*4.3.4. Accuracy of FE model interfragmentary movement predictions validated with PUfoam based synthetic bones*

The FE model predicted the movement of the fragment in screw fixations with a good accuracy. The values predicted by the FE simulation were within one standard deviation of the experimental measurements (Figure 19) in the horizontal and lateral directions. The predicted vertical direction movement was a little bit greater than the upper limit of the experimental measurement (mean + 1 SD) but still very close to it as the difference was 0.05 mm.

The optimized screw positions were found to be on the two diagonal corners of the fragment. When the virtual screws were placed in this manner, the stability of screw fixations improved dramatically to the level that is comparable to plate fixation (Figure 20). The fragment movements in the horizontal and lateral directions were smaller than the average movements in the plate fixation in these directions. Although the movement in the vertical direction was bigger than the upper limit of the experimental measurement, the difference was small 0.1mm.

Use of Polyurethane Foam in Orthopaedic Biomechanical Experimentation and Simulation 195

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

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

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

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

In this chapter we discussed the use of polyurethane in orthopaedic biomechanical experiments. Due to the similarity of polyurethane foam with cancellous bone in terms of microstructure and material properties, polyurethane has found a unique and important position in orthopaedic biomechanics studies. The main use of PU-foams was in experimental studies to find optimum values in various surgical procedures and to test stability of fracture or joint replacement implants. Due to the uniformity and consistency in material properties, PU-foam based synthetic bones are capable of generating reproducible results that are so difficult to obtain when using human cadaver bones. Therefore we used PU-foam materials in designing bone graft harvesters and obtaining validation data for FE model predictions in fracture load and stability. Although material properties of PU-foams are not identical to natural bone, they are able to generate comparable results that can provide important insight into surgical procedures or function of implants or devices. Moreover thanks to the advent of new composite bones made up of PU-foams and other relevant materials that mimic the geometry, structure and material properties of human

navigation should be an option considered by trauma surgeons if available.

model.

optimization of screw positions.

as testing hypothesis.

**5. Concluding remarks** 

**Figure 19.** Comparison between FE prediction and experimental measurement
