Rotator cuff structure
Anatomic overview
The rotator cuff consists of four muscle-tendon units: the subscapularis, infraspinatus, supraspinatus, and teres minor ( Fig. 50.1 ). These four muscles have their origins on the body of the scapula, a thin sheet of bone, and insert through their tendons onto the greater and lesser tuberosity of the proximal humerus. Coordinated action of these muscle-tendon units allows for articulation of the glenohumeral, scapulothoracic, sternoclavicular, and acromioclavicular joints and consequent smooth motion of the shoulder and arm during activity. The tendons of the rotator cuff muscles envelop the humeral head and are continuous with the underlying glenohumeral joint capsule except at the rotator interval and axillary recess. The distal end of the clavicle articulates with the medial aspect of the acromion, a bony process on the scapula, to form the acromioclavicular joint. The anterior undersurface of the acromion and the coracoacromial ligament, a strong triangular band extending between the acromion and the coracoid, form the coracoacromial arch. The subacromial and subdeltoid bursae, often referred as subacromial-subdeltoid bursa, are located superficial to the rotator cuff and underneath the deltoid muscle and coracoacromial arch. The bursa varies in size and extends laterally from the subacromial space to the proximal humeral metaphysis.
The rotator interval is the triangular region between the upper border of the subscapularis and the anterior border of the supraspinatus that contains the coracohumeral ligament, the long head of the biceps tendon, and superior glenohumeral ligament. The coracohumeral ligament originates at the lateral base of the coracoid and is the most superficial of the rotator interval structures. Its insertion is variable but is centered at the confluence of the supraspinatus tendon insertion, the rotator interval, and the subscapularis tendon insertion. The long head of the biceps tendon originates at the supraglenoid tubercle and passes intra-articularly across the glenohumeral joint and into the humeral bicipital groove under the transverse humeral ligament. The superior glenohumeral ligament originates from the superior labrum and forms a pulley around the long head of the biceps tendon as it inserts into the humerus. Finally, the axillary recess is a large pouch of capsular folds across the inferior glenohumeral joint, bounded by the anterior and posterior bands of the inferior glenohumeral ligament at its anterior and posterior extent. Like the rotator interval, the axillary recess lacks myotendinous coverage by the rotator cuff.
Vascular supply
The major vascular supply to the rotator cuff is contributed from the ascending branch of the anterior and posterior humeral circumflex arteries, the acromial branch of the thoracoacromial artery, and the suprascapular and suprahumeral arteries ( Fig. 50.2 ). A cadaveric study reported that the thoracoacromial artery contributed to the rotator cuff’s blood supply in 76% of specimens, the suprahumeral in 59%, and the subscapular in 38%. The acromial branch of the thoracoacromial artery is closely related with the coracoacromial ligament and can be encountered when the coracoacromial ligament is released during acromioplasty. Recent studies have suggested a decrease in vascularity with age, consistent with the increasing prevalence of rotator cuff pathology with aging. ,
Innervation
The rotator cuff is innervated by the subscapular, suprascapular, and axillary nerves ( Fig. 50.3 ). These nerves receive fibers from the brachial plexus, which originates in the neck from anterior divisions of the spinal nerves C5 to T1. The suprascapular nerve (C5, C6) stems from the upper trunk of the brachial plexus and passes through the suprascapular notch just medial to the base of the coracoid process to supply the supraspinatus muscle. It then passes through the spinoglenoid notch to supply the infraspinatus. The upper and lower subscapular nerves (C5, C6) and axillary nerve (C5, C6) stem from the posterior cord of the brachial plexus. The upper subscapular nerve innervates the upper portion of the subscapularis muscle by directly inserting into it, while the lower subscapular nerve bifurcates into two branches, with the cranial branch innervating the lower half of the subscapularis muscle. The axillary nerve travels anteriorly below the subscapularis muscle and axillary recess before exiting posteriorly through the quadrangular space; the intermuscular space bordered by the teres minor and teres major muscles superiorly and inferiorly, respectively; and the lateral head and long head of the triceps muscle laterally and medially, respectively. The axillary nerve and posterior humeral circumflex vessels both wrap around the posterior aspect of the surgical neck of the proximal humerus after leaving the quadrangular space, with the posterior branch of the axillary nerve supplying the teres minor muscle.
