Prostaglandin E1 and recombinant bone morphogenetic protein effect on strength of hydroxyapatite implants

Although combinations of hydroxyapatite (HAP) and bone morphogenetic protein (BMP) are expected to provide potent alternatives to autogenous bone grafts, it is still anticipated that substances that act synergistically with BMP will be found because the inducing potential of purified BMP in bone is not strong enough. We already have shown that prostaglandin (PG) E1 has a strong and dose-dependent synergistic effect on the osteoinductive activity induced by recombinant human (rh) BMP and that it enhances osteoconduction even when used alone. In this study, porous HAP rods were treated as follows: (1) without PGE1 or rhBMP (control group); (2) with varying concentrations of PGE1; and (3) with varying concentrations of PGE1 combined with 1 microg of rhBMP-2. The rods were subperiosteally implanted on the cranial bone of rabbits to evaluate the effect of these treatments on the mechanical strength of the implanted HAP rods. The HAP rods were removed 3, 6, or 9 weeks after implantation and subjected to mechanical strength determinations. The control group (no addition of BMP to the rods) showed no significant increase in three-point bending strength or in compression strength compared to pre-implantation. On the other hand, PGE1 combined with rhBMP had a strong and dose-dependent effect on the mechanical strength of HAP, increasing it significantly, especially compression strength. PGE1 also increased mechanical strength even when used alone. Histological examination revealed that PGE1, whether or not it was combined with rhBMP, increased bone formation into the pores of HAP and consequently increased the mechanical strength of porous HAP.