Promoted Bone Healing at a Rabbit Skull Gap Between Autologous Bone Fragment and the Surrounding Intact Bone with Biodegradable Microspheres Containing Transforming Growth Factor-β1

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Abstract
This study is a trial to promote repairing of the rabbit skull bone gap between an autologous bone flap and the intact bone with biodegradable gelatin microspheres containing transforming growth factor-β1 (TGF-β1). A 10-mm diameter bone defect was prepared in rabbit skulls by drilling out a bone flap of 6 mm in diameter. After a surrounding gap defect of 2 mm was created and treated with 0.5 μg of free TGF-β1 and gelatin microspheres containing 0.5 μg of free TGF-β1, the circular autologous bone flap was placed in the center. Significant bone healing at the gap defect was observed 3 weeks after implantation of the TGF-β1-containing gelatin microspheres. The bone mineral density (BMD) was significantly higher than that of other experimental groups. On the contrary, when applied with free TGF-β1, a fibrous tissue initially infiltrated into the gap defect, resulting in impairing bone healing. The tissue response was similar to that at the defect implanted with empty gelatin microspheres and TGF-β1-free phosphate-buffered saline solution alone. There was more space in the gap-filling bone in the 16-week view than the 3-week view. It is possible that this was an intermediate step along the way toward normal healing and formation of cancellous bone. We conclude that gelatin microspheres containing TGF-β1 show promise as an agent to promote bone regeneration of subcritical size defects between surgically positioned autologous bone flaps and surrounding host bone.