A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 2.

A finite element model was developed to investigate the effect of loading regimes caused by various daily activities on the mechanical behaviour of the head-neck taper junction in modular hip replacements. The activities included stair up, stair down, sit to stand, stand to sit, one leg standing and knee bending. To present the real mechanical environment of the junction, in addition to the force components, the frictional moments produced by the frictional sliding of the head and cup were applied to a CoCr/CoCr junction having a 12/14 taper with a proximal mismatch angle of 0.024°. This study revealed that stair up with the highest fretting work per unit of length (1.62 × 104 J/m) was the most critical activity, while knee bending and stand to sit with 1.96 × 103 J/m were the least critical activities. For all the activities, the superolateral region of the neck was identified as the most critical region in terms of having larger values of fretting work per unit of area. This study showed also that the relative micro-motions and contact stresses occurring at the head-neck interface for all the studied activities are mostly in the range of 0-38µm and 0-350MPa, respectively. These ranges may be accordingly employed for conducting relevant in-vitro tests to more realistically represent the mechanical environment of taper junctions with the same materials and geometry studied in this work.