Although MOM hip prostheses produce significantly lower wear rates than conventional UHMWPE-on-metal couples, and lower wear rates than highly crosslinked UHMWPE-on-metal hip prostheses, there are concerns associated with these bearings. Wear particles have been reported to be in the nanometer size range, an order of magnitude smaller than UHMWPE particles. Therefore, despite a lower wear volume than UHMWPE bearings the number of particles produced are estimated to be greater, possibly between one and ten million particles per step. These small particles have the potential to distribute throughout the body via the lymphatic system, with particles found in the lymph nodes, liver, spleen and bone marrow. As MOM hip prostheses are indicated for younger more active patients these particles will be present at these sites for a long period of time, possibly 30-40 years.