[Progressive intra-acetabular dislocation of bipolar hip prostheses: four cases].

Certain complications, such as acetabular erosion or cup dissociation, are specific to bipolar prostheses. Progressive intra-acetabular dislocation has not been reported to date. We report 4 cases. Four elderly women developed progressive intra-acetabular dislocation after implantation of a bipolar prosthesis for femoral neck fractures. The metal-backed cup verticalized progressively and the ball gradually dislocated into the acetabulum, eroding it. Revision was undertaken with a hemispheric bone ingrowth cup and partial grafting. Follow-up was then uneventful. This phenomenon is different from the acute intra-acetabular dislocation that may occur after rupture of the anti-dissociation mechanism (polyethylene ring) incorporated in the design of new implant models, or after dissociation between the ball and the cup during closed reduction of a dislocation, the cup catching on the acetabular rim. It is not due to a variation in the neck-head angles (we observed varus, valgus, and neutral angles) nor to a problem between the modular head and the neck (at the morse cone taper). For us, the cause of this progressive intra-acetabular dislocation is poor cup design associated with a weak retention system. If the centers of the inner and outer spheres are superimposed, the cup has a natural tendency to drop into varus because of its weight. This becomes even worse if the center of the outer sphere is medial to the ball center. The cup should be designed so that the center of the outer sphere (bipolar cup) lies lateral to the center of the inner sphere (ball) creating a valgus torque for the cup. Designing a bipolar cup is not as a simple matter as it may seem. We emphasize the importance of the position of the rotation centers.