The subject of this study was an investigation of a technique to non-invasively monitor the material properties (Young's modulus and Poisson's ratio) of long bone fracture callus when external fixation hardware is employed for fracture fixation. The technique employs a six degree-of-freedom loadcell integral with the fixator sidebar to measure the forces transferred to the sidebar in response to a prescribed set of applied loads. The reaction force data set is used as input to a computational model comprised of a finite element model and a search routine. The material properties are estimated by supplying inital values for the properties of interest and using the finite element model to predict the forces occurring at the site of the 6-DOF loadcell. Based on the differences between the measured and predicted reactions the search routine supplies the model with new estimates for the properties.
The performance of this technique was investigated under the idealized conditions of the laboratory. Calibrated finite element models were used with the estimation procedure to predict the material properties of the bone simulant. Under the best case conditions with well characterized mechanical models, this technique was able to predict the value for Young's modulus within 5% of the measured value.
This technique was then applied to an experimental animal model. It was found that there is a non-linear dependence of the predicted properties on the values used in the finite element model for the bone geometry and the bone modulus indicating the necessity of obtaining very accurate data for use in the model. The overall results indicate that this is a powerful technique for investigating the contribution of fixator stiffness to fracture healing by allowing a single set of animals to be monitored over the full duration of the post-opertaive period and is worthy of further study
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