A common reference frame for describing rotation of the distal femur

The understanding of rotational alignment of the distal femur is essential in total knee replacement to ensure that there is correct placement of the femoral component. Many reference axes have been described, but there is still disagreement about their value and mutual angular relationship. Our aim was to validate a geometrically-defined reference axis against which the surface-derived axes could be compared in the axial plane. A total of 12 cadaver specimens underwent CT after rigid fixation of optical tracking devices to the femur and the tibia. Three-dimensional reconstructions were made to determine the anatomical surface points and geometrical references. The spatial relationships between the femur and tibia in full extension and in 90 degrees of flexion were examined by an optical infrared tracking system. After co-ordinate transformation of the described anatomical points and geometrical references, the projection of the relevant axes in the axial plane of the femur were mathematically achieved. Inter- and intra-observer variability in the three-dimensional CT reconstructions revealed angular errors ranging from 0.16 degrees to 1.15 degrees for all axes except for the trochlear axis which had an interobserver error of 2 degrees . With the knees in full extension, the femoral transverse axis, connecting the centres of the best matching spheres of the femoral condyles, almost coincided with the tibial transverse axis (mean difference -0.8 degrees , sd 2.05). At 90 degrees of flexion, this femoral transverse axis was orthogonal to the tibial mechanical axis (mean difference -0.77 degrees , sd 4.08). Of all the surface-derived axes, the surgical transepicondylar axis had the closest relationship to the femoral transverse axis after projection on to the axial plane of the femur (mean difference 0.21 degrees , sd 1.77). The posterior condylar line was the most consistent axis (range -2.96 degrees to -0.28 degrees , sd 0.77) and the trochlear anteroposterior axis the least consistent axis (range -10.62 degrees to +11.67 degrees , sd 6.12). The orientation of both the posterior condylar line and the trochlear anteroposterior axis (p = 0.001) showed a trend towards internal rotation with valgus coronal alignment.