The clinical significance of metal ion release from cobalt-chromium metal-on-metal hip joint arthroplasty

Metal-on-metal (MOM) bearings offer extremely low wear and the avoidance of polyethylene but generate metallic wear particles. Although their total volume is dramatically smaller than polyethylene debris, these particles are in the nanometre size range and are many times more numerous. Metallic particles are ingested by macrophages or may be disseminated via lymphatics to the reticuloendothelial system. They corrode, and metal ions are present in the circulation and concentrated in erythrocytes. Excretion of metal ions via the kidneys seems to balance their generation in patients with MOM implants. However, highly sensitive detection methods can be used to show that levels of circulating cobalt and chromium ions are several times the normal level. These concentrations are well within the limits identified as dangerous to health in workers exposed to industrial chemicals, and also considerably lower than the levels found to cause cell toxicity in vitro. The local concentrations of particles and metal ions in the synovial tissue may occasionally exceed these limits and cause tissue necrosis. Clinical experience of lysis is rare in association with MOM bearings, as are hypersensitivity reactions and MOM bearings have had an excellent record over four decades and have a favourable benefit to risk ratio. Further reduction in risk will be achieved by improvement of materials, engineering, and accuracy of insertion.