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) login to view 2 more bullets microtrauma (posterior instability) login to view 4 more bullets seizures and electric shock login to view 3 more bullets 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 login to view 4 more bullets Kim test login to view 2 more bullets posterior stress test login to view 2 more bullets posterior load & shift test login to view 2 more bullets Imaging Radiographs recommended views AP login to view 2 more bullets axillary lateral login to view 1 more bullet 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 login to view 1 more bullet most dislocations reduce spontaneously technique login to view 2 more bullets 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 login to view 3 more bullets outcomes login to view 3 more bullets open or arthroscopic posterior capsular shift and rotator interval closure indications login to view 1 more bullet posterior glenoid opening wedge osteotomy indications login to view 2 more bullets open reduction with subscapularis transfer (McLaughlin) or lesser tuberosity transfer to the defect (Modified McLaughlin) indications login to view 2 more bullets hemiarthroplasty indications login to view 4 more bullets total shoulder arthroplasty indications login to view 1 more bullet 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 login to view 2 more bullets 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 login to view 2 more bullets 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 login to view 1 more bullet iliac crest bone graft login to view 1 more bullet disimpaction and bone grafting of the defect login to view 1 more bullet opening wedge glenoplasty login to view 1 more bullet 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 increased risk without appropriate control of an associated seizure disorder 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