THE ROTATOR CUFF OPPOSES SUPERIOR TRANSLATION OF THE HUMERAL HEAD

To determine the influence of rotator cuff muscle activity on humeral head migration relative to the glenoid during active arm elevation we studied five fresh cadaveric shoulders. The shoulder girdles were moun ted in an apparatus that simulated contraction of the deltoid and rota tor cuff muscles while maintaining the normal scapulothoracic relation ship. The arms were abducted using four different configurations of si mulated muscle activity: deltoid alone; deltoid and supraspinatus; del toid, infraspinatus, teres minor, and subscapularis; and deltoid, supr aspinatus, infraspinatus, teres minor, and subscapularis. For each sim ulated muscle configuration the vertical position of the humeral head in relation to the glenoid was determined at 30 degrees, 60 degrees, 9 0 degrees, and 120 degrees of abduction using digitized anteroposterio r radiographs. Both muscle activity and abduction angle significantly influenced the glenohumeral relationship, With simulated activity of t he entire rotator cuff, the geometric center of the humeral head was c entered in the glenoid at 30 degrees but had moved 1.5 mm superiorly b y 120 degrees. Abduction without the subscapularis, infraspinatus, and teres minor muscles caused significant superiorly directed shifts in humeral head position as did abduction using only the deltoid muscle. These results support the possible use of selective strengthening exer cises for the infraspinatus, teres minor, and subscapularis muscles in treatment of the impingement syndrome.