Structure of Grafts

Cortical bone grafts are used primarily for structural support, and cancellous bone grafts are used for osteogenesis. Structural support and osteogenesis may be combined; this is one of the prime advantages of using bone graft. These two factors vary, however, with the structure of the bone. Probably all or most of the cellular elements in grafts (particularly cortical grafts) die and are slowly replaced by creeping substitution, the graft merely acting as a scaffold for the formation of new bone. In hard cortical bone, this process of replacement is considerably slower than in spongy or cancellous bone. Although cancellous bone is more osteogenic, it is not strong enough to provide efficient structural support. When selecting the graft or combination of grafts, the surgeon must be aware of these two fundamental differences in bone structure. When a graft has united with the host and is strong enough to permit unprotected use of the part, remodeling of the bone structure takes place commensurate with functional demands.