A metal-on-metal hip replacement used metal bearing surfaces for both the ball (femoral head) and socket (acetabulum) components of the prosthetic hip joint. The theoretical advantage of using a metal-on-metal bearing surface was to reduce wear of the joint over time since metal is thought to be a smooth bearing surface that would have much less wear than plastic (polyethylene). The hope was that there would be less particle accumulation in the joint from friction of the bearing surfaces. This in turn would reduce bone loss from osteolysis, and ultimately prosthetic failure. Unfortunately, the metal-on-metal hip replacements released metal ions that entered into the blood stream. The marked increase of metal ions in the blood was concerning for potential negative whole body (systemic) effects. Metal-on-metal joint replacement also caused local inflammation and swelling around the hip joint. Considering these adverse effects, metal-on-metal hip replacements have been largely recalled from the market and are no longer a preferred device for total hip replacements.