| dc.description.abstract | Morse tapers are widely used in modular total hip joint replacements (THRs) to attach the femoral head onto the femoral stem. Despite the success of THR procedures, the taper junction has been associated with failures due to damage at the interface generated by fretting and corrosion. Taper design parameters including taper clearance, length, diameter, and assembly conditions have been previously shown to influence the generation of micromotions associated with taper damage. However, the role of taper geometry and surface topography on the mechanisms that influence damage is not well understood. 3D FE models of CoCr alloy femoral heads assembled onto Ti alloy trunnions were developed to investigate the contact environment and the relative motions generated in the taper interface during loading. The maximum accumulated micromotions (49 µm) over a walking cycle were found in tapers with an oval trunion and high clearance. The components of the micromotions (pistoning, normal and tangential) varied with the taper geometry and loading activity. The largest contribution of micromotion in the oval taper arise from tangential motion whilst for a round taper the relative motions were dominated by normal and pistoning. The effect of surface topography (form, waviness and roughness) on the micromotions generated were studied using 2D and 3D models. Contact conditions in the taper interface as a result of the interaction of different surface topographies at different assembly forces were studied. Idealised taper surfaces were found to be different from real, measured surfaces when assembled and generated different contact conditions. Power Spectrum Density (PSD) analysis showed that idealised surfaces comprised of only one spatial wave frequency while measured data contained 3 - 7 spatial wave frequencies. When surfaces where assembled, the measured surfaces showed flattening of the roughness peaks and large plastic strains which reached values that indicated material failure (>0.6). In measured bore surfaces, intermittent contact of deformed trunnion peaks was identified as the assembly force increased. The interaction of the surfaces in the 2D models led to the estimation of a global coefficient of friction (COF); surface roughness and adhesion had a significant effect on this estimation, for example when using a local COF of 0.21 to simulated adhesion, the estimated global COF ranged from 0.21 to 0.46. Using a value of friction from this estimated global COF in the 3D taper model under walking conditions influenced the results obtained; a high COF (0.46) decreased the magnitude of resultant micromotions but increased the magnitude of normal and tangential relative motions magnitudes by 15% and 115% respectively and decreased pistoning by 145% compared to motions generated with a global COF of 0.21. Findings from these studies suggest that the surface topography variations comprising roughness, waviness and form determine the taper performance. These findings help to understand the role of surface design in tapers and highlight the importance of manufacturing processes which will significantly affect a taper’s performance. | en_US |
