*4.3.5. Feasibility of the use of synthetic PU-foam based bone in validating FE predictions of fracture stability*

We have developed a new and efficient way of simulating interfragmentary movement in acetabular fractures using a FE model. We validated our method with a biomechanical experiment involving PU-foam based synthetic bones. There are numerous studies that employed PU-foam based synthetic bones in measuring fracture stability as discussed in Section 3. However this data has not been used in validating FE model predictions of fracture stability. The use of FE models in fracture analysis is not new. However, the major focus has been to analyze the stress distribution on the implant or the overall stiffness of bone/implant composite after fracture fixation (Eberle et al., 2009, Stoffel et al., 2003). Therefore we have performed a biomechanical experiment with PU-foam based synthetic bones to measure interfragmentary movements in 3D and used the result to validate our FE model.

The mechanical experiment with synthetic pelves showed that the displacement between fragment and bone was relatively small for both plate and screw fixations, indicating that screw fixations in single fragment fractures may be a good alternative to the current gold standard of plate fixation. In particular the excellent stability displayed by the screw FE model with the optimized screw positions indicate that screw fixations along with computer navigation should be an option considered by trauma surgeons if available.

The FE model showed a great potential for use in analyzing fracture fixation techniques. Our model was able to predict the movement of the fragment with a reasonable accuracy. Although we have not modeled screws explicitly, our modeling approach was able to accurately predict the fragment movement, which was the main aim of the model. Moreover the computational efficiency of the approach allowed us to perform a parametric study for optimization of screw positions.

Since we have used PU-foam based synthetic pelves in our study it is not known if our model predictions will be as accurate when cadaver bones are used. However the use of synthetic bones has some advantages due to their uniformity and consistency (Nabavi et al., 2009). Moreover the main aim was to make comparisons between different osteosynthesis techniques and between experimental and FE simulations and the ASTM standard states that it is "an ideal material for comparative testing (American Society for Testing and Materials, 2008a)." In this regard, the use of PU-foam based synthetic bones in comparative studies in orthopaedic biomechanics can provide useful data for FE model validation as well as testing hypothesis.
