Summary Posterior shoulder instability and dislocations are less common than anterior shoulder instability and dislocations, but are much more commonly missed. Diagnosis is made radiographically in the setting of acute dislocations. Chronic instability can be diagnosed with presence of positive posterior instability provocative tests and confirmed with MRI studies showing posterior labral pathology. Treatment may be nonoperative or operative depending on chronicity of symptoms, recurrence of instability, and the severity of labrum and/or glenoid defects. Epidemiology Incidence 2% to 5% of all unstable shoulders 50% of traumatic posterior dislocations seen in the emergency department are undiagnosed Risk factors bony abnormality glenoid retroversion or hypoplasia is a less common cause of instability ligamentous laxity Etiology Pathophysiology mechanism trauma (posterior dislocation) 50% of cases that present for evaluation usually dramatic presentation microtrauma (posterior instability) may lead to a labral tear, incomplete labral avulsion, or erosion of the posterior labrum may lead to gradual stretching of capsule and patulous posterior capsule common in lineman, weight lifters, overhead athletes usually insidious onset and presentation seizures and electric shock tetanic muscle contraction pulls the humeral head out anterior instability and dislocations are still more common with seizures however, posterior dislocations are unlikely to occur without significant trauma (ie. seizures) biomechanical forces flexed, adducted, and internally rotated arm is a high-risk position Associated conditions see table below Lesions Associated with Posterior Instability Avulsion of posterior band of IGHL Associated with acute subluxations Posterior Bankart lesions Characterized by detachment of posterior inferior capsulolabral complex Reverse Hill-Sachs lesions Associated with locked and difficult to reduce dislocations Posterior labral cyst Associated with chronic reverse Bankart lesion Posterior glenoid rim fracture Associated with chronic reverse Bankart lesion Lesser tuberosity fracture Associated with acute posterior dislocation Large capsular pouch Can see with MRI with contrast, often with chronic posterior instabiltiy Anatomy Glenohumeral anatomy Primary stabilizers of the posterior shoulder posterior band of IGHL primary restraint in internal rotation subscapularis primary dynamic restraint in external rotation primary dynamic restraint against posterior subluxation superior glenohumeral ligament and coracohumeral ligament primary restraint to inferior translation of the adducted arm and to external rotation primary static stabilizer to posterior subluxation with shoulder in flexion, adduction, and internal rotation Static restraint labrum deepens the glenoid by 50% Classification Acute versus chronic Acute = trauma, seizure, electric shock with dramatic presentation Chronic = microtrauma from repetition such as offense football lineman with insidious onset and presentation Voluntary versus involuntary Presentation History trauma or microtrauma with the arm in a flexed, adducted, and internally rotated position chronic instability often presents with insidious onset, and vague symptoms (usually pain and not instability as opposed to anterior instability) often in sporting or occupational activities that require repetitive pushing with the arm in forward flexed position foot ball lineman, weight lifters, etc Symptoms pain with flexion, adduction, and internal rotation of the arm Physical exam inspection prominent posterior shoulder and coracoid for acute posterior dislocation may be normal from chronic posterior instability from microtrauma motion limited external rotation for acute posterior dislocation shoulder locked in an internally rotated position common in undiagnosed posterior dislocations pain on flexion, adduction and internal rotation for posterior instability provocative tests - performed in the setting of chronic posterior instability Jerk test place arm in 90° abduction, internal rotation, elbow bent apply an axial force along axis of humerus and adduct the arm to a forward-flexed position a ‘clunk’ is positive for posterior subluxation 97% sensitive for posterior labral tear when combined with a Kim test Kim test performed by having the patient seated, arm at 90° abduction, followed by flexing the shoulder to 45 forward flexion while simultaneously applying axial load on the elbow & posterior-inferior force on the upper humerus. test is positive when pain is present posterior stress test stabilize scapula and look for posterior translation with a posterior direct force pain is elicited often, but this is not a specific finding posterior load & shift test place patient supine with arm in neutral rotation with 40 to 60° abduction and forward flexion, load humeral head and apply anterior and posterior translating