Bone induction by BMPs/OPs and related family members in primates.

BACKGROUND: In a series of studies in the primate Papio ursinus, we have examined the capacity of bone morphogenetic proteins (BMPs/OPs) delivered in a variety of biomaterial carrier systems to elicit bone formation in heterotopic and orthotopic sites. In this review, we compare the osteoinductive effects of different biomaterial delivery systems that have or have not been pretreated with BMPs/OPs. In particular, we focus on the geometric induction of bone formation by sintered porous hydroxyapatite (SPHA) discs with concavities on their planar surfaces, which elicit bone formation without exogenously applied BMPs/OPs.

METHODS: Heterotopic bone formation was examined by bilaterally implanting 100-mg pellets of a collagenous carrier containing BMPs/OPs in the rectus abdominis muscle of the adult baboon. Orthotopic bone formation was examined by implanting 1 g of a collagenous carrier containing BMPs/OPs into two full-thickness critical-sized 25-mm-diameter defects on each side of the calvaria of adult baboons. The BMPs/OPs whose effects were examined included recombinant human osteogenic protein-1 (rhOP-1), recombinant human transforming growth factor-beta1 (rhTGF-beta1), rhTGF-beta2, and porcine platelet derived transforming growth factor-beta1 (pTGF-beta1). Tissue from the rectus abdominis muscle was harvested 30 or 90 days after implantation. Tissue from the orthotopic calvarial model was examined at 1, 3, 6, 9, and 12 months after implantation. To demonstrate the effect of surface geometry on bone induction, hydroxyapatite powders were sintered to form solid discs with a series of concavities on the planar surfaces of the SPHA discs. The discs were either pretreated with exogenous rhOP-1 or not treated with exogenous OP-1. They were then implanted heterotopically or orthotopically into calvarial defects. Bone formation was evaluated histologically in undecalcified sections stained with Goldner's trichrome stain or 0.1% toluidine blue.

RESULTS: Naturally derived BMPs/OPs or rhOP-1 in a collagenous carrier elicit heterotopic bone formation and the complete healing of 25-mm-diameter critical-sized defects by day 90 following implantation. Binary applications of TGF-beta1 together with rhOP-1 in the collagen carrier induced massive endochondral ossicles in heterotopic sites and bone formation in calvarial defects. pTGF-beta1, rhTGF-beta1, and rhTGF-beta2 are powerful inducers of heterotopic endochondral bone formation but elicit limited bone formation in calvarial defects. SPHA discs pretreated with rhOP-1 elicited extensive bone formation in both heterotopic and orthotopic sites. However, SPHA without rhOP-1 also elicited bone formation in heterotopic and orthotopic sites and complete healing of the calvarial defects.

CONCLUSION: We have prepared SPHA discs with concavities on their planar surfaces that induce bone formation in heterotopic or orthotopic critical-sized calvarial defects without exogenously applied BMPs/OPs. This biomaterial induces bone formation by intrinsic osteoinductivity regulated by the geometry of the substratum. The incorporation of specific biological activities into biomaterials by manipulating the geometry of the substratum, defined as geometric induction of bone formation, may make it possible to engineer morphogenetic responses for therapeutic osteogenesis in clinical contexts.

CLINICAL RELEVANCE: We have implemented a clinical trial using naturally derived BMPs/OPs extracted and purified from bovine bone matrices and implanted in craniofacial defects in humans. In addition, the discovery that specific geometric and surface characteristics of sintered hydroxyapatites can induce intrinsic osteoinductivity in primates paves the way for formulation and therapeutic application of porous substrata designed to obtain predictable intrinsic osteoinductivity in clinical contexts.