The Rotator Cuff Opposes Superior Translation of the Humeral Head

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Abstract
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 mounted in an apparatus that simulated contraction of the deltoid and rotator cuff muscles while maintaining the normal scapulothoracic relationship. The arms were abducted using four different configurations of simulated muscle activity: deltoid alone; deltoid and supraspinatus; del toid, infraspinatus, teres minor, and subscapularis; and deltoid, supraspinatus, infraspinatus, teres minor, and subscapularis. For each simulated muscle configura tion the vertical position of the humeral head in relation to the glenoid was determined at 30°, 60°, 90°, and 120° of abduction using digitized anteroposterior radio graphs. Both muscle activity and abduction angle sig nificantly influenced the glenohumeral relationship. With simulated activity of the entire rotator cuff, the geo metric center of the humeral head was centered in the glenoid at 30° but had moved 1.5 mm superiorly by 120°. Abduction without the subscapularis, infraspina tus, and teres minor muscles caused significant supe riorly directed shifts in humeral head position as did ab duction using only the deltoid muscle. These results support the possible use of selective strengthening ex ercises for the infraspinatus, teres minor, and sub scapularis muscles in treatment of the impingement syndrome.

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