Definitions

Shoulder instability represents a spectrum of disorders ranging from shoulder subluxation, in which the humeral head partially slips out of the glenoid fossa, to shoulder dislocation, which is a complete displacement of the humeral head out of the glenoid. It is classified as anterior, posterior, or multidirectional and on the basis of its frequency, etiology, direction, and degree. Instability can result from macrotrauma, such as shoulder dislocation, or repetitive microtrauma associated with overhead activity, and it can occur without trauma in individuals with generalized ligamentous laxity [1–4].

The glenohumeral joint has a high degree of mobility at the expense of stability. Static and dynamic restraints combine to maintain the shoulder in place with overhead activity. Muscle action, particularly of the rotator cuff and scapular stabilizers, is important in maintaining joint congruity in midranges of motion. Static stabilizers, such as the glenohumeral ligaments, the joint capsule, and the glenoid labrum, are important for stability in the extremes of motion [2].

Traumatic damage to the shoulder capsule, the glenohumeral ligaments, and the inferior labrum is a result of acute dislocation. Repeated capsular stretch, rotator cuff, and superior labral injuries are associated with overuse injury resulting in anterior instability in athletes who participate in overhead sports; a loose patulous capsule is the primary pathologic change with multidirectional instability, and patients may present with bilateral symptoms [4–6]. Shoulder instability affects, in particular, young individuals, females, and athletes, but it may also affect sedentary individuals, with an incidence of 1.7% in the general population [1,7,8].

Traumatic instability often occurs when the individual usually falls on an outstretched, externally rotated, and abducted arm with a resulting anterior dislocation. A blow to the posterior aspect of the externally rotated and abducted arm can also result in anterior dislocation. Posterior dislocation usually results from a fall on the forward flexed and adducted arm or by a direct blow in the posterior direction when the arm is above the shoulder [5].

Recurrent shoulder instability after a traumatic dislocation is common, particularly when the initial event happens at a young age. In these individuals, it may occur repeatedly in association with overhead activity, and it may even happen at night, while changing position in bed, in those with severe instability. The patients may initially require visits to the emergency department or reduction of recurrent dislocation by a team physician; but as the condition becomes more chronic, some may be able to reduce their own dislocations [9,10].

Patients with neurologic problems such as stroke, brachial plexus injury, and severe myopathies may develop shoulder girdle muscle weakness, scapular dysfunction, and resultant shoulder instability.

Symptoms

With atraumatic instability or subluxation, it may be difficult to identify an initial precipitating event. Usually, symptoms result from repetitive activity that places great demands on the dynamic and static stabilizers of the glenohumeral joint, leading to increased translation of the humeral head in overhead sports and occupational activities. Pain is the initial symptom, usually associated with impingement of the rotator cuff under the coracoacromial arch. Patients may also report that the shoulder slips out of the joint or that the arm goes “dead,” and they may report weakness associated with overhead activity [1–3,9–11].

Patients with neurologic injury present with pain with motion and shoulder subluxation as well as scapular and shoulder girdle muscle weakness. In the case of a patient with acute shoulder injury, factors to identify include the patient’s age, dexterity or dominant side, sports and position, activity level, mechanism of injury, and any associated symptoms such as neurologic or functional deficits. In cases of acute dislocation, the age at first dislocation is a prognostic indicator in view of a recurrence of 75% to 100%, especially in skeletally immature individuals [11].

Physical Examination

The shoulder is inspected for deformity, atrophy of surrounding muscles, asymmetry, and scapular winging. Individuals are observed from the anterior, lateral, and posterior positions with the shoulder in neutral position on the side of the body as well as with flexion and abduction motion. Palpation of soft tissue and bone is systematically addressed and includes the four joints that compose the shoulder complex (sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic), rotator cuff, biceps tendon, and subacromial region.

Passive and active range of motion is evaluated. Differences between passive and active motion may be secondary to pain, weakness, or neurologic damage. Repeated throwing may lead to an increase in measured external rotation accompanied by a reduction in internal rotation; tennis players may present with an isolated glenohumeral internal rotation deficit [12]. These changes may be secondary to posterior capsule tightness, humeral torsion, and glenohumeral laxity that may lead to internal impingement [12].

Manual strength testing is performed to identify weakness of specific muscles of the rotator cuff and the scapular stabilizers. The supraspinatus muscle can be tested in the scapular plane with internal rotation or external rotation of the shoulder, and the external rotators are tested with the arm at the side of the body. The subscapularis muscle can be tested by the lift-off test, in which the palm of the hand is lifted away from the lower back (Fig. 14.1). The scapular stabilizers, such as the serratus anterior and the rhomboid muscles, can be tested in isolation or by doing wall pushups. Sensory examination of the shoulder girdle is performed to rule out nerve injuries.

Testing the shoulder in the position of 90 degrees of forward flexion with internal rotation (Hawkins maneuver) or in extreme forward flexion (until 180 degrees) with the forearm pronated (Neer maneuver) can assess for rotator cuff impingement and may reproduce symptoms of pain [10] (Fig. 14.2). Glenohumeral translation testing for ligamentous laxity or symptomatic instability should be documented. Apprehension testing can be performed with the patient sitting, standing, or in the supine position. The shoulder is stressed anteriorly in the position of 90 degrees of abduction and external rotation to reproduce the feeling that the shoulder is coming out of the joint (apprehension test). A relocation maneuver that reduces the symptoms of instability also aids in the diagnosis but appears to be less specific than apprehension testing [11] (Fig. 14.3). The causation of posterior shoulder pain (rather than symptoms of instability) with apprehension testing may be associated with internal impingement of the rotator cuff and posterior superior labrum [13–19] (Fig. 14.4).