Testing of silicon nitride ceramic bearings for total hip arthroplasty

Modern ceramic bearings used in total hip arthroplasty (THA) consist of a femoral head (ball) articulating inside a hemispherical acetabular cup (socket); the ball and socket are made of alumina (Al₂O₃) and Al₂O₃-based composite materials. In the present study, total hip bearings were made from a different ceramic material, silicon nitride (Si₃N₄), by sintering and hot isostatic pressing of powders. The resulting material had improved mechanical properties over modern Al₂O₃ THA bearings, with a flexural strength of 920 ± 70 MPa, a Weibull modulus of 19, and a fracture toughness of 10 ± 1 MPa m^1/2. Unlike zirconia-based ceramics that have also been used in THA, accelerated aging of Si₃N₄ did not adversely affect the flexural strength. In simulated wear tests, Si ₃N₄ acetabular cups produced low-volumetric wear whether articulating against Si₃N₄ or cobalt-chromium (CoCr) femoral heads. The results of this investigation suggest that Si ₃N₄ may allow improved THA bearings that combine the reliability of metal femoral heads with the low wear advantages of ceramic materials.