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

Anterior Dislocation of the Humeral Head: Top image shows humeral head displaced from glenoid and lying inferior to the coracoid process (red arrow); the middle image demonstrates a defect along the posterolateral aspect of the head, which is the Hill-Sach's deformity (green arrow). The lower image is the scapular Y view (blue line outlines scapula). The head lies in a subcoracoid (i.e. anterior location). The white arrows point to the acromion.

• 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

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.

• 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

Luxatio Erecta (Inferior Dislocation). The humeral head (white arrow) lies below (inferior) to the
glenoid (black arrow) and the arm is fixed in abduction.

• Superior shoulder dislocation (<1%)
  
  
  • Humeral head driven upward through rotator cuff
  • May be associated with fracture of humerus, clavicle or acromion