Muscle anatomy
Subscapularis
The subscapularis muscle is a triangular-shaped muscle originating from the medial two-thirds of the subscapularis fossa on the anterior surface of the scapula and passing laterally beneath the coracoid and the scapular neck. It blends with the fibers of the glenohumeral capsule, inserts onto the lesser tuberosity of the proximal humerus, and then extends and fuses with the supraspinatus tendon to form a sheath that surrounds and stabilizes the long head of the biceps tendon in the proximal bicipital groove. The subscapularis is the largest and most powerful of the rotator cuff muscles, with its origin occupying greater than 90% of the anterior scapular surface. Its attachment on the lesser tuberosity is trapezoidal in shape with a broad and wide superior attachment which narrows inferiorly. This superior attachment, often described as the upper rolled border of the tendon, is the thickest region of the tendon with the largest cross-sectional area and has been shown to be the strongest section of the tendon insertion biomechanically, with higher stiffness and ultimate load compared with the inferior tendon region. The subscapularis muscle has a dual innervation from both the upper and lower subscapular nerves that arise from the posterior cord of the brachial plexus with the upper nerve innervating a greater portion of the muscle. The blood supply is provided by the subscapular artery arising from the third part of the axillary artery.
Supraspinatus
The supraspinatus muscle originates from the medial two-thirds of the supraspinatus fossa and inserts along the greater tuberosity of the proximal humerus. The footprint of the supraspinatus on the greater tuberosity is triangular, with an average maximum medial-to-lateral length of 6.9 mm and an average maximum anterior-to-posterior width of 12.6 mm. Cadaveric studies have shown that the area of the supraspinatus tendon just proximal to its insertion is markedly less vascularized compared with the remainder of the tendon. Other studies have shown that the deep surface of the supraspinatus insertion is relatively hypovascular compared with its superficial aspect. The supraspinatus muscle is innervated by the suprascapular nerve from the upper trunk of the brachial plexus and supplied by the suprascapular artery that accompanies the nerve. Previous cadaveric studies have shown that the first motor branch of the nerve enters the supraspinatus muscle usually either under the transverse scapular ligament or 1 mm distal to it at the suprascapular notch. In a few cases this branch enters the muscle proximal to the ligament and passes superficial to the ligament and the suprascapular notch.
Infraspinatus
The infraspinatus and teres minor muscles make up the posterior portion of the rotator cuff. The infraspinatus muscle originates from the medial two-thirds of the infraspinatus fossa of the scapula and inserts onto the middle facet of the greater tuberosity of the proximal humerus. The footprint of the infraspinatus on the greater tuberosity is trapezoidal, with an average maximum medial-to-lateral length of 10.2 mm and an average maximum anterior-to-posterior width of 32.7 mm. The infraspinatus muscle is supplied by the suprascapular nerve and artery, which continue through the spinoglenoid notch after giving off branches to the supraspinatus. Previous cadaveric studies have shown that the infraspinatus muscle receives three to four branches from the suprascapular nerve in 50% of specimens.
Teres minor
The teres minor muscle takes origin from the upper two-thirds of the dorsolateral border of the scapula and inserts in the lower facet of the greater tuberosity and surgical neck of the proximal humerus. A cadaveric study has shown that approximately half of the 31 cadaver shoulders had a distinct fascial compartment surrounding the teres minor muscle and the other half had a combined fascia surrounding both the infraspinatus and teres minor muscles. The axillary nerve and both posterior circumflex humeral and circumflex scapular arteries supply the teres minor.
Tendon, ligament, and capsule structure
The four rotator cuff muscles described earlier are distinct structures whose tendinous portions coalesce to envelope the glenohumeral joint capsule and insert onto the proximal humerus. Seminal cadaveric studies have thoroughly described the confluence of the rotator cuff tendons and underlying capsular and ligamentous elements at their common insertion on the greater and lesser tuberosities. ,
Gross anatomy
Macroscopically, fibers from both the subscapularis and infraspinatus tendons interdigitate with respective fibers of the supraspinatus. Specifically, the supraspinatus and infraspinatus tendons join with each other proximal to their insertions on the greater tuberosity. In a study of 113 cadaveric specimens, the footprint of the tendon insertions to the greater tuberosity was shown to be predominately occupied by the infraspinatus tendon, with the supraspinatus insertion much smaller than previously believed. The subscapularis and supraspinatus tendons fuse to form a sheath that surrounds the long head of the biceps tendon at the proximal end of the bicipital groove of the humerus ( Fig. 50.4 ). Fibers from both the teres minor and infraspinatus tendons also merge with each other proximal to their insertions on the greater tuberosity. The teres minor and the subscapularis also have muscular insertions on the surgical neck of the proximal humerus inferior to their tendinous attachments onto the tuberosities.
