Dislocation of bipolar hemiarthroplasty following breakage of the polyethylene ring

Dislocation after bipolar hemiarthroplasty is rare. In classical dislocations of bipolar prosthesis, the prosthetic femoral head escapes from the natural acetabular cavity. Two cases of a bipolar hip prosthesis are presented in which the dislocation occurred between the cup and the metallic ball head 4 and 12 months postoperatively. The reason for the dislocation was fracturing of the edge of the polyethylene ring - either due to impingement at the full flexion position of the hip or to dislocating forces in extreme positions of the metallic ball head. To avoid this complication we recommend installation of the notch of the polyethylene ring be at the peripheral half of the acetabular cup.