Traumatic Conditions (Dislocations): Posterior Gleno-humeral Dislocation

Acute posterior dislocations (Fig 1) are relatively rare but are often missed as the shoulder can still look surprisingly normal. There is an association with epileptic seizures, and the obtunded state of these patients in the ED may contribute to the missing of these types of dislocation. They occur with forced internal rotation and adduction of the shoulder and characteristically the patient loses the ability to externally rotate.

The ‘light bulb sign’ (Fig 2) seen on an AP view of the shoulder (Fig 3) is characteristic. These injuries are seen best in the axillary view (Fig 4) (or modified axial), and failure to take this view will inevitably result in missed injuries, and consequent litigation.

Occasionally a ‘reverse Hill Sachs’ deformity, known as the trough sign, is present (Fig 5).

Learning bite

Posterior shoulder dislocations are associated with seizures and electrocution, and the incidence of bilateral posterior dislocations is higher than bilateral anterior dislocations, which are very rare, though they do occur following seizures. For this reason, a low threshold for x-ray investigation of both shoulders should be maintained in these clinical settings.


Fig 1.

Dislocation copy

Fig 2.


Fig 3.


Fig 4.


Fig 5.