The Shoulder

CHAPTER 16 The Shoulder

CHAPTER OBJECTIVES

At the completion of this chapter, the reader will be able to:

Describe the anatomy of the joints, ligaments, muscles, blood, and nerve supply that comprise the shoulder complex.
Describe the biomechanics of the shoulder complex, including the open- and close-packed positions, muscle force couples, and the static and dynamic stabilizers.
Describe the relationship between muscle imbalance and functional performance of the shoulder.
Describe the purpose and components of the tests and measures for the shoulder complex.
Perform a comprehensive examination of the shoulder complex, including history, systems review, palpation of the articular and soft-tissue structures, specific passive mobility tests, passive articular mobility tests, and special tests.
Evaluate the key findings from the examination data to establish a physical therapy diagnosis and prognosis.
Summarize the various causes of shoulder dysfunction.
Describe and demonstrate intervention strategies and techniques based on the clinical findings and any established goals.
Evaluate the intervention effectiveness to determine progress and modify an intervention as needed.
Plan an effective home program and instruct the patient in its use.

OVERVIEW

The shoulder is the most rewarding joint in the body because when a limited or painful movement is found, the finding is seldom ambiguous and often implicates the offending structure.

—James Cyriax, MD (1904–1985)

The primary function of the shoulder complex is to position the hand in space, thereby allowing an individual to interact with his or her environment and to perform fine motor functions. An inability to position the hand results in profound impairment of the entire upper extremity.¹

Secondary functions of the shoulder complex include the following:

Suspending the upper limb.

Providing sufficient fixation so that motion of the upper extremity or trunk can occur.

Serving as a fulcrum for arm elevation. Three types of arm elevation are recognized: an upward motion of the upper extremity in the scapular plane (scaption) and the motions in either the coronal plane (abduction) or in the sagittal plane (flexion).

The shoulder is endowed with a unique blend of mobility and stability. Optimal functioning of the shoulder and arm can only take place if a delicate balance between the mobility and stability is maintained. The degree of mobility is contingent on a healthy articular surface, intact muscle–tendon units, and supple capsuloligamentous restraints. The degree of stability is dependent on intact capsuloligamentous structures, proper function of the muscles, and the integrity of the osseous articular structures.¹

ANATOMY

The shoulder complex functions as an integrated unit, involving a complex relationship between its various components. The components of the shoulder joint complex consist of (Fig. 16-1):

three bones (the humerus, the clavicle, and the scapula);

three joints (the sternoclavicular [S-C], the acromioclavicular [A-C], and the glenohumeral [G-H] joints);

one “pseudojoint” (the articulation between the scapula and the thorax);

one physiological area (the suprahumeral or subacromial space).

For optimal shoulder function, cervicothoracic junction motions, and motions at the connections between the first three ribs and the sternum and spine must be available.

GLENOHUMERAL JOINT

The G-H joint is a true synovial joint that connects the upper extremity to the trunk, as part of the upper kinetic chain. The G-H joint is described as a ball and socket joint—the humeral head forms roughly half a ball or sphere (Fig. 16-1), whereas the glenoid fossa forms the socket.

The head of the humerus faces medially, posteriorly, and superiorly with the axis of the head forming an angle of 130–150 degrees with the long axis of the humerus (Fig. 16-2).² In the frontal plane, the head of the humerus is angled posteriorly (retroverted) by 30–40 degrees.^3,4 The joint capsule of the G-H joint is lax inferiorly to permit full elevation of the arm.

FIGURE 16-2 Superior aspect of shoulder showing angle of scapula. (Reproduced, with permission, from Chapter 30. Shoulder and Axilla. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy*. New York, NY: McGraw-Hill; 2011.)

