The effect of axial rotation between the femoral neck and ankle joint (Total Rotation: TR) remains unclear. The purpose of this study was to clarify whether the TR affects the kinematics in normal knees.
A total of 38 male volunteers were examined. The volunteers performed squats under single fluoroscopic surveillance in the sagittal plane. To estimate the spatial position and orientation of the femur and tibia, a 2D-to-3D registration technique was used. This technique uses a contour-based registration algorithm, single-view fluoroscopic images, and 3D computer-aided design models. The volunteers were divided into three groups (large (L), intermediate (I), and small (S)) using hierarchical cluster analysis based on the TR at the standing extension. The TR was measured using 3D bone models created from CT. The femoral neck axis (FNAx) was defined as the line connecting the head center of the femur obtained by the centroid of the best-fitted sphere and the centroid of the best-fitted circle of the most distal part of the femur neck. The ankle joint axis (AJA) was defined as the line connected between the medial and lateral malleoli. TR was defined as the angle between the FNAx and AJA. External rotation of the AJA relative to the FNAx indicated a positive value. Also, a femoral neck angle (FNAn) and tibial torsion angle (TTA) were measured. The FNAn was defined as the angle between the FNAx and surgical epicondylar axis. The TTA was defined as the angle between the proximal posterior-most line and AJA. The correlation between the FNAn, TTA, TR, and axial rotation of the knees at extension were evaluated. Axial rotation, varus-valgus alignment, and anteroposterior (AP) translation of the femur relative to the tibia were evaluated.
FNAn was highly correlated with TR and femoral axial rotation relative to the tibia at extension. Also, TTA was highly correlated with TR. The TR in group L was larger than those in groups I and S. Also, the TR in group I was larger than that in group S (L: 36.6 ± 6.0°, I: 23.2 ± 3.0°, S: 13.8 ± 5.1°; p=0.01). All groups displayed femoral external rotation with flexion. The external rotation in group S was larger than those in groups L and I beyond 50° of flexion. Regarding varus-valgus alignment, there was no significant movement and difference between the three groups. In AP translation, the medial side of the femur in all groups moved anteriorly up to 40° of flexion followed by posterior movement with flexion. The medial side in group L was more anteriorly located than those in groups I and S from 40° to 110° of flexion. On the other hand, the lateral side of the femur in all groups moved posteriorly with flexion. The lateral side in group S was more posteriorly located than those in groups L and I. In group L, a central pivot pattern from 0–40° of flexion with a bicondylar rollback beyond 40° of flexion was observed. In groups I and L, a medial pivot pattern from 0–40° of flexion with a bicondylar rollback beyond 40° of flexion was observed.
The larger TR group had a more anterior medial translation of the femur relative to the tibia with a central pivot. Whereas the smaller TR group had larger external rotation and more posterior lateral translation of the femur relative to the tibia.