A single percutaneous injection of recombinant human bone morphogenetic protein-2 accelerates fracture repair

Background: Recombinant human bone morphogenetic protein-2 (rhBMP-2), surgically implanted with a matrix material, has been shown to induce bone formation and enhance fracture repair. The purpose of this investigation was to test the hypothesis that a single, local, percutaneous injection of rhBMP-2 would accelerate fracture-healing in a standard rat femoral fracture model. Methods: Fractures were created, following intramedullary pinning, in the femora of 144 male Sprague-Dawley rats. The animals were divided into three groups of forty-eight each. Six hours after the fracture, one group received an injection of 80 μg of rhBMP-2 in 25 μL of buffer vehicle, one received an injection of 25 μL of buffer vehicle alone, and one did not receive an injection. Twelve animals from each of these three groups were killed at one, two, three, and four weeks after treatment, and the femora were harvested for torsional biomechanical testing. An additional cohort of seventy-two animals, in which a fracture was also created, was divided into the same three treatment groups; six animals from each of these groups was killed at one, two, three, and four weeks, and the femora were processed for qualitative histological analysis. Results: Torsional biomechanical testing indicated that the stiffness of the rhBMP-2-treated fractures was twice that of both control groups at the two, three, and four-week time-points. The strength of the rhBMP-2-treated fractures was 34% greater than that of the buffer-treated controls (p = 0.03) at three weeks and, at four weeks, was 60% and 77% greater than that of the buffer-treated controls and that of the untreated controls, respectively (p < 0.005). At four weeks, the stiffness and strength of the rhBMP-2-treated fractures were equal to those of the intact contralateral femora, whereas the buffer-treated and untreated fractures were significantly weaker than the intact femora. At two and three weeks, large areas of bone formation, typically spanning the fracture, were observed histologically in the rhBMP-2-treated sites. In contrast, the control fractures exhibited primarily soft cartilaginous callus at these time-points. By four weeks, remodeling of the hard callus and recorticalization were observed in the rhBMP-2-treated fracture sites, whereas cartilage and/or soft tissue was still present in the control fracture sites. Conclusions: These data demonstrate that a single, local, percutaneous injection of rhBMP-2 accelerates fracture repair in this rat femoral fracture model. This effect appears to result from a combination of the induction of bone formation at the fracture site and acceleration of the rate at which the fracture callus matures. Clinical Relevance: The ability of an injection of rhBMP-2 to accelerate fracture repair provides a rationale for its use in fractures that do not require operative treatment or that are to undergo operative treatment without direct exposure of the fracture site. In addition, the finding that an injectable compound may produce a bone-graft-like effect suggests the need for additional studies to explore its application to other types of skeletal surgery.