Imaging Case of the Week 209 Answer

The x-rays show a posterior dislocation of the shoulder.

On the frontal view, the humeral head has a light bulb appearance due to the internal rotation of the shoulder. There is also abnormal overlapping of the humeral head on the glenoid leading to a loss of the normal half moon sign, as seen below.

A reverse Hill-Sachs lesion can also be seen on the AP view, which is due to an impaction fracture of the anteromedial humeral head against the posterior glenoid. This is also known as a McLaughlin lesion.

The Y view shows the humeral head posterior to the glenoid fossa.

Posterior shoulder dislocation

normal humeral head overlapY view shoulder

Posterior shoulder dislocations, approx. 20% of which are bilateral, are associated with epileptic seizures, electrocution, and high energy trauma. It is a frequently missed injury both clinically and radiographically.