Knee wear simulation under conditions of highly demanding daily activities – Influence on an unicompartmental fixed bearing knee design

The objectives of our in vitro study were to evaluate a knee wear simulation based on patient daily activities in combination with artificial ageing of polyethylene inserts to create an optimised simulation of in vivo wear modes.A wear simulation was performed on fixed bearing unicompartmental knee arthroplasty (UKA) devices in a direct comparison of level walking (as given by the ISO 14243-1:2002(E) profiles) and in a customised test configuration based on activities for level walking (10%), stairs ascending (40%), stairs descending (40%), chair rising (8%) and deep squatting (2%).The cumulative gravimetric wear was estimated to be 15.3 mg for level walking (ISO) and 69.6 mg for high demanding activities (HDA). The gravimetric wear rate of the ISO group was 3.0 mg/million cycles, compared to 11.7 mg/million cycles for the HDA protocol. Level walking wear testing conditions (ISO) and artificial ageing alone is not sufficient to reproduce in vivo failure modes. After 3 million cycles all gliding surfaces of the HDA group developed in the tibio-femoral articulation markable areas of structural material fatigue and delamination.In conclusion a combination of artificial ageing to clinical relevant oxidation grades and a sequence of various high demanding daily patient activities is necessary to represent a revised in vitro behaviour of abrasive–adhesive wear and delamination in artificial knee replacements.