Molecular methods of enhancing lumbar spine fusion

OBJECTIVE: An optimal method for spinal fusion would induce rapid growth of bone via an osteoconductive and osteoinductive implant. This study examines the spinal fusion enhancement potential of some osteoconductive and osteoinductive biomaterials. METHODS: Four similar canines received unilateral posterolateral fusions on the left side at T13-L1 and L4-L5 and on the right side at L2-L3 and L6-L7. The experiments were grouped as follows: Group A, autogenous bone harvested from the iliac crest; Group B, autogenous bone and collagen; Group C, no implant; and Group D, autogenous bone, collagen, and recombinant human bone morphogenetic protein-2. Radiographic assessment, three-dimensional computed tomographic volumetric analysis, and biomechanical testing were performed at each level. RESULTS: For Groups A and B, the fusions demonstrated moderate bone formation at 6 and 12 weeks postoperatively. Group D fusions exhibited earlier and more dramatic increases in volume and radiodensity and eventually were comparable in size to the vertebral bodies. Average fusion volumes computed from three- dimensional computed tomographic analysis were: Group A = 1.243 cc, Group B = 0.900 cc, Group C = 0.000 cc, and Group D = 6.668 cc (P = 0.003 compared to Group A). Group D exhibited flexion and extension biomechanical properties much greater than controls. The addition of recombinant human bone morphogenetic protein-2 consistently yielded the strongest fused segments and, on average, enhanced extension stiffness by 626% and flexion stiffness by 1120% over controls. CONCLUSION: The most advantageous spinal fusion implant matrix consisted of recombinant human bone morphogenetic protein-2, autogenous bone, and collagen. Future investigators, however, need to examine the appropriate quantities of the individual components and clarify the efficacy of the matrix for the various types of spinal fusion approaches.