Bone regeneration at rabbit skull defects treated with transforming growth factor-β1 incorporated into hydrogels with different levels of biodegradability

Object. Skull bone regeneration induced by transforming growth factor-beta 1 (TGF beta 1)-containing gelatin hydrogels (TGF beta 1-hydrogels) was investigated using a rabbit skull defect model. Different strengths of TGF beta 1 were examined and compared: different TGF beta 1 doses in gelatin hydrogels with a fixed water content, different water con tents in gelatin hydrogels with a fixed TGF beta 1 dose, and TGF beta 1 in solution form. In addition, regenerated skull bone was observed over long time periods after treatment. Methods. Soft x-ray, dual energy x-ray absorptometry, and histological studies were performed to assess the time course of bone regeneration at a 6-mm-diameter skull defect in rabbits after treatment with TGF beta 1-hydrogels or other agents. The influence of TGF beta 1 dose and hydrogel water content on skull bone regeneration by TGF beta 1-hydrogels was evaluated. Gelatin hydrogels with a water content of 95 wt% that incorporated at least 0.1 mu g of TGF beta 1 induced significant bone regeneration at the rabbit skull defect site 6 weeks after treatment, whereas TGF beta 1 in solution form was ineffective, regardless of dose. The in vivo degradability of the hydrogels, which varied according to water content, played an important role in skull bone regeneration induced by TGF beta 1-hydrogels. In our hydrogel system, TGF beta 1 is released from hydrogels as a result of hydrogel degradation. When the hydrogel degrades too quickly, it does not retain TGF beta 1 or prevent ingrowth of soft tissues at the skull defect site and does not induce bone regeneration at the skull defect. It is likely that hydrogel that degrades too slowly physically impedes formation of new bone at the skull defect. Following treatment with 0.1-mu g TGF beta 1-hydrogel (95 wt%), newly formed bone remained at the defect site without being resorbed 6 and 12 months later. The histological structure of the newly formed bone was similar to that of normal skull bone. Overgrowth of regenerated bone and tissue reaction were not observed after treatment with TGF beta 1-hydrogels. Conclusions. A TGF beta 1-hydrogel with appropriate biodegradability will function not only as a release matrix for the TGF beta 1, but also as a space provider for bone regeneration. The TGF beta 1-hydrogel is a promising surgical tool for skull defect repair and skull base reconstruction.