Stability of the Giliberty bipolar hip: report of three cases.

The Giliberty Bipolar Hip Prosthesis consists of an unbonded acetabular cup which snap-fits over the head of the femoral prosthesis. Because of the free movement permitted at the cup-pelvic junction as well as at the snap-fit interface, there has been a great deal of concern over the potential for dislocation. Three patients in whom this unit was implanted sustained significant skeletal trauma subsequent to surgery. None of them dislocated. In each case the prosthetic head was known to be in a relatively vertical orientation with respect to the transverse axis of the pelvis prior to the injury. In 2 of the 3 cases radiographically identifiable movement of the cup was present before and after the injury. These observations suggest that the persistently mobile, vertically positioned unbonded cup remain stable despite the stress of significant trauma. It is probable that the potential for cup movement, even where on the X-ray it appears to have stopped, acts as a safety valve in absorbing force that might otherwise dislocate the hip or cause fractures of the femur or the pelvis.