There are a large number of hip implant devices on the market. Each manufacturer has different models but each style falls into one of four basic material categories:metal on plastic polyethylene or UHMWPEmetal on metal MoMceramic on plastic UHMWPEceramic on ceramic CoCThese category names reference the materials used for the implant bearings. The stem and ball fit into and articulate against the cup or acetabulum. Each component can be made of one of several materials.

via Hip Replacement Implant Materials.