Use of collagen sponge incorporating transforming growth factor-beta1 to promote bone repair in skull defects in rabbits

The objective of this study was to evaluate the potential of collagen sponge incorporating transforming growth factor-beta1 (TGF-beta1) to enhance bone repair. The collagen sponge was prepared by freeze-drying aqueous foamed collagen solution. Thermal cross-linking was performed in a vacuum at 140 degrees C for periods ranging from 1 to 48 h to prepare a number of fine collagen sponges. When collagen sponges incorporating 125I-labeled TGF-beta1 were placed in phosphate-buffered saline (PBS) solution at 37 degrees C, a small amount of TGF-beta1 was released for the first hour, but no further release was observed thereafter, irrespective of the amount of cross-linking time the sponges had received. Collagen sponges incorporating 125I-labeled TGF-beta1 or simply labeled with 125I were implanted into the skin on the backs of mice. The radioactivity of the 125I-labeled TGF-beta1 in the collagen sponges decreased with time; the amount of TGF-beta1 remaining dependent on the cross-linking time. The in vivo retention of TGF-beta1 was longer in those sponges that had been subjected to longer cross-linking times. The in vivo release profile of the TGF-beta1 was matched with the degradation profile of the sponges. Scanning electron microscopic observation revealed no difference in structure among sponges subjected to different cross-linking times. The TGF-beta1 immobilized in the sponges was probably released in vivo as a result of sponge biodegradation because TGF-beta1 release did not occur in in vitro conditions in which sponges did not degrade. We applied collagen sponges incorporating 0.1 microg of TGF-beta1 to skull defects in rabbits in stress-unloaded bone situations. Six weeks later, the skull defects were covered by newly formed bone, in marked contrast to the results obtained with a TGF-beta1 free empty collagen sponge and 0.1 microg of free TGF-beta1. We concluded that the collagen sponges were able to release biologically active TGF-beta1 and were a promising material for bone repair.