STERNOCLAVICULAR JOINT

The sternoclavicular (SC) joint is a spheroidal joint between the medial end of the clavicle and both the manubrium and the first costal cartilage. An intraarticular fibrocartilaginous disk stabilizes the joint and prevents medial displacement of the clavicle. The joint capsule is reinforced by the anterior and posterior SC ligaments.

ACROMIOCLAVICULAR JOINT

The acromioclavicular (AC) joint is a spheroidal joint between the lateral end of the clavicle and the acromion process of the scapula (Figure 2-1). A small, intraarticular fibro cartilaginous disk divides the joint into two compartments. A subcutaneous, noncommunicating bursa may be present over the joint. The stability of the AC joint depends on the capsule and the superior and inferior AC ligaments. The coracoclavicular ligament (conoid and trapezoid parts) extends between the distal clavicle and the coracoid process of the scapula (Figure 2-2). It suspends the scapula, stabilizes both the clavicle and the scapula, and maintains a close relation between the two bones during shoulder movements, thus limiting scapular rotation around the AC joint. The AC and SC joints augment the range of shoulder movements, particularly abduction and rotation. The joints also allow slight axial rotation of the clavicle, as well as elevation/depression and forward/backward thrusting of the shoulder.

FIGURE 2-1 THE SHOULDER.

FIGURE 2-2 THE SHOULDER (SYNOVIAL MEMBRANE AND OUTPOUCHINGS).

GLENOHUMERAL JOINT

The glenohumeral (GH) joint, the main articulation of the shoulder complex, is a multiaxial, ball-and-socket synovial articulation between the glenoid fossa of the scapula and the humeral head (Figure 2-1). The lax articular capsule and the small area of contact between the shallow glenoid fossa and the spheroidal humeral head permit a wide range of motion. The stability of the joint depends on a number of static and dynamic stabilizers. Static stabilizers include negative intraarticular pressure; GH bone geometry; the capsule; the glenoid labrum; the superior, middle, and inferior GH ligaments; and the coracohumeral ligament. The capsule, which fuses in part with the tendons of the rotator cuff, has two apertures: one for the long biceps tendon (origin from the supraglenoid tubercle) and one for the subscapularis bursa. The labrum, a ring of fibrocartilage that surrounds and deepens the glenoid cavity, contributes significantly to GH joint stability. Through a bumper effect, it functions as a “chock block” to prevent translational forces.

The inferior GH ligament complex is the primary ligamentous stabilizer of the abducted GH joint and serves to prevent anteroinferior shoulder dislocation. The middle GH ligament is tensioned at 45° of abduction, and the superior GH ligament is tight in adduction.

Dynamic stabilizers play an important role in the stability of the shoulder. They include two musculotendinous layers: 1) an inner stratum, made of the rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) and the long biceps tendon (origin from supraglenoid tubercle from glenoid fossa), and 2) an outer stratum, composed of the deltoid, teres major, pectoralis major, latissimus dorsi, and trapezius muscles.

The muscles of the inner stratum stabilize and retain the humeral head in the glenoid cavity during shoulder movements (cavity-compression mechanism), while simultaneously providing abduction (supraspinatus—origin from the supraspinatus fossa of scapula and insertion into the superior part of the greater tuberosity), external rotation (infraspinatus and teres minor—origin from the infraspinatus fossa and axillary border of the scapula, respectively, and insertion into the posterior aspect of the greater tuberosity), and internal rotation (subscapularis—origin from the subscapularis fossa and insertion into the lesser tuberosity). At the initiation of shoulder abduction, both the rotator cuff and the long biceps tendon depress and stabilize the humeral head against the glenoid cavity to counteract the upward pull of the more powerful deltoid muscle. The mechanism whereby these two groups of muscles combine to produce abduction, the one (deltoid muscle) elevating and the other (rotator cuff and biceps tendons) stabilizing the humeral head, is termed force-coupling. The muscles of the outer stratum are the prime movers of the shoulder. These provide abduction, flexion, extension, adduction, and some degree of rotation.

The coracoacromial arch—made up of the coracoid process, coracoacromial ligament, and acromion—acts as a protective, secondary socket for the humeral head, under which the rotator cuff tendons and long biceps tendon glide, with the subacromial bursa lying in between. The arch prevents upward displacement of the humeral head and protects the head and rotator cuff from direct trauma. The undersurface of the acromion is commonly flat (type 1); less frequently, it is downwardly curved (type 2) or hooked (type 3), but these conditions are more commonly associated with subacromial impingement.

