Disassembly of the inner head of a bipolar hip prosthesis.

We report an extremely rare case of inner head displacement from the stem of a bipolar hip prosthesis (BHP). An 88-year-old woman underwent BHP implantation for right femoral neck fracture. However, severe right hip joint pain occurred 12 days after surgery. A plain radiogram film revealed displacement of the inner head from the neck of the stem, accompanied by sinking of the stem. At reoperation, the inner head was disassembled from the stem, and Vancouver type A1 fracture was confirmed. Disassembly may have been caused by the pumping phenomenon or micromovement of the stem due to periprosthetic fracture. To our knowledge, this is the first report about disassembly of the BHP inner head, probably due to periprosthetic fracture.