The biology of bone grafts

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 osteogenesis and as a mechanical support. Autografts, both cancellous and cortical, are implanted fresh, provide a source of osteoprogenitor cells, and are 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. All bone grafts are initially resorbed; cancellous grafts are completely replaced in time by creeping substitution, whereas cortical grafts remain an admixture of necrotic and viable bone for a prolonged period of time. The three-dimensional framework that supports invasion of the bone grafts by capillaries and osteoprogenitor cells, termed "osteoconduction", is another important function of both autografts and allografts. Because fresh allographs evoke both local and systemic immune responses that diminish or destroy the osteoinductive and conductive processes, freezing or freeze-drying of allografts is used clinically to improve incorporation. Graft incorporation is 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.