[Biomechanical principles in diarthroses and synarthroses. II: The humerus articulation as a ball-and-socket joint].

The shoulder joint does not represent a simple ball-and-socket joint with three degrees of freedom unlike widely believed. Series of X-ray pictures show that in abduction the momentary rotational axis does not meet the center of the humerus. Morphological measurements yield the result that the radius of curvature of the cavitas is slightly greater (2 mm) than the humeral one. This physiological incongruity of the articulating surfaces is functionally necessary. Thus, the shoulder joint represents a force-locked dimeric ball-and-socket joint. In abduction e.g. the humerus simultaneously rotates around the humeral center and around the cavitas center in opposite direction. The resultant momentary rotational axis is therefore shifted towards the scapula as in vivo measurements confirm. Fundamental kinematical and statical properties of the joint are derived which explain substantial experimental finding in literature.