Posterior instability—epidemiology, history, physical examination, imaging, and nonoperative treatment

Introduction and epidemiology

Posterior shoulder instability occurs on a spectrum ranging from painful posterior labral tears with minimal instability to symptomatic posterior subluxation, all the way to locked dislocation events. These phenomena may be either voluntary or involuntary. With regard to posterior dislocation, Sir Astley Cooper is credited with the first description in a Guy’s Hospital report from 1839. There he details a dislocation of the os humeri on the dorsum scapulae in a patient with an epileptic seizure disorder. He goes on to provide a clinical description, stating that the patient “could not elevate his arm from his side” and “the shoulder had an anterior void or flatness and a posterior fullness.”

With regard to the type of posterior instability, it is now known that posterior shoulder instability events are composed primarily of recurrent subluxation events, as opposed to the more acute dislocations, which account for only 3% to 8% of total events. ^, Complete posterior dislocations occur much less frequently than anterior dislocations, with the percentage of posterior shoulder dislocations accounting for 2% to 6% of all shoulder dislocations. ^, Posterior shoulder subluxations occur commonly in the collegiate athlete population, with particular at-risk sports including wrestling, rugby, and football, whereas posterior shoulder dislocations are classically described in the setting of a traumatic injury mechanism such as seizure, electrocution, or a fall onto an outstretched hand. Furthermore, it is also known that male athletes experience posterior instability twice as commonly as female athletes.

Recent accounts have suggested that these relative percentages of posterior instability may have increased in the young and active populations. For example, Song et al. suggest that posterior and combined instability patterns may account for 43% of all cases of shoulder instability. In this study, the authors performed a retrospective review of the surgical pathology of 231 patients (mean age, 26 years) indicated for “shoulder instability” based on history and physical examination. When classifying instability as anterior, posterior, or combined based on the intraoperative injury patterns, they found that 24.2% of patients had pathology consistent with isolated posterior instability, and 18.6% had injury patterns consistent with combined instability. This chapter explores the history, physical exam, imaging, and nonoperative treatment of posterior shoulder instability.

History

A thorough patient history is essential to the treatment of posterior instability patients. History and initial presentation may vary dramatically depending on whether the instability event is an acute dislocation versus chronic subluxation. For dislocation events, the patient may present with a description of a seizure episode, electrocution, or a discreet fall onto an outstretched hand with subsequent pain and inability to move the arm from an adducted, internally rotated position. On the contrary, patients with chronic posterior subluxations may present with shoulder pain alone in the absence of an identifiable mechanism. Indeed, knowledge of these topics is essential in the correct diagnosis and treatment of posterior instability, as up to 42% of posterior instability patients will present only with a painful shoulder. Given the high propensity for misdiagnosis, treatment for posterior instability is often delayed, which can lead to patient dissatisfaction in the setting of continued pain and weakness. Posterior shoulder instability ought to be ruled out in young athletes presenting with a complaint of shoulder pain. Patients with posterior subluxations often present with vague complaints of pain and general weakness rather than a frank description of instability or dislocation.

A complete history is essential for all patients and should be conducted in a highly consistent and reproducible manner. Given the propensity for this pathology to occur in young athletes, a complete sports history is essential, including position played and timing of season. For all patients, it should be documented in what arm position the patient experiences pain and instability (e.g., a lineman describing posterior shoulder pain when a posteriorly directed force is applied to the fully extended arm during a block). Given that the patient may have a baseline genetic propensity for posterior instability, contralateral shoulder injuries, history of laxity in other joints, and familial association with laxity must also be assessed. A history of overhead motion or activity should also be specifically solicited, as overhead athletes are at highest risk for recurrent posterior shoulder instability. With regard to posterior dislocation, a history of seizure disorder or previous electrocution should also be obtained. Finally, for patients with multiple instances of posterior instability, the exact circumstances and treatment for all prior instances should be discussed, including reduction efforts (both open and closed) and previous radiographic imaging. All previous treatments, both operative and nonoperative, should be detailed, including the timing and type of any surgical interventions.

