Dynamic contributions to superior shoulder stability

It has been suggested that superior decentralization of the humeral head is a mechanical factor in the etiology of degenerative rotator cuff teals. Th is superior decentralization may be caused by muscular imbalance. The objec tive of this study was to investigate the contribution of individual should er muscles to superior stability of the glenohumeral joint. In 10 fresh fro zen cadaver shoulders the tendons of the rotator cuff, teres major, latissi mus, pectoralis major. deltoid and biceps were prepared. The shoulders were tested in a shoulder-loading device in 0 degrees, 30 degrees, 60 degrees a nd 90 degrees of glenohumeral abduction. A constant superior force of 20 N was applied to the humerus. Tensile loads were applied sequentially to the tendons in proportion to their cross-sectional alc as and translations of t he humeral head relative to the glenoid were recorded with a 3Space(TM) Fas trak system. Depression of the humeral head was most effectively achieved b y the latissimus (5.6 +/- 2.2 mm) and the teres major (5.1 +/- 2.0 mm). Fur ther studies should elucidate their possible in vivo role in the frontal pl ane force couple to counter balance the deltoid. The infraspinatus (4.6 +/- 2.0 mm) and subscapularis (4.7 +/- 1.9 mm) showed similar effects while th e supraspinatus (2.0 +/- 1.4 mm) was less effective in depression. Therefor e, the infraspinatus and subscapularis should be surgically repaired whenev er possible. The supraspinatus may be of less importance for superior stabi lity than previously assumed. Published by Elsevier Science Ltd on behalf o f Orthopaedic Research Society.