Development and validation of a technique for strain measurement inside polymethyl methacrylate

There are many instances where it is necessary to measure strains inside polymethyl methacrylate. For instance, most cemented prosthetic devices fail owing to cement failure. Thus, it is important to measure the cement stresses in vitro, in order to optimize the design. Measuring internal strains with an embedded gauge is extremely difficult. The techniques proposed in the literature either are inaccurate or require large-scale models. In the present work a technique is proposed where a single strain gauge rosette is used. The strain gauge is calibrated to correct the errors deriving from the pressure acting on the grid. Thus accurate measurements can be obtained with a small sensor. The errors associated with this technique are estimated. A pilot study is presented (a) to verify whether the procedure can be applied to the actual geometry of the prosthesis, (b) to verify whether the gauges could safely be inserted into the femur, (c) to estimate the variability of this preparation and (d) to test the robustness of the preparation to repeated loading. Promising results were found in terms of accuracy, precision and robustness of the technique.