The subscapularis muscle alone inserts on the lesser tuberosity of the humerus, and is responsible for internal rotation of the arm (Figs. 22.3 and 22.4). The supraspinatus, infraspinatus, and teres minor all insert on the greater tuberosity (Figs. 22.5, 22.6, 22.7, and 22.8). The supraspinatus tendon passes through the subacromial space beneath the subacromial bursa to its insertion site on the superior facet of the greater tuberosity. This insertion site facilitates abduction of the upper extremity, but also is a component to the frequent impingement of this tendon. Beyond 70° of abduction, the greater tuberosity has a tendency to come into contact with the acromion, resulting in the impingement of both the tendon and the subacromial bursa [10, 11]. The infraspinatus and teres minor tendons insert on the posteroinferior facet of the greater tuberosity and working conjunction to facilitate external rotation.

Tendons are primarily composed of type I collagen, which comprises approximately 85 % of their dry weight. There is also a high concentration of proteoglycans and elastin. Proteoglycans are responsible for the viscoelasticity found in tendons [12–14].

The muscles of the rotator cuff derive their blood supply from the branches of the axillary artery (Fig. 22.9). The axillary artery is generally divided in to three segments, based on the borders of the pectoralis minor. Proximally, the thoracoacromial artery emerges from the axillary artery at the level of the upper border of the pectoralis minor. The artery pierces the clavipectoral fascia, and then divides into four branches that supply the muscles of the shoulder and proximal humerus. Of these four branches, the deltoid (or humeral) branch and acromial branch are the primary blood suppliers to the scapulohumeral muscles.

At the lateral border of the pectoralis minor, the subscapular artery emerges to pass between the radial and median nerves and descends caudally to supply the subscapularis muscle. Eventually, the subscapular artery will give rise to the circumflex scapular artery whose branches form an anastomosis with the suprascapular and dorsal scapular arteries. Distally, the anterior and posterior circumflex arteries emerge and encircle the humerus, and provide blood supply to the glenohumeral joint capsule and rotator cuff tendons via its terminal branches [15, 16].

The muscles of the rotator cuff are innervated by the brachial plexus, which is formed from the branches of spinal roots C5–T1. In terms of surgical anatomy, the most important nerves are the suprascapular nerve and the axillary nerve. The suprascapular nerve arises from the superior trunk of the brachial plexus and passes through the suprascapular notch to enter into the supraspinatous fossa where it gives off two motor branches to innervate the supraspinatus muscle. It is as it passes under the superior transverse ligament that the nerve becomes most susceptible to injury via compression and shearing forces [5]. The nerve then continues to travel around the lateral border of the scapular spine and into the infraspinatous fossa via the spinoglenoid notch. The subscapular nerve arises from the posterior cord of the brachial plexus, and divides into an upper and lower nerve. The upper nerves insert directly into the subscapularis muscle, while the lower nerve continues to innervate the inferior portion of the subscapularis. The remaining innervation of the teres minor comes from the axillary nerve (Figs. 22.10 and 22.11) [17].

The motion of the shoulder is complex and takes into account motion at the acromioclavicular and sternoclavicular joints, the glenohumeral joint, and the scapulothoracic articulation. There have been numerous methods developed to describe the motion that occurs at each joint. For research purposes, the motion at the glenohumeral joint is often referred to in regard to rotation around three axes to give forward elevation, elevation in the plane of the scapula (or abduction), and humeral rotation. The term “scapulothoracic rhythm” was first used by Codman to describe the complex, coordinated movements of both the scapulothoracic articulation and the glenohumeral joint during arm motion. Further proving that this motion is coupled, pathology of the glenohumeral joint, such as the pain of a full-thickness RC tear, affects the kinematics of the scapulothoracic articulation [18].

During humeral elevation, a greater degree of motion occurs at the glenohumeral joint during the early arc of motion (first 30°). Scapular and clavicular motion are minimal with the arm in less than 90° of elevation. Beyond 90° of humeral elevation, there is upward rotation, posterior tilting, and external rotation of the scapula to allow for the full arc of motion in the upper extremity [19–21]. During this, the clavicle retracts, elevates, and rotates backward to accommodate this motion. The humerus must externally rotate to avoid impingement of the greater tuberosity under the acromion [22].

There is a lack of linearity to the ratio of glenohumeral to scapulothoracic motion with a ratio of 4:1 during the initial 25° of motion, which almost equates at 5:4 after that to average approximately 2:1 for the entire arc of elevation [23–26]. This coupled motion has been further elucidated with dual fluoroscopic imaging systems in an in vitro model [27].

