Background/Objectives Previous work has demonstrated that recombinant human bone morphogenetic protein-2 (rhBMP-2) can significantly aid in spinal fusion. Further findings have indicated that the presence of various constituents from the physical environment surrounding the cellular fusion bed, including muscle tissue and vertebral disk cells, may inhibit the osteoinductivity of rhBMP-2. However, the specific cellular conditions responsible for this inhibition have not been described. The goal of this study was to determine (1) which cellular and biochemical agents play a direct role in the inhibition of rhBMP-2 mediated fusion and (2) the overall nature of this inhibition.
Methods Fibroblasts, annulus fibrosus (AF), nucleus pulposus (NP), and muscle cells were harvested from Lewis rats and cultured in DMEM. Separate rats underwent a posterolateral intertransverse process L4-L5 fusion using 0.032 mg/mL of rhBMP-2 in absorbable collagen sponge combined with 5 × 106 of the various aforementioned cell types. Other Lewis rats underwent a similar procedure, but this time 1.25 × 106 inorganic, inactive polyethylene beads were implanted. In vivo and ex vivo radiographs were taken every 2 weeks. Rats were sacrificed at 8 weeks. Fusion was measured by manual palpation.
Results At 8 weeks, fusion was significantly inhibited when rats had been implanted with AF (1/6), NP (0/6), fibroblasts (0/6), or muscle cells (0/6). Rats with polyethylene beads showed bone formation at 4 weeks and at 6 weeks; spinal fusion was clearly observed.
Conclusions Only living cells could largely inhibit fusion, while inactive beads did not inhibit bone formation or spinal fusion. Results point towards an active, biochemical mechanism of fusion inhibition. These data show for the first time that simple physical mass presence of debris may not inhibit fusion.
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