The synovium of the shoulder lines the inner surface of the capsule. It has two extracapsular outpouchings, the tenosynovial sheath of the long biceps tendon and the bursa beneath the subscapularis tendon (Figure 2-2). A communicating infraspinatus bursa is sometimes present. The subcoracoid bursa lies between the shoulder capsule and the coracoid process, but it rarely communicates with the joint.

SCAPULOTHORACIC MOVEMENTS

The so-called scapulothoracic articulation is not a true joint but functions as an integral part of the shoulder complex. The scapula, which is connected to the posterior aspect of the chest wall by the axioappendicular muscles, provides the origin for the rotator cuff muscles and deltoid, and the trapezius inserts into its superior aspect. Scapulothoracic movements that include rotation, elevation, depression, protrusion, retraction, and circumduction are important for the normal functioning of the shoulder. The scapulothoracic bursa is located between the serratus anterior and the chest wall, just medial to the inferior angle of the scapula.

NERVE SUPPLY TO THE SHOULDER JOINT

The shoulder joint derives its nerve supply from three branches of the brachial plexus: suprascapular, axillary, and lateral pectoral nerves (C5/C6). The axillary nerve and the posterior circumflex humeral artery pass through the quadrilateral or quadrangular space, which lies inferoposterior to the GH joint, bounded by the teres minor superiorly, the teres major inferiorly, the long head of triceps medially, and the shaft of the humerus laterally (Figure 2-3).

FIGURE 2-3 ARRANGEMENT OF THE BRACHIAL PLEXUS AND ITS TRUNKS, CORDS, AND TERMINAL BRANCHES.

(From Rockwood CA, Matsen FA, Wirth MA, et al., eds.:The Shoulder, 4th ed. Philadelphia: Saunders, 2009.)

SHOULDER PAIN AND HISTORY TAKING

Shoulder pain is a common symptom of diverse causes (Table 2-1). The pain may originate in the GH or AC joint or in periarticular structures, or it may be referred from the cervical spine, brachial plexus, thoracic outlet, or infradiaphragmatic structures. Important points in the history include age, hand dominance, occupational and sport activities involving heavy lifting or overhead repetitive movements, history of trauma, onset, location, character, duration, radiation of the shoulder pain, aggravating and relieving factors, presence of night pain, and the effect on shoulder function. Associated symptoms—shoulder stiffness, restriction of movement, grinding, clicking, instability, or weakness—may also provide useful diagnostic clues.

TABLE 2-1 DIFFERENTIAL DIAGNOSIS OF SHOULDER PAIN

AC, acromioclavicular; GH, glenohumeral; OA, osteoarthritis; PsA, psoriatic arthritis; RA, rheumatoid arthritis

It is also important to determine whether the shoulder pain is isolated or associated with other stiff, painful, or swollen joints. Shoulder pain may be a feature of a more systemic arthritis. Other joint history, and a history of systemic features, may need to be taken into consideration.

ROTATOR CUFF PATHOLOGY

The spectrum of rotator cuff pathology ranges from mild rotator cuff tendinopathy to partial and complete rotator cuff tears. If the tear increases in size, a massive rotator cuff tear (< 5 cm) may develop. This can lead to the proximal migration of the humeral head and secondary GH osteoarthritis (cuff tear arthropathy).

Causative factors include repetitive low-grade trauma or unaccustomed activities, excessive overhead use in sport or work, lack of conditioning, aging, and compromise of the rotator cuff space by osteophytes on the undersurface of the AC joint, type 2 or 3 acromion, or an os acromiale (unfused acromial epiphysis). Abnormal tensile stresses that exceed the elastic limits of tendons can lead to cumulative microfailure of the molecular links between tendon fibrils, called fibrillar creep. With aging, tendons become less flexible and less elastic, making them more susceptible to injury and tears. A short-ended musculotendinous unit, from lack of regular stretching exercises, is also prone to injury.

