Influence of Taper surface topographies on contact deformation and stresses.

The role of bore and trunnion surface topography on the failure rate of total hip joint replacements due to trunnionosis is not clear despite significant variations in the design of taper components between manufacturers. Taper surface topography, along with other taper design parameters such as clearance, diameter, and assembly force, determine the initial interlock of the contacting surfaces after assembly; this has been related to relative motions that can cause fretting and corrosion at the taper interface. However, in most in-silico parametrical taper studies associated with taper micromotions, the bore and trunnion surfaces have been simplified using a flat surface and/or sinusoidal functions to mimic the surface roughness. The current study tests the hypothesis that the use of simple geometrical functions for the taper surface topography can predict the surface mechanics developed in assembled tapers. Measured and simulated surfaces of bores and trunnions were characterised using common roughness parameters and spectral density estimations. Using the same characterised surface profiles, 2D Finite Element (FE) models of CoCr alloy femoral heads and Ti alloy trunnions were developed. Models simulated assembly conditions at different resultant forces ranging from 0.5 to 4.0 kN, contact conditions were determined and associated with their topographical characteristics. Measured surfaces of bore and trunnion components comprise up to seven dominant spatial frequencies. Flattening of the trunnion microgrooved peaks was observed during the assembly of the taper. When the femoral head bore and trunnion topography were both considered a reduced number of microgrooved peaks were in contact, from 51 in an idealised taper surfaces to 35 in measured surfaces using an assembly reaction force of 4 kN. The contact points in the models developed high plastic strains, which were greater than that associated with failure of the material. Results showed that line and sine wave functions over estimate contact points at the taper interface compared to those surfaces that consider roughness and peak variation. These findings highlight the important role of modelling the full surface topography on the taper contact mechanics, as surface variations in the roughness and waviness change the performance of tapers.

Influence of taper design and loading on taper micromotion.

Debris originating at the bore-trunnion interface of modular total hip replacements has been identified as one of the causes of hip replacements failure. Friction associated with fretting and corrosion represents a potential generator of these harmful particles. Understanding the motions at the interface will help to interpret the different damage patterns found in retrieval studies and minimize the risk of fluid ingress/egress into the taper interface. Accordingly, the present study is designed to characterise the nature of the relative motions generated at the bore-trunnion interface of different taper designs during typical loading profiles. 3D Finite Element (FE) models of a CoCr femoral head assembled onto a Ti alloy trunnion were generated and variables including taper clearance, deviation from roundness, assembly force and loading conditions were introduced. Resulting micromotions relative to the trunnion surface, separation and contact area of the bore-trunnion interface were shown to be affected by both taper design and types of activities. Results indicated that, in some cases, the largest component of motion corresponded to that developed normal to the trunnion surface. Furthermore, out of roundness as small as 6 µm across the diameter, in particular orientations, significantly changed the contact mechanics and magnitude of relative motions. From the resulting parameters at the bore-trunnion interface, a pump type of motion was identified during walking, jogging and stairs up activities. The components of relative motions at the bore-trunnion interface were found to be different regardless of the resultant magnitude of the relative motions. The findings highlight the importance of high-quality manufacturing processes as small changes in the trunnion component will significantly affect the clinical performance of this common type of modular approach in total hip replacements.