Wear Behaviour of Metal-on-Metal Hip Joints With Different Clearances by Using an ISO Hip Joint Simulator and Evaluation of the Optimal Clearance

1. Introduction

Metal-on-metal (MOM) hip joints have regained a favor in arthroplasty since they own excellent wear resistance. In this study, wear tests by using a hip joint simulator were conducted with MOM bearings of specified 40 mm femoral heads. The influence of clearance on the wear behavior was investigated. Furthermore, an optimized radial clearance was estimated by lubricant film thickness and contact pressure analysis.

2. Materials and methods

Co-27Cr-5Mo-0.13N-0.05C (hereafter CCMN) alloy (mass %) was used. The ingots were vacuum induction melted, homogenized and hot forged successively. The microstructure shows equiaxed crystal grains with abundant annealing twins but no carbides.

Two groups of bearings, indicated as c_r 1 and c_r 2, were designed. The radial clearances for c_r 1 and c_r 2 were 37.9 and 148.7 μm, respectively. Wear tests were conducted in a hip joint simulator (INSTRON 8870) in Hanks' solution at 37±2°f. The force and 3-axile angle of movement were applied on the articulation according to ISO 14242-1 for 1.5 million cycles (Mc).

The contact pressures on the hip joints were also analyzed by using ABAQUS. The femoral heads were set 40 mm with radial clearances of 0–200 μm. Half models were set up and only the maximum force of 3 kN converted as pressure was applied as boundary condition.

3. Results and discussion

In general, the wear scratches paralleling to the flexion-extension direction tend to the greatest [Fig. 1]. For c_r 1, shallow scratches and few plowing-grooves were observed. While for c_r 2, deep plowing-grooves (5–10 μm) with obvious plastic deformation were generated in large quantities by abrasive wear.

The wear rate [Fig. 2] for c_r 2 was approximately 29.5 mm³/Mc more than the one for c_r 1, probable caused by different lubrication mode and contact pressure for different clearances. From the prediction of the lubricant film thickness, the lubrication regime transformed from full film lubrication to boundary lubrication for c_r 1 and mixed film lubrication to boundary lubrication for c_r 2. The simulation results [Fig. 3] also show that as the clearance increase, the contact pressure would increase leading to aggravating wear. On the other hand, if the clearance is too small, the contact pressure also increases by wedging and equatorial contact of the articulation. For a MOM articulation with a 40 mm femoral head, there is a minimum contact pressure when the radial clearance is between 40 and 50 μm.

4. Conclusions

To summarize, for a 40 mm diameter MOM articulation, abrasive wear was the dominant wear mechanism. The wear decreased as the radial clearance decreased, due to a thicker lubricant film thickness which can separate the surfaces' direct contact. However, the function of the contact pressure and radial clearance was not monotonous. It could be concluded that an optimized radial clearance ranging in 40–50 μm is appropriate for this MOM hip joint.