Fig. 1.

The biomechanical concept of the dual mobility cup consists of a double articulation—between femoral head and liner and between liner and cup. The first motion occurs between the small femoral head and the inside of the polyethylene liner, until the neck of the femoral stem comes into contact with the liner. The secondary motion occurs between the outside of the polyethylene liner and the metal acetabular cup, when a larger range of motion is required. Here the polyethylene liner acts as a large femoral head