In young persons, rotator cuff tendinopathy is often caused by a sport-related injury; for example, from use of the arm in an overhead position in baseball, racquetball, tennis, or swimming. In older individuals, an antecedent history of repetitive movements above the shoulder level or of strenuous or unaccustomed arm activity is common. Symptoms include aching pain in the shoulder, lateral aspect of the upper arm, and deltoid insertion; pain with movement, particularly abduction and internal rotation; night pain when rolling onto the affected side; restriction of shoulder movements; and sometimes weakness caused by a rotator cuff tear. The patient typically experiences shoulder pain on active abduction, especially between 60° and 120°, and difficulty with overhead work, lifting, or reaching behind the back when dressing. Clinical findings include a painful arc between 60° to 120° of abduction, limitation of active movement by pain, and tenderness localized to the rotator cuff and greater tuberosity. The supraspinatus test, Neer impingement test, Neer impingement sign, and Hawkins impingement sign (see Special Tests of Shoulder, p. 13) are often positive.

Rotator cuff tears can be partial or complete, acute or chronic, small or massive. In young adults, acute tears often result from direct trauma or a sport-related injury. In older patients, minor trauma, superimposed on cuff tendon that is already frayed from chronic impingement and age-related attritional changes, can lead to tears.

Clinical features include shoulder pain on abduction, night pain, varying degrees of weakness of abduction and external rotation, local tenderness, wasting of the supraspinatus and/or infraspinatus muscles, and loss of range of motion with difficulty elevating the arm to greater than 90° without shrugging the shoulder (positive shrug sign). The supraspinatus test, Neer impingement sign, and Hawkins impingement sign are usually positive. Rupture of the long biceps tendon may also be present. In complete tears, the drop-arm sign is positive. The diagnosis of rotator cuff tears can be confirmed by ultrasonography, magnetic resonance imaging (MRI), or arthroscopy.

BICIPITAL TENDINITIS

Bicipital tendinitis often results from chronic subacromial impingement occurring in association with rotator cuff tendinitis and rotator cuff tears. Primary isolated bicipital tendinitis is rare and develops as an overuse injury resulting from repetitive stresses applied to the tendon in certain sports, such as weight lifting and ball throwing. Anterior shoulder pain that is increased by overhead activities, shoulder extension, and elbow flexion is the main symptom. There is localized tenderness over the tendon in the bicipital groove, the Yergeson sign (see Tests for Biceps Tendon, p. 16) is present, and the speed test is often positive. Passive extension of the shoulder or resisted flexion of the elbow may also reproduce the pain. Signs of chronic impingement and GH instability are often present. Rupture of the long biceps tendon is associated with a positive Popeye sign.

Subluxation of the bicipital tendon is caused by traumatic rupture of the intertubercular (transverse humeral) ligament. It is associated with anterior shoulder pain, a clicking sensation of the shoulder as it “goes out and pops back in,” tenderness in the bicipital groove, and a positive transverse humeral ligament test.

ADHESIVE CAPSULITIS

Adhesive capsulitis, also known as frozen shoulder, is characterized by progressive global restriction of shoulder movements and is associated with pain and functional disability. A period of immobility of the shoulder is the most common predisposing factor. The capsulitis may be secondary to shoulder trauma, rotator cuff tendinitis or tears, bicipital tendinitis, or GH arthritis, or it may coexist with diabetes mellitus, hypothyroidism, or cerebrovascular events. An initial synovitis phase is followed by fibrous thickening and contracture of the capsular folds, axillary recess, rotator cuff interval, and coracohumeral ligament. The shortening of the coracohumeral ligament and rotator cuff interval acts as a tight checkrein, limiting external rotation. Capsular adhesions are rare.

The clinical course can be divided into four overlapping stages. In stage I, there is painful limitation of active and passive shoulder movements with diffuse synovitis on both arthroscopy and biopsy. Stage II is a painful “freezing” phase; shoulder pain, tenderness, and progressive, painful, global restriction of movements are present, as well as characteristic limitation of external rotation in the absence of GH arthritis. Synovial inflammation and a tight, thickened capsule are observed on both arthroscopy and biopsy. In stage III, an adhesive or “frozen” phase, there is minimal pain; movements are markedly restricted, and the patient is unable to elevate the arm to 90° without shrugging the shoulder (positive shrug sign). Disuse atrophy of the deltoid and scapular muscles is common. A thickened, contracted capsule and fibrotic synovitis are observed on both arthroscopy and biopsy. In stage IV, a resolution or “thawing” phase, pain is minimal with an increasing range of motion.