Some of the rotator cuff tendons are reinforced both superficially and deeply near their insertions on the proximal humerus by the coracohumeral ligament. The coracohumeral ligament extends as a fan-like sheet from the coracoid process and fuses superficially with the supraspinatus tendon, extending posteriorly to the infraspinatus and merging laterally with the periosteum of the greater tuberosity ( Fig. 50.5 ). Slips of the coracohumeral ligament also pass deep along the surface of the joint capsule and into the rotator interval between the subscapularis and supraspinatus tendons, attaching to both tuberosities deep to the insertions of the tendons. Another deep extension of the coracohumeral ligament, termed the “rotator cable” by Burkhart et al., further reinforces the supraspinatus and infraspinatus tendons. The fibers of the rotator cable run deep and perpendicular to the tendon fibers, extending in a semicircular pattern from within the anterior supraspinatus tendon at the intertubercular groove to the posterior aspect of the greater tuberosity within the infraspinatus tendon. The rotator cable is intimately interwoven with the joint capsule and supraspinatus and infraspinatus tendons and acts as a “transverse band” spanning the supraspinatus and infraspinatus insertions ( Fig. 50.6 ).
The glenohumeral joint capsule is a fibrous sheath that is intimately associated with the rotator cuff, running along the deep articular surface of the rotator cuff tendons and muscles and surrounding the glenohumeral joint. The capsule originates from the rim of the glenoid and firmly attaches to the deep surface of the rotator cuff tendons near their insertions into the surgical neck of the proximal humerus. The superior, middle, and inferior glenohumeral ligaments are thickenings of the glenohumeral joint capsule that radiate from the glenoid margin of the scapula and attach to the proximal humerus. The superior glenohumeral ligament is a slender band that originates immediately anterior to the attachment of the long head of the biceps tendon on the glenoid. It travels parallel to the long head of the biceps tendon, merges with the anterior edge of the coracohumeral ligament beneath the subscapularis tendon, and inserts near the upper end of the lesser tuberosity of the proximal humerus. The middle glenohumeral ligament may be cord-like, thin, or even absent. When present, the middle glenohumeral arises from the midglenoid rim, anterior to the labrum and reaches the proximal humerus at the front of the lesser tuberosity and just inferior to the insertion of the subscapularis muscle. The inferior glenohumeral ligament consists of an anterior band, a posterior band, and a diffuse thickening of the capsule between these bands that is referred to as the axillary pouch. The anterior band originates from the scapula directly below the notch in the anterior glenoid rim between the 2 to 4 o’clock positions on the glenoid face, while the posterior band originates in the 7 to 9 o’clock position of the glenoid. The inferior glenohumeral ligament travels to the underside of humeral neck, where it inserts in one of two distinct configurations: a collar-like attachment, in which the entire ligament attaches just inferior to the articular edge of the humeral head, or in the shape of a “V” with the anterior and posterior bands attaching adjacent to the articular edge of the humeral head and the axillary pouch attaching at the apex of the “V” distal to the articular edge. The border between the middle and inferior glenohumeral ligaments is often indistinct.
The coracoacromial ligament spans from the undersurface of the acromion to the lateral aspect of the coracoid and is continuous with the less dense clavipectoral fascia that also arises from the coracoid. The coracoacromial arch is the concave surface created by the undersurface of the anterior acromion and the coracoacromial ligament. The coracoacromial arch provides a strong ceiling for the shoulder joint, along which the rotator cuff tendons glide during all shoulder movements ( Fig. 50.7 ). Passage of the rotator cuff tendons and the proximal humerus under this arch is facilitated by the subacromial-subdeltoid bursa, which are two serosal surfaces in contact with each other, one on the undersurface of the coracoacromial arch and deltoid and the other on the surface of the rotator cuff. These sliding surfaces are lubricated by the bursal and synovial fluid to decrease friction and allow free motion of the rotator cuff.
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