A bipolar endoprosthesis assembly which provides for a wide-range of motion between a bone and a socket (e.g., a femur and an acetabular socket), comprising a metal outer shell element, a ultra-high molecular weight, low-friction insert formed to fit within the shell element, and a locking ring designed to lock the assembly together with a femoral head prosthesis. The outer shell has a generally spherical shape on its outer surface to be received by a human socket such as the acetabulum. The inner surface of the outer shell is faceted to inhibit rotation of a correspondingly shaped inner insert. The insert has an outside surface shaped to mate with the inside surface of the outer shell element. The locking ring is slotted to facilitate insertion of a femoral head prosthesis. The locking ring also has dual sets of locking barbs shaped to mate with recesses on the inner surface of the outer shell element. The redundant locking mechanism of the locking ring inhibits accidental in vivo disassembly of the device yet allows for relatively easy intraoperative disassembly without specialty tools or fixtures.