Mechanical Testing of Bone Regenerated with BMP-7 in TCP Scaffolding

Introduction: The gold standard for bone grafting is still autologous cortical and cancellous bone. This is due to the combined osteogenic, osteoinductive and osteoconductive properties of the graft. Autograft availability is, however, limited and often associated with donor site morbidity. The current trends are exploring methods that move away from using autografts to repair these defects by using various bone substitutes. Objectives: To assess the physical properties of the bone regenerated in a critical-size unilateral mandibular defect in the rabbit model treated with bone morphogenetic protein 7 (rhBMP-7) on a tricalcium phosphate (TCP) scaffolding. Methods: Nine adult New Zealand white rabbits (3.0-4.0kg) were used; in six cases the critical-size defect was filled with the rhBMP-7 on the TCP scaffolding (Group 1), and in three cases the TCP was used alone (Group 2). Three months post-operatively the cases were sacrificed and the mandibles were surgically removed. The mandibles were divided into hemi-mandibles at the symphysis and mounted within custom made aluminium blocks, that were mounted into a BOSE Elecroforce® 3300 test instrument. The mechanical properties of the regenerated bone in the surgical sites were then tested by applying the principle of cantilevers. The maximum load that occurred at fracture was recorded and the distance between the loading point and the site of failure on the dorsal surface of the specimen was measured. The failure moment was then calculated. Results: The failure moments for group 2 were found to be very low (0-48mNm) while those for group 1 were higher but there was considerable variation between the cases (55-2115mNm). Some of the cases in group 1 achieved failure moments comparable to normal untreated bone. Conclusions: The repair of critical-size mandibular defects with rhBMP-7 in TCP scaffolding could result in the regeneration of bone with mechanical properties similar to those of natural bone.