Recombinant human bone morphogenetic protein-4 (rhBMP-4) enhanced posterior spinal fusion without decortication.

In posterior spinal fusion, insufficient decortication may decrease the number of bone marrow derived ostoprogenitor stem cells and affect the success of bony fusion. The finding of bone formation through interaction between rhBMP-4 and non-marrow derived mesenchymal cells constituted the basis of the current study. The aim is to investigate the possibility of molecular enhancement of posterior spinal fusion by site-specific application of rhBMP-4 with or without surgical decortication. Eighteen adult rabbits underwent single level bilateral posterior intertransverse process spinal fusion at L5-L6. one side with decortication, and the other side without decortication. Two animals underwent sham operation without bone grafts, the other 16 animals were randomly allocated into three groups, using hydroxyapatite-tricalcium phosphate (HA-TCP) ceramic blocks augmented with 0, 0.125 and 0.5 mg of rhBMP-4 respectively. Spinal fusion morphology was evaluated with sequential X-ray, microradiography and histomorphology. At week 7, complete bony fusion was achieved in none of the groups without rhBMP-4 irrespective of whether the bony contact surface was decorticated or not. In the groups with low dose rhBMP-4, complete fusion occurred in two of six un-decorticated sites (33%) and in three of six (50%) decorticated sites. 100% complete fusion was found in the high dose rhBMP-4 group independent of surgical decortication. The dorsal cortices of the un-decorticated transverse processes were replaced by newly formed trabecular bone through biological remodeling. This study suggested that rhBMP-4 can induce non-marrow derived mesenchymal cells to differentiate into osteogenic cells and thus enhance the high success rate of pesterior spinal fusion in both the decorticated and un-decorticated model.