Hip implant devices fall into one of 4 categories: metal on plastic, metal on metal, ceramic on plastic, or ceramic on ceramic. The categories refer to the materials used in the bearings, or the ball and socket of the implant which articulate the joint. There is no consensus as to what materials are best and the choice usually comes down to the surgeon's preference. Metal on metal implants are now less commonly used, since it was discovered that friction and wear caused by rubbing released metal ions into the bloodstream.