Wear Performance of Large Diameter Differential Hardness Hip Bearings

Modern metal-metal (M-M) hip bearings produce low amounts of wear debris. However, concerns of metal-ion generation prompt ongoing efforts to further reduce the amount of metal wear debris. The present study compared the wear of 3 types of 54-mm, large-diameter, hard bearing systems manufactured from cast/heat-treated and wrought CoCrMo alloy and alumina ceramic materials. The hypothesis of this study was that C-M and M-M differential hardness bearing systems (bearings with hardness differential between femoral head and shell) would produce less metallic wear debris and hence reduced metal-ion release in vivo when compared with M-M bearing systems of like hardness. The 3 systems tested were composed of (1) standard M-M bearings, (2) differential hardness M-M, and (3) C-M bearings. A simulated gait profile (triple peak Paul type), which varies from 200 to 2000 N, was applied to the bearings at a frequency of 1 Hz for 5 million cycles. All the bearings were tested in anatomically inverted position in 90% bovine serum. Wear was quantified gravimetrically. Differential hardness bearing systems produced lower run-in wear rate (10-27 times), steady state wear rate (2-6 times), and total metal wear (68%-83%) when compared with the bearing systems with like hardness.