THE EFFECT OF ASSEMBLY FORCE AND ANGLE ON CONTACT PRESSURES AND MICROMOTIONS AT THE TAPER JUNCTION OF MODULAR HIP IMPLANTS

Introduction Recent reports implicate fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. Fretting observed at modular junctions is thought to be a type of ‘mechanically assisted’ corrosion phenomenon, initiated by mechanical factors that lead to an increase in contact stresses and micromotions at the taper interface. These may include: intra-operative taper assembly, taper contamination by debris or body fluids, patient weight and ‘toggling’ of the head or increased frictional torque in a poorly functioning bearing. We adopted a finite element approach to model the head-taper junction, to analyze the contact mechanics at the taper interface. We investigated the effect of assembly force and angle on contact pressures and micromotions, during loads commonly used to test hip implants. Materials and methods Models of the Biomet Type-1 taper, a 60 mm head and a taper adaptor were created. These models were meshed with a mesh size based on a mesh density convergence study. Internal mesh coarsening was applied to reduce computational cost. Elastic-plastic material properties based on tensile tests were assigned to all titanium components. The contact conditions used in the FE analyses were validated against push-on and pull-off experiments, resulting in a coefficient of friction of 0.5. To analyze micromotions at the taper-adaptor connection, the models were loaded with 2300N (ISO 7206-4) and 5340N (ISO 7206-6), after being assembled with 2-4-15 kN, axially and under a 30o angle. This ISO standard is commonly used to determine endurance properties of stemmed femoral components. Micromotions and contact pressures were analyzed by scoring them to an average micromotion and average contact pressure for the surface area in contact. Results For the higher loads (5340N) the average contact pressure decreased when a higher assembly force was used (Figure1a), as a result of the fact that the loads were distributed over a larger contact area. The average contact pressure increased when tested at the 30o angle. Figure1b shows that the average micromotion decreased when a higher assembly load is applied, except when the adaptor is assembled at a 30o angle. When assembled at a 30o angle with 15 kN the average micromotion is 1.5 times higher (11.1–7.4 µm). Discussion The location and patterns of the micromotions were consistent with the patterns and locations of wear found on retrieved tapers described in the literature and those generated in an in vitro test model (Figure 2a-b). Increased impaction loads reduced the average amount of micromotion and therefore, fretting. For more realistic results, we intend to apply more complex loading regimes in future analyses, enabling to study the effect of phenomena such as edge loading and frictional torque. Moreover, the mechanical outcome as presented here will be used in a wear model, to simulate volumetric wear.