Creation and preclinical evaluation of a novel mussel-inspired, biomimetic, bioactive bone graft scaffold: direct comparison with Infuse bone graft using a rat model of spinal fusion

OBJECTIVE Infuse bone graft is a widely used osteoinductive adjuvant; however, the simple collagen sponge scaf-fold used in the implant has minimal inherent osteoinductive properties and poorly controls the delivery of the adsorbed recombinant human bone morphogenetic protein-2 (rhBMP-2). In this study, the authors sought to create a novel bone graft substitute material that overcomes the limitations of Infuse and compare the ability of this material with that of In-fuse to facilitate union following spine surgery in a clinically translatable rat model of spinal fusion. METHODS The authors created a polydopamine (PDA)-infused, porous, homogeneously dispersed solid mixture of extracellular matrix and calcium phosphates (BioMim-PDA) and then compared the efficacy of this material directly with Infuse in the setting of different concentrations of rhBMP-2 using a rat model of spinal fusion. Sixty male Sprague Dawley rats were randomly assigned to each of six equal groups: 1) collagen + 0.2 Ng rhBMP-2/side, 2) BioMim-PDA + 0.2 Ng rhBMP-2/side, 3) collagen + 2.0 Ng rhBMP-2/side, 4) BioMim-PDA + 2.0 Ng rhBMP-2/side, 5) collagen + 20 Ng rhBMP-2/side, and 6) BioMim-PDA + 20 Ng rhBMP-2/side. All animals underwent posterolateral intertransverse process fusion at L4-5 using the assigned bone graft. Animals were euthanized 8 weeks postoperatively, and their lumbar spines were analyzed via microcomputed tomography (NCT) and histology. Spinal fusion was defined as continuous bridging bone bilaterally across the fusion site evaluated via NCT. RESULTS The fusion rate was 100% in all groups except group 1 (70%) and group 4 (90%). Use of BioMim-PDA with 0.2 Ng rhBMP-2 led to significantly greater results for bone volume (BV), percentage BV, and trabecular number, as well as significantly smaller trabecular separation, compared with the use of the collagen sponge with 2.0 Ng rhBMP-2. The same results were observed when the use of BioMim-PDA with 2.0 Ng rhBMP-2 was compared with the use of the col-lagen sponge with 20 Ng rhBMP-2. CONCLUSIONS Implantation of rhBMP-2-adsorbed BioMim-PDA scaffolds resulted in BV and bone quality superior to that afforded by treatment with rhBMP-2 concentrations 10-fold higher implanted on a conventional collagen sponge. Us- ing BioMim-PDA (vs a collagen sponge) for rhBMP-2 delivery could significantly lower the amount of rhBMP-2 required for successful bone grafting clinically, improving device safety and decreasing costs.