Mandible reconstruction and autogenous frozen bone graft: experimental study on rats

The aim of this study was to evaluate the biological behaviour of a frozen bone graft in orthotopic and heterotopic sites in the rat. The previous experimental study on this subject was published 25 years ago without sufficient detail about the histology and comparison between the orthotopic and ectopic sites. Therefore, being very important for future clinical application, we decided to evaluate the frozen bone graft using rats. The procedure was performed on two groups of five rats each (Charles River). After wide dissection of the inferior border of the mandible from the surrounding muscle, an inferior segmental resection 4 mm in length was performed, taking care not to fracture the superior part and to maintain mucosal integrity. This segment was placed in liquid nitrogen for two periods of 10 minutes each with a third period to allow it to reach room temperature. In the first group (A), the frozen segment was placed ectopically in a gluteal muscle pocket, and in the second group (B), the frozen bone was fixed in the same position in the same mandible. After 1 month of follow-up, the animals were killed, the bone graft was removed, and histology was performed. Results were consistent in both groups. In group A, the segment was surrounded by strong inflammatory reaction, with no vital cells or bone cells, but some vascular penetration. We concluded that there was no bone deposition and no bone rehabitation. In group B, the initial segment was strongly fixed to the remaining mandible, there was an increase of the macroscopic dimension that paralleled the increase in the dimension of the remaining mandible and the growth of the animal. The cortical part had thinned down, the medullary part presented signs of bone deposition as well as bone resorption and vascular penetration. The periosteum from the adjacent normal mandible was growing and covering the frozen bone graft, offering additional stimulus to the bone deposition. In conclusion, the frozen bone graft acts as a normal bone graft. It needs to be placed in contact with vascularised bone and surrounded by well vascularised soft tissue to allow deposition of new bone. If the frozen graft is placed ectopically, it will be surrounded by chronic inflammatory reaction with no bone deposition.