantation compared with normal bone, although 20-weeks survival time is relatively short for the final evaluation of bone material properties.