# Fixation Stability and Stiffness of Two Implant Systems for Proximal Femoral Variation Osteotomy
## 4 Discussion  
In this biomechanical comparative study, we investigated the torsional stiffness for internal and external rotations and the proximal strength of the osteectomized synthetic femora for two implant systems (PL and CBP). The average torsional stiffness of the F-PL group was higher than that for the G-CBP group. Differences in internal and external rotations were found between the two both groups. Figure 4: Box plots of the load to failure of the proximal femora for both tested groups. Figure 3: Box plots of the torsional stiffness for (**A**) internal, (**B**) external rotations, and (**C**) proximal femoral strength for both subgroups. However, this difference was not significant (Figure 3A,B). Moreover, the F-PL system has proximal locking screws to ensure that bending moments were further reduced during loading. The proximal femoral strength of the osteectomized synthetic femora was tested with a setup to simulate a sideways fall onto the great trochanter region. All specimens (both groups) showed plastic deformation of the implants at the end of the testing. No fractured synthetic bones or torn screws were seen in any group. The mean force where implant failure occurred was significantly higher in the F-PL group than in the G-CBP group. That the F-PL plate has a significantly higher rigidity and load to failure can be due to its construction. With 19 mm, the F-PL is 8 mm wider in the proximal and highly stressed gap area than the G-CBP. Both plates have approximately the same 3-mm thickness. This results in a higher moment of inertia of the F-PL, which is reflected in the results of the proximal strength. That there were no significant differences in torsion can be due to the fact that the G-CBP has a cannulated blade plate. A recent study has shown that the design of the blade plate is relevant for stability [16]. After the addition of a proximal screw, the pullout strength increased even if the axial loading properties of the blade plate did not increase [16]. Considering these and our results, we suspect that design modifications might influence biomechanical results, and we hypothesize that adding a screw to the blade plate will not necessarily lead to better results. Further investigation might be helpful. The fourth-generation synthetic femoral sawbones were used for testing. In a previous study, these femoral sawbones showed generally good agreement in terms of mean anatomy and macroscopic mechanical properties [13]. They are standardized and have low variance because of their biomechanical behavior. As regards testing, we expect comparable good results in vivo, as previously described [4; 9]. However, most variation operations are performed in growing children [8; 11; 12]. The diversity of the growing skeleton cannot be represented by the sawbones used. Due to ethical consideration, we would disclaim interventional clinical trials analyzing stability with pediatric patients. Further studies, for example, on failed implants after osteotomy of the femur (in vivo) might help us better understand the stability and stiffness of the implants used. A typical indication for the operation is coxa valga and excessive anteversion in spastic CP [2; 3; 12]. In severe cases, the hip is subluxated or luxated, and additive soft tissue operation is necessary [3]. Younger patients are often between 4 and 8 years [3]. The synthetic femora used cannot be representative for this pathologic anatomic situation. Furthermore, these patients have osteoporotic bone and reduced state of health, causing problems during treatment. This has to be investigated further in vivo. In this study, comparable results were described in both groups. Using the tested implants in severe osteoporotic bones might show other results. Torn screws or fractured bones might occur. Concerning our results, we decided to use the F-PL in osteoporotic bones and in cases of revision operations. However, further biomechanical and in vivo tests in osteoporotic bones are necessary.