The glenoid fossa of the scapula faces laterally, superiorly, and anteriorly at rest and inferiorly and posteriorly when the arm is in the dependent position (Fig. 16-3).⁵ The glenoid fossa is flat and covers only one-third to one-fourth of the surface area of the humeral head. This arrangement allows for a great deal of mobility but little in the way of articular stability. However, the glenoid fossa is made approximately 50% deeper (doubling the depth of the glenoid fossa across its equatorial line)⁶ and more concave by a ring of fibrous cartilage⁷ and dense fibrous collagen⁸ called a labrum. The labrum forms a part of the articular surface and is attached to the margin of the glenoid cavity and the joint capsule.⁹ It is also attached to the lateral portion of the biceps anchor superiorly.¹⁰ In addition, approximately 50% of the fibers of the long head of the biceps (LHB) brachii originate from the superior labrum (the remainder originates from the superior glenoid tubercle),⁸ with four different variations identified,¹¹ and continue posteriorly to become a periarticular fiber bundle, making up the bulk of the labrum.^10,12 The labrum enhances joint stability by increasing the humeral head contact areas to 75% vertically and 56% transversely.^5,9 The humeral–glenoid contact area provides two primary functions:¹³ it spreads the joint loading over a broad area, and it permits movement of opposing joint surfaces with minimal friction and wear.¹⁴ However, because the humeral head is larger than the glenoid (Fig. 16-3), at any point during elevation, only 25–30% of the humeral head is in contact with the glenoid, with the greatest contact occurring during elevation rather than at the extremes. Contact between the humeral head and glenoid fossa is significantly reduced when the humerus is positioned in^15–17:

FIGURE 16-3 Glenohumeral joint. (Reproduced, with permission, from Chapter 30. Shoulder and Axilla. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy.* New York, NY: McGraw-Hill; 2011.)

adduction, flexion, and internal rotation (IR);

abduction and elevation;

adducted at the side, with the scapula rotated downward.

Although the labrum provides some stability for the G-H joint, additional support is provided by both dynamic and static mechanisms. The dynamic mechanisms include the muscles of the rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis muscles) (Fig. 16-4) and a number of muscle force couples described later. The static mechanisms, which include reinforcements of the joint capsule, joint cohesion and geometry, and ligamentous support, are also described later.

FIGURE 16-4 Rotator cuff muscles. (Reproduced, with permission, from Chapter 30. Shoulder and Axilla. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy*. New York, NY: McGraw-Hill; 2011.)

The scapula (Fig. 16-1) functions as a stable base from which G-H mobility can occur. The scapula is a flat blade of bone that is oriented to contribute to stability: it lies along the thoracic cage at 30 degrees to the frontal plane (Fig. 16-2), 3 degrees superiorly relative to the transverse plane to augment functional reaching motions above shoulder height,¹⁸ and 20 degrees forward in the sagittal plane.^19–21 This orientation results in arm elevation occurring in a plane that is 30–45 degrees anterior to the frontal plane. When elevation of the arm occurs in this plane, the motion is referred to as scapular plane abduction or scaption.

The scapula’s wide and thin configuration allows for its smooth gliding along the thoracic wall and provides a large surface area for muscle attachments both distally and proximally.²⁴ In all, 16 muscles gain attachment to the scapula (Table 16-1). Six of these muscles, including the trapezius, rhomboids, levator scapulae, and serratus anterior, support and move the scapula, while nine of the other ten (the omohyoid is not included) are concerned with G-H motion.^25–27

TABLE 16-1

Muscles of the Scapula

Trapezius	Subscapularis
Levator scapulae	Coracobrachialis
Long and short head of the biceps	Pectoralis minor
Rhomboid major	Serratus anterior
Rhomboid minor	Long head of triceps
Supraspinatus	Teres major
Infraspinatus	Teres minor
Deltoid	Omohyoid

Posteriorly, the scapula is divided by the elevated scapula spine into two unequally sized muscle compartments. The supraspinous fossa is small and serves as the site of origin for the supraspinatus muscle (see Fig. 16-4). The infraspinous fossa gives attachment for the downward-acting infraspinatus and teres minor muscles (see Fig. 16-4), important muscles for the stabilization of the humeral head (see “Muscles of the Shoulder Complex” section). The spine of the scapula (see Fig. 16-1) provides a continuous line of attachments for the supporting trapezius muscle along its upper border, whereas the deltoid muscle, which suspends the humerus, gains origin from its lower border. The undersurface of the scapula is covered by the subscapularis muscle (see Fig. 16-4), which also assists in stabilizing the humeral head against the glenoid fossa.

