**2.1. Structural anatomy**

#### *2.1.1. Rotator cuff, interval, crescent, and cable*

The rotator cuff is composed of the musculotendinous units that bound the glenohumeral joint. Its components are supraspinatus (SS), infraspinatus (IS), teres minor (TM), and subscapularis (SSC) muscles [5, 6] (**Figure 1**). The supraspinatus, which is most commonly involved in rotator cuff tears, originates on the superior aspect of the scapular body, in the supraspinous fossa, and inserts onto the anterior-superior aspect of the greater tuberosity of the humerus. The infraspinatus originates on the posterior scapular body, from the infraspinous fossa, and inserts on the posterior-superior aspect of the greater tuberosity. The teres minor, which is rarely involved in rotator cuff tears, originates from the lateral lower-half of the scapular body, inferior to the infraspinatus, and inserts on the posterior – inferior aspect of the great tuberosity and humeral head. The subscapularis, which is the largest muscle of the rotator cuff group, originates from the anterior scapular body (the subscapular fossa), runs deep to the coracoid process, and inserts onto the lesser tuberosity of the humerus. Innervation to the rotator cuff comes from the C5-6 nerve roots, with the suprascapular nerve supplying the supraspinatus and infraspinatus, axillary nerve supplying teres minor, and the upper and lower subscapular nerves supplying the subscapularis. The close interplay and confluence of the different parts of the rotator cuff creates several structures important for glenohumeral joint stability and function. These include the rotator interval, crescent, and cable.

The rotator interval (**Figure 1A**) is the anterior triangular space between the anterior border of the supraspinatus and superior border of the subscapularis, and contains the anterior glenohumeral joint capsule, the coracohumeral ligament (CHL), and the superior glenohumeral ligament (SGHL). The interval helps maintain the biceps tendon within the bicipital groove, and also contributes to stability of the glenohumeral joint [7, 8]. The rotator interval is also often implicated in the adhesions and contractures that occur in adhesive capsulitis of the shoulder. The rotator crescent is a thin sheet of rotator cuff tendon, comprising the distal portions of the SS and IS insertions. The crescent is proximally bound by a thick bundle of fibers—the rotator cable—that runs perpendicular to the SS and IS fibers. Burkhart et al. described a biomechanical

**Figure 1.** Arthroscopic views of the rotator cuff tendons (left shoulder). (A) Intraarticular view from the posterior portal, showing the humeral head (HH), supraspinatus (SS), subscapularis (SSC), the long-head of the biceps tendon (LHBT), the middle glenohumeral ligament (MGHL), and rotator interval (RI). (B) Bursal view of the superior rotator cuff (SS and IS)..

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for treatment of these tears have not demonstrated consistently good outcomes [2–4]. These include attempts at relieving pain by way of debridement with or without biceps tenotomy; balancing the anterior/posterior force couples by way of partial repair; restoring cuff integrity by way of interpositional grafting; and tendon transfers. Reverse shoulder arthroplasty has been gaining popularity and demonstrates good outcomes as a treatment option for patients with rotator cuff arthropathy, but is typically reserved for the elderly patients. Massive and irreparable rotator cuff tears in younger and more active individuals, especially without sig-

Recently, a new surgical procedure called superior capsular reconstruction (SCR) was described by Mihata et al., who reported promising short-term clinical outcomes in 24 shoulders (23 consecutive patients) with symptomatic irreparable rotator cuff tears [2]. Although the procedure has a strong appeal for physicians treating patients with this difficult problem and has been quickly gaining popularity, caution regarding widespread use is warranted, as large-scale and long-term data is still lacking. This chapter reviews the anatomy and function of the rotator cuff and shoulder capsule; patho-etiology of rotator cuff tears; and rationale, techniques, outcomes, and future direction of superior capsule reconstruction for irreparable

