Study Design. Efficacy of a bovine-derived osteoinductive growth factor was studied in a rabbit model and in a nonhuman primate model of posterolateral lumbar spinal fusion. Objectives. To determine the minimum effective dose of growth factor and the Influence of different carrier material on the outcome of intertransverse process lumbar fusion. Summary of Background Data. Bone morphogenetic proteins and related growth factors are becoming increasingly available in purified extract or genetically engineered forms and are capable of inducing new bone formation in vivo. Osteoinductive growth factors to enhance lumbar spinal fusion have not been well studied in models of posterolateral intertransverse process:fusion. Because of the diminished potential of bone regeneration in primates (including humans) compared with phylogenetically lower animals, extrapolations regarding dose and efficacy cannot be made directly from results obtained in experiments performed on phylogenetically lower animals. Experiments on nonhuman primates are a critical step before attempting to use these growth factors in humans. Methods. One hundred fifteen adult New Zealand white rabbits and 10 adult rhesus macaques underwent single level posterolateral intertransverse process lumbar spinal arthrodesis to evaluate different doses and carrier materials fora bovine-derived osteoinductive bone protein extract, Rabbit fusion masses were evaluated 5 weeks after arthrodesis by manual palpation, radiography, biomechanical testing, and light microscopy. Monkey fusion masses were evaluated 12 weeks after arthrodesis by radiography and light microscopy. Results. Successful posterolateral intertransverse process spinal fusions were achieved in the rabbit model using an osteoinductive growth factor with three different carriers (autogenous iliac bone, demineralized allogeneic bone matrix, and natural coral). There was a dose-dependent response to the osteoinductive growth factor in the rabbit model, indicating that a threshold must be overcome before bone formation is induced. The methodology for biologic enhancement of spinal fusion developed in the rabbit model was transferred successfully to the rhesus monkey, where the use of the osteoinductive growth factor with a demineralized bone matrix carrier resulted in spinal fusion in 12 weeks. Conclusions. These experiments provide an essential building block in the understanding of the biology of spinal fusion and the use of osteoinductive growth factors to enhance a posterolateral intertransverse process spinal fusion. The achievement of posterolateral spinal fusion in the rhesus monkey using an osteoinductive growth factor is a significant step toward the biologic enhancement of spinal fusion in humans.