A prominent feature of the scapula is the large overhanging acromion (see Fig. 16-3), which, along with the coracoacromial ligament and the previously mentioned labrum, functionally enlarges the G-H socket. The position of the acromion also places the deltoid muscle in a dominant position to provide muscular support during elevation of the arm. Bigliani et al.^21,28 introduced the following acromion types:

Type I has a relatively flat undersurface

Type II is slightly convex

Type III is hooked

The distance, or angle, between the scapula and the clavicle is variable and depends on function. While the shoulder is protracted, the angle is 50 degrees. At rest, the angle is approximately 60 degrees, and with retraction the angle increases to 70 degrees.³

Along the medial border of the scapula arise three muscles: the two rhomboid muscles and the serratus anterior (Fig. 16-5), all of which aid with scapular stability during arm elevation (see later). The coracoid process (see Fig. 16-3) projects forward like a crow’s beak, for which it is named. This forward position provides an efficient lever whereby the small pectoralis muscle can help to stabilize the scapula. In addition, the process serves as a point of origin for the coracobrachialis and the short head of the biceps muscle.

FIGURE 16-5 Rhomboid muscles and serratus anterior. (Reproduced, with permission, from Chapter 1. Back. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy.* New York, NY: McGraw-Hill; 2011.)

The lateral attachment of the voluminous joint capsule of the G-H joint attaches to the anatomical neck of the humerus (Fig. 16-6).⁶ Medially, the capsule is attached to the periphery of the glenoid and its labrum. The overall strength of the joint capsule bears an inverse relationship with the patient’s age; the older the patient, the weaker the joint capsule. The fibrous portion of the capsule is very lax and has several recesses, depending on the position of the arm. At its inferior aspect, the capsule forms an axillary recess, which is both loose and redundant. The recess permits normal elevation of the arm, although it can also be the site of adhesions when the shoulder is immobilized in adduction. The anterior aspect of the joint capsule is reinforced by three ligaments (Z ligaments), which are described in the next section. The tendons of the rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis) reinforce the superior, posterior, and anterior aspects of the capsule, as does the LHB tendon.

FIGURE 16-6 Glenohumeral joint and ligaments. (Reproduced, with permission, from Chapter 30. Shoulder and Axilla. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy*. New York, NY: McGraw-Hill; 2011.)

An inner synovial membrane lines the fibrous capsule and secretes synovial fluid into the joint cavity. The synovium typically lines the joint capsule and extends from the glenoid labrum down to the neck of the humerus. It also forms variously sized bursae, the largest of which, the subacromial or subdeltoid bursa, lies on the superior aspect of the joint (see later).

The greater and lesser tuberosities (see Fig. 16-1), which serve as attachment sites for the tendons of the rotator cuff muscles, are located on the lateral aspect of the anatomical neck of the humerus, an imaginary line that separates the humeral head from the rest of the humerus. The lesser tuberosity serves as the attachment for the subscapularis. The greater tuberosity serves as the attachment for the supraspinatus, infraspinatus, and teres minor. The greater and lesser tuberosities are separated by the intertubercular groove (see Fig. 16-1), through which passes the tendon of the LHB on its route to attach to the superior rim of the glenoid fossa. This groove has wide variance in the angle of its walls, but 70% fall within a 60–75-degree range.²⁹ Certain shoulder disorders, including rotator cuff and bicipital tendinopathy, have been associated with anomalies of this groove.³⁰ As the tendon of the LHB (Fig. 16-7) passes over the humeral head from its origin, it makes a right-angle turn to lie in the anterior aspect of the humerus. This abrupt turn may permit abnormal wearing of the tendon at this point. The roof of this groove is formed by the transverse ligament. The transverse humeral ligament (Fig. 16-7), which runs perpendicular over the biceps tendon, was once thought to function as a restraint to the biceps tendon within the intertubercular groove. However, this appears to be the role of the coracohumeral ligament.³⁰

