Flexion at the knee moves the lower leg posteriorly. The knee also undergoes small rotations and translations. As in other joints, external rotation at the knee moves the anterior aspect of the distal segment away from the midline and internal rotation produces the opposite effect. Knee rotations in either direction depend upon the amount of knee flexion. On average, at 30 degrees of flexion, the knee can internally rotate to about 30 degrees and externally rotate to around 40 degrees.
Translations are minimal in the normal knee and limited to only a few millimeters.
Translation increases in the presence of various pathologies and is therefore assessed in clinical examinations of knee injury. For example, the anterior drawer test measures the amount of anterior translation of the tibia relative to the femur and is increased when the anterior cruciate ligament is ruptured. Why this occurs should become apparent as we examine the structural arrangement of the knee and its ligaments. The knee has several unique features which we will examine in detail. As already discussed, the principal motion at the knee is flexion, which can be defined as sagittal plane rotation of the femoral condyles upon the tibial plateau.