 Higher metal ion levels have been found in whole blood, serum and urine of patients with MoM hip arthroplasties THA and HRA compared to preoperative values, and to THA with other bearing surfaces metal-on-polyethylene, ceramic-on-polyethylene, ceramic-on-ceramic. Although MoM hip arthroplasty has been shown to produce less volumetric wear compared to metal-on-polyethylene, the wear debris consists of more numerous, small, nanometre size particles, which are ingested by macrophages. Contrary to polyethylene, metal particles and ions are not chemically inert but may have directly toxic, biological effects and may elicit hypersensitivity reactions in addition to the macrophage-driven, innate, foreign-body immune responses to particulate debris of any material. Consequently, concerns have been raised about the physiological consequences of metal release from MoM hip prostheses into the peri- prosthetic tissue and systemic circulation. 