FIGURE 16-7 Soft-tissue structures of the shoulder. (Reproduced, with permission, from Chapter 30. Shoulder and Axilla. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy.* New York, NY: McGraw-Hill; 2011.)

The region below the greater and lesser tuberosities, where the upper margin of the humerus joins the shaft of the humerus, is referred to as the surgical neck (Fig. 16-1). The axillary nerve and posterior humeral circumflex artery lie in close proximity to the medial aspect of the surgical neck.

Ligaments

A number of structures function as static stabilizers during motion of the arm to reciprocally tighten and loosen, thereby limiting translation and rotation of the G-H joint surfaces in a load-sharing fashion.³¹ These include the G-H ligaments and the posterior capsule. Further static stabilization is provided by the glenoid labrum. The posterior capsule is under tension when the shoulder is in flexion, abduction, IR (in particular, the superior and middle segments), or in any combination of these.

At the anterior portion of the outer fibers of the joint capsule, three local reinforcements are present: the superior, middle, and inferior G-H ligaments. Together with the coracohumeral ligament, these ligaments form a Z-pattern on the anterior aspect of the shoulder.³² In the midrange of rotation, the G-H ligaments are relatively lax and stability is maintained primarily by the action of the rotator cuff muscle group compressing the humeral head into the conforming glenoid articulation.³¹ The G-H ligaments appear to produce a major restraint during shoulder flexion, extension, and rotation³³:

The anterior G-H ligament is under tension when the shoulder is in extension, abduction, and/or external rotation (ER).

The posterior G-H ligament is under tension in flexion and ER.

The inferior G-H ligament is the most important of the G-H ligaments. It is under tension when the shoulder is abducted, extended, and/or externally rotated. In addition, this ligament is a primary restraint against both anterior and posterior dislocations of the humeral head, and is the most important stabilizing structure of the shoulder in the overhead athlete.³⁴ At 0-degree abduction, the subscapularis muscle, the labrum, and the superior G-H ligament are the primary restraints against anterior translation. At 45-degree abduction, the subscapularis muscle and the middle and inferior G-H ligaments, along with labrum, prevent anterior translation. When the arm is abducted more than 90 degrees, which is the most common position of anterior dislocation, the anterior fibers of the inferior G-H ligament are the primary restraint against anterior movement.^9,35

The middle G-H ligament is under tension when the shoulder is flexed and externally rotated. In addition, the middle G-H ligament and the subscapularis tendon limit ER from 45–75 degrees of abduction, and are important anterior stabilizers.

Other ligaments help provide stability to the G-H joint. These include the following:

The coracohumeral ligament. The coracohumeral ligament (see Fig. 16-6) arises from the lateral end of the coracoid process and runs laterally, where it is split into two bands by the presence of the biceps tendon. The posterior band blends with the supraspinatus tendon to insert near the greater tuberosity, and the anterior band blends with the subscapularis tendon to insert near the lesser tuberosity. The coracohumeral ligament covers the superior G-H ligament anterosuperiorly and fills the space between the tendons of the supraspinatus and subscapularis muscles, uniting these tendons to complete the rotator cuff in this area. Tears of the cuff usually extend longitudinally between the supraspinatus and coracohumeral ligament so that the hood action of the cuff is lost. It is generally agreed that the posterior band of the coracohumeral ligament limits flexion, whereas the anterior band limits extension of the G-H joint.³⁶ Both the bands also limit inferior and posterior translation of the humeral head, strengthening the superoanterior aspect of the capsule.^36,37

The coracoacromial ligament. The coracoacromial ligament (see Fig. 16-6) is often described as the roof of the shoulder. It is a very thick structure that runs from the coracoid process to the anteroinferior aspect of the acromion, with some of its fibers extending to the A-C joint. The ligament consists of two bands that join near the acromion, and it is ideally suited, both anatomically and morphologically, to prevent separation of the A-C joint surfaces. The coracoclavicular ligaments and the costoclavicular ligaments are described in the A-C joint section and the S-C joint section, respectively.

