[kinematic consideration of the shoulder girdle and its consequences on common surgical methods]

Motility of the upper extremities is the sum of movements of an open kinematic chain made up of the hands and the lower and upper arms in the humero-scapular joints which are integrated in the closed kinematic chains of the bilateral symmetrical shoulder girdles. Each shoulder girdle assembles three kinematic links (scapula, clavicle, hemithorax), with three joints (sternoclavicular, acromioclavicular, thoraco-scapular), creating a three-bar linkage. This linkage may be characterized as an effective transformer of motion with 11 degrees of freedom ensuring wide ranges of motions. This motion-transformer acts strictly as a guided linkage, allowing a precise reference between initial motion and functional result. The operative transfixation of acromioclavicular joints in the case of either dislocation or arthrodesis, also of the sternoclavicular joint reduces the degree of kinematic freedom from 11 to 8 and alters the shoulder as a motion-transformer into a rigid girdle with a variable length of support, with degeneration of the shoulder linkage. Surgical treatment of acromioclavicular dislocation with coraco-clavicular fusion using a screw as described by Bosworth produces osseus strain and a girdle under twofold static in determination (F = -2) with pressure forces causing bone deformation, loosening of the screw and fatigue fractures. This operation therefore cannot yet be recommended. Additional cadaver experiments with means of strain gauge cells have shown that lengthening of the clavicle by 0.5 cm increases the forces transmitted from the elevated arm to the hemithorax by 16%. Shortening of the osteotomized clavicle by only 1 cm leads to an increase of these forces by about 40%.(ABSTRACT TRUNCATED AT 250 WORDS)