Stability of external fixators used for knee arthrodesis after failed knee arthroplasty

Secondary arthrodesis as treatment for failed knee arthroplasty is being used increasingly. The usual method is sustained rigid external fixation. The fusion rate is low in cases with considerable metaphyseal bone loss and poor bone quality. Currently, the Hoffmann- Vidal fixator or comparable designs are preferred to the less stable Charnley single frame fixator. The standard Hoffmann- Vidal device uses only transverse percutaneous fixating pins and gives an anteroposterior bending stiffness that is considerably lower than the lateral bending stiffness. In the present study, external fixators were tested for stability in a material testing machine under standardized conditions using synthetic bones. The anteroposterior bending stiffness was significantly improved when sagittal pins connected to a ventral compression rod were added to a Hoffmann- Vidal fixator. Stiffness was still more improved when sagittal pins were connected to ventral frames as in a modification of the Hoffmann- Vidal or the similar Ace-Fischer fixator.