BMP-2 for intramuscular bone induction: effect in squirrel monkeys is dependent on implantation site

Demineralized bone matrix (DBM) reproducibly induces extraskeletal bone formation in rodents, but its effects in dogs and primates are negative or uncertain. In previous studies on the squirrel monkey, DBM did not induce bone, although the same implants were effective in rats. DBM implants augmented with recombinant human bone morphogenetic protein-2 (rhBMP-2) induced intramuscular bone formation in squirrel monkeys. However, the amount of induced bone was often minimal and sometimes absent. One explanation of this weak and unpredictable effect could be reactions to cellular components of the allogeneic DBM that was used as carrier. Therefore, we now repeated the experiment, using bovine type 1 collagen as carrier. 48 collagen discs (10 mm diameter), containing 0, 10, 40 or 200 microgram rhBMP-2, were implanted in 6 monkeys and 6 rats. The BMP-2 implants induced dose-dependent amounts of intramuscular bone in rats whereas, in squirrel monkeys, macroscopic bone induction occurred in less than half of the BMP-2 implants. There was no dose-dependency. Whether bone was formed or not was significantly influenced by individual variation among the monkeys, and by implant location within the muscle. Implants close to the muscle aponeurosis more often induced bone than did purely intramuscular ones. In this small series, we could not demonstrate a significant effect of BMP-2, as compared to control implants. Presumably there were too few cells expressing BMP-2 receptors in the only minimally traumatized muscle of these monkeys.