Dolph Houben

34 CHAPTER 2 Biology of Vascularized Bone Grafts (VBG) Living bone has significant physiologic advantages to many other reconstructive methods in restoring limb form and function. Survival of osteocytes cannot be expected in autogenous bone depending upon diffusion of oxygen from surrounding tissue. This is particularly true in limb salvage surgery, where the transferred autograft is large and bulky. Vascularized bone flaps remain viable due to the maintenance of its endosteal and/or periosteal circulation provided by microvascular repair of its transplanted vascular supply [37, 42, 43] . Remaining viable and dynamic in its new site, a VBG does not undergo the gradual replacement of dead trabeculae by creeping substitution. Creeping substitution is a process, required for any non-vascularized or ‘conventional’ graft. In this process, osteoclasts precede the osteogenesis, with osteopenia and loss of mechanic strength as result [44-46] . This increases the risk of late stress fractures. Not only does the intrinsic blood flow of a VBG maintain strength, but it also increases and improves the rate of healing. It is capable of responding to applied stress by remodeling and undergoing hypertrophy if not excessively shielded by internal fixation or protection from weight-bearing [47] . Up to 80% of structural vascularized bone grafts will show significant hypertrophy over time in response to stress loading by 24 months following reconstructive surgery [48, 49] . In many cases, however, fibular flaps require protection from mechanical loads during the hypertrophic process, particularly when placed to span a larger and stronger bone. Instead, the mechanical load should be gradually increased over time to promote bone remodeling and hypertrophy and minimize the risk of early stress fractures. Indications / Contraindications Autogenous free tissue transfershavebecomea relatively common technique for the reconstruction of complex bone and soft tissue defects. The use of these more complex reconstructive options should be considered only when simpler reconstructive methods have failed or are likely to fail. Generally, it is reasonable to consider a vascularized bone graft as a reconstructive option for bone defects larger than 6-8cm due to limb-sparing tumor surgery, but also due to traumatic loss, infection, or congenital pseudarthrosis [50-52] . Other indications include; a bone defect of smaller size which has failed to heal with non-vascularized bone grafts, a previously infected non- union with a segmental defect, a non-union with or without a defect due to radionecrosis, spinal and sacral defects, failed primary reconstruction, pathologic fractures due to osteonecrosis, or reconstructions which are likely to result in “biologic failure” [33, 34, 52, 53] [54] . The biologic failure can be caused by a damaged or poor soft tissue envelope, infection, previous irradiation, scar formation, and vascular injury. The position and fixation of the fibula in which the fibula is placed depends upon the location of the recipient site. In large diameter bones, the fibula is often combined with a large allograft, placed either within the medullary canal or as an onlay spanning the defect. Cases of persistent bony non-union, radionecrosis, and pathologic fractures can be treated by the onlay technique. Vascularized grafts may also be considered after failed attempts to obtain union by conventional techniques [55, 56] . Relative contraindications for harvesting a free fibula graft are those patients for whom the harvest would cause insufficient lower extremity blood flow.