The factors contributing to a delayed union or nonunion are many. In general they may be divided into three major categories: deficiencies in vascularity and angiogenesis, deficiencies in the robustness of the chondroosseous response, and deficiencies in stability, strain, or physical continuity. Frequently, deficiencies in more than one category are present, thus complicating the approach to therapy. For a bone grafts to enhance fracture healing, it must provide or stimulate that which is deficient. Autogenous fresh cancellous and cortical bone most frequently are used, but other common grafts include allogeneic frozen, freeze dried, or processed allogeneic cortical, corticocancellous and cancellous grafts, and demineralized bone matrix. These grafts have varying capacities to provide active bone formation, to induce bone formation by cells of the surrounding soft tissue, and to serve as a substrate for bone formation. However, the graft cannot exert its biologic activity in isolation, dependent as it is on the surrounding environment for cells to respond to its signals and, in some cases, for blood supply. The mechanical environment of the graft site is also important. Successful graft incorporation requires that an appropriate match must be made between the biologic activity of a bone graft, the condition of the perigraft environment, and the mechanical environment. The task of the clinician who performs a bone grafting procedure for the enhancement of fracture healing is to choose the right graft or combination of grafts for the biologic and mechanical environment into which the graft will be placed.

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