Coracoacromial Arch

The coracoacromial arch (Fig. 16-6) is formed by the anteroinferior aspect of the acromion process, and the coracoacromial ligament, which connects the coracoid to the acromion and the inferior surface of the A-C joint.^38–40 A number of structures are located in the subacromial or suprahumeral, space (Fig. 16-7) between the coracoacromial arch superiorly and the humeral head inferiorly, and the coracoid process, anteromedially. These include (from inferior to superior) the following:

the head of the humerus,

the long head of biceps tendon (intra-articular portion),

the superior aspect of the joint capsule,

the supraspinatus and upper margins of subscapularis and infraspinatus,

the subdeltoid–subacromial bursae,

the inferior surface of coracoacromial arch.

In normal individuals, the subacromial space averages 10–11 mm in height with the arm adducted to the side.^41,42 Elevating the arm in the plane of the scapula decreases this space, and the space is at its narrowest between 60 and 120 degrees of scaption.⁴³ During overhead motion in the plane of the scapula, the supraspinatus tendon, the region of the cuff most involved in overuse syndromes of the shoulder, can pass directly underneath the coracoacromial arch. If the arm is elevated while internally rotated, the supraspinatus tendon passes under the coracoacromial ligament, whereas if the arm is externally rotated, the tendon passes under the acromion itself.⁴⁴

Muscle imbalances or capsular contractures can cause an increase in superior translation of the humeral head, narrowing the subacromial space. For example, if the rotator cuff muscles are weak or injured, increased translation occurs between the humeral head and the glenoid labrum.⁴⁵ This increase in translation may lead to increased wear on the labrum, increased reliance on the static restraints (e.g., ligaments, capsule), and eccentric overloading of the dynamic (muscle) restraints, which in turn can result in instability and/or a condition termed “subacromial impingement syndrome (SIS).”^46,47

Bursae

Approximately eight bursae are distributed throughout the shoulder complex. The subdeltoid–subacromial bursae (see Fig. 16-7) are usually collectively referred to as the subdeltoid bursa because they are often continuous in nature. The subdeltoid bursa is one of the largest bursae in the body and provides two smooth serosal layers, one of which adheres to the overlying deltoid muscle and the other to the rotator cuff lying beneath. This bursa is also connected to the acromion, greater tuberosity, and coracoacromial ligament. As the humerus elevates, it permits the rotator cuff to slide easily beneath the deltoid muscle. There are also smaller bursae interposed between most of the muscles in contact with the joint capsule:

The subcoracoid bursa. This bursa is located under the coracoid process.

The subscapular bursa. This bursa is located between the subscapular muscle tendon and the anterior neck of the scapula and protects the tendon as it passes under the coracoid process.

Neurology

The shoulder complex is embryologically derived from C5 to C8, except the A-C joint, which is derived from C4 (Fig. 16-8). The sympathetic nerve supply to the shoulder originates primarily in the thoracic region from T2 down as far as T8.⁴⁸

FIGURE 16-8 Neurological and vascular structures of the shoulder. (Reproduced, with permission, from Chapter 30. Shoulder and Axilla. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy*. New York, NY: McGraw-Hill; 2011.)

One study⁴⁹ attempted to determine the variability of the course and the pattern of these nerves and found that the peripheral nerves contributing to the anterior shoulder joint included the axillary (C5–6), subscapular (C5–6), and lateral pectoral (C5–6). The same study found that the nerves contributing articular branches to the posterior joint structures are the suprascapular nerve (C5–6) and small branches of the axillary nerve.⁴⁹ The pathways of these nerves are described in Chapter 3.

