[Biomechanics of the ring fixator--contributions of individual structural elements]

The mechanical behaviour of the circular external fixation device using thin, pre-tensioned wires is difficult to predict, because the individual elements, such as rings, rods and wires, can be assembled in a large variety of configurations. This behaviour is of particular importance in the treatment of large segmental defects and in bone lengthening, because most of the load is transmitted through the fixator. In this study, the mathematical basis of the widely varying definitions of stiffness under axial, bending and torsional loads is presented. The principal variables influencing a particular configuration and their effects are indicated. Particular consideration must be given to the fact that the wires under tension generate a non-linear behaviour of the frame. The information needed for reporting on different fixator systems is indicated. Finally, the effect of individual structural elements, such as number, direction and tension of the wires, number of threaded rods and distance between the planes of the rings on the different stiffnesses, is presented and discussed with respect to clinical significance.