Avoiding the material nonlinearity in an external fixation device

Background. External fixation devices are widely used for treating unstable bone fractures because of their attractive features including minimal invasiveness, maximum tailorability, and extreme versatility. In one type of these devices (i.e., the fine-wire fixators), these unique features are made possible by the use of tensioned wires to support bone fragments. The major problem with these wires is their yielding. Once the wires yield, the fracture healing process will be adversely affected. A recent study showed that the nonlinear behavior observed in these tensioned wires can be geometric and material, and the geometric nonlinearity will stiffen the wires while the material nonlinearity will cause the wires to yield. This study is to investigate if it is possible to avoid the material nonlinearity in order to retain the elastic and repeatable performance for the wires. Methods. Nonlinear and large deformation finite element analyses were conducted. Models of a bone segment transfixed by pairs of cross-aligned wires subjected to various levels of pre-tension were developed. The bone segment was subjected to a vertical load, and the load-displacement curves, wire tensions and wire tensile stresses were obtained under a full cycle of loading and unloading regime. Findings. 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, we can avoid the material nonlinearity. Doing so we will be able to stiffen the fixation device and retain elastic and predictable mechanical performance at the same time. Interpretation. The findings will lead to a new paradigm toward enhancing the performance of external fixation devices.