Posterior shoulder instability represents an array of disorders, ranging all the way from symptomatic subluxation (symptomatic, excessive translation of the humeral head on the glenoid) to multiple frank dislocation events. Posterior shoulder instability events are primarily subluxation events in nature, with complete dislocations comprising only 3% to 8%. , Posterior dislocations are significantly less common than anterior, with the percentage of posterior events accounting for 2% to 6% of all shoulder dislocation events. ,
With advances in imaging and arthroscopy, the diagnosis of posterior shoulder instability has improved. In addition, there are some recent reports suggesting an increase in the prevalence of posterior shoulder instability based upon arthroscopic findings in surgical shoulder instability cohorts. However, the presence of a posterior labral tear does not necessarily denote posterior instability, as 36% of patients with instability are noted to have combined labral lesions at arthroscopy. Robinson reported the largest known series of posterior dislocations findings. A recent report on posterior instability in a collegiate athlete population showed that intercollegiate athletes were at highest risk, and those that participated in wrestling, rugby, and football had the highest rates of instability. Male athletes experience posterior instability at twice the rate of female athletes. As will be discussed, a complete history and physical examination are essential in the diagnosis of posterior instability, as 42% of posterior “instability” patients do not report instability symptoms, but rather pain only.
Posterior shoulder instability has become an increasingly appreciated shoulder condition. Effective treatment of posterior instability patients is often delayed by misdiagnosis, which can lead to continued patient dissatisfaction or prolonged feelings of pain and weakness. Posterior instability is more difficult to diagnose than anterior instability owing to a wide spectrum of nonspecific clinical complaints. Patients will often present with vague symptoms such as pain, muscle weakness, or mechanical symptoms. A delay in recognition of posterior instability can complicate the timing of necessary surgical treatment. To mitigate this delay in treatment, clinicians must use a combination of high suspicion, detailed histories, complete physical examinations, and appropriate diagnostic imaging.
A thorough history and high level of suspicion are imperative for accurate diagnosis of posterior instability. Although both recurrent dislocations and subluxations both fall under the spectrum of instability, it is important to differentiate between the presentation of the two disorders when evaluating a patient’s history. Recurrent posterior subluxations often present with clinical complaints of pain and general weakness rather than frank instability, which increases the difficulty of making a proper diagnosis. Indeed, pain may be the only complaint. As previously noted, young, athletic populations who participate in sports with posterior loading activities or dynamic overhead motion are at highest risk for recurrent posterior shoulder instability. Overuse and repetitive microtrauma often cause instability and pain in overhead throwers, contact athletes like football or rugby players, and weight lifters.
A comprehensive physical examination and select diagnostic imaging are essential to further promote diagnostic accuracy of posterior shoulder instability. Ensuring the correct diagnosis will allow for a more precise treatment plan and ultimate surgical intervention. In a systematic review by Xu et al., it was determined that most of the cases that had delayed treatment were improperly diagnosed as “frozen shoulder” or “shoulder sprain.” Differentials for vague shoulder pain because of posterior instability can include disorders such as rotator cuff tears, frozen shoulder, nerve entrapment, osteoarthrosis, congenital laxity, and subacromial impingement; however, with appropriate provocative testing on examination, a clinician should be able to make a more accurate diagnosis. Owing to the potential subtle variations in clinical presentation, bilateral shoulder examinations can be helpful for clinicians to clarify differences in symptoms between each side, including asymmetry, muscle atrophy, scapular winging, and tracking. Tenderness to the posterior glenohumeral joint line and normal, symmetrical range of motion (ROM) are often found on physical examination, with some exceptions of cases with increased external rotation and decreased internal rotation. Specific posterior instability testing such as the load-and-shift test, the posterior apprehension test, the Kim test, and the jerk test can identify degree and direction of joint instability and should be included to ensure an accurate diagnosis. In particular, the Kim test (patient’s humerus is abducted and moved posteriorly with a posterior directed force) has been shown to be one of the most sensitive (80%) and specific (94%) tests.
Recurrent posterior dislocation is less common than recurrent posterior subluxation. Patients presenting with this condition often present with their shoulder held in a position that is adducted and internally rotated. Xu et al. found that the misdiagnosis rate of frank posterior dislocations is 73.2%, with an average 5.88-month delay in treatment. Failure to identify acute posterior dislocations has been associated with adverse conditions such as avascular necrosis of the humeral head or degenerative osteoarthritis, recurrent dislocations, and even permanent disability. Therefore early, accurate diagnosis of posterior shoulder dislocations is crucial and requires a high level of suspicion, especially in patients presenting with shoulder pain after seizure, electrocution, direct trauma with internal rotation and forced adduction, or a fall on an extended and internally rotated arm.
