An AP shoulder image is shown below. What is the arrowed crescent shaped structure?

Even if you have no idea what this is, you could probably have a guess as to whether it is bone, fat, water or air density. A fat density would be a fair guess but an air density would be even better. This is known as vacuum phenomenon and is caused by a mechanism akin to removing the lid from a soft drink bottle (fizzy drink, carbonated drink or pop drink depending on where you are in the world).

This patient presented to the Emergency Department after she lost her footing and fell whilst walking along the footpath/sidewalk/verge/roadside. As she fell, she tried to break her fall by grabbing onto a pole resulting in a traction force to her humerus. This in turn caused a reduction in the pressure in her gleno-humeral joint, releasing nitrogen out of solution. What you are seeing is a part of the articular cartilage of the gleno-humeral joint contrasted by gas.

Is this important for a Radiographer to know? Recognising vacuum phenomenon can be useful in cases where the referring doctor may not be familiar with this appearance and requests further views or comparison views. Further views to clarify this appearance are not required.


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