Do Modality or Landmark Selection Affect Femoral Torsion Angles? A Retrospective Study Comparing Four Commonly Used Measurement Methods on MRI and CT

Introduction Assessment of femoral torsion on preoperative hip imaging is recommended. However, it is still unclear whether magnetic resonance imaging (MRI) is as accurate as computed tomography (CT) and how different measurement methods affect the resulting angles. We compared MRI- and CT-based assessment of femoral torsion using four commonly used measurement methods.

Material and Methods A retrospective comparative study on 46 patients (57 hips) with hip pain (mean age: 19 ± 5 years; 18 [39%] men) who underwent low-dose CT and 3-T MRI of the hip including sequences of the pelvis and distal condyles between 2017 and 2018. Four different landmarks for the proximal femoral reference axis were compared.

Results Interrater and intrarater reliability was excellent for each of the four CT- and MRI-based measurement methods (range of intraclass correlation coefficients: 0.97–0.99). No systematic bias (all p > 0.05) was observed between CT and MRI for any of the four measurement methods. Mean torsion angles increased by 17.6 degrees for CT and 18.7 degrees for MRI between the most proximal (i.e., Lee method CT: 11.4 ± 14.7 degrees; MRI: 9.3 ± 13.5 degrees) to the most distal measurement methods (Murphy method CT: 29 ± 14.5 degrees; MRI: 27.9 ± 15.2 degrees).

Conclusion MRI- and CT-based femoral torsion measurements are comparable in terms of accuracy, reliability, and reproducibility but depend on the level of selected landmarks used to define the proximal reference line.