Principles and mechanisms of automatic rotation during terminal extension in the human knee joint.

The mechanisms solely responsible for automatic rotation during terminal extension were investigated by means of destruction experiments on knee joint structures. These are the posterior cruciate ligament (PCL), the anterior cruciate ligament (ACL) and the curvature of the medial femoral condyle. The first mechanism is based on the extraordinary obliquity of the constantly taut PCL guiding bundle, which produces torque in the final extension stage. The second mechanism depends on the fact that the ACL becomes 'too short' towards the final extension stage, which in turn is due to the shape of the articular surfaces. The third mechanism is caused by the deflection of the medial femoral condyle by the intercondylar eminence of the tibia. Each of the collateral ligaments can, in their own right, prevent excessive automatic rotation. On the one hand, the mechanism of the automatic rotation is reversed during initial flexion by the medial collateral ligament (this would be 'too short' in the case of nonreversal). On the other hand, it is reversed by the popliteus muscle. It is suggested that automatic rotation is not related to securing an amuscular stance (locking mechanism).