Axis location of tibial rotation and its change with flexion angle.

The magnitude and location of the axis of tibial rotation were measured at 15 degrees increments between 0 degree and 90 degrees flexion using 24 normal anatomic knee specimens, and their changes with flexion angle were investigated. The magnitude of tibial rotation was small (8.3 degrees) at 0 degree flexion, but increased rapidly as the flexion angle increased and reached a maximum rotation (31.7 degrees) at 30 degrees flexion. It then decreased again with additional flexion (24.8 degrees at 90 degrees flexion). The location of the axis was close to the tibial insertion of the anterior cruciate ligament at 0 degree flexion, gradually moving toward insertion of the posterior cruciate ligament (observed at 45 degrees and 60 degrees flexion), and then moved anteriorly again with additional flexion: the axis was approximately equidistant from the two cruciate insertions at 90 degrees flexion. The results showed that a relatively large degree of tibial rotation was possible in a normal knee and that the location of the axis remained approximately in the area between the two cruciate ligament insertions throughout the range of flexion. However, the location of the axis changed with the flexion angle within this area according to the changes in direction and tension of the cruciate ligaments and the surrounding soft tissues.