Be aware of the “O” sign in the bipolar cup dissociation during closed reduction of bipolar dislocation: A case report

RATIONALE: Bipolar 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 CONCERNS: We 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. DIAGNOSES: The patient was diagnosed with early bipolar cup dissociation. INTERVENTIONS: The patient underwent revision surgery to replace and reassemble the femoral head component. OUTCOMES: No further dislocation occurred following the surgery. LESSONS: To 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 EVIDENCE: Level V, case report.