[Biomechanical principles in diarthroses and synarthroses. I: Basic concepts in diarthroses]

The incongruity of the articulating surfaces is one of the main constructional characteristics of human joints. Unfortunately, this has virtually been neglected in studying of the mechanism of joints. Only in incongruent articulating surfaces does the force locking, which is permanently produced by gravity and/or compressively acting muscular forces (muscular tonus), engender mechanically stable states of balance of the connected limbs. From the incongruity one can conclude that the dimeric link chain represents the most simple constructional element of the human joints. They are force locked, constrained gear systems composed by dimeric link chains. In this context the overlapped dimeric link chain deserves special attention since it is the only mechanically stable device in the presence of compressive forces. We present fundamental terms and characteristics of the gear mechanisms of human joints.