Introduction and Aims: Metal-on-metal (MOM) hips offer an attractive solution for hip arthroplasty. However, concerns remain over the optimum metallurgical condition of the cobalt chrome alloy and also the optimum bearing diameter to produce to least possible wear.

Method: Hip bearings of 28, 40 and 56mm high carbon cobalt chrome diameter were tested, all were HIPed and Solution annealed, apart from four 40mm bearings were left ‘as cast’. Radial clearances were controlled at 110 microns, initial surface finish was Sa 0.05 microns, and the sphericity deviation was 4–8 microns. A multi-directional biaxial rocking hip joint simulator was used. All bearing couples were initially subjected to three million cycles of standard physiological walking (2450 N max, 1 Hz) followed by four million cycles of severe gait tests, i.e. slow walking (0.62 Hz) and fast jogging (4500 N max, 1.75 Hz).

Results: In the diameter study, the 56mm bearings produced the lowest wear rates for all patient activities simulated followed by the 40 and 28mm bearings. However, the running-in wear was greatest for the 56mm bearings followed by the 28 and 40mm bearings. All surface wear patterns showed very similar characteristics, suggesting similar wear processes. This would indicate that larger diameters are indeed better in terms of wear in the long term, but may produce slightly more wear initially, which has been reflected in increased ion release in short term studies.

Comparing the 40mm as cast and heat-treated bearings, running-in wear was observed for both material groups in the first million cycles, generating wear rates of 2.3 mm3/million cycles and 2.4 mm3/million cycles for the HIPed/solution heat treated and as cast components respectively, indicating no statistical difference (p > 0.9). Under steady-state wear conditions, the combined normal walking wear rate was also similar for both groups, showing 0.48 mm3/106 cycles (p > 0.2). Under simulated fast jogging cycles, the results again showed no statistical difference in wear performance between the two groups (p > 0.3), generating approximately a 10-fold increase in volumetric wear compared to normal walking, showing 4.4 mm3/106 cycles.

Conclusion: This wear testing program, which used both standard and adverse testing conditions, has shown that heat treatments (HIP and solution anneal) do not affect the wear rates of cast cobalt chrome alloy. Further, this testing has confirmed that larger diameter bearings do indeed produce less wear in the long term.