Femoral Torsion: Impact of Femur Position on CT and Stereoradiography Measurements.

OBJECTIVE: The purpose of this study is to evaluate stereoradiographic measurements of femoral torsion with different femoral positions, in comparison with CT measurements, with use of the current standard axial-slice technique. We hypothesize that CT measurements vary with femoral spatial positioning because of the resulting projection onto the CT plane, whereas stereoradiographic measurements, which are derived from a 3D reconstruction of the femur, remain constant. MATERIALS AND METHODS: Both in vitro and in vivo studies were conducted. CT and stereoradiographic examinations were performed using 30 dry femurs in the following six femoral positions: neutral position (with the femoral mechanical axis aligned with the longitudinal axis of the CT scanner or stereoradiography system), 10° of abduction, 10° of adduction, 5° of flexion, 10° of flexion, and 5° of extension. The impact of femoral position on torsion measurement was assessed using paired t tests. In addition, 18 patients (mean [± SD] age, 42.3 ± 19.9 years) who underwent both CT and stereoradiography examinations were retrospectively assessed. The correlation between femoral positioning and torsion measurement was determined using the Pearson correlation coefficient. RESULTS: Flexion and extension statistically significantly affected CT measurement of femoral torsion (p < 0.01) but not stereoradiography measurement (p > 0.21). A strong correlation existed between hip flexion and the difference between femoral torsion measured by CT and stereoradiography (r = -0.80). CONCLUSION: The accuracy of femoral torsion determined by axial CT depends on the position of the femur. Hip flexion significantly reduced the femoral torsion angle measured by CT. Conversely, the accuracy of stereoradiography was independent of femur positioning. Thus, stereoradiography is preferable to CT for accurate measurement of femoral torsion, while it also substantially reduces the radiation dose.