The influence of hip rotation on femoral offset in plain radiographs

BACKGROUND AND PURPOSE: Adequate restoration of femoral offset (FO) is critical for successful outcome after hip arthroplasty or fixation of hip fracture. Previous studies have identified that hip rotation influences the projected femoral offset (FO(P)) on plain anteroposterior (AP) radiographs, but the precise effect of rotation is unknown. PATIENTS AND METHODS: We developed a novel method of assessing rotation-corrected femoral offset (FO(RC)), tested its clinical application in 222 AP hip radiographs following proximal femoral nailing, and validated it in 25 cases with corresponding computed tomography (CT) scans. RESULTS: The mean FO(RC) was 57 (29–93) mm, which differed significantly (p < 0.001) from the mean FO(P) 49 (22–65) mm and from the mean femoral offset determined by the standard method: 49 (23–66) mm. FO(RC) correlated closely with femoral offset assessed by CT (FO(CT)); the Spearman correlation coefficient was 0.94 (95% CI: 0.88–0.97). The intraclass correlation coefficient for the assessment of FO(RC) by AP hip radiographs correlating the repeated measurements of 1 observer and of 2 independent blinded observers was 1.0 and 1.0, respectively. INTERPRETATION: Hip rotation affects the FO(P) on plain AP radiographs of the hip in a predictable way and should be adequately accounted for.