There are several, commonly used, definitions for axes when describing knee kinematics, e.g..: mechanical axis, anatomical axis, epicondylar axis, transverse axis, posterior condylar line, Whiteside's line, and axes of rotation of the knee. In order to incorporate both translation and rotation motion of the knee one needs to describe the motion as a combination of a translation and a rotation of the two rigid bodies, i.e. a 6DOF (degrees of freedom) motion between the tibia and the femur.
For our Biomechanics work we use advanced kinematics tools for the description and analysis of motions. We describe the momentary kinematics axis of the knee using momentary screw parameter. The screw axis along with its associated pitch provides vital information regarding the specific knee behavior, and enables to perform biomechanical analysis of the joint in higher dimensional mathematical spaces.