Mechanical effects of metal plate fixation. In vitro investigation on intact and osteotomized human and rabbit tibiae.

In order to study the in vitro stress-protecting effect on intact bone, and the rigidity of plate fixation on 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 metal plate application. The deformation was measured with a linear voltage differential transformer. In the human specimens a median stiffness increase of 31 per cent was obtained in the plated bone segment after application of a tibial plate (140 x 12 x 4 mm) and of 43.8 per cent after application of a femoral plate (140 x 16 x 5 mm). In the rabbit specimens a median stiffness increase of 31.9 per cent was obtained after application of a thin plate (45 x 5 x 1 mm). In osteotomized human tibiae with tibial plate fixation, a median elastic stiffness of 51.4 per cent of the intact bone was found, compared to 40.4 per cent in osteotomized rabbit tibiae plated with the thin plate. There was good agreement between the results in human tibiae plated with a tibial plate, and rabbit tibiae with the small plate. In previous studies of the stress-protecting effect of internal plate fixation on rabbit tibiae more rigid plates have been used. These plates seem to have been overdimensioned. In further studies in rabbits plates of similar size as the one tested in this study should be applied.