A low wear rate is believed to be critical for extending the implant life of a prosthetic joint, and wear volumes produced by MOM articulations have been estimated to be 40-100 times lower than metal-on-polyethylene bearings. The wear of MOM prostheses is known to be highly dependent upon the materials, tribological design and finishing technique. Clinical studies of retrieved first and second-generation MOM hip prostheses have shown linear penetrations of approximately 5 μm/year. This is equivalent to a wear volume of approximately 1 mm3/year, two orders of magnitude lower than conventional polyethylene acetabular cups.