Dolph Houben

178 CHAPTER 9 Vascularized fibula flaps used for the reconstruction of a large segmental defects alone demonstrate hypertrophy as a response to stress loading in 80% of cases at 24 months after surgery [7] . When a VBG is combined with an CBA, the VBG substantially improves the biological properties of the reconstruction due to the independent vascularity. In short-term, the VBG induces fusion and incorporation into the recipient bone. In long-term, the VBG interacts with the allograft by inducing revascularization of endosteal surface of the CBA [34] . The endosteal surface of the CBA otherwise remains avascular and therefore represents an element of lesser mechanical resistance [24, 37] . Computed tomography studies of a combined VBG and CBA reconstruction demonstrate this effect in three different remodeling patterns depending upon the variation of the load on the reconstruction. If the CBA remains intact, the mechanical stress on the vascularized fibula remains mild and constant. In response, the VBG will show an increase in diameter without cortical thickening (pattern 1). Osteo-inductive processes have been found in areas where the growing vascularized bone comes into contact with the endosteal surface of the CBA. The second pattern is associated with a stress fracture of the CBA. Due to the suddenly increased axial strain on the VBG, the plastic properties of the hypertrophying VBG becomes heightened. As a result, an increase in diameter is observed together with significant cortical thickening. The osteo-inductive processes found in these cases are more intensely activated at the CBA level. This leads to a complex process of osteogenic substitution [34] , but hypertrophy of the VBG together with revascularization of the CBA is preferable. It has been described in the literature that in some cases of combined VBG and CBA reconstruction, the complete CBA shell had been reabsorbed. The reabsorbing process had occurred simultaneously with increase hypertrophy of the VBG [34] . The biological behavior of the VCA with short-term immunosuppression and surgically-induced neo-angiogenesis was compared to the biological behavior of VBGs, CBAs or a combination of VBG and CBA. We found in our experimental VCA study that no fractures occurred during the 20-week survival period and all proximal host-transplant interfaces demonstrated complete union. In our porcine tibia VCA defect model, we used rigid internal fixation with dual locked LCP plates. Only one uni-cortical locking screw was used to hold the allotransplant in place. Inability to limit activity in our pig hind-limb model is a potential explanation for the three cases of incomplete distal union. We observed periosteal new bone arising from the VCA sufficient to form a bridging callous. This healing process is comparable to the healing process of autogenous VBGs [7] . Change of mechanical bone properties have been reported after both VBG and CBA reconstruction [38] that. Bone material properties within the allotransplants changed 20 weeks after transplantation compared to normal (contra-lateral) bone while bone mineral density (BMD) was maintained. Mechanical protection of the allotransplants by the internal fixation and limited follow-up are possible explanations for the changed mechanical properties. Implantation of a VBG within a CBA has been reported to increase the biologic properties of the reconstruction and improve outcomes [12, 34] . Biologically, the revascularization of the CBA by a VBG can be compared to autogenous revascularization of a VCA with AV bundle implantation, wherein the VBG acts as the AV bundle. Living bone allotransplantation provides immediate bulk