The rotator cuff is composed of the musculotendinous units that bound the glenohumeral joint. Its components are supraspinatus (SS), infraspinatus (IS), teres minor (TM), and subscapularis (SSC) muscles [5, 6] (**Figure 1**). The supraspinatus, which is most commonly involved in rotator cuff tears, originates on the superior aspect of the scapular body, in the supraspinous fossa, and inserts onto the anterior-superior aspect of the greater tuberosity of the humerus. The infraspinatus originates on the posterior scapular body, from the infraspinous fossa, and inserts on the posterior-superior aspect of the greater tuberosity. The teres minor, which is rarely involved in rotator cuff tears, originates from the lateral lower-half of the scapular body, inferior to the infraspinatus, and inserts on the posterior – inferior aspect of the great tuberosity and humeral head. The subscapularis, which is the largest muscle of the rotator cuff group, originates from the anterior scapular body (the subscapular fossa), runs deep to the coracoid process, and inserts onto the lesser tuberosity of the humerus. Innervation to the rotator cuff comes from the C5-6 nerve roots, with the suprascapular nerve supplying the supraspinatus and infraspinatus, axillary nerve supplying teres minor, and the upper and lower subscapular nerves supplying the subscapularis. The close interplay and confluence of the different parts of the rotator cuff creates several structures important for glenohumeral joint stability and function. These include the rotator interval, crescent, and

nificant arthritic changes of the glenohumeral joint, remain a clinical conundrum.

**2. Anatomy, biomechanics, and functions of the rotator cuff**

tears.

cable.

**2.1. Structural anatomy**

128 Advances in Shoulder Surgery

*2.1.1. Rotator cuff, interval, crescent, and cable*

**Figure 1.** Arthroscopic views of the rotator cuff tendons (left shoulder). (A) Intraarticular view from the posterior portal, showing the humeral head (HH), supraspinatus (SS), subscapularis (SSC), the long-head of the biceps tendon (LHBT), the middle glenohumeral ligament (MGHL), and rotator interval (RI). (B) Bursal view of the superior rotator cuff (SS and IS)..

The rotator interval (**Figure 1A**) is the anterior triangular space between the anterior border of the supraspinatus and superior border of the subscapularis, and contains the anterior glenohumeral joint capsule, the coracohumeral ligament (CHL), and the superior glenohumeral ligament (SGHL). The interval helps maintain the biceps tendon within the bicipital groove, and also contributes to stability of the glenohumeral joint [7, 8]. The rotator interval is also often implicated in the adhesions and contractures that occur in adhesive capsulitis of the shoulder.

The rotator crescent is a thin sheet of rotator cuff tendon, comprising the distal portions of the SS and IS insertions. The crescent is proximally bound by a thick bundle of fibers—the rotator cable—that runs perpendicular to the SS and IS fibers. Burkhart et al. described a biomechanical

model of rotator cuff tears using 20 cadaver shoulders, where the rotator cable acts as a stress shield for the crescent, and the two structures form a "suspension bridge." According to this model, tears in the crescent have minimal effects on shoulder function, while those that involve the cable impair its ability to distribute the load and tension between the anterior and posterior rotator cuff and therefore its role as a dynamic stabilizer of the humeral head [9]. This concept has clinical implications as it helps guide decision making in identifying tears that can be managed non-operatively, versus those that require surgical fixation.

The shoulder capsule provides static stability, serving to prevent excessive translation of the humeral head relative to the glenoid [5, 10]. The anterior capsule prevents anterior translation, while the posterior prevents posterior translation. The function of the superior capsule was previously poorly understood and continues to be studied. Ishihara et al. demonstrated in a biomechanical study that the superior capsule plays an important role in passive stability in all directions, and cutting it significantly increases abnormal translation, especially superiorly [10]. This can lead to a decrease in the acromiohumeral distance—a finding commonly seen in patients with chronic massive superior cuff tears as well as cuff-tear arthropathy [2] (**Figure 2**).

