While the majority of the orthopedic literature focuses on anterior glenohumeral instability, posterior shoulder instability can also lead to significant disability. The presentation and mechanism of injury for posterior shoulder instability is distinct from anterior pathology. Often, posterior glenohumeral instability may constitute a subluxation event as opposed to a frank dislocation. Additionally, patients often report a more repetitive, chronic injury pattern as opposed to a single traumatic event. Many patients will report only pain rather than distinct episodes of instability.

In general, the position of posterior instability occurs when the shoulder is forward flexed and internally rotated.¹ However, posterior instability may occur in athletes participating in a variety of sports and the mechanism is often specific to the sport. For example, overhead motions from baseball and racquet sports can result in posterior instability from the follow-through motion. The leading arm used for batting or golfing can also be subject to posterior shoulder instability. Weightlifting, specifically with activities such as bench press or push-ups, may result in posterior shoulder instability.^1,2 In football, offensive linemen are at particular risk due to blocking. In gymnastics, events such as the parallel bars and rings can put athletes at risk.^1,2 Finally, swimming is another sport in which athletes can develop posterior shoulder instability.

The presentation of posterior shoulder instability in athletes may present as either acute or chronic pathology. Both acute and chronic injuries can lead to either soft tissue (capsulolabral) deficiency and/or bony deficiency (glenoid and humeral defects). However, posterior instability is not limited to the athletic population. Traumatic injuries due to electrocution and seizure events have been linked to posterior shoulder dislocations. Finally, underlying soft tissue disorders, hyperlaxity, and dysplasia due to glenoid or humeral head retroversion can predispose patients to recurrent subluxation events.

Posterior shoulder instability represents a minority of glenohumeral instability cases—approximately 10% of all instability events—and posterior instability occurs at a reported rate of 1.1 to 4.64 per 100,000 person-years.^{2, 3 and 4} However, the true incidence may be higher given the often subtle presentation due to the more chronic, repetitive nature of the pathology compared with anterior instability. The highest incidence occurs in men between the ages of 20 and 49 years. However, there may be a bimodal distribution, with another peak in male patients over 70 years of age (Figure 40-1).² While posterior shoulder instability rates have been cited between 2% and 8% of all shoulder instability cases, the rates may be higher in certain high-risk populations such as athletes and military service members.^5,6 Given the overall lower prevalence, there are fewer studies that report the outcomes of recurrent posterior instability as compared with anterior instability, which makes evidence-based counseling with regard to management of posterior instability all the more challenging.

Reports of posterior shoulder dislocations date back to the 1700 and 1800s, but the earliest descriptions of surgical management of posterior shoulder dislocations were published in the mid- to late 1900s.⁴ Described techniques over this time period included the McLaughlin procedure, open staple capsulorrhaphy, the reverse Putti-Platt procedure, glenoid osteotomy, modified capsular shift, humeral rotational osteotomy, thermal capsulorrhaphy, labral repair, and intra-articular bone block procedures.⁴

Patients presenting with posterior shoulder instability may not report a distinct dislocation event, so obtaining a careful history and physical examination is paramount.^1,12 Patients may describe recurrent subluxation and symptoms such as pain, weakness, fatigue, clicking, and popping. Asking about the activities and specific positions that recreate the symptoms can also be helpful. For example, the patient’s pain may be anterior or posterior during the subluxation episodes. Additionally, patients should be asked about a history of seizure disorders and any risk factors that may predispose them to seizure activity such as known epilepsy, substance abuse, or diabetes.

For physical examination, as with any suspected shoulder pathology, the examination should begin with the evaluation of the cervical spine, evaluating range of motion and any signs of radiculopathy. Visual inspection may reveal any muscular asymmetries and a prominent coracoid may be appreciated on the affected side in the setting of an acute posterior dislocation. Scapulothoracic motion should be evaluated and compensatory scapular winging may be present in the setting of posterior instability. Scapular winging may be secondary to posterior instability or may be the primary cause. Range of motion should also be assessed. In the setting of an acute posterior dislocation, the shoulder may be locked with the arm in a position of increased internal rotation and limited external rotation.

