Histology and stability study of cortical bone graft influence on titanium implants

Objective The mechanisms for integrating titanium implants in recipient bone are still not well defined, and it is not known whether the process of bone formation around implants inserted into grafts is the same as that described for titanium implants in nongrafted cortical bone. This study compared the histology, stability, and osseointegration of titanium implants inserted in cortical bone with and without a simultaneous autologous cortical bone graft in an experimental animal model. Methods Thirty titanium implants were inserted in 3 sheep. Half of the implants were inserted to fix the graft to the recipient bone, and the remainder were inserted in the distal part of the tibial metaphysis as controls. The animals were humanely killed at 2, 6, and 8 months after surgery. A stability test (unscrewing torque) was performed immediately on 12 fresh specimens (6 grafted implants and 6 control implants). The remaining unscrewed implants, both grafted and not grafted, were subjected to histomorphometric analysis. Results After osseointegration, the unscrewing force exceeded the fracture limit of the titanium fixtures in both the grafted samples and controls, demonstrating their optimal stability but failing to demonstrate an improvement in the grafted bone. Histomorphometric analysis demonstrated newly formed tissue that extended from the contact area inside the graft, beginning at 6 months. At 8 months, the implant threads in the graft were surrounded by a large amount of newly formed bone mixed with necrotic fragments. Conclusions Our results show that onlay cortical grafts on cortical bone enhance the osteogenic potential of the host bone, ensuring solid, viable bone tissue support that results in a high rate of integration of the titanium fixtures. The loading forces affect the bone-healing process after implant insertion; bone matrix was deposited unequally, being greater proximally (90% versus 40%), which is perpendicular to the maximal load tension lines when the sheep are standing. This implies that in the human jaw, where the tension lines parallel the axis of the implants, the implants can play an important role in guiding new bone formation during osseointegration.