[Rotational displacement of eccentric cups cemented in the acetabulum (author's transl)].

The authors have operated on 15 hips using a total prosthesis comprising a metallic head and plastic cup cemented in the acetabulum. The acetabular component is thicker at its upper part to accommodate eventual wear in the plastic. In all cases the cup rotated through 180 degrees so that, after several months, the thicker part was situated at the lower pole of the acetabulum. In two cases, a fracture of the cup was found. The cause of this abnormal rotation can be explained by a torque action taking place at each step because of the eccentric design of the cup. This was demonstrated by a prosthesis inserted into a cadaver and tested by simulated walking. 45 kilometres of walking were sufficient to provoke a rotation of 90 degrees. Conclusions are given on the use of such eccentric cups which should be avoided.