Elements of a general theory of joints. 5. Basic mechanics of the knee.

In the sagittal plane the interplay of the knee joints, the patellofemoral and the tibiofemoral joint, are depicted as a closed kinematic chain consisting of 3 dimeric link chains. Kinematically the system is without constraint and possesses 3 (abstract) degrees of freedom. Only the application of external forces fixes the single dimeric link chains in mechanically stable positions and thus determines the momentary functional state of the knee. Without external loads the single links can snarl up like those of a bike's chain. The crucial ligaments play a minor role for knee bending. A qualitative analysis of the force distribution within the knee is presented. In the frontal plane the tibiofemoral joint represents a linkquadrangle with positive drive (one abstract degree of freedom) which also becomes mechanically stable by external compressive forces.