Assessment of effects of rhBMP-2 on interbody fusion with a novel rat model

The effects of using off-label recombinant human bone morphogenetic protein (rhBMP)-2 for interbody fusion are controversial. Although animal models of posterolateral fusion are well-established, establishing animal models to validate the safety and efficacy of interbody fusion is difficult, which may contribute to the inconsistent clinical results. To develop a novel animal model of interbody fusion in rat coccygeal vertebrae without destroying bony endplates. An experimental animal study. Forty-five male Sprague-Dawley rats underwent coccygeal interbody fusion without violating vertebral endplates. The animals were divided into three different groups based on the materials that were implanted into the interbody space (1) allogeneic iliac bone (IB) alone (IB group), (2) IB and 3 µg of rhBMP-2 (BMP low-dose group), or (3) IB and 10 µg of rhBMP-2 (BMP high-dose group). Fusion rates were investigated using microcomputed tomography 6 weeks after the operation. The incidence of adverse events, including soft-tissue swelling, delayed wound healing, osteolysis, and ectopic bone formation were evaluated. The total number of adverse events (using the adverse event score) in each group and the swelling ratio (calculated using the surgical site tissue volume [TV; TV on postoperative day 1/preoperative TV]) were also evaluated. The fusion rates in the BMP low- and high-dose groups (33.3% and 46.7%) were not significantly different, but both were significantly higher than that in the IB group (0%) (p=.042 and .006, respectively). Significant differences in the incidence of osteolysis, adverse event scores, and swelling ratios were observed only between the BMP high-dose and IB groups (p=.043, .006 and .014, respectively). We developed a novel rat model of interbody fusion in which the vertebral endplates were not violated, reflecting the normal clinical setting. rhBMP-2 use increased the fusion rate, but a higher dose of rhBMP-2 did not lead to a higher fusion rate than that for low-dose rhBMP-2; conversely, it led to an increase in the occurrence of adverse events. This novel rat model of coccygeal interbody fusion that preserved bony endplates has clinical significance for validating the effectiveness of biologics or bone graft substitutes before clinical trial.