The effect of head impaction on contact stress and micromotions at the head-taper connection

Introduction: Recent reports have implicated fretting corrosion at the head-stem taper junction as a potential cause of failure of some large diameter metal-on-metal (MOM) devices. While it has been suggested that larger MOM heads, may induce greater frictional torques at the taper connection, the exact mechanisms underlying fretting corrosion remain poorly understood.

It is likely that the onset of the corrosion process is caused by mechanical factors, such as contact stresses and micromotions occurring at the interface. These stresses and micromotions depend on the fixation of the head onto the stem and may be affected by blood, fat, bone debris or other contaminations. The fixation of the head is achieved intraoperatively through impaction.

To further study this phenomenon, we adopted a finite element approach in which we modeled the head-taper junction fixation mechanics. In this model, we analyzed the effect of impaction force on the micromotions occurring at the head-stem interface.

Materials and methods: We created a model of a BIOMET Type-1 taper and an adapter that is typically used for larger heads.

Titanium alloy material properties were assigned to both components, and frictional contact (μ = 0.5) was simulated between the adapter and the taper.

To ensure that the model accurately represented the contact mechanics, we first simulated experiments in which the head was assembled on the taper in a load-controlled manner, at different load (4 and 15 kN), after which it was disassembled axially. The disassembly loads predicted by the FEA simulations were then compared to the experimental values.

After ensuring a correct prediction of the disassembly loads, we used various impaction loads (2, 4, and 15 kN) to assemble the taper, after which a 2.3 kN load (ISO 7206-4) was applied to the adapter/taper assembly. This loading regime is commonly used to determine endurance properties of stemmed femoral components. Under these loading conditions, we then analyzed the contact stresses and micromotions, and the effect of impaction load on these quantities.

Results: Both the contact stresses and the micromotions were most prominent in the medial part of the taper (Figure 1). The simulations indicated the highest contact stresses (up to 112 MPa) occurring at the distal part of the taper (Figure 2), which was also the region where the highest micromotions occurred (Figure 3). We found that the micromotions were lowest (1.5 μm) when the head was impacted with 15 kN, while the highest micromotions were found in the 2 kN case (7.5 μm).

Discussion: The location in which the highest contact stresses and micromotions occurred is consistent with the predominant location of damage on tapers found in retrieval studies[1]. Experimental data will be used to further validate the current model after which we can expand the model by applying more complex and realistic hip joint loads of activities of daily living. In this manner we will be able to give a more complete overview of the wear patterns observed in retrievals and to propose solutions to minimize wear at this interface.