Efficacy of interspinous process lumbar fusion with recombinant human bone morphogenetic protein-2 delivered with a synthetic polymer and β-tricalcium phosphate in a rabbit model.

INTRODUCTION: As a powerful bone-inducing cytokine, rhBMP-2 has been used as a bone graft substitute in combination with animal-derived collagen to achieve interbody or posterolateral spinal fusion. Successful interspinous process fusion using rhBMP-2 in combination with synthetic carrier materials would offer a safe, minimally invasive spinal fusion option for the treatment of spinal disorders. The aims of the present study were to achieve interspinous process fusion by implanting rhBMP-2-retaining degradable material instead of bone grafting and to evaluate efficacy for vertebral stabilization.
MATERIALS AND METHODS: A polymer gel (200 mg), β-tricalcium phosphate powder (400 mg), and rhBMP-2 (0, 30, 60 or 120 μg) were mixed to generate a plastic implant, which was then placed during surgery to bridge the L5-6 interspinous processes of 58 rabbits. Control animals received implants either without rhBMP-2 or with autogenous bone chips from the iliac crest. L5-6 vertebrae were recovered 8 weeks postoperatively. Interspinous process fusion was evaluated by radiography, biomechanical bending test, intradiscal pressure (IDP) measurement, and histology.
RESULTS: In bending tests, strength of fusion was significantly greater in BMP60 and BMP120 groups than in sham, BMP0, BMP30 or autogenous bone groups. IDP at L5-6 was significantly reduced in BMP60 and BMP120 groups compared to sham, BMP0, BMP30, and autograft groups. Histologically, coronal sections of the fusion mass showed a bone mass bridging both spinous processes.
CONCLUSION: Solid interspinous process fusion was achieved in rabbit models by 8 weeks after implanting the biodegradable bone-inducing material. These results suggest a potential new less-invasive option without bone grafting for the treatment of lumbar disorders.