Tribological performance of a metal-on-metal total hip arthroplasty was evaluated as a function of the diametral clearance between the femoral head and acetabular cup. Frictional torque results, measured by a pendulum apparatus, suggested that a threshold clearance existed. Below the threshold clearance, frictional torque increased nearly linearly as the clearance decreased. Beyond the threshold, the clearance effect was minimal. The wear results, measured using a hip simulator, showed that a running-in wear was present in the first million cycles. The subsequent steady-state wear rate was very low in all tested clearances. The highest running-in wear was associated with the largest clearance. These results demonstrated the usefulness of combining a pendulum test and a simulator wear test to fully characterize the clearance effect.

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