Bone graft substitutes.

Surgeons involved in skeletal repair, reconstruction, and oncology commonly encounter or create bone defects that are unlikely to heal if treated by fixation alone. Cancellous and cortical autografts have been used for decades to treat skeletal defects, but the amount of autograft is limited, and morbidity related to autograft harvesting can be considerable. Allograft can “extend” autograft, but has limited biological and mechanical properties, carries with it the potential for disease transmission, and has not been accepted for other reasons in some societies. The need for autograft substitutes has led to the development of a wide variety of skeletal substitute materials that are being introduced rapidly into skeletal surgery and are thus likely to be visualized with increasing frequency by radiologists and pathologists. These materials vary with respect to composition, biologic properties, mechanical strength, and radiographic appearance. Several important terms have been developed to describe the biologic properties of bone graft substitute/extender materials [1–3]. A material is osteoconductive if, due to its composition, shape, or surface texture, it promotes bone formation along its surface when it is placed into bone. Purely osteoconductive materials, such as hydroxyapatite, are usually not associated with bone formation when placed outside bone. A material is osteoinductive if it induces bone to form when placed into an extraskeletal site, usually by inducing stromal cells to differentiate into osteoblasts or osteoblast precursors. Osteoinductive materials, such as demineralized bone matrix or selected bone morphogenetic proteins, also form bone within the skeleton, but, as originally defined by Marshal Urist, a material is demonstrated to be osteoinductive by the formation of bone in an extraskeletal site such as skeletal muscle. A material is osteogenic if it causes bone formation because of the implantation of viable cells, such as autograft, aspirated or enriched bone marrow osteoprogenitor cells, or by the combination of those cells and some other synthetic substrate. While these three terms are thought to describe the principal mechanisms of many bone graft substitute materials, other mechanisms of enhancing bone formation also exist, such as the simple provision of a local source of calcium ions that, by re-precipitation as a carbonated apatite, may serve as a nidus for bone mineralization. Some investigators have introduced terms such as “osteopromotive” or “osteostimulatory” into the literature, in part via marketing studies, and they use those terms to imply enhanced bone formation by some other, usually undefined (and sometimes mythical), mechanism. It should also be noted that the terms osteoconductive, osteoinductive, and osteogenic are relative, not absolute. Bone graft materials can be classified in many different ways, including based on composition, biologic or mechanical properties, cost, or radiographic appearance. The table inserted below is not comprehensive, but it lists some examples of bone graft substitutes/extenders according to composition (Table 1). The appearance of these materials on imaging studies depends on the composition of the material, the extent to which it has become “incorporated” into bone, and, of course, the nature of the imaging study itself. It is important for radiologists to recognize that some of the preparations of bone graft substitutes are mineralized, or partially mineralized, at the time they are inserted into the bone. Skeletal Radiol (2007) 36:1105–1107 DOI 10.1007/s00256-007-0377-4