Lumbar interbody fusion with porous biphasic calcium phosphate enhanced by recombinant bone morphogenetic protein-2/silk fibroin sustained-released microsphere: an experimental study on sheep model.

Biphasic calcium phosphate (BCP) has been investigated extensively as a bone substitute nowadays. However, the bone formation capacity of BCP is limited owing to lack of osteoinduction. Silk fibroin (SF) has a structure similar to type I collagen, and could be developed to a microsphere for the sustained-release of rhBMP-2. In our previous report, bioactivity of BCP could be enhanced by rhBMP-2/SF microsphere (containing 0.5 µg rhBMP-2) in vitro. However, the bone regeneration performance of the composite in vivo was not investigated. Thus, the purpose of this study was to evaluate the efficacy of BCP/rhBMP-2/SF in a sheep lumbar fusion model. A BCP and rhBMP-2/SF microsphere was developed, and then was integrated into a BCP/rhBMP-2/SF composite. BCP, BCP/rhBMP-2 and BCP/rhBMP-2/SF were implanted randomly into the disc spaces of 30 sheep at the levels of L1/2, L3/4 and L5/6. After sacrificed, the fusion segments were evaluated by manual palpation, CT scan, biomechanical testing and histology at 3 and 6 months, respectively. The composite demonstrated a burst-release of rhBMP-2 (39.1 ± 2.8 %) on the initial 4 days and a sustained-release (accumulative 81.3 ± 4.9 %) for more than 28 days. The fusion rates, semi-quantitative CT scores, fusion stiffness in bending in all directions and histologic scores of BCP/rhBMP-2/SF were significantly greater than BCP and BCP/rhBMP-2 at each time point, respectively (P < 0.05). These findings indicate that the SF microspheres containing a very low dose of rhBMP-2 improve fusion in sheep using BCP constructs.