On the role of ligaments in the guidance of the human knee passive motion

Equivalent spatial mechanisms already proved to replicate the passive motion of the knee, i.e. the relative motion of the femur and tibia in virtually unloaded conditions, with a high accuracy. These models are defined by a direct representation of the anatomical structures that, according to experimental evidence, guide the passive motion, namely the articular surfaces and the isometric ligaments (anterior cruciate, posterior cruciate and medial collateral ligaments). This paper is addressed to understand the role that the other ligaments of the knee have during passive motion. These ligaments do not contrast the passive motion, but at the same time it is quite improbable that they are all slack, since their main function (i.e. resisting external loads) would be achieved with a reduced efficacy. In particular, the role of the lateral collateral ligament is analysed, due to its importance for the knee stability. Starting from a previous equivalent mechanism, this analysis is performed by defining three topologically similar mechanisms that feature the articular surfaces, the lateral collateral ligament and two isometric ligaments. The motion generated by these alternate mechanisms is compared with the experimental passive motion of a specimen, in order to verify if the lateral collateral ligament can actually guide the same passive motion. The results prove that the lateral collateral ligament is a redundant constraint for the passive motion, and the knee is an overcontrained system, guided by the cruciates and the collateral ligaments.