Measurement of femoral antetorsion and tibial torsion by magnetic resonance imaging.

Several methods for measuring femoral and tibial torsion by CT have been described since 1980. Alternative methods of measurement to CT are needed to avoid irradiating young patients facing derotation osteotomy. In this study, MRI was used to quantify femoral and tibial torsion in normal adult volunteers, firstly with transverse sections analogous to those of CT in order to establish reference values; and secondly, with sections along the axis of the neck of femur to assess the influence of the orientation of the slices on the measured values. The images were acquired using fast T1 weighted gradient echo sequences in 0.2 and 1 T magnets. Average femoral antetorsion (CT-analogous method) was 10.4 degrees (average side difference = 4.6 degrees); average tibial torsion was 41.7 degrees (average side difference = 6.1 degrees). A steeply inclined slice along the axis of the femoral neck gave a mean measurement of the angle of antetorsion that was significantly higher than the mean obtained from either slightly inclined or transverse sections (16.7 degrees vs 12.1 degrees and 11.2 degrees, respectively, p < 0.001). In conclusion, MRI provides an alternative to CT in the measurement of femoral and tibial torsion. MRI enables one to orientate the slice along the axis of the femoral neck, thus obtaining a single cross-section of the entire neck. However, the normal range of measurements will vary according to the plane of image.