In concept, autogenous bone (that is, bone borrowed from another part of oneâ€™s body) would be the best material to restore or improve the craniofacial skeleton, because it has the potential to be revascularized and, then, incorporated into the facial skeleton. In time, it could be biologically indistinguishable from the adjacent native skeleton. Practically, the use of autogenous bone is limited for several reasons. The morbidity incurred to the donor site, time, and operating room costs associated with autogenous bone graft harvest can be significant. Furthermore, the revascularization of bone grafts turns out to be detrimental to the persistence of the bone graft. In other words, bone grafts placed on the surface of the facial skeleton tend to lose their shape over time once they become revascularized. Revascularization (the growing of blood vessels into the bone) allows osteoclasts (cells which dissolve bone) to act in response to the deforming forces of the overlying soft tissue envelope â€“ primarily the muscles. The ability of these forces to alter the shape of the bone is referred to as mechanotransduction.
The change in bone shape and volume in response to the load placed on it was first described by the 19th century German anatomist Julius Wolff. It has long been described as Wolffâ€™s Law of Bone. People who have broken their legs are familiar with the effects of Wolffâ€™s law. After removal of a long leg cast both the soft tissues and the underlying bone one sees that the leg has shrunk.Â When casted, the muscles are not being used causing them to atrophy. Lack of muscle forces on the bone also causes the bone to atrophy. With increasing exercise the muscle gets bigger and stronger causing the bone to increase in shape and volume also. The same thing happens when a bone graft is placed on the facial skeleton. It is hypothesized that genetically, the facial soft tissues and underlying skeleton are designed to have a specific relationship. The craniofacial surgeon Linton Whitaker termed this phenomenon â€œbiologic boundariesâ€. A bone graft placed on the facial skeleton disrupts the biologic boundary. The bodyâ€™s response is to restore the genetically assigned relationship between the soft tissue envelope and the underlying skeleton. This causes the bone graft to be gradually resorbed with gradual loss of the desired new contour.
The inevitable resorption of the bone graft, as well as the poor handling characteristics of autogenous bone grafts also limit the quality and predictability of the aesthetic result. With the exception of interposition grafts used to reconstruct segmental load-bearing defects of the maxilla and mandible, the majority of craniofacial skeleton replacement and, particularly, facial skeleton augmentation is done with alloplastic implants. Alloplast implants are never vascularized and therefore immune to Wolffâ€™s and Whitakerâ€™s laws allowing them to maintain their shape and volume over time.