THE HEALING OF SEGMENTAL BONE DEFECTS, INDUCED BY RECOMBINANT HUMAN BONE MORPHOGENETIC PROTEIN (rhBMP‐2): A RADIOGRAPHIC, HISTOLOGICAL, AND BIOMECHANICAL STUDY IN RATS

Subcutaneous implants of a recombinant human form of the bone-inducing protein rhBMP2 (recombinant human bone morphogenetic protein-2) in rats have resulted in the local induction of endochondral bone formation. To test the osteoinductive activity of rhBMP-2 in an osseous location, we created five-millimeter segmental defects in the femora of forty-five adult male SpragueDawley rats. Two doses of lyophilized rhBMP-2 (1.4 or 11.0 micrograms) were implanted in each defect, together with guanidine-hydrochioride extracted demineralized rat-bone matrix as a carrier, and the results were compared with those in rats that had implantation of guanidine-hydrochioride extracted demineralized rat-bone matrix only. The formation and healing of bone were determined by radiographic, histological, and mechanical analysis. Both doses ofrhBMP-2 induced formation of endochondral bone in the osseous defects in a dose-related manner. Implantation of 11.0 micrograms of rhBMP-2 yielded significant (p < 0.05) bone formation, resulting in radiographic, histological, and mechanical evidence of union. Despite new-bone formation in the defects that had received 1.4 micrograms of rhBMP-2, no instances of union were observed. CLINICAL RELEVANCE: RhBMP-2 is the first growth factor available in recombinant form to demon*One or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will he directed to a research fund or foundation, educational institution, or other non-profit organization with which one or more of the authors are associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were Grants AR07281-ll, AR38520-03, and AR25791 -12 from the National Institutes of Health:Grant 9()-539 from the Orthopaedic Research and Education Foundation: the Greenwall Foundation: and the Genetics Institute. tThe University ofTexas M. D. Anderson Cancer (‘enter, University ofiexas, 1515 Holcombe Boulevard, Houston,Texas 77030. Ibe Hospital for Special Surgery.535 East 70th Street, New York, N.Y. 1(8)21. §Orthop#{228}dische Universitatsklinik Wien,AKI-I.Garnisongasse 13. A-l09() Wien, Austria. 9lGenetics Institute, 87 Cambridge Park Drive, Cambridge. Massachusetts 02140). strate osteoinductive activity leading to successful union in an orthotopic site. The results of this study support the potential application of rhBMP-2 as a bone-graft substitute of unlimited availability for clinical situations when local bone formation is needed. The formation, maintenance, and regeneration of bone is a complex process involving the interactions of many cellular elements with systemic and local regulatons. These regulators include growth and differentiation factors, hormones, cytokines, and extracellular matrix components, all of which have direct or indirect effects on bone’””3t’4. Urist” noted that demineralized bone matrix could induce de nova formation of cartilage and bone when it was implanted in extraskeletal sites. Subsequent investigations identified the active component of the matrix as proteinaceous and demonstrated that it could be extraded from the bone matrix48’49. Urist named this Osteoinductive component bone morphogenetic protein (BMP). Putative osteoinductive factors that demonstrate the formation of bone have been extracted from bone in both humans”2’ and animals2’4’24’25’ ” ””952’54’ as well as from osteosarcomas”38 ”43. Experiments in animals and humans have confirmed the osteoinductive activity of these crude and partially purified bone-derived preparations by demonstrating de novo formation of bone in ectopic and orthotopic sites’ b8,22,2)31,57,4S,4749,S2#{149} The sequence of cellular events that is induced locally by these extractable protein components of bone matrix has been charactenized extensively and has been observed to be similar to that seen during fracture repair and during formation of embryonic endochondral bone”27. However, the precisc molecular and cellular mechanisms underlying bone induction remain to be elucidated. Until recently, only chemically extracted bonederived substances had been described. Preparations resulting from various purification procedures have yielded a number ofproteins that possess different physical properties and a wide range of dose-response relationships. In previous experiments, these differences in composition and purity have confounded the charac-