A BIOMECHANICAL ASSESSMENT OF PLATE FIXATION, WITH INSUFFICIENT BONY SUPPORT

The use of an internal fixation plate in the presence of a bone defect was studied using a theoretical model of an idealized long bone having a circular cross section and loaded using a combination of axial and bending loads. The analysis showed that the 'bending-open' loading mode does not occur if, in the normal unplated bone, the line of action of the resultant axial load passes within the outer cortex at the location where the plate is to be applied. If bony contact cannot be achieved, lower plate stress is always encountered when the plate is attached to the compression side instead of the tension side. In vivo verification of the model was addressed in a pilot experiment using instrumented metal plates applied bilaterally to the femora of one dog. The results and analysis of this study suggest that when applying plates to diaphyseal fractures were bony stability cannot be achieved, it may be acceptable or even desirable to attach the plate to the compression side of the bone.