The Effect of Plate Working Length on Interfragmentary Movement in a Distal Femoral Fracture: A Biomechanical Study

Background Distal femoral fractures are typically treated by bridging with locking plates. The mechanical environment is influenced by working length (WL), with larger WL theoretically increasing the interfragmentary movement (IFM). However, considering the strength of modern locking plates, the effect of WL is questionable. The aim of this study was to quantify the effect of WL on IFM in distal femoral fractures at an approximated physiological load. Materials and methods Ten fourth-generation composite femurs with a 10 mm transverse fracture gap were uniformly fixed to a 13-hole locking plate with a short (95 mm) and long (175 mm) WL. The constructs were mounted in an Instron machine (Instron, Norwood, MA) and axially loaded from 50 to 750 N, 50,000 times. The micromotion was measured with the digital image correlation method of the implant-femur construct under cyclic loading. A simplified Bernoulli-Euler beam model analysis was performed to evaluate the contribution of plate deformation on the observed micromotion and serve as a control of the results. Results In the cyclic loading analysis, the median (min-max) IFM was 0.81 mm (0.66-1.73) and 1.64 mm (0.26-2.35), respectively, for the short and long WL group (p = 0.421). The median lateral movement (shear) was 0.89 mm (0.71-1.52) and 2.81 mm (2.16-3.94), respectively, in the short and long WL group (p = 0.007). The range of deformation remained constant throughout the 50,000 cycles, irrespective of the WL. In the beam model, we found the contribution of axial plate compression to the observed deformation was negligible. Using the beam model, we calculated an expected axial IFM of 0.82 mm for the short WL and 1.52 mm for the long WL. Conclusion During an approximated physiological loading, we found less than 1 mm difference in axial IFM between a short and long WL. We found substantially higher levels of shear movement in the long WL. We have provided a simple model to predict IFM based on WL and femur characteristics.