forces noting subluxation Posterior Load & Shift Grading 1+ Apparent translation but not to rim 2+ Translation to glenolabral rim 3+ Translation over glenolabral rim 4+ Translation with complete dislocation Imaging Radiographs recommended views AP unreliable may show a 'lightbulb' sign axillary lateral best view to demonstrate a dislocation optional Velpeau view if patient is unable to abduct arm for axillary view CT indications analyze the extent and location of bone loss in a chronic dislocation (>2 to 3 weeks) MRI indications chronic posterior instability without history of acute posterior dislocation evaluate for suspected posterior labral tear, reverse Hill-Sach's lesion, or associated rotator cuff tear may show Kim lesion (concealed avulsion of the deep posteroinferior labrum, with apparently intact superficial labrum) Treatment Nonoperative acute reduction and immobilization in external rotation for 4 to 6 weeks indications should be initially attempted for all acute traumatic posterior dislocations most dislocations reduce spontaneously technique immobilize in 10-20 degrees of external rotation with elbow at side after 6 weeks advance to physical therapy (rotator cuff strengthening and periscapular stabilization) and activity modification (avoid activities that place arm in high-risk position) physical therapy may be a first line treatment for chronic posterior instability with rotator cuff strengthening, periscapular stabilizers may be considered for the in-season athlete Operative open or arthroscopic posterior labral repair (Bankart) indications recurrent posterior shoulder instability despite appropriate course of physical therapy continued pain with loading of arm in forward flexed position (bench press, football blocking) negative Beighton score outcomes 80% to 85% success at 5- to 7-year follow-up after open repair similar outcomes with arthroscopic repair after shorter follow-ups avoidance of excessive shoulder flexion, adduction, and internal rotation in the immediate post-operative period open or arthroscopic posterior capsular shift and rotator interval closure indications positive Beighton score posterior glenoid opening wedge osteotomy indications excessive congenital glenoid retroversion limited studies assessing outcomes with this approach open reduction with subscapularis transfer (McLaughlin) or lesser tuberosity transfer to the defect (Modified McLaughlin) indications chronic dislocation < 6 months old reverse Hill-Sachs defect < 40% hemiarthroplasty indications chronic dislocation > 6 months old severe humeral head arthritis collapse of humeral head during reduction reverse Hill-Sachs defect > 40% of articular surface total shoulder arthroplasty indications significant glenoid arthritis in addition to one of the hemiarthroplasty indications Techniques Open or arthroscopic posterior labral repair and capsular shift goal is to repair any labral detachment or capsular tears, and/or reduce the posterior capsule volume approach arthroscopic approach to shoulder high lateral portal may be better than standard portal for posterior labral work (ie drilling trajectory for suture anchors) lateral decubitus position may allow for improved visualization for arthroscopic stabilization posterior capsular shift may be performed in addition to labral repair, capsular shift may be less desirable in throwing athletes closure of rotator interval augments posterior capsular shift controversial thermal shrinkage of capsule (historical) contraindicated due to complications mechanism breaks collagen cross links critical temp (65 to 75° C) complications recurrence capsular necrosis axillary nerve injury postoperative care immobilizer with arm in neutral position (external rotation sling) or standard sling early range of motion and strengthening full heavy labor and contact sports after 6 month Open reduction with subscapularis with or without tuberosity transfer to defect approach deltopectoral approach technique to repair defect subscapularis transfer (McLaughlin procedure) subscapularis with lesser tuberosity transfer used by most (modified McLauglin procedure) iliac crest bone graft can be used for any glenoid bone loss disimpaction and bone grafting of the defect if < 3 weeks the surgeon can try disimpaction and bone grafting of the defect opening wedge glenoplasty may be indicated with congenital glenoid retroversion complications stiffness AVN osteoarthritis Complications Stiffness most common complication after labral repair Recurrence 2nd most common (7% to 50%) posterior glenoid bone loss as little as 11% of glenoid diameter is a risk factor for recurrence after isolated labral repair without bony augmentation Degenerative joint disease 3rd most common Adhesive capsulitis Overtightening of posterior capsule may lead to anterior subluxation or coracoid impingement Nerve injury axillary or suprascapular