BIOMECHANICS OF THE CIRCULAR EXTERNAL FIX ATION DEVICE - CONTRIBUTIONS OF INDIVIDUAL STRUCTURAL ELEMENTS

The mechanical behaviour of the circular external fixation device usin g thin, pre-tensioned wires is difficult to predict, because the indiv idual elements, such as rings, rods and wires, can be assembled in a l arge variety of configurations. This behaviour is of particular import ance in the treatment of large segmental defects and in bone lengtheni ng, because most of the load is transmitted through the fixator. In th is study, the mathematical basis of the widely varying definitions of stiffness under axial, bending and torsional loads is presented. The p rincipal variables influencing a particular configuration and their ef fects are indicated. Particular consideration must be given to the fac t that the wires under tension generate a non-linear behaviour of the frame. The information needed for reporting on different fixator syste ms is indicated. Finally, the effect of individual structural elements , such as number, direction and tension of the wires, number of thread ed rods and distance between the planes of the rings on the different stiffnesses, is presented and discussed with respect to clinical signi ficance.