In terms of provocative tests, the load and shift test, the Jerk test, and the Kim test can be helpful in the evaluation of posterior labral tears and posterior shoulder instability. The load and shift test can be performed for both anterior and posterior instability. The patient is placed supine on the examination table with the scapula stabilized on the table but with the glenohumeral joint free. With the arm placed in 40° to 60° of abduction and neutral rotation, the examiner loads the humeral head into the glenoid fossa and then applies an anterior force to the humerus (anterior load and shift) and subsequently a posterior force to the humerus (posterior load and shift) to assess the amount of translation (Figure 40-2). This should be compared with the contralateral side. Grading of humeral head translation can be categorized as 1+ for translation to the glenoid rim, 2+ for translation over the glenoid rim but with spontaneously reducibility, and 3+ for translation over the glenoid rim with locking (requires manual reduction). The Jerk test has been described with the patient either in a seated or in a lateral decubitus position. The examiner should use one hand to forward elevate the arm to 90°, rotate the arm to neutral, and maintain neutral abduction. The examiner should use the contralateral hand to stabilize the scapula. Then, an axial load is applied, loading the glenohumeral
joint, and the humerus is translated posteriorly. The test is considered positive if there is a visible or palpable subluxation and reduction event and/or if there is pain and a click associated with the maneuver. The Kim test is performed with the patient sitting upright. The examiner puts the arm in a position with 90° abduction and then forward elevates the shoulder to 45°. An axial load is then applied to the glenohumeral joint, and a posteroinferior force is placed on the humerus.¹³ These tests may have utility in being performed together, as the load and shift alone is only 14% sensitive for posterior instability, but the Kim test and Jerk tests together have a sensitivity of 97% for identifying posteroinferior labral tears.^1,13

Additionally, generalized laxity should be assessed in patients with shoulder instability. In addition to the aforementioned provocative tests, the sulcus sign and Beighton score should be determined to assess whether the patient has MDI as opposed to isolated posterior shoulder instability. While these tests do have utility in guiding clinical decision making in the outpatient setting, it is important to note that, if surgery is indicated, repeating the assessments of humeral head translation and laxity in the operating room with the patient under anesthesia may provide a more accurate metric.

Imaging for posterior shoulder instability should begin with plain radiographs followed by 3D imaging. Standard radiographic views including anteroposterior (AP), true glenoid AP (Grashey), scapular Y, and axillary views can allow for the assessment of glenohumeral joint reduction, glenoid morphology and dysplasia, bone loss, and presence of a reverse Hill-Sachs lesion. Specialized views such as the West Point and Stryker notch view can also be utilized but may be less helpful in the evaluation of posterior instability compared with anterior instability. If there is suspected or indeterminate bone loss, a computed tomography (CT) scan may help to more accurately assess bone loss.¹⁴ Additionally, CT may allow for assessment of other important posterior shoulder instability risk factors including glenoid retroversion,^6,15,16 defect morphology,¹⁷ and acromial morphology.¹⁸ In a retrospective study by Dominik et al, the authors found that anatomic factors such as greater posterior acromial height (>23 mm), greater posterior acromial tilt, and lower anterior and posterior acromial coverage angles were significantly associated with posterior instability.¹⁸ Magnetic resonance imaging (MRI) can allow for the evaluation of isolated labral tears and capsuloligamentous complex injuries (eg, reverse humeral avulsion of the inferior glenohumeral ligament) and can also allow for quantification of glenoid and/or humeral head bone loss.

The authors’ preferred technique for performing an open posterior capsular shift procedure is as follows. The patient is placed in a beach chair position and the arm placed in an arm holder. The procedure can also be performed from a lateral decubitus position.¹⁹ First, an examination under anesthesia is performed to assess range of motion, anterior and posterior translation with the load and shift test, and the sulcus sign. Next, a diagnostic arthroscopy is performed to evaluate the glenohumeral joint cartilage, to inspect the labrum for the presence of a tear, and to further evaluate capsular laxity. Specifically, arthroscopic inspection can be helpful for the evaluation of posterior labral tears as they are sometimes difficult to appreciate even with direct visualization from an open posterior approach.²⁰