Tribology of Metal-on-Metal Bearings at High Inclination Angles

Although metal-on-metal hip bearings generally experience low wear in vivo and in simulator testing, high cup inclination angle has been shown to dramatically increase wear. A recent study has shown that metal-on-metal (MoM) bearings converge to a specific contact area regardless of bearing size, clearance, or even contact mode. This evidence points to a relationship between contact pressure and wear rate such that as the contact pressure is reduced (due to the formation of a conforming surface contact, aka. the wear scar) the wear rate will approach a low-steady state value. This research suggested that the run-away wear that leads to extremely high MoM wear may be due to the inability of the specific bearing to reach a low contact pressure.

Rim loading prevents the formation of the conformance area in a symmetrical manner which may lead to high contact pressures. Building on previous research which proposes the interdependencies of the wear rate, total wear, and contact pressures of bearings at proper inclination angles, this study will determine the effect of high inclination angle on the wear rate, total wear, and contact pressure behavior of MoM bearings.

Finite element analysis (FEA) was used to determine the contact pressures of MoM bearings with increasingly larger wear patches simulating increasing wear volume. This analysis was performed with various cup angles to determine the effect of inclination angle on the contact pressure of the MoM bearing through its wear process.

This study confirms the hypothesis that high cup inclination angle leads to run-away wear. Additionally, large diameters reduced the effect of high inclination angles. Low bearing clearance does not affect the ability of the bearing to reach low contact pressures but reduced the amount of wear volume required to reach low contact pressures. The results of this study agrees with the clinical results regarding high inclination angles for MoM bearings and illustrates a method to engineer MoM bearings for good tribological performance.