Disassembly of bipolar cup with self-centering system: a report of seven cases.

Disassembly of bipolar cups with a self-centering system occurred in six patients (seven hips; five women, one man) and the cause of the failure was evaluated. The mean age of the patients at the time of arthroplasty was 49.7 years (range, 27-85 years), and mean weight was 48.4 kg (range, 37-65 kg). The mean time to failure was 7.5 years (range, 4.8-9.2 years). Before failure, all implants functioned well and none of the patients had sustained trauma. Retrieval study showed that the cause of failure of the locking mechanism with a self-centering system was severe polyethylene abrasion at the rim attributable to impingement and deformity of the locking ring. After the locking ring detached, the inner head dislocated from the outer head resulting in a varus position of the outer head in the acetabulum. If the deformed locking ring did not detach, the inner head could dislocate, with the varus outer head remaining in the acetabulum. The incidence of this failure was 11%. This disassembly is not a rare occurrence numerous years after a well-functioning bipolar hemiarthroplasty, even with the self-centering system.