**4.3. Development and validation of finite element predictions of the stability of fracture fixation with PU-foam based synthetic bones**

The posterior wall fracture is the most common fracture type of the acetabulum(Baumgaertner, 1999). Depending on the fragment size, open reduction and internal fixation (ORIF) is performed especially when the fracture involves more than 50% of the posterior wall. But ORIF requires considerable exposure that often leads to major blood loss and significant complications(Shuler et al., 1995). Percutaneous screw fixations, on the other hand, have become an attractive treatment option as they minimize exposure, blood loss and risk of infection. As such, they have been advocated by some authors (Parker and Copeland, 1997) for the treatment of minimally displaced acetabular fractures without comminution or free fragment in the joint. However the biomechanical stability of percutaneous fixation has not been studied thoroughly, especially in terms of interfragmentary movement. In particular, the stability of percutaneous fixation in acetabular fractures has not been compared with the more conventional ORIF involving a plate with screws. There have been previous biomechanical studies that compared different types of stabilization in posterior wall fractures (Goulet et al., 1994, Zoys et al., 1999). But the main focus of such studies was to compare the strength of several types of osteosynthesis. However it is interfragmentary movement that exerts major influences on the primary stability and fracture healing (Klein et al., 2003, Wehner et al., 2010). As discussed in Section 3, PU-foam based synthetic bones have been used extensively in testing stability of various osteosynthesis techniques. Therefore we have further developed our FE model capable of prediction acetabular fractures to simulate stability in osteosynthesis. Specifically, we have developed a fast and efficient way of predicting the interfragmentary movement in percutaneous fixation of posterior wall fractures of the acetabulum and validated with a matching biomechanical experiment using PU-foam based synthetic pelves.
