Shoulder #5
Findings
The contour of the humeral head is abnormally rounded; the light bulb sign. The joint space is increased. The appearances suggest posterior dislocation and a further view should be obtained to confirm this. 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
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.
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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Used by permission of Dr A C Downie (a.downie@umds.ac.uk)
Guy's & St Thomas' Hospitals & UMDS
