Finite element modelling of a unilateral fixator for bone reconstruction: Importance of contact settings

When reconstructing a large segmental bone defect by means of a porous scaffold, a fixator is used to stabilize the reconstruction. The fixator stiffness is an important factor as it will influence the biomechanical environment to which scaffold and regenerating tissues are exposed. A finite element (FE) model can be used to predict the fixator stiffness. The goal of this study was to develop and validate a detailed 3D FE model of a custom-developed unilateral external fixator. In particular, it was hypothesized that the contact interfaces between the different fixator components play a major role for the prediction of the fixator stiffness. In vitro mechanical testing of the entire fixator as well as of separate fixator components was performed in order to measure the stiffness. The mechanical test set-ups were simulated by means of detailed FE models that considered different levels of refinement of the various contact interfaces. The error on predicted fixator stiffness in comparison to measured stiffness was reduced from 121% to 16% by refining the contact settings of the FE model. The individual sources of error between the measured and predicted fixator stiffness could be quantitatively assessed as well. In conclusion, this study warrants for a careful modelling of the geometry and contact settings, when using FE models for the prediction of fixator stiffness.