These shoulder x-rays are from a 45 year old man presenting with left shoulder pain who has just recovered from a generalised tonic clonic seizure. What do you note and what 3 signs can you appreciate in the x-ray?

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The x-rays show posterior dislocation of the shoulder. The abnormality around the left ACJ is due to an old injury.

The 3 signs that can be seen are:

• Absent Half Moon Sign – This is due to lack of overlap of the humeral head with the glenoid. Normally, part of the glenoid fossa is overlapped by the medial part of the head of the humerus, giving rise to half moon appearance. This is absent in posterior shoulder dislocation.

• Light bulb sign – The head of the humerus appears like a light bub in the AP view due to an associated internal rotation of the humerus.

• Trough sign – This is caused by reverse Hill Sach’s Lesion. The reverse Hill Sach’s lesion is due to a compression fracture in the anteromedial humeral head sustained when the internally rotated head is driven against the posterior glenoid rim. The trough is the zone between the medial aspect of the humeral head and the margin of the impaction fracture.

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