VOLUME MAINTENANCE OF ONLAY BONE-GRAFTS IN THE CRANIOFACIAL SKELETON - MICROARCHITECTURE VERSUS EMBRYOLOGIC ORIGIN

The superior volume maintenance of membranous over endochondral bone h as been shown in several studies and provides the basis for its prefer red clinical use as an onlay grafting material in the craniofacial ske leton. The scientific rationale for this seeming embryologic advantage , however, has never been proven. Our hypothesis is that the pattern o f onlay bone graft resorption is primarily determined by a graft's mic ro-architecture (relative cortical and cancellous composition) rather than its embryologic origin (membranous versus endochondral). Twenty-f ive adult New Zealand, White rabbits were used for this study. Eight a nimals were killed at 3 weeks, eight animals at 8 weeks, and nine anim als at 16 weeks. Three graft types were placed onto each rabbit craniu m: cortical bone graft of membranous origin and cortical and cancellou s bone graft of endochondral origin. Fluorochrome markers were injecte d into all living rabbits at 1, 6, and 14 weeks. Microcomputed tomogra phy scanning was performed on all of the bone grafts to determine post sacrifice volumes and to obtain detailed information regarding the bon e graft's trabecular architecture. In addition, specimens were examine d histologically. Volume analysis showed a statistically greater resor ption rate in the cancellous endochondral bone graft than in either th e endochondral or membranous cortical bone grafts (p < 0.05) for all t ime points. In addition there was no significant difference in the res orption rates between the endochondral and membranous cortical bone gr afts. A post-test power analysis (alpha = 5 percent) of the volume dat a comparing the two types of cortical bone grafts showed that a differ ence in resorption of 8.9 percent would be detected with a 90-percent probability. Previous studies, which have shown a seeming superiority of membranous over endochondral bone grafts, used composite grafts com posed of both cortical and cancellous portions. By separating these co mponents, we have shown that cortical bone grafts maintain their volum es significantly better than cancellous bone grafts. In addition, we f ound no statistical difference in the resorption rates between the two cortical onlay bone grafts of different embryologic origins, a findin g that has never been previously published. From our results, we belie ve cortical bone to be a superior onlay grafting material, independent of its embryologic origin. We believe these results challenge the cur rently accepted theories of bone graft dynamics and may lead to a chan ge in the way clinicians approach bone graft selections for craniofaci al surgery.