Sitting Sideways Causes Different Femoral-Tibial Rotations in Each Knee

Abstract

Purpose According to a previous study, asymmetrical kneeling, such as sitting sideways, does not exhibit asymmetrical movements. Rotational analyses of each femur and tibia help explain why rotational knee kinematics while sitting sideways do not exhibit asymmetrical movement. We aimed to assess the rotation of the femur and tibia in normal knees while sitting sideways. Methods Each volunteer sat sideways under fluoroscopy. Two-dimensional and three-dimensional registration techniques were used. After evaluating the femoral rotation angle relative to the tibia at each flexion angle, the femoral and tibial sole rotation angles at each flexion angle were compared between the ipsilateral and contralateral knees. Results While sitting sideways, both knees showed femoral external rotation relative to the tibia with flexion. In the ipsilateral knees, the femurs exhibited an external rotation of 26.3 ± 8.0°, from 110° to 150° of flexion. Conversely, the tibia exhibited an external rotation of 12.2 ± 7.8°, from 110° to 150° of flexion. From 110° to 150° of flexion, femoral external rotation was significantly larger than tibial external rotation. In the contralateral knees, the femurs exhibited an internal rotation of 23.8 ± 6.3°, from 110° to 150° of flexion (110°, p < 0.001; 120°, p < 0.001; 130°, p < 0.001; 140°, p < 0.001; and 150°, p < 0.001). Contrastingly, the tibia exhibited an internal rotation of 30.4 ± 8.8°, from 110° to 150° of flexion, which was significantly larger than femoral internal rotation (110°, p = 0.002; 120°, p < 0.001; 130°, p < 0.001; 140°, p < 0.001; and 150°, p < 0.001). Conclusions Although bilateral knees exhibited femoral external rotation relative to the tibia while sitting sideways, the ipsilateral and contralateral knees showed femoral and tibial sole rotations in opposite directions. In particular, the contralateral knees might show a strained movement because both femurs and tibias exhibited internal rotation with flexion. Patients who have undergone guided-motion total knee arthroplasty (TKA) or medial-pivot TKAs might be advised to avoid sitting sideways.

Keywords: axial rotation; kinematics; kneeling; normal knee; sitting sideways.

Figure 1. The global coordinate system.

A vertical line from the tibial origin was established as the Y-axis. A line passing through the centre of medial and lateral eminences and the ankle centre was projected onto the plane perpendicular to the Y-axis, and the projection was established along the Z-axis. A line perpendicular to both Y- and Z-axes was denoted as the X-axis.

Figure 2. Femoral and tibial sole rotation.

In the AP view, clockwise and anticlockwise rotations of the right and left knees, respectively, were defined as the external rotation and represented as positive. AP: Anteroposterior.

Figure 3. Rotation angle while sitting sideways.

The markers indicate femoral rotation relative to the tibia. *, significant differences between the ipsilateral and contralateral knees (p < 0.05).

Figure 4. Sole rotation of the femur and tibia in the ipsilateral knees while sitting sideways.

The markers indicate femoral and tibial rotation relative to the global coordinate system. The anteroposterior view of each flexion angle is shown. *, significant differences between the femur and tibia (p < 0.05).

Figure 5. Sole rotation of the femur and tibia in the contralateral knees while sitting sideways.

The markers indicate femoral and tibial rotation relative to the global coordinate system. The anteroposterior view of each flexion angle is shown. *, significant differences between the femur and tibia (p < 0.05).