Friction in hip bearings under continuous normal walking conditions: Influence of swing phase load and patient weight

Due to the known osteolytic potential of polyethylene, pre-clinical testing of total hip replacements (THR) has been much focused on articulating wear in the past. However, friction related effects (e.g. taper wear or mechanical implant loosening) are reported for modern THR. Established material combinations made of cross-linked polyethylene, ceramics and metals are measured in a modified hip simulator to investigate friction for two different walking profiles: ISO 14242-1 and data from patient measurements. In addition, the patient profile was adapted for different loads during the swing phase, a variation in patient weight and walking speed. Ceramic-on-Ceramic bearings show the lowest overall frictional torques over a gait cycle for all investigated testing profiles while there is no difference between the head material (metal, ceramics) when combined with polyethylene. Taper torques of under 2 Nm are reported for continuous walking. Friction increases with higher patient weight and lower walking speed. In addition, the swing phase load only seems to have an influence on polyethylene bearings while hard-on-hard (combinations of ceramics and metal only) are less sensitive to a variation of load in the last 40% of the gait cycle. Friction measurements offer a powerful tool to increase the performance of pre-clinical testing. Beside the continuous “ideal” conditions presented here, more challenging profiles are proposed.