BIOMECHANICAL PRINCIPLES IN DIARTHROSES A ND SYNARTHROSES .2. THE SHOULDER JOINT AS A DIMERIC BALL-AND-SOCKET JOINT

The shoulder joint does not represent a simple ball-and-socket joint w ith three degrees of freedom unlike widely believed. Series of X-ray p ictures show that in abduction the momentary rotational axis does not meet the center of the humerus. Morphological measurements yield the r esult that the radius of curvature of the cavitas is slightly greater (2 mm) than the humeral one. This physiological incongruity of the art iculating 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 moment ary rotational axis is therefore shifted towards the scapula as in viv o measurements confirm. Fundamental kinematical and statical propertie s of the joint are derived which explain substantial experimental find ing in literature.