SURFACE STABILITY OF CERAMIC-CERAMIC BEARINGS ON HIP SIMULATOR WEAR TESTING

INTRODUCTION The reported revision rate for THA is below 10% at 10 years. Major factors for revision are aseptic loosening or dislocation of the articulating components. CoC bearings in total hip arthroplasty (THA) have demonstrated very low wear rates. Due to producing the least number of wear particles of any articular bearing used for THA, osteolysis is very rarely observed. Zirconia-platelet toughened alumina (ZPTA) has improved toughness and bending strength while maintaining all other advantageous properties of alumina. Consequently, its clinical fracture rate is minimal and wear resistance is superior to alumina. OBJECTIVES Since a trend exists towards the usage of larger bearings the aim of this study was to compare the tribological behavior of different ZPTA/ZPTA THAs with respect to their ball head diameter. METHODS Wear tests according to ISO14242-1 were performed in a servo-hydraulic hip simulator with ZPTA bearings from 28 to 48mm up to 5 million cycles (mc). Wear was measured every mc and surfaces were inspected at 5mc. The tests were carried out in 20g/l protein serum. RESULTS All ZPTA articulations showed a characteristic running in period followed by a steady decrease in wear rate up to 5mc. The 48mm bearing showed highest running in wear. For all bearing sizes wear decreased to about 0.05mm³/mc or less throughout the testing period. For all couplings minimal differences in the wear rate was found. CONCLUSION The current study supports the extremely low wear rates for ZPTA/ZPTA bearings and showed that increasing the diameter up to 48mm has only a small influence on the wear rate when tested under ISO14242-1 conditions. Larger diameter bearings increase the range of motion and joint stability and reduce the risk of impingement or dislocation. Thus, larger diameter ZPTA bearings seem a safe solution for active patients due to higher dislocation resistance and reduced wear. Other loading conditions such as subluxation are known to contribute to the clinical wear rate and will be investigated in future tests.