Other nerves innervate the muscles that act upon the shoulder. These include the long thoracic nerve (C5–7) (Fig. 16-8), which innervates the serratus anterior, the spinal accessory nerve (cranial nerve XI and C3–4), which innervates the sternocleidomastoid (SCM) and trapezius muscles, and the musculocutaneous nerve (C5–7) (Fig. 16-8), which innervates the coracobrachialis, biceps brachii, and brachialis, before dividing into its cutaneous branches.

Vascularization

The vascular supply to the shoulder complex is primarily provided by branches off the axillary artery (Fig. 16-9A), which begins at the outer border of the first rib as a continuation of the subclavian artery. The axillary artery is commonly divided into three parts: above, behind, and below the pectoralis minor muscle. The axillary artery meets the more deeply placed brachial plexus in the neck, and here they are encased in the axillary sheath, together with the axillary vein.⁵² The lateral, posterior, and medial cords of the brachial plexus descend behind the first portion of the artery and then take up their respective locations at the second portion of the artery (behind the pectoralis minor muscle). The branches of these cords also maintain their respective positions. Compression of the axillary artery (or, to a lesser degree, the axillary vein) can result in shoulder dysfunction and most commonly occurs in the posterior fossa around the shoulder and against the humerus with shoulder elevation.⁵² The G-H joint receives its blood supply from the anterior and posterior circumflex humeral as well as the suprascapular and circumflex scapular vessels (Fig. 16-9B-C).⁵³ The vascular supply to the labrum arises mostly from its peripheral attachment to the capsule and is from a combination of the suprascapular circumflex scapular branch of the subscapular and posterior humeral circumflex arteries (Fig. 16-9C).⁸

The brachial artery (Fig. 16-9A) provides the dominant arterial supply to each of the two heads of the biceps brachii. The artery travels in the medial intermuscular septum and is bordered by the biceps muscle anteriorly, the brachialis muscle medially, and the medial head of the triceps muscle posteriorly.⁵⁴

The microvasculature of the rotator cuff has been the subject of much discussion and consists of three main sources: the thoracoacromial, suprahumeral, and subscapular arteries.⁵³ The supraspinatus receives its primary supply from the thoracoacromial arteries. The subscapularis receives its supply from the anterior humeral circumflex (Fig. 16-9A) and the thoracoacromial arteries (Fig. 16-9A). The posterior rotator cuff muscles, the infraspinatus and teres minor, receive their blood supply from the posterior humeral circumflex and suprascapular artery (Fig. 16-9C). The circulation of the rotator cuff is unidirectional with no flow traversing the tide mark at the insertion of the supraspinatus.⁵⁵ The supraspinatus and biceps tendons appear to be particularly vulnerable to areas of relative avascularity, referred to as critical zones. The vascular compromise of the supraspinatus is thought to be due to a number of factors:

It can be directly compressed by the subacromial structures. With the arm adducted to the side, the vessels within the supraspinatus tendon are poorly perfused.⁵⁶ Other arm positions, such as raising the arm above 30 degrees, have been shown to increase intramuscular pressure in the supraspinatus muscle to an extent that may impair normal blood perfusion.^56,57

Its blood vessels travel parallel to the tendon fibers, which make them vulnerable to stretch.⁵⁸ Avascularity appears to increase with age beginning as early as 20 years.⁵⁹

The presence of a critical zone just proximal to the supraspinatus insertion point.⁵⁶ Two early studies noted a critical zone that lies slightly proximal to the supraspinatus insertion point.^60,61 Since then, it has been determined that the critical zone is more likely a zone of anastomoses between the vessels supplying the bone and the tendon and is not less vascular except in certain positions.^56,62,63

Although it is possible that sustained isometric contractions, prolonged adduction of the arm or increases in subacromial pressure⁶⁴ may reduce the microcirculation, it is unlikely that frequent abduction or elevation of the arm would produce selective avascularity of the supraspinatus or biceps tendon.

