Implant materials

Factors that impact result

Clinical experience and laboratory investigation have revealed that the fate of non-vascularized onlay bone grafts placed in the craniofacial skeleton depends on three interrelated factors: the architecture of the graft, its revascularization, and the recipient site.

Graft architecture and revascularization

Onlay grafts harvested from the calvarium are superior to grafts harvested from the iliac crest in volume maintenance. This difference was initially attributed to the embryonic origin of the graft, prompting use of the terms membranous and endochondral grafts as opposed to the anatomic terms calvarial and iliac crest grafts. Kusiak et al. reported that calvarial bone grafts become revascularized earlier than iliac grafts, and attributed this volume maintenance to their early revascularization. However, this explanation was questioned when Lin et al. and Sullivan et al. reported more extensive revascularization in iliac crest grafts.

Lin evaluated the volume persistence of calvarial and iliac crest grafts fixed to the rabbit snout with lag screws ( Fig. 3.1 ). Early revascularization of the grafts occurred only in the cancellous portion of both grafts suggesting that the cortical portion of the grafts acted as a mechanical barrier to ingrowing blood vessels ( Fig. 3.2 ). This observation prompted Chen’s study.

Using a model similar to Lin’s, Chen et al. also evaluated the volume persistence of calvarial and iliac crest grafts fixed to the rabbit snout with lag screws. In addition, Chen correlated graft revascularization and graft resorption with graft architecture in the cortical and cancellous regions of both calvarial and iliac grafts. Revascularization was evaluated using latex vascular cast injection, while osteoclastic activity was evaluated using a tartrate-resistant acid phosphatase stain. As expected, he found that at 70 days the calvarial grafts demonstrated greater volume maintenance than the iliac bone (72% vs. 32%, P < .025). He found significantly greater revascularization and osteoclastic activity in the cancellous portion of both the calvarial and iliac crest bone grafts by the 10th day after onlay grafting. Minimal activities were present in the cortical bone ( Figs. 3.3 and 3.4 ). Because calvarial grafts contain more cortical bone, the superior volume maintenance can be understood by the influence of bone architecture on revascularization and resorption. Cortical bone is more slowly revascularized, and therefore less available to osteoclastic activity. The amount of cortical bone in an onlay graft determines its volume maintenance. The importance of graft architecture, as opposed to its embryologic origin, to the volume persistence of onlay bone grafts has been supported in later studies by Gosain.

Rigid fixation

Chen’s study also explained the observation of others that rigid fixation improves graft volume maintenance. Lag screw fixation obliterates the gap between the graft and the recipient bed and therefore may reduce neovascularization of the graft at this interface. Rigid fixation may therefore improve the volume maintenance of onlay bone grafts by limiting, rather than facilitating, revascularization of the graft.

Recipient site

The volume maintenance of a bone graft depends on where it is placed. Zins et al. originally proposed that bone grafts placed in depository fields on the growing facial skeleton would have improved volume maintenance. Gosain et al., using an adult animal model, found complete or near-complete resorption of onlay bone grafts over 1 year (despite the use of rigid fixation and irrespective of their depository or resorptive recipient beds). Rather than its embryologic role in facial growth, the recipient site impacts the fate of a bone graft by the functional stresses it places on the graft. These stresses are determined by regional musculoskeletal forces and by deforming forces of the overlying soft tissue envelope.

The description of this phenomenon is attributed to the German anatomist Julius Wolff, and has long been referred to as Wolff’s Law. It was described for the axial skeleton but is equally applicable to the craniofacial skeleton. When used as an interposition or inlay graft to fill a defect in a location with significant musculoskeletal forces acting on it, for example the mandible, functional stresses will result in the graft’s eventual remodeling into a size and shape similar to the segment it is replacing. When the bone replaced provides a more protective function with little stress on it, for example the cranial vault, the graft loses volume due to osteolysis, as well as revascularization and osteoclastic resorption. Similarly, the volume persistence of an onlay bone graft will be determined by the musculoskeletal forces working on them and the deforming forces of the overlying soft tissue envelope. This translation of mechanical forces into chemical or electrical signals to elicit cell activity is termed “mechanotransduction,” where osteocytes are believed to be the primary mediators. When stressed, osteocytes convert the mechanical forces into molecular signals, which result in bone remodeling.

In summary, the fate of an onlay bone graft is unpredictable but depends on its revascularization and factors that impact its subsequent remodeling. If a bone graft does not become revascularized, it acts like a sequestrum, which remains inert and can maintain its volume indefinitely. This realization prompted the senior author’s use of certain alloplastic materials to replace or augment the facial skeleton. The use of alloplastic materials also has the advantage of avoiding the morbidity, time, and cost associated with autogenous graft harvest.

Autogenous bone graft has long been considered the gold standard material for facial skeleton reconstruction. The term “gold standard” is derived from economics, where the standard economic unit of account is based on a fixed quantity of gold. Gold was chosen because, of all major metals, it is the most resistant to corrosion, and therefore maintains its volume and value over time. Ironically, as noted above, volume retention is not an attribute of autogenous bone when used as an onlay graft.