A comparative analysis of the microarchitecture of cortical membranous and cortical endochondral onlay bone grafts in the craniofacial skeleton.

Previous work in this laboratory established that an onlay bone graft's survival is determined primarily by its relative cortical and cancellous composition rather than its embryologic origin. A volumetric analysis of external bone graft resorption, however, does not explain the internal microarchitectural changes that may be occurring as these grafts become incorporated. To expand the knowledge of bone graft dynamics beyond volumetric parameters, a better understanding of the internal processes of bone graft remodeling is needed. In this comparative study of cortical onlay bone graft microarchitecture, the authors propose to show that cortical onlay bone grafts undergo measurable internal microarchitectural changes as they become incorporated into the surrounding craniofacial skeleton. In addition, the authors propose to further demonstrate similarities between the internal microarchitecture of cortical onlay bone grafts of different embryologic origin over time. Twenty-five adult New Zealand White rabbits were used for this study. They were divided into two groups of eight animals and one group of nine. The groups were killed at 3, 8, and 16 weeks. Cortical membranous and endochondral bone grafts were placed subperiosteally onto each rabbit's cranium. In addition, five ungrafted cortical endochondral and membranous bone specimens were used as controls. Microcomputed tomography (MCT) scanning and histomorphometric analysis were performed on all of the specimens to obtain detailed information regarding the microarchitecture of the cortical bone grafts. The parameters of bone volume fraction, bone surface area to volume, mean trabecular number, and anisotropy were used to give quantitative information about a bone's micro-organization. The results showed that there is no statistically significant difference between the cortical endochondral and the cortical membranous bone grafts for bone volume fraction, bone surface to volume, mean trabecular number, and anisotropy measurements for all time points. There were, however, statistically significant differences when comparing the control and 3-week groups to the 16-week group for all parameters. The advanced MCT technology and histomorphometric techniques proved to be effective in providing a qualitative and quantitative ultrastructural comparison of cortical endochondral and membranous onlay bone grafts over time. In this study, a statistically significant change in the internal microarchitecture of cortical onlay bone grafts of different embryologic origins was seen as they were remodeled and resorbed at all time points. Specifically, the onlay cortical bone grafts developed a less dense, more trabecular, and less organized internal ultrastructure. In addition, no difference in the three-dimensional ultrastructure of cortical endochondral and membranous bone was found. These results challenge some of the currently accepted theories of bone-graft dynamics and may eventually lead to a change in the way clinicians approach bone-graft selection for craniofacial surgery.