Binding to COMP Reduces the BMP2 Dose for Spinal Fusion in a Rat Model.

STUDY DESIGN The aim of this study is to test the effect of cartilage oligomeric matrix protein (COMP) on enhancing rhBMP-2 induced spinal fusion in a prospective 8-week interventional trial of spinal fusion in rats. OBJECTIVE To determine whether the amount of bone morphogenetic protein-2 (BMP-2) required to achieve spinal fusion in a pre-clinical model can be reduced by the addition of COMP. SUMMARY OF BACKGROUND DATA BMPs are applied clinically at supraphysiological doses to promote spinal fusion by inducing osseous growth, but dose-related limitations include ectopic bone formation and local inflammatory reactions. COMP is a matricellular BMP-binding protein expressed during endochondral ossification and fracture healing. In vitro studies demonstrate enhanced activity of BMP bound to COMP. We hypothesized that BMP bound to COMP could achieve equivalent spinal fusion rates at lower doses and with fewer complications. METHODS Posterolateral intertransverse process spinal fusion at L4 to L5 was performed in 36 Lewis rats. COMP (10 μg) was tested with or without "low-dose" rhBMP-2 (2 μg), and the results were compared with the "low-dose" (2 μg rhBMP-2) and "high-dose" (10 μg rhBMP-2) groups. All groups utilized insoluble collagen bone matrix carrier (ICBM). Fusion was evaluated by radiology, histology, and manual palpation. BMP release kinetics were evaluated in vitro. RESULTS Fusion grading of microCT images demonstrated that the fusion rate with the COMP+LoBMP was statistically equivalent to HiBMP, and significantly better than LoBMP without COMP. These results were confirmed with radiographs and manual palpation. BMP release kinetics suggest that COMP increased local concentrations of BMP due to decreased growth factor retention on the scaffold. CONCLUSION COMP enhances BMP-induced bone formation, enabling lower doses of BMP to achieve the same level of spinal fusion. COMP may function by affecting the availability and biological presentation of BMP-2. A decrease of BMP-2 required for fusion may reduce dose-related adverse effects, surgical costs, and improve clinical outcomes. LEVEL OF EVIDENCE N/A.