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Rotator Cuff Muscles

The RTC is composed of the supraspinatus, the infraspinatus, the subscapularis, and the teres minor muscles. The tendinous portion of the supraspinatus interdigitates with the subscapularis and the infraspinatus to form a common, continuous insertion on the humeral head, enveloping approximately 75% of the GH articulation and with a mean area of insertion on the greater tuberosity of approximately 6 cm2.

The mean distance across the insertion is 14.7 mm, and the thickness of the terminal 2 cm of the RTC ranges from 9 to 12 mm. The size and area of supraspinatus insertion (1.55 cm2) is less than that of the infraspinatus insertion (1.76 cm2).

The RTC often serves more than one function simultaneously. The muscles act as prime movers if their line of action is within the intended direction of motion. In addition, they act as joint stabilizers by opposing the action of the deltoid, thereby centering the humeral head against the glenoid during shoulder motion.

Loss of RTC function, either from fatigue or from frank tears, can lead to superior translation of the humeral head during arm elevation, because the deltoid is left unopposed.

The thin, crescent-shaped sheet of RTC comprising the distal portions of the supraspinatus and infraspinatus insertions is termed the rotator crescent, and it is bounded on its proximal margin by a thick bundle of fibers called the rotator cable.

The rotator cable averages 2.6-fold the thickness of the rotator crescent that it surrounds, and it shields the RTC tendons from excessive stress and is readily seen arthroscopically from within the GH joint.

The supraspinatus muscle is active with any motion involving elevation, and it is the most commonly torn tendon of the cuff . Investigators have described distinct anterior and posterior portions of the supraspinatus tendon. The anterior portion has a larger physiological cross-sectional area, making it better suited to withstand greater mechanical loads .

The supraspinatus tendon inserts as a footprint, thickening at its insertion, with an average of 1.55 mm of bone between the cartilage edge and the tendon insertion approximately 2 mm medial to the greater tuberosity. This is important in determining the size of partial-thickness RTC tears.

The posterior portion of the RTC is made up of the infraspinatus and teres minor muscles. The infraspinatus has a pennate architecture with a central raphe that should not be confused with the intermuscular interval between it and the teres minor.

The subscapularis internally rotates the humerus and acts as a passive stabilizer to anterior subluxation and external rotation. The lower fibers of the subscapularis also contribute to GH stability, resisting the shear forces and superior pull of the deltoid.Tears in the upper portion of the subscapularis can result in dislocation of the long head of the biceps tendon because other structures composing the sling are torn as well.Multiple vessels contribute to the vascularity of the RTC . In addition, the deltoid muscle and scapular rotators play a role in shoulder motion and stability.

Several bursae lie within the soft tissues surrounding the shoulder joint. The subacromial and subdeltoid bursae are found superficial to the tendons and separate it from deltoid. The bursae vary in size and extend laterally from the subacromial space to the proximal humeral metaphysis.

The subacromial bursa becomes thickened in disease states, and it has numerous free nerve endings, indicating that it may be involved with pain perception in pathologic conditions. The subscapularis bursa lies between the tendon and neck of the scapula, just inferior to coracoid process, and protects the tendon as it courses along the scapular neck and coracoid. This bursa communicates with the GH joint capsule and can harbor intra-articular loose bodies.

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