Long-Term Contact-Coupled Wear Prediction for Total Metal-on-Metal Hip Joint Replacement

Abstract

Wear debris has been recognised as constraining factor for man-made hip joints to stay long enough in the body. The major long-term failure identified for metal-on-UHMWPE is aseptic loosening. Metal-on-metal hip joint implant has been found to have minimal wear compared to metal-on-UHMWPE. The purpose of this study was to apply the computational simulation in predicting wear of metal-on-metal total hip joint replacement. Finite element model of half ball and the 45° inclined cup was developed to represent the femoral head and acetabular cup, respectively. A simple Archard’s equation was used to simulate the wear process combining with the finite element model. Two constant wear factors taken from experimental hip simulator were used, representing the running-in and steady state phases. All motions, flexion-extension, abduction-adduction and internal-external rotation of hip joint, and a vertical load at the centre of the femoral head with a maximum value of 2200N during gait were considered. The wear simulation of 28mm femoral head with 60μm diametral clearance was carried out for up to 50 million cycles, and subsequently analysed. The contact pressure decreased dramatically from the initial cycle to the first million cycles accompanied by an increase of contact area; however the decrease of contact pressure was very small between 30th to 50th year. Biphasic linear and volumetric wear were observed due to the use of two different wear factors for running-in and steady state periods. There was relatively good agreement in volumetric wear between the present and hip simulator study. The total predicted volumetric wear for 50 yeas was 4.2 mm3. It was found that the post-wear bearing surface would be advantageous to the hip implant by increasing the lubricating film and thus decreasing wear.