Bipolar head dissociation: radiographic assessment of bipolar hemiarthroplasty dislocation and the role of the "O" sign.

Bipolar hemiarthroplasty dislocation is an uncommon but clinically significant complication of closed reduction that often necessitates revision, causing prolonged recovery, increased morbidity, and higher costs. A recently described radiographic marker, the "O sign"-a concentric circle appearance of the prosthesis on radiographs-has been proposed as an indicator that closed reduction may be safely attempted. While potentially useful for pre-reduction assessment, its predictive value remains uncertain. We report a case evaluating the diagnostic value of the O sign in bipolar hemiarthroplasty dislocation. A patient underwent both radiography and computed tomography (CT) prior to closed reduction. Radiographic features, including concentricity of the external shell and central positioning of the inner femoral head, were assessed for alignment and eccentricity. On anteroposterior radiographs, the external shell displayed a concentric "O" appearance. However, closer inspection revealed unequal circumferential shell thickness suggesting anterior rotation. CT imaging confirmed eccentric positioning of the femoral head within the outer shell, consistent with early bipolar head dissociation. This case demonstrates that while a concentric O sign on radiographs may suggest closed reduction is feasible, subtle misalignments such as asymmetric shell thickness or eccentric head positioning can be missed without CT. The predictive value of the O sign therefore depends not only on external shell symmetry but also on internal head alignment. Failure to meet either criterion may indicate early dissociation and warrants caution before attempting closed reduction. By integrating CT into assessment, the O sign may evolve from a binary marker to a more refined tool for guiding management of bipolar hemiarthroplasty dislocations.