The description of the relative motion between rigid bodies is called kinematics. The knee joint is the largest joint of the human body and has an intricate anatomy, and thus its kinematics have intrigued researchers for a long time1. Apart from direct visual observation, the most popular tool for studying the joint has been radiography. Historically, the knee has been treated as if it were a planar mechanism 2. In other words, the movement of the knee was reduced to a two-dimensional projection of a three-dimensional reality. In recent years, the limitations of this methodology have become clear, with the major flaw being the inability to ascertain the location of the axes of rotation before performing kinematic analyses3. In 1983, Grood and Suntay presented a joint coordinate system that provided a geometric description of the three-dimensional rotational and translational motion between two rigid bodies, and they applied this system to the knee joint4. With use of this model, the described joint displacements became independent of the order in which the component rotations and translations occur. The new mathematical insights led to the concept of the helical axis and opened the door for a correct scientific description of the kinematics of the knee, thus allowing for six degrees of freedom5. However, as the mathematical accuracy improved, the complexity increased and the model appeared to be impractical and difficult to apply to the clinical setting (i.e., the clinicians failed to understand the engineers). Hollister et al., and later Churchill et al., tried to bridge the gap, reducing the descriptive model to essentially two degrees of freedom6,7. In the model of Hollister et al., knee motion was described as pure rotations occurring around two axes: the so-called flexion-extension axis and …