• Seung-Bo Sim, Sang-Woo Son, and Bum-Jin Shim.
• Department of Orthopedic Surgery, Kyungpook National University Hospital, Jung-gu, Daegu, South Korea.
• Medicine (Baltimore). 2023 Sep 15; 102 (37): e35234e35234.

RationaleBipolar cup dissociation following hip hemiarthroplasty is a rare complication of which only a few cases have been reported, and it usually requires revision surgery because of difficulties in closed reduction.Patient ConcernsWe report the case of a 57-year-old man who underwent bipolar hemiarthroplasty 2 months ago caused by a left femoral neck fracture. Postoperatively, bipolar dislocation occurred thrice, and the patient showed bipolar cup dissociation during closed reduction maneuver at the recent visit. At the time of this event, no consideration was given to the shape of the prosthesis on the radiograph.DiagnosesThe patient was diagnosed with early bipolar cup dissociation.InterventionsThe patient underwent revision surgery to replace and reassemble the femoral head component.OutcomesNo further dislocation occurred following the surgery.LessonsTo avoid dissociation of the components during closed reduction, it would be helpful to have knowledge of the "O" sign, a concentric circle shape of the prosthesis on the radiograph.Level Of EvidenceLevel V, case report.Copyright © 2023 the Author(s). Published by Wolters Kluwer Health, Inc.

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