Diagnosis

diagnosis

The condition often escapes diagnosis.

• delayed diagnosis is seen in 50-79% of patients

• delay may be caused by
  • subtle clinical examination findings e.g. - minimal palpable humeral displacement and patients often display enough motion in the joint
  • inadequate initial imaging
    • in an anteroposterior view, posterior dislocation may look normal
    • an orthogonal view will decrease the rate of delay in diagnosis

Specific history and physical examination findings will aid in identifying the condition

• maintain a high index of suspicion, based on mechanism of injury e.g. -
  • indirect trauma, with the shoulder in a position of flexion; adduction and internal rotation, with a coaxial force applied on the arm
  • extreme muscular contraction e.g. - seizure or electrocution
    
    

• patients will usually present with
  • the affected shoulder in internal rotation in the in adducted position and tend to cradle it with the other arm.
  • inability to externally rotate (active or passive)
    • difficulty in daily activities e.g. - combing the hair and washing the face
  • severe pain if acute
    
    

• physical examination finding are often subtle and may include
  • abnormal shoulder contour
  • prominent coracoid process anteriorly
  • palpable posterior positioned humeral head

• other shoulder injuries are often present along with posterior dislocation e.g. - fractures (34%), reverse Hill-Sachs injuries (29%), and rotator cuff tears (2-13%) (1,2)

Radiography

• two orthogonal views - anterioposterior plus axillary, velpeau, or scapular Y, are required to detect posterior dislocation (1)
  
  

• several radiographic signs have been described suggesting the diagnosis of posterior dislocation
  • light-bulb appearance
  • rim sign - there is more than 6 mm between the anterior glenoid rim and the humeral head
  • trough line - vertical line made by the impaction fracture of the humeral head

Note:

• in chronic cases, patients may be referred to the orthopaedic surgeon with the diagnosis of a frozen shoulder, or post-traumatic stiff shoulder (2)

• a delay in diagnosis will often result in an impression fracture on the anterior aspect of the humeral head (reverse Hill-Sachs lesion). Prolonged dislocation may cause the fracture to enlarge and propagate with damage to the articular cartilage. This will proceed to osteoarthritis and eventual avascular necrosis with subsequent shoulder arthroplasty (1)

Reference:

• (1) Jacobs RC, Meredyth NA, Michelson JD. Posterior shoulder dislocations. BMJ. 2015;350:h75.

• (2)Cicak N. Posterior dislocation of the shoulder. J Bone Joint Surg Br. 2004;86(3):324-32.