Comparison of wear of ultra high molecular weight polyethylene acetabular cups against alumina ceramic and chromium nitride coated femoral heads

Over 80% of artificial hip joints utilise polyethylene acetabular cups articulating on metal or ceramic femoral heads. Alumina ceramic heads have previously been shown to reduce polyethylene wear. However, there are concerns about the brittle nature of ceramics and this places design constraints on neck length and head size. It is therefore of interest, to investigate ceramic-like coatings on metallic heads. The aim of this study was to compare the wear of polyethylene against alumina ceramic and chromium nitride (CrN) coated CoCr femoral heads in a hip joint simulator. Three 28-mm diameter Biolox™ Forte alumina ceramic heads and three CrN coated CoCr heads were articulated against UHMWPE GUR 1020 GVF (gamma irradiated in a vacuum foil pack), acetabular cups in the Leeds II hip joint simulator. The CrN heads were coated at Ionbond Limited using arc evaporative physical vapour deposition. The wear of polyethylene against alumina ceramic was 17 mm3/million cycles and against CrN was 23 mm3/million cycles. The difference was not statistically significant. Deliberate scratching of the femoral heads after 5 million cycles did not alter the wear rate. The coated heads had a slightly higher surface roughness and a higher contact angle than the ceramic heads. The contribution of both factors to the wear rate is currently being investigated.