Recent literature in biomechanics has demonstrated that motion of the human knee joint in virtually unloaded conditions is guided in a single and complex path by the passive structures of the joint alone. It has been deduced that the joint behaves as a single degree of freedom mechanism. Early models, dated on beginning of this century, showed this in the sagittal plane only. The three dimensional motion has been recently modelled by means of equivalent parallel mechanisms, based also on experimental observations on knee specimens of two separate frictionless contacts and three isometric ligament fibres. The model of the joint is a key tool for a deeper understanding of the knee motion and the design of reliable and efficient prostheses. It also provides useful information for the planning of surgical intervention on the basic structures of the knee. The progress of this modelling is reported in this paper. A medical and biomechanical rationale for these studies is provided, together with a number of relevant publications. The pioneering planar four-bar linkage has been initially advanced to a single degree of freedom equivalent spatial mechanism characterised by plane-to-sphere contacts. The closure equations for this mechanism have been progressively simplified. Extensions to more complex articular surfaces have been also performed. The results reported for all these models demonstrate that the original model assumptions were valid and that refinements in articular surface descriptions are justified to a certain extent
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