Ligaments and other soft tissues, as well as articular contacts, all contribute to the stability of the knee joint. However, precisely how these structural components of the knee joint affect joint stability is not fully understood. The extent to which geometric changes to said structure, resulting from surgical reconstruction and or replacement procedures, may affect joint movement is unclear. This paper was designed to explain connections between the instantaneous motion of the knee and restraining forces in the components that are engaged with it and responsible for such motion (i.e. the anterior cruciate ligaments (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), as well as bony contacts in the medial and lateral compartments). This instantaneous motion is characterized by a rotation about the instantaneous screw axis (ISA), governed by restraining forces along the wrench axes. Said restraining forces may be systematically explored using the principle of reciprocity [Hunt, 1990], which has previously been used to successfully explain the twists that a body may execute whilst maintaining, and not breaking, contact with other bodies. In this way, the motion coordinates may be linked to the force ones, within a novel unified screw coordinate construct. Thus, two approaches which were previously treated as separate entities may now be explained together and in simpler terms to clinicians and other practitioners.

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