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- β1 (TGFβ1)-containing gelatin hydrogels (TGFβ1-hydrogels) was investigated using a rabbit skull defect model. Different strengths of TGFβ1 were examined and compared: different TGFβ1 doses in gelatin hydrogels with a fixed water content, different water contents in gelatin hydrogels with a fixed TGFβ1 dose, and TGFβ1 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 TGFβ1 -hydrogels or other agents. The influence of TGFβ1 dose and hydrogel water content on skull bone regeneration by TGFβ1-hydrogels was evaluated. Gelatin hydrogels with a water content of 95 wt% that incorporated at least 0.1 μg of TGFβ1 induced significant bone regeneration at the rabbit skull defect site 6 weeks after treatment, whereas TGFβ1 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 TGFβ1-hydrogels. In our hydrogel system, TGFβ1 is released from hydrogels as a result of hydrogel degradation. When the hydrogel degrades too quickly, it does not retain TGFβ1 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-μg TGFβ1- 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 TGFβ1-hydrogels. Conclusions. A TGFβ1-hydrogel with appropriate biodegradability will function not only as a release matrix for the TGFβ1, but also as a space provider for bone regeneration. The TGFβ1-hydrogel is a promising surgical tool for skull defect repair and skull base reconstruction.