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Case 76

Posterior dislocation of the shoulder

A painful shoulder after a fall

This patient complained of pain and restricted movement in the shoulder, having blacked out and fallen over. What abnormality is demonstrated? What other view would be useful? And if the patient was unable to abduct the arm, what other view can be performed?

Findings

Posterior dislocation is much less common than anterior dislocation (approximately 4% of dislocations), and is frequently much less obvious on the AP view alone, requiring a further view for confirmation. This may be either an axial view of the shoulder, or a tangential view of the scapula if the patient can not raise their arm sufficiently for the former.

Diagnosis

Posterior dislocation of the shoulder

Discussion

Signs of posterior dislocation on the AP view:

• Light bulb sign: The humeral head takes on a rounded, more symmetrical shape on the AP view, like a light bulb due to rotation of the humerus. The humerus is internally rotated, with the articular surface facing posteriorly.
• Rim sign: The distance between the glenoid and the articular surface of the humerus increases to over 6mm. This occurs because the humeral head impacts on the posterior rim of the glenoid which holds it away from the glenoid articular surface. This posterior rim is often fractured.
• Loss of parallelism: The articular surfaces of the glenoid and humerus should normally run parallel, but this relationship may be lost in dislocation.

Posterior dislocation is often a result of epileptic seizures, occuring while the limbs are in extreme internal rotation. Unexplained bilateral posterior dislocations with no history of trauma should suggest the onset of epilepsy.

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