Elements of a general theory of joints. 1. Basic kinematic and static function of diarthrosis

The kinematicaly open, dimeric link chain represents the simpliest module of a diarthrosis. The simpliest synarthrosis is represented by a (visco-)elastic disk that connects two rigid bodies. Diarthroses and synarthroses are statically distinguished by the character of force transfer. Because of their adhesive connection to the corresponding bony structures synarthroses can transfer shear stresses while diarthroses can only be loaded by forces which act normal to their functional surfaces. Therefore, the interface between the body structure and the elastic synarthrosis can be considered as a surface of infinite static (adhesive) friction, while the contacting surfaces in a diathrosis possess vanishing static friction. Kinematically, the difference between diarthroses and synarthroses is established by the fact that diarthroses possess axes of rotation which are approximately fixed relatively to the morphology of the corresponding functional surfaces while in synarthroses the positions of rotational axes depend on the geometric structure of the applied and transferred force system. The first part entire paper presents the most simple module of a diarthrosis, the kinematically open dimeric link chain.