Bone defects in the craniomaxillofacial skeleton vary from the small (few millimeters) periodontal defects to the large segmental defects resulting from trauma, surgical excision, or cranioplasty. Such defects typically have complex three-dimensional structural needs, which are difficult to restore. In cranial vault defects, the underlying brain needs permanent protection. Segmental jaw defects require restoration of mechanical integrity, temporomandibular joint function, and intermaxillary dental occlusion. Maintaining acceptable facial esthetics is another unique consideration in the treatment of facial defects, which cannot be underestimated. Bone grafts remain the gold standard for reconstructing segmental bone defects. We will overview the status of bone grafting techniques for craniofacial reconstruction, their biological foundation, as well as future directions.

The earliest report of a bone grafting procedure came in an 1682 book by Job Janszoo van Meekeren, a surgeon in Amsterdam. In this account, the author reported a case in Russia, where the surgeon restored a cranial defect using a cranial bone graft from a dead dog. In 1881, Sir William MacEwen of Rothesay, Scotland, published the first case report of successful interhuman transfer of bone grafts. He used tibial bone wedges excised from three donors, during surgical correction of skeletal deformity, to reconstruct a humeral defect in a 3-year-old child. Subsequent clinical reports helped establish the efficacy of autogenous bone grafts in defect reconstruction.

MECHANISM OF ACTION OF BONE GRAFTS
A bone graft is defined as any implanted material that promotes bone healing, whether alone or in combination with other material. Augmentation of bone healing at the recipient site occurs through one or more of the following mechanisms: osteoconduction, osteoinduction, and osteogenesis. An osteoconductive material simply allows, or directs, new bone formation along its surfaces. Examples include bone graft matrix and synthetic osteoconductive polymers. An osteoinductive graft supplies recruitment and/or differentiation factors for bone-forming cells at the recipient site.

An osteogenic graft supplies induced, or inducible, bone-forming cells to the recipient site. Accordingly, an ideal bone graft is the one that functions through all three mechanisms by providing a template that directs three-dimensional bone growth (osteoconduction), recruits and induces differentiation of resident bone-forming cells, and supplies more bone-forming cells to the recipient site. Such grafts include cancellous and vascularized bone grafts.

Bone grafts can be employed for functions other than to stimulate bone formation within a defect. An onlay graft laid over facial bone surfaces could augment the cheek prominence or restore facial contour. In this case, more emphasis is directed toward the rate of graft resorption. Those grafts that are known for their slow resorption, such as calvarial and cortical bone, or nonresorption, such as synthetic materials, are preferred. Such grafts might also be used for their mechanical properties wherever mechanical support or immediate protection of vital structures is required, as in reconstructing orbital floor or calvarial defects.

Slow resorption is a disadvantage if the graft is used to augment bone formation at the recipient site. Graft incorporation is inversely proportional to how solid the graft is and how slow it resorbs. Therefore, osteoconductive graft materials with interconnected internal spaces that reach the outer surface are better scaffolds for directing three-dimensional bone invasion of the graft. This architecture provides more surface area along which native osteoclasts can attach themselves and start dissolving the graft, which is the first stage in graft incorporation.