MUSCLES OF SHOULDER AND UPPER ARM
The main function of the four musculotendinous units that contribute to the rotator cuff is to compress the humeral head into the glenoid to provide a fulcrum for rotation. Whereas each muscle aids in specific motions, it is this concavity compression that is essential for the proper function of the other muscles that affect the glenohumeral joint.
The supraspinatus muscle occupies the supraspinatous fossa of the scapula. It takes its origin from the medial two thirds of the bony walls of this fossa. The tendon blends deeply with the capsule of the shoulder joint and inserts on the highest of the three facets of the greater tubercle of the humerus. The supraspinatus muscle aids the deltoid in the first 90 degrees of forward flexion and abduction. Partial or full-thickness tears of this tendon are not uncommon and may be well tolerated if the remaining intact cuff can compensate. This is particularly true if the tear involves the crescent portion of the supraspinatus tendon rather than the cable portion of the tendon (see Plates 1-6 and 1-42). Tears involving the anteriormost portion of the supraspinatus and, in particular, the anterior cable result in a larger amount of muscle weakness, tendon retraction, and muscle atrophy than tears isolated to the central crescent portion of the tendon. Large two-tendon tears involving more than the supraspinatus can lead to superior migration of the humeral head, owing to the unopposed contraction of the deltoid. The supraspinatus muscle is innervated by the suprascapular nerve (C5, C6) from the superior trunk of the brachial plexus. The nerve may become entrapped as it enters the supraspinatous fossa through the scapular notch, where it passes under the superior transverse scapular ligament. The suprascapular artery accompanies the nerve but it passes over the transverse scapular ligament.
The infraspinatus muscle arises from the infraspinatous fossa of the scapula and inserts on the middle facet of the greater tubercle of the humerus. Deeply, its fibers blend with those of the capsule of the shoulder joint. This muscle acts to externally rotate the arm. Pronounced weakness is demonstrated by the external rotation lag sign, in which the patient cannot maintain passive external rotation at the side (see Plate 1-40). The suprascapular nerve and artery continue through the spinoglenoid notch after giving off branches to the supraspinatus. Ganglion cysts can be seen in this area in conjunction with glenohumeral labral tears and may compress the nerve (see Plate 1-51).
Teres Minor Muscle
The teres minor muscle arises from the upper two thirds of the lateral border of the scapula. Its tendon passes upward and lateralward to insert in the lower facet of the greater tubercle and surgical neck of the humerus. It also blends deeply with the capsule of the shoulder joint. The muscle is invested by the infraspinatus fascia and is sometimes inseparable from the infraspinatus muscle. The teres minor muscle contracts with the infraspinatus to aid in external rotation of the humerus. A branch of the axillary nerve ascends onto its lateral margin at about its midlength. The teres minor muscle is separated from the teres major by the long head of the triceps brachii and by the axillary nerve and posterior circumflex humeral vessels. It is pierced by branches of the circumflex scapular vessels along the lateral border of the scapula.
The subscapularis muscle originates from the medial two thirds of the subscapularis fossa on the anterior surface of the scapular body. The tendon passes across the anterior surface of the capsule of the shoulder joint to end in the lesser tubercle of the humerus. The tendon is separated from the neck of the scapula by the large subscapular bursa. The subscapularis muscle is the principal internal rotator of the arm but also acts in adduction. The upper half of the subscapularis has been shown to carry over 70% of the muscle fibers, tension, and strength of the entire muscle. As a result of this, distribution tears of the upper portion of the subscapularis are associated with more disability than tears involving the inferior half of the muscle. Dysfunction of the subscapularis muscle results in weakness best defined with the abdominal compression test and the internal rotation lift off test (see Plate 1-43). The muscle is innervated on its costal surface by the upper and lower subscapular nerves.