MUSCLES OF SHOULDER AND UPPER
ARM
ROTATOR CUFF
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.
Supraspinatus Muscle
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.
Infraspinatus Muscle
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.
Subscapularis Muscle
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.