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While a significant number of rotator cuff tear cases present to the physician after a traumatic episode, most tears do not occur in a setting of a normal tendon. Preexisting degenerative changes are usually found in the torn tendons, and the injury that leads to clinical presentation is likely the "straw that breaks the camel's back." A number of both intrinsic and extrinsic pathways and risk factors are thought to contribute to chronic degeneration and weakening of

**Figure 2.** Anterior-posterior plain radiograph of a left shoulder with rotator cuff arthropathy. Note the "high-riding" humeral head, with a decreased acromiohumeral distance, and interrupted Shenton's line at the inferior aspect of the

The main intrinsic mechanism pathway is thought to be tenocyte apoptosis and inflammation resulting from chronic microtrauma to the rotator cuff tendons. Advancing age is the most common reason for this mechanism, and age has been found to be the strongest risk factor for rotator cuff disease. This is thought to be due to the combination of age-related degenerative changes and accumulation of microtrauma and macrotrauma over the course of an individual's lifetime [3, 4]. Older patients are also more likely to develop larger tears; Gumina et al.

**3. Etiology of rotator cuff tears**

glenohumeral joint.

the cuff tendons, as described below (**Table 1**).

#### *2.1.2. Shoulder capsule*

While the rotator cuff is the main dynamic stabilizer of the glenohumeral joint, the glenohumeral joint capsule acts as a static stabilizer. It is a thin membranous structure located deep to the rotator cuff; it originates medially from the glenoid neck and inserts laterally to the anatomical neck of humerus.

The capsule is thicker anteriorly than posteriorly. The anterior capsule contains focal thickened bundles, which are called superior, middle, and anterior-inferior glenohumeral ligaments (GHL). The posterior capsule has an inferior thickening called the posterior-inferior GHL, but does not have separate ligaments further superiorly. Directly inferiorly, between the anterior-inferior and posterior-inferior glenohumeral ligaments, the capsule forms the axillary pouch, which tightens in abduction, and relaxes in adduction [3–5, 10]. Contracture and loss of normal axillary pouch volume is frequently seen in adhesive capsulitis, whereas a patulous capsule with an enlarged pouch is often seen in multi-directional shoulder instability.

The superior capsule is thin and was previously less well-studied. It originates from the glenoid neck along with its anterior-posterior counterparts, courses directly underneath the SS and anterior part of the IS, and attaches to 30–61% of surface area of the greater tuberosity (GT) [5, 10]. Nimura et al. measured superior capsule attachments in cadaveric shoulders. They reported thicker footprint at the anterior edge of SS and posterior edge of IS (5.6 ± 1.6 mm and 9.1 ± 1.7 mm, respectively), whereas the attachment was thinner at the middle area of the rotator cuff, near the posterior margin of SS (4.4 ± 1.2 mm). The authors concluded that the thinnest point of the capsule could contribute to the etiology of the initiation of degenerative rotator cuff tears [5]. The superior capsule is closely associated with the SS and IS, and typically tears together with complete tears of these tendons [1–4].

## **2.2. Function**

The muscles of the rotator cuff help initiate movement of the shoulder joint, and also serve as the main dynamic stabilizer of this joint. Supraspinatus aids in abduction of the humerus, particularly in the scapular plane; external rotation is provided by infraspinatus (more active in adduction), and teres minor (more active in abduction); and internal rotation is the function of subscapularis. Furthermore, SS prevents abnormal inferior-superior translation of the humeral head, particularly during active arm elevation, by compressing the head into the glenoid fossa. The balancing forces between SSC anteriorly and IS and TM posteriorly provide stability in the sagittal plane, and the upward force of the deltoid is balanced by that of IS, TM, and SSC in the coronal plane [11, 12].

The shoulder capsule provides static stability, serving to prevent excessive translation of the humeral head relative to the glenoid [5, 10]. The anterior capsule prevents anterior translation, while the posterior prevents posterior translation. The function of the superior capsule was previously poorly understood and continues to be studied. Ishihara et al. demonstrated in a biomechanical study that the superior capsule plays an important role in passive stability in all directions, and cutting it significantly increases abnormal translation, especially superiorly [10]. This can lead to a decrease in the acromiohumeral distance—a finding commonly seen in patients with chronic massive superior cuff tears as well as cuff-tear arthropathy [2] (**Figure 2**).

**Figure 2.** Anterior-posterior plain radiograph of a left shoulder with rotator cuff arthropathy. Note the "high-riding" humeral head, with a decreased acromiohumeral distance, and interrupted Shenton's line at the inferior aspect of the glenohumeral joint.
