In vitro effects of external fixation on intact and osteotomized tibiae. A biomechanical study.

In order to study the in vitro mechanical effects of external fixation on intact and osteotomized bone, human and rabbit tibiae were tested in three-point bending in an Instron testing machine. Intact tibiae were loaded in the elastic range before and after application of an external fixation device. In the human specimens the Vidal-Adrey double frame was used, and in the rabbit specimens the external mini-fixation. The bone deformation in the anteroposterior direction was measured with a linear voltage differential transformer. After application of external fixation a median bone elastic stiffness of 110.2 per cent in human tibiae and of 106.1 per cent in rabbit tibiae was found, in relation to the stiffness of intact tibiae without external fixation. The stiffness increase was significant in human specimens, but not significant in rabbit specimens. In osteotomized human tibiae fixed with the Vidal-Adrey double frame a median stiffness of 8.5 per cent of the stiffness of intact bones was found when compression was not applied, compared to 19.9 per cent when compression was applied. In rabbit tibiae a median stiffness of osteotomized bones with external mini-fixation was 8.9 per cent of the stiffness of intact bones. There was a good agreement between the results in rabbit tibiae with the mini-fixation and human tibiae with external fixation without compression. The external mini-fixation thus seems to be a suitable model for the study of bone healing in rabbit tibiae.