Effect of Nonlinearity in Tensioned Wires of an External Fixation Device

External fixation is widely used in the treatment of unstable fractures, limb lengthening, and congenital and pathological orthopedic deformities because of its attractive features such as minimal invasiveness, maximum tailorability, and extreme versatility. These features are made possible by the use of tensioned wires to support bone fragments. These seemingly simple wires actually fulfill a very complex duty. One major problem with these wires is their yielding. Once the wires yield, the fracture healing process will be compromised. Thus, to maximize the benefit of these wires, it is necessary to know their fundamental characteristics. This chapter provides an in-depth look at the cause of the nonlinear behavior observed in these tensioned wires using a computational approach. It illustrates that the nonlinear behavior of the wires originates not only from the material hardening and yielding but also from the induced large deformation. Pre-tensioning the wires is beneficial for stiffening a fixation device but is disadvantageous to maintaining the wire elasticity. By limiting the level of the pre-tension, one can avoid the material nonlinearity, which is the main cause for material yielding, hence the loss of tension in the wires and the loss of functionality of the fixation device.