The amount of metallic ions released post operatively will be assessed by the concentration of Cobalt, Chromium and Molybdenum present in urine and analysed by I.C.P.M.S. (Inductive Coupled Plasma Mass Spectroscopy). The amounts of metallic ions released will be related to the hip function and range of motion measured pre-operatively and post-operatively. Secondary performance of the metal-on-metal articulation will be determined by radiographic evaluation and incidence of complications / revision rates.