Design of hard-on-hard bearing couples has traditionally been characterized by the material of the bearing couple, clearance between the bearing surfaces, sphericity of the components, surface roughness, and the radii of the components. All of these factors play a role in the lambda ratio and fluid film thickness calculations. However, the fluid film for hard on hard bearings can be interrupted by issues like the presence of 3rd body particles, intermittent walking, jogging, and subluxation. Only recently have researcher begun to simulate some of these disruptions in the fluid film for hard on hard bearings.

Recent laboratory testing has looked at the effects of utilizing different materials and methodologies to evaluate hard-on-hard bearings. Ceramic-on-metal is a unique combination of components that is currently available. Several authors have shown that this combination can reduce the amount of metal wear generated during the test by a factor of 4–100. However, an occasional anomaly has shown up in some of these tests where a wear couple in a steady state wear mode will have a several-fold increase in wear for a short duration.

For bearing couples that have a metal component, ion analysis of the serum lubricant can be utilized to monitor the amount of wear. This technique can provide real-time data on the amount of wear seen in simulator testing without removing the specimens from the machine. Further, there are some designs of metal cups that cannot be removed from the simulator without causing damage to the component. Data from a ceramic-on-metal simulator test confirmed that the short-term anomaly in gravimetric wear correlated with an increase in metal ion levels.

Distraction testing evaluates the change in wear due to the unintended subluxation of the hip. This may occur during a standard walking gait if the hip is loose, during impingement, or during deep-knee bends, squatting, or rising from a chair. Distraction testing has various effects on wear depending on the material of the bearing couple. UHMWPE is insensitive to this additional mode of simulator testing. Metal-on-metal and ceramic-on-ceramic can increase in wear by up to an order of magnitude. The utilization of Biolox-delta rather than Biolox-forte can reduce the amount of wear seen during distraction testing. Diamond-on-diamond is insensitive to this wear mode and showed immeasurable wear.

Other issues during testing of hard-on-hard bearings are still being explored. It is well known that 3rd body particles will disrupt fluid films and can increase wear. But the results from adding particles is variable. Metal-on-metal tests can have one specimen with very little increase while another specimen has an order of magnitude increase. Deformation of the shell caused by insertion during surgery has been shown to occur. Currently, this deformation has not been able to be replicated in a simulator, therefore, its effects are unknown.

The design and laboratory testing of hard-on-hard bearings has improved significantly over the past decade. Further research is still needed to evaluate designs that may potentially increase resistance to failure modes other than standard walking gait cycles.