When simplified to a single plane, the center of rotation lies within 6 mm of the geometric center of the humeral head [28]. This tight focus regarding the center of rotation is largely due to the small amounts of translation that occur during initial elevation. The center of rotation is also affected by the integrity of the rotator cuff as well as the long head of the biceps tendon [29, 30].

Forces at the glenohumeral joint depend on multiple factors including the overall condition of the muscle, the size of the muscle as measured by the cross-sectional area, and the position of the joint. In general, a muscle is strongest at the midpoint of its excursion compared to when it is fully contracted or extended.

If the muscle is atrophic due to muscular or neural pathophysiology, it will not function as otherwise expected based on the biomechanics alone. Cross-sectional area of the muscle has been measured to determine a muscle’s volume and subsequent forces, which it is able to generate [31]. The orientation of the muscles surrounding the shoulder to the glenoid creates a joint reaction force with a large component perpendicular to the glenoid, further aiding humeral head compression [32]. The location of the muscle and tendon unit with regard to the joint also becomes important when setting up the testing apparatus during biomechanical testing. Tears of the RC affect the overall amount of force that can be produced in the abducted arm. When a tear was created that involved 1/3 or 2/3 of the supraspinatus, the force decreased only 5 %, and a complete, retracted tear of the supraspinatus caused only a loss of torque of 58 % [33].

The position of the arm also effects the direction of pull of the muscle, with the most obvious example being the supraspinatus, which can perform abduction or external rotation based on the arm’s position (Fig. 22.2). The position of the arm also affects the morphology of the rotator cuff, as was shown in an MRI study evaluating supraspinatus tendon during different positions of rotation and abduction of the humerus. Abduction over 30° was found to shorten the tendon, while external and internal rotations caused elongation of the anterior and posterior portions of the tendon, respectively [34]. The size of the individual rotator cuff muscle can be compensated for by positioning that creates a more effective moment arm. The subscapularis and infraspinatus muscles generate forces of two to three times more than the supraspinatus; however, a more effective moment arm causes the supraspinatus to be a much more effective abductor [35].

The rotator cuff complex functions overall in three broad categories: rotation of the humerus about the scapula, compression of the humeral head into the glenoid, and providing muscle balance to the glenohumeral joint. The subscapularis functions to internally rotate the humerus, while the infraspinatus and teres minor are external rotators. The supraspinatus functions to abduct as well as provide weak external rotation with the arm in adduction [36].

Compression of the humeral head into the glenoid has been researched by determining reaction force testing. This is accomplished by using a dynamic shoulder testing apparatus that has determined that joint forces increase throughout abduction and peak at approximately 90° of motion. Firing of the supraspinatus increases the joint forces, and simulated paralysis of the supraspinatus results in a significant decrease in compression [37].

There remains controversy on how much muscle balance and stability to the glenohumeral joint is provided by the rotator cuff complex. Different in vivo studies have been performed in an effort to gain more realistic information on the stabilizing effects of the rotator cuff. Loads to the rotator cuff were given, and the strain of the inferior glenohumeral ligament was measured, showing that the infraspinatus and teres minor were most responsible for aiding to the stability of the GH joint [38]. It was also shown that the subscapularis stabilized anteriorly with the arm in abduction; however, with external rotation, the humerus became less important as the posterior musculature became of primary importance. The rotator cuff musculature has been shown to have a greatest effect on glenohumeral stability in the midrange of motion when the capsule-labral complex is lax [39].

The moment arm and orientations of the rotator cuff muscles change with the abduction angle of the arm. Different authors have measured this through radiographic studies [40, 41]. Not surprisingly, the anterior and middle deltoids as well as the supraspinatus have the largest moment arm.

Much of the work done to corroborate the biomechanical studies of the rotator cuff has utilized electromyography and selective nerve blocks to include or exclude certain muscles and record their activity. This type of research has led to a greater understanding of the proportion of individual rotator cuff muscle involvement with certain motions. The percentage of involvement of the supraspinatus with external rotation was quantified with this pattern of research, which has led to an increased recognition of physical exam tests to detect RC tears [42].

It has been proposed that the rotator cuff tendon tears are related to hypoperfusion. A study utilizing Doppler flowmetry to analyze blood flow to the rotator cuff failed to identify any “critical” zone of hypoperfusion in a normal rotator cuff. Blood flow was found to be highest at the edges of torn rotator cuff tendons and lowest in tendons suffering from chronic impingement [15].

In cadaveric studies utilizing electromagnetic tracking devices, a 2 cm rotator cuff tear was created to determine the effect on kinematics in vitro. EMG data was used to apply force through cables sewn into the muscles. The defect resulted in posterior angulation of the plane of elevation, most notably throughout the midpoint of abduction [43].