BMP depletion occurs during prolonged acid demineralization of bone: characterization and implications for graft preparation.

Demineralization of allograft bone increases the bioavailability of matrix-associated bone morphogenetic proteins (BMPs), rendering these grafts osteoinductive. While osteoinductivity is related to BMP content, little is known about how the demineralization protocol, in particular, extended demineralization times, affects graft BMP levels. We characterized the BMP-7 content of <710 μm bovine bone powder demineralized under various conditions. Using 1 g of bone per 50 ml of 0.125 N, 0.25 N, or 0.5 N HCl, demineralization was performed at room temperature for 5 min to 24 h. Minimum residual calcium levels were obtained within 90 min and were <1 wt % using the 0.25 N and 0.5 N baths and 17 wt % using the 0.125 N bath. Measured peak BMP-7 levels were also obtained within 90 min and were 161-165 ng g(-1) using the 0.25 N and 0.5 N baths and 55.2 ng g(-1) using the 0.125 N bath. This compares to 5.1 ng g(-1) for undemineralized bone. Further acid bath exposure to 24 h resulted in BMP-7 decline to about 50% of the peak value, which was significant. The BMP-7 half-life was estimated to be 26 h. It is likely that the decline was due to diffusion of BMP-7 from the bone matrix into the acid. These results suggest the importance of not over demineralizing bone grafts and should stimulate further research that can be incorporated into the processing methodology followed by tissue banks.