Callus formation and fixation rigidity: a fracture model in rats.

To produce different amounts of callus in rats, we devised a procedure to fix leg fractures using intramedullary nails that differ in bending rigidity. We inserted a silicone cannula into the intact diaphysis of the tibia of rats that had already been killed and then fractured the tibia by a three-point bending technique. The fracture was stabilised by insertion of either a stainless-steel or polypropylene nail into the silicone cannula. Biomechanical testing showed that the initial stiffness of the fractures differed between the two nail types by a factor of 16. In vivo, 16 Wistar rats were operated on by the same technique to study the formation of callus. Four weeks after fracture, the size of the callus differed significantly between the steel-nailed and polypropylene-nailed fractures. In conclusion, mechanically differing internal fixation devices led to different callus responses in rats when all other factors were kept equal.