Chromium nitride coating for large diameter metal-on-polyethylene hip bearings under extreme adverse hip simulator conditions

Large diameter total hip replacements using polyethylene liners have been proposed due to low wear and oxidative stability observed in the latest generation of this material. Concerns exist with large diameter metal bearing surfaces and ceramic heads are generally expensive to manufacture. A ceramic chromium nitride (CrN) coating on a metal head may be an alternative bearing surface, maintaining low polyethylene wear and minimising cobalt release. Vitamin-E blended highly crosslinked polyethylene liners (52 mm diameter) paired with electron beam physical vapour deposited (EBPVD) CrN coated and uncoated CoCrMo heads were tested in a hip simulator. Under standard conditions no difference was observed in polyethylene wear rates (9.2 and 9.5 mm3/mc) but the coating prevented cobalt release. Alumina particles produced substantial damage on the uncoated heads but did not damage the coated heads. Further testing without abrasive particles increased the polyethylene wear (469 mm3/mc) and cobalt release (847 ppb/mc) in the uncoated bearings yet remained low in the coated components (13 mm3/mc wear, 17 ppb/mc cobalt). Additionally, the coating reduced the generation of nanometre sized polyethylene particles by an order of magnitude under all adverse test conditions. This CrN coating may have the potential to reduce clinical wear allowing for large diameter components.