Unexpected absence of effect of rhBMP-7 on distraction osteogenesis

We asked whether locally applied rhBMP-7 with a Type I collagen carrier could enhance the consolidation phase in distraction osteogenesis. We performed unilateral transverse osteotomy of the tibia in 28 immature male rabbits. In half the animals (14 rabbits), 70 microg rhBMP-7 (28.5 mg rhBMP-7/CC, OP-1) was applied locally on the day of the osteotomy; the control group (14 rabbits) had no local application. After 7 days, 3 weeks of distraction was begun at a rate of 0.5 mm every 12 hours. Starting with Week 2 of distraction, we assessed radiographic, ultrasonographic, and densitometric parameters once per week. Animals were sacrificed after 3 weeks of distraction, and after 1, 2, and 3 weeks of additional consolidation. Radiographic evaluation revealed a tendency for increased bony union and bone differentiation in the control group compared with the rhBMP-7/CC group. The bone mineral content was higher in the control group at each time. Ultrasonographic evaluation revealed the development of a cyst in the regenerate in 92% of the animals in the rhBMP-7/CC group versus 0% in the control group. Ultrasonography and histomorphometry showed delayed regenerate development in the rhBMP-7/CC group. We suggest the poor results related to the relatively large bulk of the solid carrier of the OP-1 that might have acted as a mechanical obstacle to osteogenesis.