A common reference frame for describing rotation of the distal femur

Abstract

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.

Constitutional Varus Does Not Affect Joint Line Orientation in the Coronal Plane

“…In recent publications we reported detailed values on the distribution and means of knee alignment in the coronal and horizontal planes, diverging from previous assumptions that ruled for a long time [12,28]. However, a systematic understanding of the role of joint line orientation and its association with the development of malalignment and arthritis is lacking.…”

“…A systematic understanding of normal mean values and variance in the population is lacking. We have reported detailed values on the distribution and means of knee alignment in the coronal and axial planes [2,28]. In these studies, performed on asymptomatic young individuals, we found that a substantial proportion of the population (24%) has so-called constitutional varus, defined as coronal tibiofemoral mechanical alignment of more than 3°varus.…”

Preoperative assessment of femoral rotation and its relationship with coronal alignment: A magnetic resonance imaging study

“…8,[10][11][12] In a CT-based kinematic cadaver study from 2009, 12 Victor et al used the dependent relationship between the transverse axis of the femur and tibia to explore the true, three-dimensional, rotational axis of the knee and found that of all the surface-derived landmarks, the patients with osteoarthritis and concluded that on average the PCL was only 1.68 internal rotation (IR) relative to the SEA. They identified a statistically significant difference in the relationship in different coronal plane alignments -1.08 in the varus knee, 2.18 in the neutral knee, and 2.68 in the valgus knee.…”

A new approach to implant alignment and ligament balancing in total knee arthroplasty focussing on joint loads

“…However, the use of these anatomical landmarks has to be critically discussed especially concerning their intra-and inter-observer reproducibility [31,61,74] . Greatest inter-and intraindividual variability was found for the Whiteside line, followed by the TEA [71] . Yan et al [74] found the maximum potential errors for TEA and Whiteside line to be 13 ° (3 ° internal rotation to 10 ° external rotation) and 24 ° (16 ° internal rotation to 8 ° external rotation), respectively.…”