Accuracy and Reliability of Computed Tomography and Magnetic Resonance Imaging Compared to True Anatomic Femoral Torsion

BACKGROUND Abnormal torsion of the femur is correlated to lower extremity pathologies. Although computed tomography (CT) scan is the gold standard torsional measurement, magnetic resonance imaging (MRI) is proposed as a viable alternative. Our aim was to determine the accuracy and consistency of MRI and CT femur rotational studies based on 4 described protocols. METHODS Twelve cadaveric femora were stripped of soft tissue before imaging and physical assessment of torsion. Four advanced imaging series were obtained for each specimen: CT with axial cuts of the femoral neck (CT-axial); CT with oblique cuts of the femoral neck (CT-oblique); MRI with axial cuts of the femoral neck (MR-axial); MRI with oblique cuts of the femoral neck (MR-oblique). Anatomic specimens were placed with the posterior femoral condyles flat on a dissection table for assessment of true torsion with digital images. Three independent reviewers performed all measurements, including true torsion, using imaging software. Bland-Altman analysis was repeated with the data from each reviewer. RESULTS Interobserver repeatability for all groups was high at 0.95, 0.87, 0.90, 0.97, and 0.92 for CT-axial, CT-oblique, MR-axial, MR-oblique, and true torsion, respectively. CT-axial had the lowest mean difference from clinical imaging for all three observers (all <1 degree) and held the tightest 95% limits of agreement for 2/3 observers. As torsion increases from neutral, MR-oblique linearly overestimates the rotation compared with true torsion. CT-oblique and MR-axial showed slightly greater differences from true torsion compared with CT-axial, but did not reach clinical significance. CONCLUSIONS CT-axial was both most accurate and reproducible when compared with true torsion of the femur and should be the gold standard imaging modality; however, both MR-axial and CT-oblique were accurate to a level that is likely less than clinical significance. MR-axial images should be used in clinical situations where radiation exposure needs to be limited. MR-oblique images can overestimate true antetorsion and should not be used. CLINIC SIGNIFICANCE CT-axial followed by MRI-axial is the most accurate and consistent in measuring true torsion of the femur.