The neck–malleolar angle: an alternative method for measuring total lower limb torsion that considers the knee joint rotation angle

Abstract

A greater understanding of the lower limb geometry is necessary for the correction of lower limb torsional deformities. The purpose of our study was to measure the normal values of knee torsion using CT and to introduce the neck-malleolar angle as an alternative for measuring lower limb torsion. We studied 77 consecutive CT studies performed from 2007 to 2009 in our clinic. In 67 cases, there was evidence of old trauma or surgical intervention to one limb, whereas the contralateral limb was healthy. The remaining 10 patients had no history of trauma or surgical intervention and were available for paired analysis in order to find the normal intraindividual variability. The whole limb, femoral and tibial torsion were measured according to the "Ulm method". Finally, the knee joint rotational angle and the neck-malleolar angle (the angle between the femoral neck axis and the bimalleolar axis) were measured. The average knee joint rotation angle was 2.4 ± 6.4° while the neck-malleolar angle was 13.2 ± 10.2°. Right to left side differences in healthy paired limbs for total limb rotation, knee joint rotation and the neck-malleolar angle were 6.1 ± 4.1°, 3.9 ± 2.8° and 7.5 ± 4.3° respectively. The mean absolute rotational differences between injured and healthy limbs was 14.5 ± 10.1°, whereas the mean absolute neck-malleolar differences amounted to 12.5 ± 9.9° (p = 0.013) indicating that the knee compensates for torsional asymmetries. The neck-malleolar angle takes into consideration the buffering effects of the knee joint in the transverse plane and contributes valuable additional information. Further studies including the long-term results of patients with torsional errors are important.