Whilst research has shown that high concentrations of nanometre-sized metal wear particles are cytotoxic to human fibroblasts and macrophages in vitro, there are also concerns about the release of metal ions from these small particles, and the potential effect that these ions have on cells and tissues. Elevated ion concentrations have been reported in both the blood and urine of patients with metallic implant components. In addition, contrary to what might be expected, there is no conclusive evidence that ion levels fall in vivo after the bedding in period of the MOM prostheses, during which wear of the prostheses are higher. Cobalt and chromium ions have high toxicity, and there are very real concerns about the effects of sub-lethal doses of metal ions, which have been shown to cause DNA damage. This damage takes the form of chromosome aberrations and chromosome translocations. Over long periods of exposure the worry is that this will lead to the development of certain types of cancer, such as leukaemia and lymphoma. In animal models elevated levels of cobalt, chromium and nickel have been correlated with increased carcinoma rates. However, reports in the literature of malignancies developing after total hip or knee replacement surgery are exceedingly rare. Out of eight epidemiological studies on the relative risk of cancer after TJR, only one study looked specifically at MOM implants. This study did not find an increased risk of cancer development in the subjects receiving MOM hip prostheses compared to those receiving metal-on-polyethylene hip prostheses. However, all the studies to date have been substantially underpowered in terms of patient numbers required to show a difference between the two prosthesis types.