Physical exam

The exam for patients with an acute dislocation begins with an inspection of the entire shoulder girdle. A skin exam should be performed to identify swelling or ecchymosis, which may suggest a recent dislocation and/or fracture. Close observation should be made regarding asymmetry compared with the contralateral side, which may also indicate a dislocation, nerve dysfunction, or atrophy. Patients with a dislocation may have flattening of the anterior shoulder with prominence of the coracoid, as well as a posterior prominence of the humeral head ( Fig. 34.1 ). ^, Next, a full neurovascular exam is performed. Although the incidence of neurovascular injury after anterior dislocation has been described, injury after posterior dislocation is limited primarily to case reports. One such report described a pediatric patient with an axillary artery injury and avulsion of the axillary nerve from the posterior cord after dislocation. Although rare, the consequences of missing such an injury are devastating, thus warranting a complete neurovascular exam.

The physical exam should then be tailored to the patient’s presentation. Classic findings of a locked posterior dislocation include significantly limited external rotation, abduction, and forward elevation. ^, These findings may be subtle and missed by providers, which can lead to a delay in diagnosis. Xu found a 73% rate of misdiagnosis in a meta-analysis of dislocations, and delay to diagnosis averaged 5.9 months. Often, posterior dislocations are misdiagnosed as adhesive capsulitis due to the associated decreased range of motion. In patients with missed chronic dislocations, the range of motion in abduction and external rotation may be improved relative to the initial presentation due to humeral head erosion from articulating with the posterior glenoid rim. After a dislocation has been reduced and adequate time has passed to permit a more thorough exam, attention must be paid to the rotator cuff. Although often a result of anterior dislocation in patients older than 40 years, full-thickness rotator cuff tears have been described in patients in the 20- to 40-year age group after posterior dislocation.

For patients presenting with symptomatic subluxations, both shoulders should be examined to determine a baseline from the contralateral shoulder. The basic exam includes inspection, palpation, range of motion, and provocative testing. Inspection for surgical scars, atrophy, and scapular winging begins the exam, and a neurovascular exam follows. Although range-of-motion testing is often normal, observing scapular and humeral motion during both forward elevation and adduction can reveal abnormal scapular mechanics. Patients may elicit scapular winging during forward elevation or adduction and while returning to neutral to prevent posterior subluxation of the humerus. Scapular dyskinesia may be both a causative and resultant adaptation in posterior instability. In some patients, palpation of the shoulder may elicit posterior glenohumeral tenderness. Rotator cuff testing, although often normal, should focus on the integrity of the subscapularis because this is the primary dynamic stabilizer of posterior translation. ^, The biceps anchor may also be involved and therefore deserves attention when performing the exam. Specifically, type VIII and type X superior labrum anterior and posterior (SLAP) tears based on the Maffet subclassification of Snyder are those that involve the posterior labrum. We typically perform a Speed and Yergason test to evaluate for bicep pathology. It should be noted that false-positives for these tests have been reported by some groups, notably in the setting of concomitant shoulder pathology. ^, A report from Kibler et al. has shown the modified dynamic labral shear test may have the highest specificity and sensitivity for detecting SLAP tears, confirmed by arthroscopy. This test is performed with the arm in 120-degree horizontal abduction with maximal external rotation. The arm is then maximally abducted, then lowered to 60 degrees of abduction while imparting an anterior-directed force on the humerus. Pain and/or clicking is indicative of a positive test, which simulates the peel-back mechanism that is thought to lead to shearing of the bicep anchor. This study did not describe the incidence of concomitant posterior instability in their cohort. The diagnosis by physical exam of a concomitant SLAP tear with posterior instability can be challenging.