Technological advances in diagnostic imaging have made it easier for clinicians to identify classic signs of posterior dislocation or subluxation events in the context of high suspicion and a detailed physical examination. Shoulder radiographs should, at a minimum, include a true anteroposterior view, internal and external rotation views, a scapular Y-view, and an axillary view. X-rays should be performed any time there is suspected shoulder pathology or loss of external rotation of shoulder, particularly following an electrocution or seizure event. True anteroposterior views by themselves are often read as normal. However, close review of the anteroposterior radiograph will show a “lightbulb” sign with subtle overlap of the humeral head and glenoid ( Fig. 27.1A ). The axillary lateral view provides further information regarding the degree and direction of glenohumeral instability and allows clinicians to visualize reverse Hill–Sachs lesions, glenoid dysplasia, or glenoid bone loss ( Fig. 27.1B ). Bony Bankart lesions, located on the posterior glenoid rim, can be seen on the West Point axillary view. Xu et al. confirm that axillary or Y-view radiographs, as well as computed tomography (CT), substantiates diagnosis of initial posterior shoulder dislocations, reporting correct diagnosis at initial radiographic assessment in 100% of cases.
In addition to radiographs, it is useful to order magnetic resonance imaging to evaluate the condition of the capsule, posterior labrum, and other associated soft tissue structures in the shoulder that may affect surgical treatment. However, recent studies have shown that magnetic resonance arthrograms (MRAs), which include the addition of intraarticular contrast dye, can better assist clinicians in identifying crucial characteristics of symptomatic posterior shoulder instability, such as increased glenoid retroversion, glenoid dysplasia, and posterior capsular area. MRA axial views are particularly helpful to identify labral tears (which may be commonly missed without intraarticular dye), humeral head subluxations, and reverse Hill–Sachs lesions, whereas coronal views can help rule out posterior reverse humeral avulsions of the glenohumeral ligament.
Ultimately, if glenoid retroversion or dysplasia is suspected, a three-dimensional CT scan can be performed to analyze the anatomy of articular surfaces in the shoulder. CT scans can be used to determine the dimensions and location lesions such as Hill–Sachs, reverse Bankart, or bony Bankart, as well as to define the amount of posterior glenoid bone loss. Proper identification of the osseous structures and abnormalities may drive the selection of appropriate surgical interventions, therefore it is imperative to review imaging thoroughly. Although a detailed description of posterior instability surgical intervention is beyond the scope of this chapter, some authors advocate for a bony procedure (glenoid osteotomy) along with a combined soft tissue instability procedure when excess glenoid retroversion (20 degrees) is encountered.
Physical therapy is often the first line of treatment for posterior instability to improve rotator cuff and scapular stabilizer strength, but it is not always indicated. Relative contraindications to initial nonoperative management include significant bony pathology/glenoid bone loss and labral tears in high-level overhead athletes following a single instability event. Similarly, patients with multiple instances of instability events may require operative intervention. Chronic instability symptoms have the potential to keep young athletes from competing in their respective sports, and it has been reported that surgical treatment may be necessary to return them to their normal high-functioning activity level.
Surgical treatment of posterior shoulder instability can also be largely complicated by poor patient selection. Although recurrent instability after surgery can happen to any patient who reinjures his or her shoulder, clinicians should use a preoperative evaluation of their patients to identify patients who might not do well after surgery. Studies have reported that poor operative outcomes may occur in patients with multidirectional instability, patients with worker’s compensation claims, patients with previously failed repairs, and habitual or voluntary dislocators. Managing patient expectations is essential before any surgical intervention. Making a diagnosis of multidirectional instability is essential because these patients tend to do poorly compared to true posterior instability patients.
It is important to recognize positional and voluntary dislocators because their shoulder instability may be under conscious control, and these patients may present with psychological problems. Voluntary instability was first described by Carter Rowe and colleagues in 1973, who reported that these patients do not readily comply with standard treatment because of their psychological propensity to subluxate their shoulders. However, it should also be noted that study was performed with only 26 patients. In these cases, patients will develop dislocation and recurrent instability for secondary gains, which may include seeking attention or obtaining medications. A 1992 study by Hurley et al. reported that surgery is not indicated for patients with continuous, habitual voluntary instability because of a high failure rate. It should be noted that voluntary dislocators should be differentiated from positional dislocators, as positional dislocation patients subluxate or dislocate by changing the position of their scapula and may benefit from stabilization procedures. Poor postoperative outcomes in these patients can be avoided by proper preoperative evaluation and selection to avoid surgical intervention.
As shown in Table 27.1 , there are a number of described surgical interventions for posterior shoulder instability. As previously discussed, it is imperative to rely on preoperative imaging, history, and physical examination before indicating patients for any of these procedures. Although an arthroscopic posterior labral repair is commonly indicated in the presence of an isolated labral tear, it is equally important to recognize when significant bone loss may require the use of lesser tuberosity transfer (modified McLaughlin’s procedure in setting of engaging reverse Hill–Sachs lesion) or glenoid bone block/allograft (significant posterior glenoid loss). In effect, the bony pathology must also be addressed with engaging lesions to produce the best patient outcomes.