Close-Packed Position

The close-packed position for the G-H joint is 90 degrees of G-H abduction and full ER or full abduction and ER, depending on the source.

Open-Packed Position

Without IR or ER occurring, the open-packed position of the G-H joint has traditionally been cited as 55 degrees of semi-abduction and 30 degrees of horizontal adduction.⁶⁵ More recently, a cadaver study, which examined the point in the range at which maximal capsular laxity occurred in seven subjects, determined the open-packed position to be 39 degrees of abduction in the scapular plane or at the point which is 45% of the maximal available abduction range of motion (ROM).⁶⁶ This finding suggests that the open-packed position may be closer to neutral and that joint mobility testing and joint mobilizations of the G-H joint should be initiated according to a smaller angle of abduction than the traditionally cited open-packed position.⁶⁶

The zero position for the G-H joint is the arm relaxed by the side, which, relative to the scapula, averages approximately 0 degrees of abduction, 12 degrees of flexion, and 10 degrees of ER.⁶⁷

Capsular Pattern

According to Cyriax,⁶⁸ the capsular pattern for the G-H joint is that ER is the most limited, abduction the next most limited, and IR the least limited in a 3:2:1 ratio, respectively. However, this pattern only appears to be consistent with adhesive capsulitis of the shoulder. IR, rather than ER or abduction, appears to be the most limited motion in conditions with selected capsular hypomobility.⁶⁹

THE ACROMIOCLAVICULAR JOINT

The A-C joint is a synovial joint, formed by the medial margin of the acromion and the lateral end of the clavicle. The A-C joint is most often described as a gliding or plane joint, but the joint surfaces can vary from flat to slightly convex or concave, corresponding with the articulating surface of the acromion. In the early stages of development, the articular surfaces of the A-C joint are lined with hyaline cartilage, which changes to fibrocartilage at approximately 17 years of age on the acromial side of the joint, and until approximately 24 years of age on the clavicular side.⁷⁰ Within the A-C joint, there is variable presence of a thumbtack-shaped intra-articular fibrous disk that projects superiorly into, and incompletely divides, the A-C joint. This disk is subject to tearing.⁷⁰ When viewed from above, the clavicle is convex anteriorly in the medial two-thirds and convex posteriorly in the lateral one-third (see Fig. 16-1). Variability in the inclination of the joint is common and can be anywhere from 10 to 50 degrees, but the anteromedial border of the acromion usually faces anteriorly, medially, and superiorly.⁷¹ This variability may account for the differences in vulnerability to separation seen among people.⁷²

The A-C joint serves as the main articulation that suspends the upper extremity from the trunk, and it is at this joint about which the scapula moves. Specifically, the A-C joint provides the scapula with additional range of rotation on the thorax, which allows the scapula to adjust outside of its initial plane (posterior tipping and IR) to follow the changing shape of the thorax as movement occurs.^73,74 The clavicle serves as the lever by which the upper extremity acts on the torso and as an attachment site for many soft tissues.^70,75 The thin joint capsule is lined with synovium, which is strengthened anteriorly, posteriorly, inferiorly, and superiorly by A-C ligaments (see Fig. 16-6). The superior A-C ligament (see Fig. 16-6) gives support to the capsule and serves as the primary restraint to posterior translation and posterior axial rotation at the joint.⁷⁰ Other support structures for the joint include the costoclavicular, coracoclavicular (conoid and trapezoid) ligament, the pectoralis major, SCM, deltoid, and trapezius muscles (see Fig. 16-10).^75,76

FIGURE 16-10 Muscles of the shoulder. (Reproduced, with permission, from Chapter 1. Back. In: Morton DA, Foreman K, Albertine KH. eds. *The Big Picture: Gross Anatomy.* New York, NY: McGraw-Hill; 2011.)