Dislocations of the Shoulder
Posterior Shoulder Dislocation

• Types of dislocations about the shoulder
  
  
  • Glenohumeral dislocation (the most common by far)
  • Acromioclavicular dislocation (12%)
  • Sternoclavicular dislocation (uncommon)
    
    
• Types of glenohumeral dislocations
  
  
  • Anterior or subcoracoid shoulder dislocation (96%)
    • Mechanism
      • External rotation and abduction
      • 40% recurrent
    • Age
      • Younger individuals
    • May be associated with:
      • Fracture of greater tuberosity (15%)
      • Bankart lesion
        • Fracture of anterior glenoid rim
      • Hill-Sachs defect (50%)
        • Impaction fracture of posterolateral surface of humeral head due to impaction of humeral head against anterior rim of glenoid during dislocation
          
          
  • Posterior shoulder dislocation (2-4%)
    
    
    • Causes
      • Traumatic
        • Convulsive disorders or electroshock therapy
      • Nontraumatic
        • Congenital or developmental
        • May be done voluntarily, especially in children
      • Usually due to axial loading of an adducted and internally rotated arm
    • In >50% unrecognized initially and subsequently misdiagnosed as frozen shoulder
      • May be difficult to see on AP radiograph
      • Typically, a scapular Y view or transthoracic lateral of the humeral head demonstrate a posterior dislocation better
    • Imaging signs of posterior dislocation
      • Rim sign (66%) = distance between medial border of humeral head and anterior glenoid rim >6 mm
      • Humeral head is fixed in internal rotation no matter how forearm is turned – “lightbulb sign”
      • May be associated with:
        • Trough sign (75%) = "reverse Hill-Sachs" = compression fracture of anteromedial humeral head
        • Fracture of posterior glenoid rim
        • Avulsion fracture of lesser tuberosity
          • Isolated  fractures of the lesser tuberosity should raise suspicion of an associated posterior dislocation

• Inferior shoulder dislocation (1-2%)
  
  
  • Luxatio erecta
    • Extremity held over head in fixed position with elbow flexed
  • Mechanism
    • Severe hyperabduction of arm resulting in impingement of humeral head against acromion
    • Humeral articular surface faces inferiorly
  • Complications
    • Rotator cuff tear
    • Fracture of acromion with or without inferior glenoid fossa and with or without fracture of the greater tuberosity
    • Neurovascular injury
      
      
• Superior shoulder dislocation (<1%)
  
  
  • Humeral head driven upward through rotator cuff
  • May be associated with fracture of humerus, clavicle or acromion

 
Posterior Dislocation of the Shoulder. Image on left demonstrates a "trough fracture" or "reverse Hill-Sachs fracture"of the antero-medial aspect of the humeral head (red arrow) as well as widening of the glenohumeral joint space (black arrow). The scapular Y view on the right shows that the humeral head (white arrow) no longer resides in the glenoid (G) but posterior to the acromion (A), The coracoid process is marked C.