Tribological considerations in primary and revision metal-on-metal arthroplasty.

BACKGROUND: Metal-on-metal hip bearings undergo biphasic wear, starting with a short period of high wear (bedding-in) and followed by low steady-state wear. Bedding-in is the process by which the cup wears locally to conform to the geometry of the head. This process reduces the maximum contact stress and allows for appropriate lubrication. A critical area of conformance and wear is required for the bearing to reach a low steady-wear state. Cups were analyzed in this study after primary and revision wear scenarios to determine this critical area for this specific bearing.
METHODS: Forty and 56-mm cobalt-chromium resurfacing bearings with 150 and 400-microm clearances were wear tested in a hip simulator for 5 million cycles. The cups underwent an additional 5 million cycles of testing against new heads, simulating a revision scenario. The revision heads were manufactured to cause the highest mismatch with the pre-worn cups, resulting in polar or local annular contact. Cup wear area was determined from weight-loss measurements after each phase of testing.
RESULTS: All bearings experienced a biphasic wear performance with a short period of high wear followed by low steady-state wear. A consistent critical area of conformance was reached by all bearings after primary and revision testing conditions, regardless of bearing size, bearing clearance, or contact mode.
CONCLUSIONS: An area of conformity (wear) reduces contact pressures, is beneficial for lubrication, and is critical to reach a low steady-state wear rate. This study shows that this critical area is consistent regardless of bearing size, clearance, or contact mode. Bearing designs that allow the proper formation of this conformance area should bed-in and reach a low steady-state wear rate.