Natural history of autografts and allografts

The clinical outcome of bone grafting procedures depends on many factors, including type and fixation of the bone graft as well as the site and status of the host bed. Bone grafts serve one or both of two main functions, as a source of osteogenetic cells and as a mechanical support. Autografts, both cancellous and cortical, are usually implanted fresh and are often osteogenetic, whether by providing a source of osteoprogenitor cells or by being osteoinductive. The latter is a process whereby the transplanted tissue induces mesenchymal cells of the recipient to differentiate into osteoblastic cells. Cortical grafts, whether autogeneic or allogeneic, at least initially act as weight-bearing space fillers or struts. All bone grafts are initially resorbed, but cancellous grafts are completely replaced in time by creeping substitution, while cortical grafts remain an admixture of necrotic and viable bone for a prolonged period of time. The three-dimensional framework, which supports invasion of the bone grafts by capillaries and osteoprogenitor cells, termed "osteoconduction", is another important function of both autografts and allografts. Fresh allografts are more slowly and less completely replaced by host bones because they invoke both local and systemic immune responses that diminish or destroy the osteoinductive and conductive processes. Although freezing or freeze-drying of allografts improves acceptance, their failure rate is still too high. These processes are also influenced by the vascularity and composition of the host bed. Thus, the interaction of the host and the bone graft determines the success of these procedures, which ultimately is to provide a mechanically efficient support structure.