The yielding of tensioned fine wires in the Ilizarov frame

The Ilizarov frame uses tensioned fine wires to support bone fragments. The objective of this study was to determine whether these wires deformed plastically under functional load bearing and to determine the significance of such deformation on the long-term performance of frames used for treating lower limb conditions. The mechanical characteristics of the wires were determined by means of destructive tensile tests and used in the construction of a series of finite element models replicating typical frame configurations. Each model was then subjected to a single load cycle representing a single step and the residual displacement (i.e. plastic deformation) was determined. In each case a residual displacement of between 0.26 and 0.42 mm was observed giving a corresponding reduction in wire tension of between 8.3 and 32.8 per cent. These reductions in wire tension reduce the frame's overall stiffness and so compromise its ability to inhibit high-amplitude axial and shear motions at the fracture site which are deleterious to the healing outcome.