Realistic Boundary Conditions in Testing Diaphyseal Femoral Implants

Background: Considerable variation exists in the proximal and distal fixture designs used in biomechanical testing of diaphyseal femoral implants. In most of them a load is applied to the femoral head, but muscle forces are rarely represented, so bending of the diaphysis is unphysiologically high. Methods: This study investigated twelve testing configurations reported in the literature using finite element and analytical models of the femoral diaphysis and a lateral fixation plate. The models differed in proximal and distal displacement constraints, femoral orientation, and load application. Internal force distributions, as well as bone and implant stresses and bone strains, were calculated for each configuration. Results: Common experimental conditions, in which the load is applied to the femoral head, generally produced high bending moments, resulting in, what we interpret to be, unrealistically high bone and implant stresses and bone strains. In contrast, axial loading along the femoral shaft generated substantially lower bending, which we propose to be more physiological, particularly in two such cases with reduced rotational constraint. Conclusions: Their constant bending distribution should improve reproducibility and their implementation should provide a practical approach to biomechanical testing of diaphyseal femoral implants.