Artificial hip joints have been in use for a number of years; various combinations of metals and polymers have been tested both in vitro and in vivo. Modern ceramics have found application as bearings in hip replacement due to the enhanced wear and friction that they offer. It has been hypothesised that during the swing phase of gait it is possible for the Femoral head and the Acetabular cup to dislocate, before relocating during heel contact.

Severe loading such as this could cause greater levels of wear to occur in artificial hip joints.

This study provides comparative analysis between ceramic-on-ceramic hip joint pairings under both severe and standard loading profiles.

Five zirconia-toughened alumina (ZTA) 28mm diameter bearing pairs were tested on a ProSim Hip Simulator for 5.3 million cycles (MC), two under severe loading and three under standard loading conditions. Additionally a Loaded Soak Control, Soak Control and Environmental Control were used. Wear was recorded every 0.5 MC by gravimetric measurement. Surface microscopy images from a Zygo New View 100 and an Atomic Force Microscope (AFM) were taken before testing and then at, 0.5 MC. 2.5 MC, and 5.3 MC.

The standard loading profile followed ISO14242-1 standard with 2650±50N maximum force, ±10° internal-external rotation and −15–30° flexion-extension. To simulate aggressive wear condition, microseparation inferiorly and micro-lateralisation laterally were applied during the swing phase. Dual acting cylinders were used to apply a constant force of 350±50N in opposition to the standard loading profile to enable separation between the Femoral Head and the Acetabular Cup during the swing phase. This microseparation was measured by means of a Linear Variable Differential Transformer (LVDT) and the setting gave a reading of 1.2mm ± 0.1mm at the start of each 0.5 million run. The value for microlateralisation was 0.9mm whilst the inferior separation was 1.2mm.

Wear rates for the ceramic cups under severe wear condition were found to be 0.0356±0.0059mm3/ MC and for the standard wear condition to be 0.0178±0.0049mm3/MC.

The femoral heads had wear rates of 0.0164±0.0046mm3/MC for severe wear condition and no wear was detected for the standard wear condition.

The results of the present study showed almost no wear under standard gait condition and only a modest increase in wear occurred when using severe wear condition. Thus the resulting wear rates are still significantly lower than those found for alumina-alumina total hip joints [1, 2].