Next, a determination of shoulder laxity should be performed. The load and shift test is used to identify anteroposterior (AP) laxity and compared with the contralateral shoulder. It is accomplished by positioning the shoulder in 30 degrees of both abduction and forward elevation. The patient’s forearm is supported in one hand, and the examiner’s other hand is used to compress the glenohumeral joint while imparting a posterior-directed force through the humerus. It should be noted that more than 50% of Division I college athletes were noted to have asymptomatic grade 2+ posterior laxity, thus emphasizing the importance of making the distinction between baseline laxity and instability. Similarly, in a group of asymptomatic adolescents, Emery and Mullaji showed that more than 50% had clinical signs of laxity, primarily with posterior drawer testing. Originally, the posterior drawer test was described by Gerber in 1984. With the patient supine, the arm is abducted to 90 degrees and forward elevated to 30 degrees. The examiner places the thumb of his or her contralateral hand on the humeral head and translates the head posteriorly as the arm is brought into adduction and elevation. This maneuver may cause subluxation of the humeral head and, when accompanied with patient apprehension, can be a sign of instability. This test has mostly been replaced with distinct provocative tests for posterior instability, with the majority of them placing the shoulder in internal rotation, forward elevation, and adduction. This maneuver places tension on the posterior capsular structures. The posterior apprehension test is performed by placing the shoulder in 90 degrees of forward elevation, adduction, and 30 degrees of internal rotation, which should elicit pain. The jerk test is performed in 90 degrees of forward elevation, abduction, and internal rotation. The scapula is stabilized, and the humerus is moved across the body horizontally while imparting a posterior-directed load on the glenohumeral joint. A painful clunk is elicited, which is the humeral head reducing after subluxation, or even dislocation occurs. A jerk test that elicits pain is highly suggestive of failing nonoperative treatment, whereas a painless test is associated with higher success rates of nonoperative treatment ( Fig. 34.2 ). The Kim test loads the posterior-inferior labrum. It is performed in a similar starting position of forward elevation, abduction, and internal rotation; however, it differs in that the examiner uses one hand to elevate the affected arm diagonally 45 degrees in the superior and adduction directions while imparting an axial load. The other hand is used to pull the humerus inferiorly and posteriorly ( Fig. 34.3 ). A painful test is deemed positive, regardless of the presence of a clunk. Kim reported greater than 80% sensitivity and specificity for this test in the diagnosis of posterior-inferior labral tears. Combined, a positive Kim and jerk test neared 100% sensitivity for posterior shoulder instability, with arthroscopic evaluation as the gold standard. Table 34.1 lists the published sensitivities and specificities for several physical exam findings.

TABLE 34.1

Physical Exam Tests With Associated Sensitivity and Sensitivity for Posterior Instability

Exam	Sensitivity (%)	Specificity (%)
Painful jerk	89.7	85
Kim	80	94
Combined Kim and jerk	97	—

Patients with multidirectional instability (MDI) may also share positive exam findings seen in posterior instability or even elicit symptomatic posterior instability. Increased glenohumeral translation in all directions may be noted on laxity testing for patients with MDI. An examination for generalized ligamentous laxity should also be performed. One such method to accomplish this is by calculating the Beighton score. Positive findings of generalized laxity may lead to adjustments in treatment and/or rehabilitation protocols, most notably if MDI is diagnosed. The pathology and treatment of MDI are distinct from isolated posterior instability and are discussed in Chapter 37 . Voluntary positional posterior shoulder dislocation can be elicited during the history and physical exam and is often performed by the patient with the shoulder in a forward-elevated position with asymmetric muscle contraction ( ). This finding is important to note because it differs from patients with habitual dislocations. Patients with habitual dislocations are recommended to undergo psychologic treatment and rehabilitation and are often managed nonoperatively. Patients with positional dislocations can benefit from surgical intervention. ^,

Imaging

In the setting of a posterior dislocation, appropriate imaging begins with the three-view trauma series consisting of a true AP, scapular Y, and axillary lateral view. Imaging findings on the AP may be subtle in posterior dislocations. Because the humerus internally rotates when dislocated, the head faces posteriorly. When viewed on the AP, the cortical rim of the humeral head becomes outlined such that it resembles a lightbulb, the so-called “lightbulb sign” ( Fig. 34.4 ). Loss of the normal overlap between the glenoid and the humeral head may be seen due to internal rotation and subsequent lateralization of the humeral head on an oblique AP image, which has been termed “loss of the half-moon sign.” A locked dislocation may have abnormal increased overlap between the head and the glenoid rim on a true AP, described as the “crescent sign” ( Fig. 34.5 ). Humeral head impaction may occur and has been described as the “trough line.” The normal relationship of the inferior arc connecting the glenoid and the inferior humeral neck, deemed “Moloney line,” is often disrupted ( Fig. 34.6 ). Despite these signs, obtaining an isolated AP view resulted in a missed diagnosis rate of nearly 90% in one meta-analysis of patients with posterior dislocation.